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Diaphragmatic Absorber Scrutiny Needed
Old 4th August 2020
  #1
Gear Head
 

Room Treatment & Diaphragmatic Absorber Scrutiny Needed

Hello

please read the whole post - I have tried to include all and as much information as possible to make helping easier. room pics and plans attached and REW mdat is attached

I am looking for some insights or confirmation that I am on the right path with treating my room. Its been a bit of a roller coaster ride, a lot of trial and error and a lot of frustration and learning over the last year (which is a good thing I guess)

I built an out-room on my property 12 months ago with the purpose of being a mixing/recording room in one and over the past 12 months have been trying to get it sounding right.

my room is:
6,700mm (29ft) L x 4,585mm (15ft) W x 3,675mm (12.05ft) H
(the ceiling is vaulted so this is from the floor to the pinnacle of the ceiling) the average height from floor to ceiling is 3.13m (10.27ft)

The walls are plastered brick cavity walls, the 3 windows have been closed with 42mm of dry walling and sealed closed and skimmed over. the ceiling is vaulted with 30mm drywalling with 2" 703 behind it and sealed up

room volume: 96.2 m3 / 3397 ft3 (using average ceiling height)
Room Area: 30.334.63 ft²

This is the Amroc result based on my room measurements:
https://amcoustics.com/tools/amroc?l...=313.1&r60=0.6

So essentially as I understand my main axial modes are:
25.6 Hz, G0#, 1-0-0, ax
37.4 Hz, D1, 0-1-0, ax
51.19 Hz, G1#, 2-0-0, ax
54.77 Hz, A1, 0-0-1, ax

74.81 Hz, D2, 0-2-0, ax
76.79 Hz, D2# 3-0-0, ax
102.39 Hz, G2# 4-0-0, ax

The mode I hear most prominently which resonates and causes the whole room to "swell" is G1, G#1, A1 - when I play these notes or a sustained bass note, kick drum is in or close to these keys it creates a "boom" in the room.

Which is a clear indication of my low end room mode problem I am experiencing. So I have begun trying to sort this problem out using Diaphragmatic Absorption.

As it stands I am using broad band absorption panels 703 fiberglass insulation panels (4" Thick 1m W x 1.8m L) along the walls in the RF zone and at 45 deg across all the 4 front and back corners and on the front and back walls. 1.8m x 0.6m panels hanging in the ceiling from the front wall to the listening position.

The room is based off of a live end dead end approach so the back side walls and ceiling don't have any absorption and the floor is tiled (I find the back end of the room has a bit of flutter echo and is a bit too live so I will probably need to add panels in the ceiling and a carpet on the floor. I have been told by the guy who has been working with me that the broad band absorption is within the ball park of sabines equation - but my ears tell me it probably needs a little more.

Using Rod's book, RA and the master hand book of acoustics and the acoustic modelling website I set out to try solve some of my issues;

Based on Rods book;

I have built 2 diagrammatic absorbers 1.4m W x 1.8m H x 140mm deep with 4" of 703 and a 12mm MDF face as the diaphragm (8kg /m2) all sealed air tight which gives me a peak absorption around 50hz

I think expecting more results from these 2 I thought they weren't working properly so I ended up building membrane traps:

Based on RA;

I have built 4 limp membrane traps using 1"/22mm MDF sealed frames, 59"/1500mm H x 25"/650mm W x 10"/250mm D with a 5kg Density Barrier Mat and using 165mm of 703 as the absorption - the acousticmodelling.com calculator centers this trap around 49-50Hz which is what I am after

This too didn't solve the issue and I have tried placing them in various positions, in the corners, on the side walls on the back and front walls but to me it almost sounds like its coming from the ceiling? and side corners - where the wall meets the ceiling - So using a SPL meter and 50 Hz tone I went up where the ceiling meets the wall and into the middle point of the vaulted ceiling and indeed this is where I get the highest readings on the meter so I am inclined to think that I need to now move these traps into the ceiling and wall corners, and back and front walls and ceiling-wall corners?

The only problem is the barrier mat is hellishly expensive (international shipping size and weight). But I think I need more to fill up the high pressure zones - I am not sure? So with cost in mind I am thinking about reverting back to building more and smaller hard-faced MDF diaphram absorbers.

for the side wall to ceiling corners - Based on this spec (600mm L x 600mm W x 290mm D) using 6mm MDF as the face (4.28 kg/m2) whice gives me around 50hz center freq

for the front and back corners: 1800mm H x 600mm x 290mm D with 6mm MDF Facing and 165 mm of 703

For the front walls using my initial and existing hard faced MDF traps mentioned above. ( 2pcs - 1.8m H x 1.4m W)

For the back walls using 3-4 pcs 1800mm H x 600mm W x 300mm D with 6mm MDF 165 mm of 703 and possibly for the side walls too.

and using the limp membrane traps I have built (8) which I think I will need to take up into the ceiling?

Now this is where my questions come in... am I on the right track, is going after the 50hz range limited and should i be going after other modes too, is it necessary to build all of these traps or am I wasting my time and money? Do my traps seem right in their design specs? is MDF inferior to the barrier mat? To be honest I feel like I have no clue what I am doing and just need some help/guidance/assurance I have spent a lot of money and have no idea if I am going in the right or wrong direction - I feel like there could be a structured and predictable solution for all of this but I dont know what that is
Attached Files
File Type: pdf Studio Pics.pdf (1.27 MB, 38 views) File Type: mdat Jul 25 16_31_37.mdat (5.65 MB, 21 views)
Old 4th August 2020
  #2
Lives for gear
 
avare's Avatar
 

What are walls and ceiling made of? Depending on the construction and price of materials in your part of the world (hint: thwre is a reason for location in your profile) it may be cheaper to increase the mass of the walls and ceiling.

Andre
Old 4th August 2020
  #3
Quote:
Originally Posted by Anna_logue View Post
Hello

I am looking for some insights or confirmation that I am on the right path with treating my room. Its been a bit of a roller coaster ride, a lot of trial and error and a lot of frustration and learning over the last year (which is a good thing I guess)

I built an out-room on my property 12 months ago with the purpose of being a mixing/recording room in one and over the past 12 months have been trying to get it sounding right.

my room is:
6,700mm (29ft) L x 4,585mm (15ft) W x 3,675mm (12.05ft) H
(the ceiling is vaulted so this is from the floor to the pinnacle of the ceiling) the average height from floor to ceiling is 3.13m (10.27ft)

The walls are plastered brick cavity walls, the 3 windows have been closed with 42mm of dry walling and sealed closed and skimmed over. the ceiling is vaulted with 30mm drywalling with 2" 703 behind it and sealed up

room volume: 96.2 m3 / 3397 ft3 (using average ceiling height)
Room Area: 30.334.63 ft²

This is the Amroc result based on my room measurements:
https://amcoustics.com/tools/amroc?l...=313.1&r60=0.6

So essentially as I understand my main axial modes are:
25.6 Hz, G0#, 1-0-0, ax
37.4 Hz, D1, 0-1-0, ax
51.19 Hz, G1#, 2-0-0, ax
54.77 Hz, A1, 0-0-1, ax

74.81 Hz, D2, 0-2-0, ax
76.79 Hz, D2# 3-0-0, ax
102.39 Hz, G2# 4-0-0, ax

The mode I hear most prominently which resonates and causes the whole room to "swell" is G1, G#1, A1 - when I play these notes or a sustained bass note, kick drum is in or close to these keys it creates a "boom" in the room.

Which is a clear indication of my low end room mode problem I am experiencing. So I have begun trying to sort this problem out using Diaphragmatic Absorption.

As it stands I am using broad band absorption panels 703 fiberglass insulation panels (4" Thick 1m W x 1.8m L) along the walls in the RF zone and at 45 deg across all the 4 front and back corners and on the front and back walls. 1.8m x 0.6m panels hanging in the ceiling from the front wall to the listening position.

The room is based off of a live end dead end approach so the back sides of the walls and ceiling don't have any absorption and the floor is tiled (I find the back end of the room has a bit of flutter echo and is a bit too live so I will probably need to add panels in the ceiling and a carpet on the floor. I have been told by the guy who has been working with me that the broad band absorption is within the ball park of sabines equation - but my ears tell me it probably needs a little more.

Using Rod's book, RA and the master hand book of acoustics and the acoustic modelling website;

Based on Rods book;

I have built 2 diagrammatic absorbers 1.4m W x 1.8m H x 140mm deep with 4" of 703 and a 12mm MDF face as the diaphragm (8kg /m2) all sealed air tight which gives me a peak absorption around 50hz

I think expecting more results from these 2 I thought they weren't working properly so I ended up building membrane traps:

Based on RA;

I have built 4 limp membrane traps using 1"/22mm MDF sealed frames, 59"/1500mm H x 25"/650mm W x 10"/250mm D with a 5kg Density Barrier Mat and using 165mm of 703 as the absorption - the acousticmodelling.com calculator centers this trap around 49-50Hz which is what I am after

This too didn't solve the issue and I have tried placing them in various positions, in the corners, on the side walls on the back and front walls but to me it almost sounds like its coming from the ceiling? and side corners - where the wall meets the ceiling - So using a SPL meter and 50 Hz tone I went up where the ceiling meets the wall and into the middle point of the vaulted ceiling and indeed this is where I get the highest readings on the meter so I am inclined to think that I need to now move these traps into the ceiling and wall corners, and back and front walls and ceiling-wall corners?

The only problem is the barrier mat is hellishly expensive (international shipping size and weight). But I think I need more to fill up the high pressure zones - I am not sure? So with cost in mind I am thinking about reverting back to building more and smaller hard-faced MDF diaphram absorbers.

for the side wall to ceiling corners - Based on this spec (600mm L x 600mm W x 290mm D) using 6mm MDF as the face (4.28 kg/m2) whice gives me around 50hz center freq

for the front and back corners: 1800mm H x 600mm x 290mm D with 6mm MDF Facing and 165 mm of 703

For the front walls using the existing hard faced MDF traps mentioned above. ( 2pcs - 1.8m H x 1.4m W)

For the back walls using 3-4 pcs 1800mm H x 600mm W x 300mm D with 6mm MDF 165 mm of 703 and possibly for the side walls too.

and using the limp membrane traps I have built (8) I will need to take up into the ceiling.

Now this is where my questions come in... am I on the right track, is going after the 50hz range limited and should i be going after other modes too, is it necessary to build all of these traps or am I wasting my time and money? Do my traps seem right in their design specs? is MDF inferior to the barrier mat? To be honest I feel like I have no clue what I am doing and just need some help/guidance/assurance I have spent a lot of money and have no idea if I am going in the right or wrong direction
Download REW , buy a usb mic and measure.

Take everything out of room( treatment, furniture, etc) when you measure confirm what the modes are first( place a subwoofer in one corner and measuring mic in opposite corner and measure). See if it complies with theoretical measurements based on the peaks in measurement.

Then setup mic at listening position and measure your left and right speakers separately to get an idea what is happening at the listening position currently.

From there plan a course of action to see if a better speaker/listening position can be found and afterwards what treatment plan is needed for the intended goal.
Old 4th August 2020
  #4
Gear Head
 

Quote:
Originally Posted by avare View Post
What are walls and ceiling made of? Depending on the construction and price of materials in your part of the world (hint: thwre is a reason for location in your profile) it may be cheaper to increase the mass of the walls and ceiling.

Andre
The walls are Brick Cavity Walls that have been cement plastered. The Ceiling is (2x15mm) 30mm Drywalling with 2" of 703 behind them (Sealed) and skimmed over. Location is Cape Town, South Africa. - pics and REW .mdat file is now attached to original post.
Old 4th August 2020
  #5
Gear Head
 

Quote:
Originally Posted by thethrillfactor View Post
Download REW , buy a usb mic and measure.

Take everything out of room( treatment, furniture, etc) when you measure confirm what the modes are first( place a subwoofer in one corner and measuring mic in opposite corner and measure). See if it complies with theoretical measurements based on the peaks in measurement.

Then setup mic at listening position and measure your left and right speakers separately to get an idea what is happening at the listening position currently.

From there plan a course of action to see if a better speaker/listening position can be found and afterwards what treatment plan is needed for the intended goal.
Thanks for the Reply - this is something that I initially did before adding any treatment and have done at each step in adding additional treatment. The modes showing up on the Water Falls, Spectrogram and SPL levels are the same modes I get with the room dimensions on Amroc, so this all works out -

But most importantly I can very clearly hear it and its very annoying - I can almost hear it coming from the where the wall meets the celing and in the ceiling at the apex (ceiling is vaulted) which led me to get up there with a ladder and a SPL meter - now Im thinking to put a bunch in the ceiling. But generally it sounds like the entire room "swells" up and rezonates at this frequency so much so it seems like my 6 traps sound like a drop in the ocean.

I am building units to treat these but I am not getting the results I am after and the low freq modes are still too hot and the decay times too long. Like I say - not sure if I have enough or they aren't placed correctly - at the moment I am in the mind that I need to build a lot more and they need to go into the ceiling -

just need some sort of confirmation on this or if I have this totally wrong or if someone with some knowledge would be able to give me clear structured guidance based on the information & details I have provided - perhaps they would easily be able to say "based on all the information, these are your problem areas this is what you need to do/try"

- pics and REW .mdat file is now attached to original post.
Old 4th August 2020
  #6
Quote:
Originally Posted by Anna_logue View Post
just need some sort of confirmation on this or if I have this totally wrong or if someone with some knowledge would be able to give me clear structured guidance based on the information I have provided
Post the REW MDAT so it can be analyzed further.

Also what are the wall materials(is it drywall, concrete, brick or combination), also distance of listening position from front wall, speaker woofer distance from front wall and side walls.

A boosted 2-0-0 mode is usually the result of sitting too close to the center of room. If you move closer to the front wall and its still strong then you must treat the entire backwall and front wall. The best treatment is membrane traps that can target multiple modes (25.6hz/51.19hz)

If the 0-0-1 mode is strong it usually results in a null at either 54.77 or 109hz (depending on how high you sit). Again the only way to treat is to treat the entire ceiling completely as you can't treat the floor. Again membrane traps is your best bet.
Old 4th August 2020
  #7
Gear Head
 

Quote:
Originally Posted by thethrillfactor View Post
Post the REW MDAT so it can be analyzed further.

Also what are the wall materials(is it drywall, concrete, brick or combination), also distance of listening position from front wall, speaker woofer distance from front wall and side walls.

A boosted 2-0-0 mode is usually the result of sitting too close to the center of room. If you move closer to the front wall and its still strong then you must treat the entire backwall and front wall. The best treatment is membrane traps that can target multiple modes (25.6hz/51.19hz)

If the 0-0-1 mode is strong it usually results in a null at either 54.77 or 109hz (depending on how high you sit). Again the only way to treat is to treat the entire ceiling completely as you can't treat the floor. Again membrane traps is your best bet.
The walls are cement plastered brick (cavity wall) I have attached to my orignal post - a pdf with all my room pics - this has all the plans and listening position laid out etc. The REW .mdat is also there now. hopefully this paints a much clearer picture
Old 4th August 2020
  #8
Quote:
Originally Posted by Anna_logue View Post
The walls are cement plastered brick (cavity wall) I have attached to my orignal post - a pdf with all my room pics - this has all the plans and listening position laid out etc. The REW .mdat is also there now. hopefully this paints a much clearer picture
Yeah cement plastered brick is going to keep the lows from escaping, so the problems with decays and peaks are surely happening.

Membrane traps are the best recommendation for these kind of surfaces and you will have to treat not only the backwall, but front wall as well. To get a membrane trap to treat 25.6hz it will have to be deep and fixed/sealed to the wall.

I would build membrane traps to treat the enitre top half and bottom half of wall tuned to the 25.6hz and then the middle section of wall build a composite membrane trap that not only attacks 51.19hz but can absorb frequencies up to 500hz.

Just looked MDAT.

Can you measure left and right separately? Also your measurement measures high distortion levels. Check your gain staging. Limit the sweep from 20hz-20,000hz and no timing reference.

I would push your listening position forward as well as monitors.

It seems your measured 1-0-0 mode(24.3hz) is lower than your theoretical one(25.6hz).

36.39hz (0-1-0) decay is a problem so you have to treat the side walls as well.

The null at 54.6 hz could be from height and 80.3-81.7hz looks to be SBIR.

The peak at 50.7hz( 0-2-0) and 74.2hz is the 0-2-0 mode which means you need treatment on the back, front and side walls.

The nulls at 117hz, 177hz, 320hz are all SBIR.

Last edited by thethrillfactor; 4th August 2020 at 09:39 PM..
Old 5th August 2020
  #9
Gear Head
 

Thank you for the reply this is definitely the type of insight and direction I need, so I really appreciate you taking the time to look at this.

Quote:
Originally Posted by thethrillfactor View Post
Membrane traps are the best recommendation for these kind of surfaces and you will have to treat not only the backwall, but front wall as well. To get a membrane trap to treat 25.6hz it will have to be deep and fixed/sealed to the wall.
OKay so I'm not sure if you had noticed I have built 2 types of membrane traps - 1 using a 5kg/m2 Mineral Loaded Barrier mat, and the other type using an 12mm MDF facing for the diaphragm - these peak at around 50Hz

Getting a trap that will go down to 24/25 Hz while keeping the depth under 1m/3ft would require a much higher density face - now this is where the uncertainty for me creeps in - the Barrier mat is very expensive, I need to import it and comes in a maximum density of 10kg/m2 whereas the MDF is easily available here. Is there a problem with using a thick piece of MDF (18mm/0.7") - does it need to specifically be of a limp nature like the barrier matt or is the thick rigid MDF fine because of its high density (12.8kg/m2) this would allow the depth of the trap to be reasonable.

480mm/18.8" Deep: (f = 60 / (SQRT 12.8 * 0.48) = 24.2Hz
400mm/15.7" Deep: Acoustic Modelling: 220mm/8" Insulation + 7mm Air + 12.8kg/m2 Face (18mm/0.7" thick) = 24.5Hz

Quote:
Originally Posted by thethrillfactor View Post
I would build membrane traps to treat the enitre top half and bottom half of wall tuned to the 25.6hz and then the middle section of wall build a composite membrane trap that not only attacks 51.19hz but can absorb frequencies up to 500hz.
When you say the middle section of the wall - this is the middle of front and back wall - not the middle of the room where the 2-0-0 50hz pressure is? By middle do you mean vertical middle or horizontal middle?

A Composite bass trap - is this a membrane trap that has a porous absorption (703/Foam) on the front?

Quote:
Originally Posted by thethrillfactor View Post
Just looked MDAT.

Can you measure left and right separately? Also your measurement measures high distortion levels. Check your gain staging. Limit the sweep from 20hz-20,000hz and no timing reference.
OKay yes - I have been using someone who is helping me he has the beringer ECM-8000 mic - I only have a large diaphram recording mic. I have ordered the Dayton Audio EMM-6 and a SPL meter which will be arriving next week then I will be able to do this for you.


Quote:
Originally Posted by thethrillfactor View Post
I would push your listening position forward as well as monitors.
Okay I will try moving the speakers and listening position forward to see if this helps & with the nulls you mention below

Quote:
Originally Posted by thethrillfactor View Post
It seems your measured 1-0-0 mode(24.3hz) is lower than your theoretical one(25.6hz).
OKay the traps Should be good between 24-25hz - I am just not sure about the construction for this - mentioned above

Quote:
Originally Posted by thethrillfactor View Post
The null at 54.6 hz could be from height and 80.3-81.7hz looks to be SBIR.
Do you mean sitting/listening height - or the ceiling?

So my take away from this is that I need a lot more trapping targeting various modes in various area's of the room and to move my speaker and listening position forward - I just need to know if using high density/thick pieces of MDF will be okay and if my calculations are correct. it seems at some points using acoustic modeling the amount of 703 gets fairly thick
Old 6th August 2020
  #10
Quote:
Originally Posted by Anna_logue View Post
Thank you for the reply this is definitely the type of insight and direction I need, so I really appreciate you taking the time to look at this.



OKay so I'm not sure if you had noticed I have built 2 types of membrane traps - 1 using a 5kg/m2 Mineral Loaded Barrier mat, and the other type using an 12mm MDF facing for the diaphragm - these peak at around 50Hz

Getting a trap that will go down to 24/25 Hz while keeping the depth under 1m/3ft would require a much higher density face - now this is where the uncertainty for me creeps in - the Barrier mat is very expensive, I need to import it and comes in a maximum density of 10kg/m2 whereas the MDF is easily available here. Is there a problem with using a thick piece of MDF (18mm/0.7") - does it need to specifically be of a limp nature like the barrier matt or is the thick rigid MDF fine because of its high density (12.8kg/m2) this would allow the depth of the trap to be reasonable.

480mm/18.8" Deep: (f = 60 / (SQRT 12.8 * 0.48) = 24.2Hz
400mm/15.7" Deep: Acoustic Modelling: 220mm/8" Insulation + 7mm Air + 12.8kg/m2 Face (18mm/0.7" thick) = 24.5Hz
If it were me i would treat all the surfaces except the floor.

If the mdf is too stiff or dense it won't vibrate and will relegate the trap useless.

Look at at different solution instead of mdf like a metal plate.


Quote:
Originally Posted by Anna_logue View Post
When you say the middle section of the wall - this is the middle of front and back wall - not the middle of the room where the 2-0-0 50hz pressure is? By middle do you mean vertical middle or horizontal middle?

For a room with walls composed of your materials, you have to do as much of the back, front and side walls as possible. Basically all the corners that meet between back/front side walls i would add membranes/helmholtz as well.

I would break up the back wall where you have 1 big trap 4 x 8 at the bottom that meets the wall and floor and one up top that meets wall and ceiling. Across the entire middle of wall(left to right) I would build traps that attack the 2-0-0 mode and go up to 250hz.

The side walls that meet the back and front walls i would have membranes there as well from floor to ceiling targeting the 0-1-0 mode.

Quote:
Originally Posted by Anna_logue View Post
A Composite bass trap - is this a membrane trap that has a porous absorption (703/Foam) on the front?

Or it can have a diffuser in front with a membrane in back.


Quote:
Originally Posted by Anna_logue View Post
OKay yes - I have been using someone who is helping me he has the beringer ECM-8000 mic - I only have a large diaphram recording mic. I have ordered the Dayton Audio EMM-6 and a SPL meter which will be arriving next week then I will be able to do this for you.




Okay I will try moving the speakers and listening position forward to see if this helps & with the nulls you mention below



OKay the traps Should be good between 24-25hz - I am just not sure about the construction for this - mentioned above



Do you mean sitting/listening height - or the ceiling?

So my take away from this is that I need a lot more trapping targeting various modes in various area's of the room and to move my speaker and listening position forward - I just need to know if using high density/thick pieces of MDF will be okay and if my calculations are correct. it seems at some points using acoustic modeling the amount of 703 gets fairly thick
If you will build membranes its crucial that your target frequencies are correct. You don't want to build a bunch of membrane traps and then i find out they don't work properly.


If not build them so the Q is wider even though it won't be as effective as narrow. But that means you will need a lot of them.
Old 6th August 2020
  #11
Here is a slatted helmholtz that is 24" deep with slats that are 48" and 1/64" apart.

It goes down to 26hz. If you fill it up with the right amount of insulation you can broaden the peak.
Attached Thumbnails
Diaphragmatic Absorber Scrutiny Needed-screen-shot-2020-08-05-9.57.47-pm.png   Diaphragmatic Absorber Scrutiny Needed-screen-shot-2020-08-05-10.09.40-pm.jpg  
Old 8th August 2020
  #12
Gear Head
 

Quote:
Originally Posted by thethrillfactor View Post
If it were me i would treat all the surfaces except the floor.

For a room with walls composed of your materials, you have to do as much of the back, front and side walls as possible. Basically all the corners that meet between back/front side walls i would add membranes/helmholtz as well.


If the mdf is too stiff or dense it won't vibrate and will relegate the trap useless.

Look at at different solution instead of mdf like a metal plate.
I have abandoned the idea of using an MDF panel absorber as the thickness wont work as you have pointed out, and it seems to be quite unpredictable. Although the adjusted formula f=192/SRT(m*d) is adjusted for rigid panel/MDF and porous absorption (but presumably at small thicknesses which it can be activated)

I have come to the conclusion that I need trapping for various modes on the front, back, side walls corners and ceiling - so this is what I will work towards now.

I have attached a rudimentary drawing of my understanding of your placement and targeting suggestions - I am just unsure if the ceiling should trap the 54.8 Hz (0-0-1) mode over the entire ceiling or it should target the same mode found on the side walls in those high pressure zones 36hz (0-1-0) ,74hz (0-2-0), 50.7hz (2-0-0).

Quote:
Originally Posted by thethrillfactor View Post
If you will build membranes its crucial that your target frequencies are correct. You don't want to build a bunch of membrane traps and then i find out they don't work properly.

If not build them so the Q is wider even though it won't be as effective as narrow. But that means you will need a lot of them.
I know - I know this too well, I have built 4 x MLV limp membrane absorbers and 2 x Rigid Panel absorbers. I have another roll of 3mx1250 MLV here to build more. (makes 4 x 1500 x 600 absorbers)

the limp membranes I have some degree of confidence in them because Acoustic modeling calculator incorporates a limp membrane I think 6" of 703 has made them quite wide and probably less effective at the center 50Hz - not sure?

At the time I wasn't aware of the adjusted formula for rigid panel and using 12mm MDF as the face probably doesn't help much either - So I don't think this is working as it should

So I also want to get this right because have thrown a lot of money, blood sweat tears and energy into a lot of uncertainty and arguably a gamble, but out of sheer desperation to try get this right, resulting in a lot of disappointment and perhaps wasted time and money.

I feel like I have the "What" of what I need to do but not the "how"

I discovered the idea of VPR/CBA absorbers 2 days ago and I have been going down a rabbit hole gathering as much information as I can on these (Data Sheets, xls Calculators, Patents, Research documents by Fuchs etc.)

These seem like a viable option for low end absorption at smaller depths and seem to be broad band - I am still left puzzled by whether they go down to 20Hz effectively and if this would be effective for my 1-0-0 mode, how to mount them free floating and whether a 2.5mm plate is better than 1mm below 50Hz. and the CIB/basotec foam densities that should be used etc.

I have been using the calculators, and trying to get the plate modes as close to my room modes and lets say the lowest mode of the plate in the range of 24/25hz - how effective is it going to be down there and does it then make more sense to not use a VPR/CBA method, but build a deep sealed box using the conventional Rigid Panel Absorber model but using a steel plate to get down there? does steel need to be damped in this scenario too? - so much uncertainty

If you have any pointers on this it would be amazing - I have done my head in reading a fair amount of these in the past 2 days and I am going to attempt reading through the entire thread of G.E.'s at Trapping Traps

Helmholtz rezonators could be an option but I would like to avoid this if possible because of the large depths, narrow frequencies and they seem a bit complex. But I would do what ever I need to do to get where I need to be - Im just not sure on the how right now.
Attached Thumbnails
Diaphragmatic Absorber Scrutiny Needed-lowabsorberlayout.jpg  
Old 8th August 2020
  #13
If your current MDF membrane absorbers aren’t working because they missed the target frequency, could you turn them into helmholtz absorbers by drilling holes in the front of them and measuring the resonance after each hole until it they get where you want them to be?
Old 8th August 2020
  #14
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Quote:
Originally Posted by Grovestand View Post
If your current MDF membrane absorbers aren’t working because they missed the target frequency, could you turn them into helmholtz absorbers by drilling holes in the front of them and measuring the resonance after each hole until it they get where you want them to be?
Never thought about this - seems like a good idea.

I first learned about the rigid panel absorber from reading about it in Rod's book Home Recording Studio: Build It Like the Pros. Using the formula provided. I have since learned this needs to be adjusted for rigid panels and porous absorbtion - I also ended up using 12mm - the logic told me that the higher density would lower the frequency while keeping the depth reasonable (140mm) - but perhaps, and I am not 100% sure this may be too thick

Point being, that in the book it mentions using the "finger test" where you dial up a sine wave placing your finger on the panel and where it is most resonant is the center frequency - and too me the point of highest vibration is at 49/50Hz - so I dont know, this is with the trap placed in the 50hz high pressure - is there a better way to test these things - maybe REW but Im not sure on the method - from the listening position I dont see great reductions in the 50hz decay time but perhaps this is because of not having enough?
Old 8th August 2020
  #15
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Quote:
Originally Posted by Anna_logue View Post
Never thought about this - seems like a good idea.

I first learned about the rigid panel absorber from reading about it in Rod's book Home Recording Studio: Build It Like the Pros. Using the formula provided. I have since learned this needs to be adjusted for rigid panels and porous absorbtion - I also ended up using 12mm - the logic told me that the higher density would lower the frequency while keeping the depth reasonable (140mm) - but perhaps, and I am not 100% sure this may be too thick

Point being, that in the book it mentions using the "finger test" where you dial up a sine wave placing your finger on the panel and where it is most resonant is the center frequency - and too me the point of highest vibration is at 49/50Hz - so I dont know, this is with the trap placed in the 50hz high pressure - is there a better way to test these things - maybe REW but Im not sure on the method - from the listening position I dont see great reductions in the 50hz decay time but perhaps this is because of not having enough?
Building a Membrane Bass Trap
Old 8th August 2020
  #16
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Quote:
Originally Posted by Jason Foi View Post
Thanks - Yeah I have read this thread before - it has more to do with me not seeing significant results with my panel - perhaps my expectations are too high - perhaps the "finger" test doesnt work? Perhaps the Rigid Panel is too thick? My Dayton Audio Mic and SPL level should arrives this week so I will be able to so some more testing. I have been relying on someone else who has been helping me for taking room measurements so testing in my own time hasn't really been possible
Old 8th August 2020
  #17
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Quote:
Originally Posted by Anna_logue View Post
Thanks - Yeah I have read this thread before - it has more to do with me not seeing significant results with my panel - perhaps my expectations are too high - perhaps the "finger" test doesnt work? Perhaps the Rigid Panel is too thick? My Dayton Audio Mic and SPL level should arrives this week so I will be able to so some more testing. I have been relying on someone else who has been helping me for taking room measurements so testing in my own time hasn't really been possible
% of relevant surface covered is important. If your traps hit their Fc but you arent getting the results you want you just need more traps. I think i've read you want somethig like a minimum of 25% of the relevant surface area dedicaticated to each Fc. Jens Eklund works with pressure traps a lot in his designs and is a super nice guy. Perhaps reaching out to him would be beneficial.
Old 8th August 2020
  #18
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Quote:
Originally Posted by Jason Foi View Post
% of relevant surface covered is important. If your traps hit their Fc but you arent getting the results you want you just need more traps. I think i've read you want somethig like a minimum of 25% of the relevant surface area dedicaticated to each Fc. Jens Eklund works with pressure traps a lot in his designs and is a super nice guy. Perhaps reaching out to him would be beneficial.
Yeah I think this conclusion as been reached with according to thethrillfactor's suggestions above. OKay sounds interesting I will have a look into him
Old 8th August 2020
  #19
Quote:
Originally Posted by Anna_logue View Post
Never thought about this - seems like a good idea.
Oops. Forgot to give credit. Yes it is. Not my idea!
Old 8th August 2020
  #20
Quote:
Originally Posted by Anna_logue View Post
I have abandoned the idea of using an MDF panel absorber as the thickness wont work as you have pointed out, and it seems to be quite unpredictable. Although the adjusted formula f=192/SRT(m*d) is adjusted for rigid panel/MDF and porous absorption (but presumably at small thicknesses which it can be activated)

I have come to the conclusion that I need trapping for various modes on the front, back, side walls corners and ceiling - so this is what I will work towards now.

I have attached a rudimentary drawing of my understanding of your placement and targeting suggestions - I am just unsure if the ceiling should trap the 54.8 Hz (0-0-1) mode over the entire ceiling or it should target the same mode found on the side walls in those high pressure zones 36hz (0-1-0) ,74hz (0-2-0), 50.7hz (2-0-0).
The drawing is almost correct. On the corners i wouldn't straddle the trapping but have the Backwall and Side wall trapping meet at corner.

The sidewalls same as backwall but in middle portion you can build something like the BBC did here:
http://downloads.bbc.co.uk/rd/pubs/reports/1994-12.pdf


Quote:
Originally Posted by Anna_logue View Post
I know - I know this too well, I have built 4 x MLV limp membrane absorbers and 2 x Rigid Panel absorbers. I have another roll of 3mx1250 MLV here to build more. (makes 4 x 1500 x 600 absorbers)

the limp membranes I have some degree of confidence in them because Acoustic modeling calculator incorporates a limp membrane I think 6" of 703 has made them quite wide and probably less effective at the center 50Hz - not sure?

At the time I wasn't aware of the adjusted formula for rigid panel and using 12mm MDF as the face probably doesn't help much either - So I don't think this is working as it should

So I also want to get this right because have thrown a lot of money, blood sweat tears and energy into a lot of uncertainty and arguably a gamble, but out of sheer desperation to try get this right, resulting in a lot of disappointment and perhaps wasted time and money.
I've never been a fan of using limp membranes personally, I prefer hardboard front, slats(helmholtz) or using metal. I've had better luck with matching those up to the predicted formulas than limp membranes. Also the materials are easier to find(except when using metal).

Quote:
Originally Posted by Anna_logue View Post
I feel like I have the "What" of what I need to do but not the "how"

I discovered the idea of VPR/CBA absorbers 2 days ago and I have been going down a rabbit hole gathering as much information as I can on these (Data Sheets, xls Calculators, Patents, Research documents by Fuchs etc.)

These seem like a viable option for low end absorption at smaller depths and seem to be broad band - I am still left puzzled by whether they go down to 20Hz effectively and if this would be effective for my 1-0-0 mode, how to mount them free floating and whether a 2.5mm plate is better than 1mm below 50Hz. and the CIB/basotec foam densities that should be used etc.

I have been using the calculators, and trying to get the plate modes as close to my room modes and lets say the lowest mode of the plate in the range of 24/25hz - how effective is it going to be down there and does it then make more sense to not use a VPR/CBA method, but build a deep sealed box using the conventional Rigid Panel Absorber model but using a steel plate to get down there? does steel need to be damped in this scenario too? - so much uncertainty

If you have any pointers on this it would be amazing - I have done my head in reading a fair amount of these in the past 2 days and I am going to attempt reading through the entire thread of G.E.'s at Trapping Traps

Helmholtz rezonators could be an option but I would like to avoid this if possible because of the large depths, narrow frequencies and they seem a bit complex. But I would do what ever I need to do to get where I need to be - Im just not sure on the how right now.
Your problem right now is you don't want to give up the depth. We are talking about huge waveforms(in the case of 24hz you are talking 46ft).You have to understand the physical limitations as well in terms of your space. Technically if you used 3 18" subwoofers in your back corners and middle of back wall,set them up with 3 mics and flip them out of phase, with at tight non phase EQ, you could cancel out the 24hz decay. This is what the Bag EnD E trap did.
But you would still need to give up 18"-24" of depth for the setup.

On the Plate traps, they work best at their arrived size( 1 x 1.5m) according to Peter D'Antonio and up against where 2 or 3 boundaries meets like the corners. wall to ceiling, ceiling to floor. But the lowest Freq it can treat is around 31.5hz(0.61) and the peak is really from 63hz to 125hz:
https://www.rpgacoustic.com/document...-brochure.pdf/

Only issue is you need a lot of them. When they used them at Blackbird Studio C they needed 32 plate traps.

You could try hangers, but you need 36" of space.
There just no easy lunch when treating the sub low end.

If you are prepared to give up depth all around then you can seriously attack it.

My suggestion is study what the late Boggy Petrovic was able to do in his My Room designs and principles. But to implement you still need to give up some depth(at least 24 inches).
Old 9th August 2020
  #21
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Quote:
Originally Posted by thethrillfactor View Post
The drawing is almost correct. On the corners i wouldn't straddle the trapping but have the Backwall and Side wall trapping meet at corner.

The sidewalls same as backwall but in middle portion you can build something like the BBC did here:
http://downloads.bbc.co.uk/rd/pubs/reports/1994-12.pdf
This BBC unit is great! - thank you for sharing, I have seen their others before but not this particular one. Its a similar concept to a VPR using more readily available and cheaper material - I would need to use 703 we dont have a 32 density here.

I'm getting confused about what trapping should go where so I have done 2 more drawings to get a better understanding. I am guessing its the second image because you mentioned the trapping from the side walls should meet where the walls meet, but earlier you mentioned that the 25hz 1-0-0 mode should be trapped in the corners? My understanding is also that the back wall mirrors the front wall - or have I missed something here. I think once I have established where the trapping should be and which modes, we can go into detail about the actual device needed for those modes.

Quote:
Originally Posted by thethrillfactor View Post
Your problem right now is you don't want to give up the depth. We are talking about huge waveforms(in the case of 24hz you are talking 46ft).You have to understand the physical limitations as well in terms of your space. Technically if you used 3 18" subwoofers in your back corners and middle of back wall,set them up with 3 mics and flip them out of phase, with at tight non phase EQ, you could cancel out the 24hz decay. This is what the Bag EnD E trap did.
But you would still need to give up 18"-24" of depth for the setup.

On the Plate traps, they work best at their arrived size( 1 x 1.5m) according to Peter D'Antonio and up against where 2 or 3 boundaries meets like the corners. wall to ceiling, ceiling to floor. But the lowest Freq it can treat is around 31.5hz(0.61) and the peak is really from 63hz to 125hz:
https://www.rpgacoustic.com/document...-brochure.pdf/

Only issue is you need a lot of them. When they used them at Blackbird Studio C they needed 32 plate traps.

If you are prepared to give up depth all around then you can seriously attack it.
You are right, screw the real estate issue - I want to do this right so I will do what needs to be done even if it means taking up space. I was hoping that VPR's might be able to do the trick, (please excuse my ignorance - I am learning a lot here and there arent many people I am able to consult with in my country when it comes to modal problems).
Attached Thumbnails
Diaphragmatic Absorber Scrutiny Needed-lowabsorberlayout2.jpg   Diaphragmatic Absorber Scrutiny Needed-lowabsorberlayout3.jpg  
Old 11th August 2020
  #22
Quote:
Originally Posted by Anna_logue View Post
This BBC unit is great! - thank you for sharing, I have seen their others before but not this particular one. Its a similar concept to a VPR using more readily available and cheaper material - I would need to use 703 we dont have a 32 density here.

I'm getting confused about what trapping should go where so I have done 2 more drawings to get a better understanding. I am guessing its the second image because you mentioned the trapping from the side walls should meet where the walls meet, but earlier you mentioned that the 25hz 1-0-0 mode should be trapped in the corners? My understanding is also that the back wall mirrors the front wall - or have I missed something here. I think once I have established where the trapping should be and which modes, we can go into detail about the actual device needed for those modes.



You are right, screw the real estate issue - I want to do this right so I will do what needs to be done even if it means taking up space. I was hoping that VPR's might be able to do the trick, (please excuse my ignorance - I am learning a lot here and there arent many people I am able to consult with in my country when it comes to modal problems).

If you are willing to give up the depth then a whole world opens up in terms of the possibilities. If i had 3 ft of space to work with i personally would do a composite layered trap. This way you can hit the very low end(Helmholtz), low mids(insulation wall) and if needed upper mids( diffuser).
Old 11th August 2020
  #23
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Quote:
Originally Posted by thethrillfactor View Post
If you are willing to give up the depth then a whole world opens up in terms of the possibilities. If i had 3 ft of space to work with i personally would do a composite layered trap. This way you can hit the very low end(Helmholtz), low mids(insulation wall) and if needed upper mids( diffuser).
Yes I am willing to give up what ever I need to, to get where I need to be, 3ft if that's what is required. I'm committed, dedicated and serious about this - I built the room for this sole purpose and its a dream in progress.

I just need guidance into the specifics of how to do this the right way and I will do it - I apologize for my n00bness. I can follow detailed directions from a experienced professional and I am even willing to pay for a detailed consultation if need be. I am hoping to get this so I can iron out uncertainty and misunderstandings with building these devices, I dont have much experience building these composite layered traps and as you mentioned I don't want to do all the work wrong. I have some experience in constructing/sealing my previous membrane and panel traps and I get the basic idea's and principles.

My Dayton Audio EMM-6 mic and SPL meter arrived today so I have attached the mdat files to this post. Left, Right seperately 20Hz - 20KHz no timing reference. I went through and did the level calibration and tried to get the gain staging right - I dont think this had been done properly in the past with the other guy who has been helping me.

Interesting to see a bit of the 49/50hz 2-0-0 mode reduction - this is presumably because of the limp traps in the corners and on the sides - the 54Hz 0-0-1 mode is still very long - which explains why I keep hear it coming from the ceiling when those sustained notes are played! I can relate this a lot more now to what I am seeing on REW and Amroc

EDIT: deleted Mdats Because they were taken without soundcard cal done correctly see below for new ones.
Old 12th August 2020
  #24
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Quote:
Originally Posted by thethrillfactor View Post
If you are willing to give up the depth then a whole world opens up in terms of the possibilities. If i had 3 ft of space to work with i personally would do a composite layered trap. This way you can hit the very low end(Helmholtz), low mids(insulation wall) and if needed upper mids( diffuser).
Putting what you have described above into AFMG sound Flow and Acoustic Modeling these are the results I get...I had to play around with the parameters to arrive at something that reaches 24 Hz while still retaining broader band absorption, I'm not sure if these parameters are so out of whack that they will behave like this in reality? perhaps you can give me some pointers.

Acoustic Moddelings results based on this come up with:
"Dotted lines indicate that the model used to estimate absorber properties has been used outside of its applicable range. Results may therefore be less accurate than where a solid line is displayed."

Parameters:
Total Depth: 918mm
Model: Miki
Dimension 1.2m W x 2.4m H
Layer 1: Rigid Backing.
Layer 2: Absorber: 150mm, 48 kg/m3, 16000 Pa.s/m2.
Layer 3: 700mm Air
Layer 4: Perforated Panel: 18mm MDF - hole diameter (20mm, hole spacing 20mm.(78.54%) / (Slit 20mm & Slot Spacing 20mm (100%) - the perforated (pink) and slat (red) are similar - slat seems to go down a bit further.
Layer 5: Absorber: 50mm, 48 kg/m3, 16000 Pa.s/m2.
Attached Thumbnails
Diaphragmatic Absorber Scrutiny Needed-afmg-soundflow-layered-trap.jpg   Diaphragmatic Absorber Scrutiny Needed-acoustic-modelling-layered-trap.png  
Old 12th August 2020
  #25
Quote:
Originally Posted by Anna_logue View Post
Putting what you have described above into AFMG sound Flow and Acoustic Modeling these are the results I get...I had to play around with the parameters to arrive at something that reaches 24 Hz while still retaining broader band absorption, I'm not sure if these parameters are so out of whack that they will behave like this in reality? perhaps you can give me some pointers.

Acoustic Moddelings results based on this come up with:
"Dotted lines indicate that the model used to estimate absorber properties has been used outside of its applicable range. Results may therefore be less accurate than where a solid line is displayed."

Parameters:
Total Depth: 918mm
Model: Miki
Dimension 1.2m W x 2.4m H
Layer 1: Rigid Backing.
Layer 2: Absorber: 50mm, 48 kg/m3, 16000 Pa.s/m2.
Layer 3: 450mm Air
Layer 4: Perforated Panel: 18mm MDF - hole diameter (20mm, hole spacing 20mm.(78.54%) / (Slit 200mm & Slot Spacing 20mm (100%) - the perforated (pink) and slat (red) are similar - slat seems to go down a bit further.
Layer 5: Absorber: 50mm, 48 kg/m3, 16000 Pa.s/m2.

I've attached a model of a simple 24" Slotted Helmholtz panel and 12" Fluffy/John Mainsville R19 with a 1" gap. The transition between the Slotted panel and Insulation is the challenge. What can be done to either broaden the peak of the slotted panel, help the fluffy extend further down, or use something else at the front that works further from the boundary, but could with another restrained layer bump down an octave?

In terms of your model/design a few questions?
Why would you put something like Rockwool semi rigid(layer2) on the wall when it works best away from the boundary? Also why use MDF again? Its expensive for this purpose and you won't ever see it? You can use other cheaper materials especially if you are doing the entire backwall.

Lastly will your panel work? In theory its possible, but until its built in place its hard to tell to be honest. I know its a cop out answer, but just because it works on paper it may not work as well on location. Some of these composite traps only work by experimentation.

Have you remeasured space with nothing in it, except with a sub in one corner and mic at the the other( measured at different heights) to verify the modes?
Attached Thumbnails
Diaphragmatic Absorber Scrutiny Needed-screen-shot-2020-08-12-5.49.30-pm.jpg  
Old 13th August 2020
  #26
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akebrake's Avatar
 

Wrong file!

Quote:
Originally Posted by Anna_logue View Post
...My Dayton Audio EMM-6 mic and SPL meter arrived today so I have attached the mdat files to this post. Left, Right seperately 20Hz - 20KHz no timing reference. I went through and did the level calibration and tried to get the gain staging right - I dont think this had been done properly in the past with the other guy who has been helping me...
Hopefully your Soundcard don’t measure like this...
It should look like a straight line. E.g your latest mdats are in error.

(BTW It's a very common error loading a measurement file in stead of a loopback (Soundcard) measurement for calibration.
Attached Thumbnails
Diaphragmatic Absorber Scrutiny Needed-soundcard-cal-err.jpg   Diaphragmatic Absorber Scrutiny Needed-soundcard-cal-.jpg  
Old 13th August 2020
  #27
Gear Head
 

Quote:
Originally Posted by akebrake View Post
Hopefully your Soundcard don’t measure like this...
It should look like a straight line. E.g your latest mdats are in error.

(BTW It's a very common error loading a measurement file in stead of a loopback (Soundcard) measurement for calibration.
Ah right, thanks for picking this up - I am fairly new to taking measurements - I thought I had done it correctly but obviously not - thank you for helping, I will redo this and post the results
Old 13th August 2020
  #28
It’s a deep dive, kind of addictive.
Old 16th August 2020
  #29
Gear Head
 

Quote:
Originally Posted by thethrillfactor View Post
I've attached a model of a simple 24" Slotted Helmholtz panel and 12" Fluffy/John Mainsville R19 with a 1" gap. The transition between the Slotted panel and Insulation is the challenge. What can be done to either broaden the peak of the slotted panel, help the fluffy extend further down, or use something else at the front that works further from the boundary, but could with another restrained layer bump down an octave?
OKay thanks - good to know you can use the fluffy insulation which is generally cheaper - we only have one manufacturer that I know of here isover/saint gobain - they do a product called Aerolite - the data sheets show a 11kg/m2 density but there is no information on the gas flow resistivity specification. I have called them and I am trying to find this out to better model this. I guess it will be between 3-5kPa*sm/2

Isover Aerolite

Quote:
Originally Posted by thethrillfactor View Post
In terms of your model/design a few questions?
Why would you put something like Rockwool semi rigid(layer2) on the wall when it works best away from the boundary? Also why use MDF again? Its expensive for this purpose and you won't ever see it? You can use other cheaper materials especially if you are doing the entire backwall.
I did try the different ordering of the insulation and SoundFlow was giving me a better result with the insulation against the rigid backing - but what you are saying makes sense and the models show it too. The MDF was for the perforated panel but the slats seem to give a better result so wood will be better there. I think your approach makes more sense now.

I noticed the slot spacing is 1220 mm wide - I dont think we can get wood slats this wide - my guess is you can put them together with no gap and seal the join?

Quote:
Originally Posted by thethrillfactor View Post
Lastly will your panel work? In theory its possible, but until its built in place its hard to tell to be honest. I know its a cop out answer, but just because it works on paper it may not work as well on location. Some of these composite traps only work by experimentation.
Iagree, I think my understanding was a bit off.

Quote:
Originally Posted by thethrillfactor View Post
Have you remeasured space with nothing in it, except with a sub in one corner and mic at the the other( measured at different heights) to verify the modes?
I haven't, I want to do this. I don't have a sub only 6" near field monitors. I will need to find the time to remove all the panels and traps out I am not sure I will be able to get the rigid panel traps out of the door - I will try though.

Quote:
Originally Posted by akebrake View Post
Hopefully your Soundcard don’t measure like this...
It should look like a straight line. E.g your latest mdats are in error.

(BTW It's a very common error loading a measurement file in stead of a loopback (Soundcard) measurement for calibration.
I deleted the previous mdats the new ones are attahced to this post - checked the sound card calibration and it is flat now - thanks for the heads up!

I measured the gap between the door and the back walll which gives me 940mm to play with - cant be more than this with out stopping the door opening fully. front wall can be more, but I think 940mm is enough given the modelling.

What about the idea of completely filling up the back and front wall with 940mm of fluffy insulation? Seems like it might be easier? Although AFMG SoundFlow and Acoustic modelling have very different things to say about this. Not sure which to trust.

Blue Line is 3348 kpa and Other Colour is 5000kpa
Attached Thumbnails
Diaphragmatic Absorber Scrutiny Needed-940mmfluffy_sf.jpg   Diaphragmatic Absorber Scrutiny Needed-940mmfluffy_am.png  
Attached Files
File Type: mdat Aug 16 17_30_22.mdat (4.90 MB, 7 views) File Type: mdat Aug 16 20_47_34.mdat (4.90 MB, 5 views)
Old 16th August 2020
  #30
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Kyle P. Gushue's Avatar
Enjoying this thread.

Have you checked this thread for the GFR of the isover?

Common Gas Flow Resistivity numbers.

If you have no sub, you can use your speaker in the corner to excite as much modal activity as you can.

Cheers!
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