1D QRD diffusors (Can I use a well width of 0.75 inches?) Viscous Losses?!

[davecfraser]

Hi,

I've recently constructed small project studio this summer and worked on a few projects for local artists. I'm in a bit of a lull right now and thought about building some 1D QRD diffusors. Right now all I've done treatment wise to the room is 4" Roxul Basstraps on the ceiling in a cloud above the listening position and in most corners of the room.

A rough floorplan is at http://www.davidcavanfraser.com/davi.../dcfstudio.gif

The best listening position has been determined using REW graphs is basically along the wall between corners 1 and 2 on the floorplan.

My frequency response at listening position (after treatment) and using some EQ looks like:
http://www.davidcavanfraser.com/davi...cyresponse.jpg


I'm wondering if perhaps some Diffusors on the sidewall to control 1st reflections might help? I tried 2X4' 4" bass traps to kill first reflections and noticed no frequency response improvements so I ended up leaving the two side walls empty (beside the listening area).


The room isn't DEAD, but it's by no means lively. I'd love to achieve a more spacious sound than my space limitations allow.

Would four 4' 1D QRD diffusors on empty wall spaces improve the room? Would a few 2D QRDin the limited amount of available ceiling space improve the room's sound?

and my main question. Would a well width of 0.75 inches be a terrible diffusor? QRDdude tells me I'll get "serious" viscous lossess, but other people on here say, stick about 0.70 inches and you're okay. What am I compromising by going smaller than QRDdude's recommended 1 inch? In Canada when they sell you a 2 x 4, they're actually selling you a 1.5" X 3" or something like that, and a 1" X 2" is actually 0.75" X 1.5", so you can see why either 0.75 or 1.5" are both ideal for construction purposes.


Thanks in advance for your help! You guys have been invaluable throughout this process. The studio hasn't paid for itself just yet, but it's getting there, one client at a time

Maple Hill Recording Studio - Erin Guelph Toronto Ontario Canada by the way

[boggy]

Originally Posted by davecfraser

Hi,

I've recently constructed small project studio this summer and worked on a few projects for local artists. I'm in a bit of a lull right now and thought about building some 1D QRD diffusors. Right now all I've done treatment wise to the room is 4" Roxul Basstraps on the ceiling in a cloud above the listening position and in most corners of the room.

A rough floorplan is at http://www.davidcavanfraser.com/davi.../dcfstudio.gif

Non-symmetric room

Originally Posted by davecfraser

The best listening position has been determined using REW graphs is basically along the wall between corners 1 and 2 on the floorplan.

My frequency response at listening position (after treatment) and using some EQ looks like:
http://www.davidcavanfraser.com/davi...cyresponse.jpg

For your response I will use low pass shelving filter (yes, equalizer), because it isn't clear why your loudspeaker has that response. Shelving filter will solve it, as is displayed at attached modified graph (look at "arrows"). I suspect that standard acoustic treatment can solve your response... only EQ can.
But... possibly... hard soffit mount (in concrete) may help to improve your response, but I don't know why your loudspeakers has high pass shelving response... I'm not sure. It looks similar as your "baffle step" compensation (internal eq filter in loudspeaker) is switched off.

Originally Posted by davecfraser

I'm wondering if perhaps some Diffusors on the sidewall to control 1st reflections might help? I tried 2X4' 4" bass traps to kill first reflections and noticed no frequency response improvements so I ended up leaving the two side walls empty (beside the listening area).

diffusers can't help your measured frequency response as equalizer can. But you need it if you like to get hear more details.

Originally Posted by davecfraser

The room isn't DEAD, but it's by no means lively. I'd love to achieve a more spacious sound than my space limitations allow.

Diffusers can give more "spacious" sound

Originally Posted by davecfraser

Would four 4' 1D QRD diffusors on empty wall spaces improve the room? Would a few 2D QRDin the limited amount of available ceiling space improve the room's sound?

yes and yes. If they are at first reflection positions. Also keep in mind not to mount diffusers too close to you, there are dependence between minimum distance from you and diffuser (four or three wavelengths) and lowest working frequency.

Originally Posted by davecfraser

and my main question. Would a well width of 0.75 inches be a terrible diffusor?

No. I build couple of N41 QRDs with 19mm wells width... and this diffsors works well.

Originally Posted by davecfraser

QRDdude tells me I'll get "serious" viscous lossess, but other people on here say, stick about 0.70 inches and you're okay. What am I compromising by going smaller than QRDdude's recommended 1 inch? In Canada when they sell you a 2 x 4, they're actually selling you a 1.5" X 3" or something like that, and a 1" X 2" is actually 0.75" X 1.5", so you can see why either 0.75 or 1.5" are both ideal for construction purposes.

It's ok to build diffuser with 0.75" (19mm) well width... at least it works much, much better than simple flat surface

EDIT1: I attached second graph where you can see what your response may look like after shelving filter applied.
EDIT2: Your loudspeaker response may be because measurement microphone... too

[localhost127]

Originally Posted by davecfraser

Hi,
I'm wondering if perhaps some Diffusors on the sidewall to control 1st reflections might help?

whatever treatment you place at early reflection points (to attenuate early indirect specular energy from impeding the listening position) must be broadband in nature throughout the entire specular region - such that the entire specular reflection is fully attenuated. otherwise, you are merely EQ'ing/filtering/coloring the reflection.

not a terribly difficult task with absorption, but diffusion then requires considerably more attention.

the diffuser's bandwidth can change with angle of incidence. will you be mounting the diffuser flat against the side-wall such that incident energy will be non-normal, or will you mount the diffuser off-set to the sidewall such that inbound direct energy from the source is normal incident?

do you have the real estate to construct the diffuser to be effective throughout the entire specular region - or can you design a hybrid absorber/diffuser such that absorption takes place below the diffuser lower cut-off?

Originally Posted by davecfraser

I tried 2X4' 4" bass traps to kill first reflections and noticed no frequency response improvements so I ended up leaving the two side walls empty (beside the listening area).

depending on the polar response of the speaker and acoustical impedance of the boundaries, you should see a difference in the frequency response as you are minimizing or eliminating the high-gain indirect energies that combine constructively and destructively with the direct signal at the listening position.

[davecfraser]

Boggy, THANKS for your response!

Originally Posted by boggy

Non-symmetric room

Yes I built that diagonal wall to maximize room volume, as suggested by Rod Gervais and Andre.

Originally Posted by boggy

For your response I will use low pass shelving filter (yes, equalizer), because it isn't clear why your loudspeaker has that response. Shelving filter will solve it, as is displayed at attached modified graph (look at "arrows"). I suspect that standard acoustic treatment can solve your response... only EQ can.
But... possibly... hard soffit mount (in concrete) may help to improve your response, but I don't know why your loudspeakers has high pass shelving response... I'm not sure. It looks similar as your "baffle step" compensation (internal eq filter in loudspeaker) is switched off.

My monitors are BX5A M-Audio monitors. I don't believe they have a baffle step compensation switch (at least not on the outside of the speakerbox).

So you're saying cut a little of the higher frequencies using eq? (Or boost some of the base frequencies?) I suspect that perhaps the type of Fabric I used on my bass traps doesn't absorb high-frequencies as well as it could and so I get more high frequencies???

Originally Posted by boggy

diffusers can't help your measured frequency response as equalizer can. But you need it if you like to get hear more details.

How do diffusors help you hear more detail? I've never been to a studio that has proper diffusion so I don't really know what it sounds like.

Originally Posted by boggy

Diffusers can give more "spacious" sound

I think I understand why, by introducing a time differential, our ears hear a larger room because reverb time (and pre-delay) is how our ears understand the size of the room we are in.

Originally Posted by boggy

yes and yes. If they are at first reflection positions. Also keep in mind not to mount diffusers too close to you, there are dependence between minimum distance from you and diffuser (four or three wavelengths) and lowest working frequency.

So listen to QRD's minimum seating position suggestions?

Originally Posted by boggy

No. I build couple of N41 QRDs with 19mm wells width... and this diffsors works well.
It's ok to build diffuser with 0.75" (19mm) well width... at least it works much, much better than simple flat surface .

OK that's exactly what I was thinking about building so awesome. Was your N41 WRD a 2d or a 1d?

Originally Posted by boggy

EDIT1: I attached second graph where you can see what your response may look like after shelving filter applied.
EDIT2: Your loudspeaker response may be because measurement microphone... too

I used the program REW with a Behringer ECM8000 microphone with the mic calibration file installed, it's the sum of the left AND right speaker Frequency Response curve.

[davecfraser]

Localhost, thanks for your response, you bring up a lot of excellent points, I appreciate your input very much!

Originally Posted by localhost127

whatever treatment you place at early reflection points (to attenuate early indirect specular energy from impeding the listening position) must be broadband in nature throughout the entire specular region - such that the entire specular reflection is fully attenuated. otherwise, you are merely EQ'ing/filtering/coloring the reflection.

That's a very good point, something I hadn't really stopped and thought about.

Originally Posted by localhost127

not a terribly difficult task with absorption, but diffusion then requires considerably more attention.

Right, that makes a lot of sense, the only thing is I couldnt' measure an improvement by sweeping with the Bass traps on the sidewall so I kindof ruled out absorption.

Originally Posted by localhost127

the diffuser's bandwidth can change with angle of incidence. will you be mounting the diffuser flat against the side-wall such that incident energy will be non-normal, or will you mount the diffuser off-set to the sidewall such that inbound direct energy from the source is normal incident?

Unfortunately my room limitations are such that anything other that flat against the wall is impractical.

Originally Posted by localhost127

do you have the real estate to construct the diffuser to be effective throughout the entire specular region

hmmm, isn't it pretty much true that no-one in the home studio realm has the real estate to construct a diffusor that works down in the bass frequencies? You start talking about diffusors over 1' in depth and minimum seating positions of over 8'

Also what's the specular region?

Originally Posted by localhost127

- or can you design a hybrid absorber/diffuser such that absorption takes place below the diffuser lower cut-off?

Maybe? Any links / suggestions? Stuff the wells with Roxul RHT-40? I've got lots of it I was thinking of making my sidewall 1st reflection diffusors around 7" in depth with 0.75" wells so effective diffusive range would be 1kish to 9kish and scatter down to 500hz.

Originally Posted by localhost127

depending on the polar response of the speaker and acoustical impedance of the boundaries, you should see a difference in the frequency response as you are minimizing or eliminating the high-gain indirect energies that combine constructively and destructively with the direct signal at the listening position.

I think I understand what you're saying, but you're suggesting this result is achieved by absorption or hybrid diffusion / absorption?

[localhost127]

Originally Posted by davecfraser

hmmm, isn't it pretty much true that no-one in the home studio realm has the real estate to construct a diffusor that works down in the bass frequencies? You start talking about diffusors over 1' in depth and minimum seating positions of over 8'

Also what's the specular region?

specular region above the schroeder/davis cut-off frequency where energy behaves and can be modeled like light (geometry; angle of incidence=angle of reflection). below this is the modal region (LF/bass where energy has wave properties).

i was not insisting the diffusers were to be used for frequencies in the modal region.

[davecfraser]

AHHh thanks for clarifying that makes complete sense. So Sound has a duality just like light does (diffraction, particle-like behaviour AND wave-like behaviour).

What's the actual frequencies of this changeup?

[localhost127]

http://www.akutek.info/Papers/MS_Sch..._Revisited.pdf

[boggy]

Originally Posted by davecfraser

Boggy, THANKS for your response!

You are welcome

Originally Posted by davecfraser

Yes I built that diagonal wall to maximize room volume, as suggested by Rod Gervais and Andre.

Symmetry is also priority... we have two (symmetrical) ears, and don't listen (only) mono sound... but never mind... it is not a that big problem

Originally Posted by davecfraser

My monitors are BX5A M-Audio monitors. I don't believe they have a baffle step compensation switch (at least not on the outside of the speakerbox).

Ok...

Originally Posted by davecfraser

So you're saying cut a little of the higher frequencies using eq? (Or boost some of the base frequencies?)

Yes

Originally Posted by davecfraser

I suspect that perhaps the type of Fabric I used on my bass traps doesn't absorb high-frequencies as well as it could and so I get more high frequencies???

No, I don't think that your fabric is a problem

Originally Posted by davecfraser

How do diffusors help you hear more detail? I've never been to a studio that has proper diffusion so I don't really know what it sounds like.

This is about psychoacoustics... I have some explaination:
I think that diffusers form a specific reflection from its surface, which make easier to ears and brain to ignore it. Then you hear more details FROM your speakers, and not "details" which exist only in your room and nowhere else. (this is my own explanation from my own experience)

Originally Posted by davecfraser

I think I understand why, by introducing a time differential, our ears hear a larger room because reverb time (and pre-delay) is how our ears understand the size of the room we are in.

It is not only reverberation time from where our brain "decide" size of the room. With diffusers we can chorten RT60 at high frequencies because viscous effect, which you already know from QRDude, but room with more diffusers will usually has lower RT60, but brain will decide (when you close your eyes) that room is bigger than you visually expect.
You have a bit more about that topic here: http://www.bozoel.com/hosted/myroom-...hite_paper.pdf
or here AES E-Library

Originally Posted by davecfraser

So listen to QRD's minimum seating position suggestions?

No, i don't have minimum seating position suggestions, but I have minimum working frequency suggestion... I don't build diffusers which works lower than 1kHz in small rooms. You can find your distance from your diffuser, then you can decide lower working frequency (depth of well)

Originally Posted by davecfraser

OK that's exactly what I was thinking about building so awesome. Was your N41 WRD a 2d or a 1d?

1D

Originally Posted by davecfraser

I used the program REW with a Behringer ECM8000 microphone with the mic calibration file installed,

No need for calibration file if it is not exactly (literary) for your (piece of) ECM8000

Originally Posted by davecfraser

it's the sum of the left AND right speaker Frequency Response curve.

That is possibly a problem... (bolded by me) ... with high pass shelving response...
Then don't sum it... show us both left and right loudspeaker response in same graph (not summed, again)

[davecfraser]

Originally Posted by localhost127

http://www.akutek.info/Papers/MS_Sch..._Revisited.pdf

REad this paper, a bit confusing, what is V (volume in Cubic Meters) What is T (Rt60 in Seconds?) if so I calculated the Fs to be around 230hz ish for my room meaning that's where the sounds transistions from modal to spectral (if my understanding is correct). Is this the frequency I need to diffuse DOWN to ?

That seems a little extreme.

What about just diffusing from let's say 1K up to 10K and shoving roxul behind the larger wells? Anyone ?

[John White]

Originally Posted by davecfraser

... Is this the frequency I need to diffuse DOWN to ? That seems a little extreme.

Yes, it is probably extreme. There was a somewhat recent thread that addressed this anomaly. I believe Boggy was also part of that discussion. I don't feel I had a true grasp of the concept so I didn't retain the info. Perhaps someone else can recall it.

What about just diffusing from let's say 1K up to 10K and shoving roxul behind the larger wells? Anyone ?

10K is also extreme. QRD's are known to scatter well above the diffuse calculations. In answer to your original question, diffusing up to 10K would likely absorb quite a bit of energy.

[boggy]

Originally Posted by John White

Yes, it is probably extreme. There was a somewhat recent thread that addressed this anomaly. I believe Boggy was also part of that discussion. I don't feel I had a true grasp of the concept so I didn't retain the info. Perhaps someone else can recall it.

This is Schroeder's frequency, not minimum working frequency for diffuser.
Minimum listener's distance from diffuser limits this earlier. Lowest working frequency is defined with three (or four) wavelengths distance from diffuser to listener. For most common (small) rooms diffuser's best lowest working frequency defined in this way is higher that Schroeder's frequency, but it is related to Schroeder frequency. So, when room become bigger, distances from diffusers to listeners are longer, then diffusers may have deeper wells, so lowest working frequency may be even lower, similar as Schroeder's frequency become lower, when we increase volume.

QRD, diffusers for small rooms which works from 1kHz and up, is usually good designer choice.

[John White]

Yes, Thank You- room size/ listener distance. Thanks for the reminder, yet I still feel this was just the tip of the discussion. Just to play devils advocate however, perhaps there is a small room yet it is fairly narrow compared to the length. Assuming there is enough length in order to lower the effective frequency, how does this relate to the Schroeder frequency cut off limit?

Certainly there would still be a range of low frequencies not addressed by the QRD?

I realize this is probably not something to lose sleep over, but find it interesting.

[boggy]

Originally Posted by John White

Yes, Thank You- room size/ listener distance. Thanks for the reminder, yet I still feel this was just the tip of the discussion. Just to play devils advocate however, perhaps there is a small room yet it is fairly narrow compared to the length. Assuming there is enough length in order to lower the effective frequency, how does this relate to the Schroeder frequency cut off limit?

Diffusers design is related to room dimensions and shape, because psychoacoustics, or, better, human reaction to (highly) diffused reflections, not directly and tightly related to Schroeder's frequency. Schroeder frequency define "point" where response in room become statistical instead of modal... there can be some relations but not direct and not obvious.

Originally Posted by John White

Certainly there would still be a range of low frequencies not addressed by the QRD?

Well... if you have room which is big enough... like concert halls, etc..... it is all about numbers.

Originally Posted by John White

I realize this is probably not something to lose sleep over, but find it interesting.

[John White]

Great, thanks again. I think we are on the same page in my simple version.

And congratulations, they gave you your own little icon!:
It looks just like you.

Good luck to the OP. Let us know your results.

[Cegor]

@Boggy

I was going to message you personally but this is probably a good post for everyone on the thread/forum to see. How would someone go to eq their speaker output? I've only heard of people tuning their rooms.

Thanks again

[boggy]

Originally Posted by John White

..........
And congratulations, they gave you your own little icon!:
It looks just like you.
......

Originally Posted by Cegor

......How would someone go to eq their speaker output? I've only heard of people tuning their rooms.
......

I usually don't recommend that, but it can help a bit in already decently treated room. In cases when I'm using an EQ, it's always in conjuction with acoustical measurements, and I'm constantly double checking everything with great care
Some advice:
- Use standard acoustic treatment and loudspeaker positioning as most as you can.
- Fine tuning with eq may be couple of decibels only.
- Use EQ only for peaks and humps in response.
- I never have luck with automated room-eq softwares, so I do this always manually (I'm fairly skilled loudspeaker designer, so I already know how to do it)
- If you are unsure, hire a professional.