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#61
13th July 2012
Old 13th July 2012
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Quote:
Originally Posted by MTB Vince View Post
Wow. I'd love to see before and after bass decay waterfall graphs for that!

Happy Trails!
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#62
13th July 2012
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Great Stuff Tim, Thank you very much for the time and effort - I've seen these on sayers forum though never went for them.......your pdf is soooooooo damn simple!
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#63
13th July 2012
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NLP......That looks as though you've gone from a wacky 70Hz Hump right into a peak of around 30.. sure their not combs?
TLB.
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13th July 2012
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Quote:
Originally Posted by NLP View Post
"... measurement microphone is not in totally exact position, few cm shifted..."
and of course membranes have limits.
Membranes are only for bass traping.
Yes, you are correct about their limits according to your measurements.

What specific freq have you tuned those membranes according to your calculation?

Four membranes at that size in four corners isin't a great result after all and I woulden't be satisfied because there is still a HUGE null at 100Hz despite your installed treatments.
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13th July 2012
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Quote:
Originally Posted by Mctwins View Post
Yes, you are correct about their limits according to your measurements.

What specific freq have you tuned those membranes according to your calculation?

Four membranes at that size in four corners isin't a great result after all and I woulden't be satisfied because there is still a HUGE null at 100Hz despite your installed treatments.
Mic position and speaker position change frequency and intensity of nulls. Mic and speaker position do not change the frequency and length of modal decay. Even with the microphone being moved, the FR might not be exact but the decay will be close to the same almost anywhere in the room. The graph sure shows an immense trapping of bass, beyond almost anything I've seen at 40hz, 55hz, and 70hz and you say it isn't a great result because there's a null spot in his tests? You mean to tell me that 4 traps that barely take up any floor space, reducing modal ringing from what looks to be easily over 2 seconds to ~.3 seconds isn't a great result? What exactly is a 'good' result then, from panels that only take up a combined floor space of two thirds of a square meter? You are belligerent.
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13th July 2012
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Yeap..dad walked in!
#67
13th July 2012
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Great

Thanks boggy, you might want to include a link to that wavelet, which I highly recommend. Particulary when it is imported into a DAW.

Thank you NLP that is a great result. Exactly what I hoped to see.

DD
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13th July 2012
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Quote:
Originally Posted by kasmira View Post
Mic position and speaker position change frequency and intensity of nulls. Mic and speaker position do not change the frequency and length of modal decay. Even with the microphone being moved, the FR might not be exact but the decay will be close to the same almost anywhere in the room. The graph sure shows an immense trapping of bass, beyond almost anything I've seen at 40hz, 55hz, and 70hz and you say it isn't a great result because there's a null spot in his tests? You mean to tell me that 4 traps that barely take up any floor space, reducing modal ringing from what looks to be easily over 2 seconds to ~.3 seconds isn't a great result? What exactly is a 'good' result then, from panels that only take up a combined floor space of two thirds of a square meter? You are belligerent.
And you are saying????

He has still ringing below 100Hz, around 700ms.
Can you answer me this.....
What specific freq has he(NLP) tuned those membranes according to the calculation?
#69
13th July 2012
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Mum.............Dads here
#70
13th July 2012
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Quote:
Originally Posted by DanDan View Post
Thanks boggy,
You're welcome!
Quote:
Originally Posted by DanDan View Post
you might want to include a link to that wavelet, which I highly recommend. Particulary when it is imported into a DAW.
Ok, here is link to thread where we discussed it at Pro Recording Workshop
Wavelet type test tone designed for personal listening acoustical analysis
(links for test files are included too)
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--"We can never see past the choices we don't understand." (Oracle, The Matrix Reloaded)

Better control room needs:
-a much smaller desk
-speaker stands
-best possible position for loudspeakers and yourself
-a broader and thicker cloud
-a broader and thicker wall panels
-more super chunks in all corners
-a binary diffuser slats over proposed treatment
-to not expect "sensational" response
-good luck!
NLP
#71
13th July 2012
Old 13th July 2012
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I just want constructively to share my working experiences.
This 4 membranes (very similar dimensions) goes down to lowest modal frequency of this room (cca. 34Hz), surfaces are solid (concrete), so resonances can be calculated fairly accurate, there are not any help from resonating drywall etc.
In another bigger room (max dimension is 10m), also concrete, they work till 25Hz (monitoring sistem goes very deep) and then efficiency decreasing.
I am not a fan of any resonating acoustic elements (helmholtz, hard membranes...), but limp mass absorbers are somehow smaller "necessary evil"
... in little more forgiving (with better proportions or just bigger room) spaces I use rather complex porous sandwich/multilayer absorbers with layers of different densities, gas flow resistivities, depths... with excellent results.

Over the membranes we put hangers, porous absorption... so issues with 80 or 100Hz are not relevant (and are relative, like kasmira wrote). I think that those absorbers are giving excellent results, there is still enough space for other kind of treatments.

Thanks Dan and others.
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#72
13th July 2012
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Quote:
Originally Posted by boggy View Post
...Limp mass resonant absorbers CAN be pretty small...
I suppose too that 1200x600 is still small relatively to wavelength of room modes in studios which is designed by Andy Munro (not-that-small-rooms)

But 2600x1000 is BIG... big enough to mess up absorber's working range.
Thanks again Boggy.
I still can't seem to appreciate why a larger unit is detrimental. If it's a matter of resonance, why can't the unit incorporate more absorbent material? Besides, being limp does in itself have the quality of low resonance.

And just to think out loud- Still, how can a passive absorber ADD energy into a space? Perhaps at a different frequency, but it certainly couldn't preserve the amplitude of the initial signal. One can easily see an object "set in motion" by a sound wave so I can see how the decay might be influenced. However, were the unit to be set in motion to begin by a given frequency, it would seem the absorption coefficient would be great relative to the time which the object comes to rest.
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#73
13th July 2012
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Quote:
Originally Posted by John White View Post
Thanks again Boggy.
You're welcome!
Quote:
Originally Posted by John White View Post
I still can't seem to appreciate why a larger unit is detrimental. If it's a matter of resonance, why can't the unit incorporate more absorbent material? Besides, being limp does in itself have the quality of low resonance.
Resonances I talk about are purely determined by dimensions of membrane, not by membrane material (density, Young modulus etc.)... something like "membrane modes". Limp mass absorber works as mass-spring-damper system, more like piston motion than membrane modal vibrations. So I recognize that interfering two absorption mechanisms may degrade performances (of one).
Quote:
Originally Posted by John White View Post
And just to think out loud- Still, how can a passive absorber ADD energy into a space?
He cannot add energy, of course, but he can reflect it, usually modified. Place acoustic guitar in control room and use my burst tone tests (listed above)... and you will hear how guitar resonator can extend some (specific) frequencies, when they are near to string resonances. So returned energy may be smaller, but if decay is longer (e.g. undamped Helmholtz absorbers), that's not good for transients reproduction in control room.

Also, this is a very good reason why control room is no good place for keeping string instruments.
Quote:
Originally Posted by John White View Post
Perhaps at a different frequency, but it certainly couldn't preserve the amplitude of the initial signal. One can easily see an object "set in motion" by a sound wave so I can see how the decay might be influenced. However, were the unit to be set in motion to begin by a given frequency, it would seem the absorption coefficient would be great relative to the time which the object comes to rest.
Hm, there are a very good reasons why I always advise people to use porous absorbers, which make impedance which is predominantly resistive... so there are no possibilities where absorber may "return" energy back to room (reactive part of impedance), for only limited range of frequencies (not like ordinary wall) which is usual characteristic of badly designed/built resonant type of absorbers (and they make things worse)

#74
13th July 2012
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Mum..................Man the with a beard.......am i allowed to talk to him?
Boogy is his name!
#75
13th July 2012
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Quote:
Originally Posted by Mctwins View Post
And you are saying????

He has still ringing below 100Hz, around 700ms.
See your PMs, I will show you why these are so effective compared to other traps that take the same floor area. He does still have ringing at 70 and 80hz ish, but it is knocked down from being over 2 seconds. Still insanely impressive. Concrete walls make for really bad decay times in the lows, there aren't many treatments that address this much decay.

The nulls are also there in the before test, but with him moving the microphone a little bit it changed the frequency (barely) and changed the intensity of the null. His absorbers did not create it, and even if they did, at least we would know where the null is coming from and fix the problem accordingly. What his absorbers DID do, is reduce modal decay from ~2 seconds to ~.3 seconds at 40, and 55hz, and yes you were correct, ~.7 seconds at 70hz.
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#76
15th July 2012
Old 15th July 2012
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Not a very big photo, but all I can find. This room was cubic and I treated it with the limp mass absorbers. These custom made absorbers had a combination of a limp mass membrane (over cavities) and then a layer of 2" fiberglass over the front. Each large box was divided into 2 smaller different depth cavities. Worked good, although far from ideal room ratio there was no major bass problems at the listening position.

(Please don't ask for measurements, I do not have any)
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-ems1.jpg  
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Last edited by Tim Farrant; 15th July 2012 at 06:54 AM.. Reason: edit
#77
15th July 2012
Old 15th July 2012
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File

You are welcome NLP. If you still have that REW file could you Zip and post it here please. I would like to tweak the views try Overlay etc. to show up how much these devices have achieved.

Best, DD
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#78
15th July 2012
Old 15th July 2012
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No problem.
Loudspeaker = K&H300.
#79
15th July 2012
Old 15th July 2012
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Show off

Thanks NLP. Nice Speakers.
I am very impressed by your test. If I may help you and Tim to show off a little....
Name:  Screen shot 2012-07-15 at 18.40.04.jpg
Views: 4755
Size:  83.6 KB

DD
NLP
#80
15th July 2012
Old 15th July 2012
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Yes I know it is a nice result.
Hope to be of any help to other members.
... unfortunately the bigger system came (k&h 410 and 2 x k&h 810) when final acoustic treatments are done:(
Too much additional layers of absorption to see how low membranes goes.

In a few months I will project some smaller control rooms, that will incorporate similar MLM absorbers, I will post in situ measurements/results, I can measure unsealed/sealed situation with insulation and unsealed with and without porous insulation behind (like curtain).

Aha BTW I remember where I read that BBC is no longer excited with membrane absorbers (not small and modular):
http://downloads.bbc.co.uk/rd/pubs/a...icpractice.pdf
page 91/92:
"... ageing process of the membrane materials..." other infos are irrelevant.
Today we have newer limp mass materials (EPDM) or more stable and reinforced with PVC foil, different type of fibers... and limp mass membrane is fair enough broadband, this particular membranes goes from cca. 25-70Hz.
#81
16th July 2012
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I would like to see T30 graphs from before and after.

EDIT; from NLP file, of course.
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16th July 2012
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Quote:
Originally Posted by Mctwins View Post
I would like to see T30 graphs from before and after.

EDIT; from NLP file, of course.
He provided his measurement files - you want to see something look at them yourself....... you do know how to look right?

Rod
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#83
17th July 2012
Old 17th July 2012
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Quote:
Originally Posted by DanDan View Post
Thanks NLP. Nice Speakers.
I am very impressed by your test. If I may help you and Tim to show off a little....
Attachment 300667

DD
I wonder if the 100Hz dip may be an axial mode not being addressed by wall/wall corner placement of the absorbers. I would suggest adding additional membrane traps to the room at the wall/ceiling intersection at the centre of the room tuned to 100Hz may help flatten this out.

Last edited by Tim Farrant; 17th July 2012 at 02:03 AM.. Reason: clarity
#84
17th July 2012
Old 17th July 2012
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Topt

Tim, I would need to start playing with Windowing to confirm what I suspect ( or ask Jeff Hedback). i.e. The 100Hz null is probably not modal but a BIR. Modes are frequently over-rated IMO.
http://recording.org/studio-construc...en-matter.html

McT, REW provides Topt, JohnPM's own attempt at dealing with the vagaries of extracting Decay times in a non diffuse environment. It is generally similar to T30 but also generally better for the job at hand. John knows what he is doing. So I hope you Topt will work for you. However, John has chosen to stick to the ISO and other 'Standard' restrictions regarding frequency. He does not do below the 100Hz band due to the many uncertainties of measurement down there. His call.
If you download the free REW you can easily look at these files and make comparisons yourself. I reckon it is pretty obvious from the numbers even here, that making simple numerical Decay estimations in a small modal and BIR dominated room is a mugs game.
However in the Waterfall, you will find a remarkable reduction in very low frequency decay from a relatively small intrusion into the the room. It would be hard/strange to ignore what we are seeing here.
This is IMO a very valuable confirmation of Tim's claim that these things work. Again, I thank both for sharing with us.
Progress can happen here, and does.
Name:  Screen shot 2012-07-15 at 18.43.40.jpg
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DD
#85
17th July 2012
Old 17th July 2012
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Quote:
=DanDan;8075536

McT, REW provides Topt, JohnPM's own attempt at dealing with the vagaries of extracting Decay times in a non diffuse environment. It is generally similar to T30 but also generally better for the job at hand. John knows what he is doing. So I hope you Topt will work for you. However, John has chosen to stick to the ISO and other 'Standard' restrictions regarding frequency. He does not do below the 100Hz band due to the many uncertainties of measurement down there. His call.
If you download the free REW you can easily look at these files and make comparisons yourself. I reckon it is pretty obvious from the numbers even here, that making simple numerical Decay estimations in a small modal and BIR dominated room is a mugs game.
However in the Waterfall, you will find a remarkable reduction in very low frequency decay from a relatively small intrusion into the the room. It would be hard/strange to ignore what we are seeing here.
This is IMO a very valuable confirmation of Tim's claim that these things work. Again, I thank both for sharing with us.
Progress can happen here, and does.


DD
Thanks for sharing.

More questions now then before.

Who is JohnPM?
What is Topt? Difference between standard ISO?
Uncertainties below 100 Hz? Explain further.
WHY is it not measured below 90 Hz in the graphs? The lines falls straight down.

I will look into REW..........
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17th July 2012
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Some regular voices from the Acoustics forums aren't chiming in... is that because they're too busy crapping their pants?

I built a BUNCH of dense foam bass traps... like 8. And of course they intrude quite a bit into a small room because they have to straddle the corner. The idea that I could make a few smaller, cleaner traps, and just hang them on walls at the corners... very exciting.
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17th July 2012
Old 17th July 2012
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Quote:
Originally Posted by NLP View Post

Aha BTW I remember where I read that BBC is no longer excited with membrane absorbers (not small and modular):
http://downloads.bbc.co.uk/rd/pubs/a...icpractice.pdf
page 91/92:
"... ageing process of the membrane materials..." other infos are irrelevant.
Today we have newer limp mass materials (EPDM) or more stable and reinforced with PVC foil, different type of fibers... and limp mass membrane is fair enough broadband, this particular membranes goes from cca. 25-70Hz.
Since that guide is dated 1990, how much does it reflect current thinking? RD 1994-12 discusses construction of a steel stud based broadband panel with a metal membrane. http://downloads.bbc.co.uk/rd/pubs/reports/1994-12.pdf

Are there any BBC reports available after 1995?
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#88
17th July 2012
Old 17th July 2012
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Here is simplified sketch of that empty room with acoustic measurement results - acoustic mapping.
It is easier to see what the problems are, where tu put acoustic elements, which types.
Attached Files
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#89
17th July 2012
Old 17th July 2012
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I think that they are not public available.

Quote:
Originally Posted by 2manyrocks View Post
Since that guide is dated 1990, how much does it reflect current thinking? RD 1994-12 discusses construction of a steel stud based broadband panel with a metal membrane. http://downloads.bbc.co.uk/rd/pubs/reports/1994-12.pdf

Are there any BBC reports available after 1995?
#90
19th July 2012
Old 19th July 2012
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I guess they just read this forum now instead of having to do their own research.
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