Need qualified room acoustician

Nyal Mellor · Feb 15, 2011

The Bogg said:
Thanks also for your input Ethan (I had a bunch of your realtraps before I built the room with built-in bass traps). I think one of the issues in my room is too much absorption - the whole ceiling for example is absorptive. The advantage to an overly absorptive room is the pinpoint imaging when it's present in the recording. But there are definitely tradeoffs.

I just need that magical guy who can walk into my room and say "aha, your problem is xxx, and the solution is yyy"! I've speculated about comb-filtering, overdamping, etc... but I don't have the solution.

What you need is someone to measure your room. If you can't find someone local and don't want to use a remote design service then try measuring it yourself using the guides stickied on Ethan's forums, post them here and you might get Ethan, Jeff and me giving you some advice! The basics of frequency response, modal decay, reverberation time and energy time curves can be measured and have a very strong correlation with sound quality.

RBFC · Feb 15, 2011

Nyal,

So the virtually undamped reverberation below 100 Hz is not an issue? Or were you simply looking at the higher frequencies and expecting that some additional bass trapping would be employed?

Lee

microstrip · Feb 15, 2011

Nyal Mellor said:
How about 1"? We're trying to fix the upward slope in the reverberation time, not the longer reverberation time at low frequencies. Which BTW is quite ok:

You should NEVER use 1" at primary reflection points (3" is the minimum) though since you will end up with a nicely distorted reflection coming back at you. Look at these absorption curves. It should be pretty clear that a 1" thick absorber will absorb everything above 1000Hz but then increasingly less below that. So if you were to put a signal with equal energy per octave (pink noise) into the 1" absorber you would expect to see something coming back at you looking like the inverse of the absorption curve i.e. too much midrange, not enough treble.

According to some experts you should not use fiberglass board absorbers at the first lateral reflection points - it spoils the "feeling of space" and excessively attenuates high frequencies.

I have tried fiberglass 40mm fiberglass boards and RPG abbfusors at these points, but had to remove them - they killed my system.

As usual, also on room acoustics I have found that one man's food in another man poison.

Ethan Winer · Feb 15, 2011

RBFC said:
So the virtually undamped reverberation below 100 Hz is not an issue?

That sure would be an issue for me. Not just that the time is so long, but also because it's so much longer than the other bands. However, the range of acceptable decay times depends a lot on the overall size of the room.

--Ethan

RBFC · Feb 15, 2011

So, in the interest of learning what an acoustician would think when viewing that graph:

With a combination of bass trapping and 1" fiberglass panels (strategically placed), we should be able to create a relatively smooth decay rate in this room. That pretty much it?

Lee

microstrip · Feb 15, 2011

RBFC said:
So, in the interest of learning what an acoustician would think when viewing that graph:

With a combination of bass trapping and 1" fiberglass panels (strategically placed), we should be able to create a relatively smooth decay rate in this room. That pretty much it?

Lee

You can try it with a simualtor such as CARA - but I doubt you achive what you want unless you start with an already adequated room, having appropriate diffusion and reflection.
My room is rectangular and I tried this approach, both using the simulator and buying a lot of 40 mm thick fiberglass boards. But could not have success. Perhaps I could not find that strategic places.

RBFC · Feb 15, 2011

I don't want to give the impression that I have any idea how "strategic placement", etc. would be undertaken according to the graph. I'm simply trying to learn from ground zero about how measurements and graphs can guide us toward good decisions in sound reproduction!

Lee

audioguy · Feb 15, 2011

Nyal Mellor said:
You should NEVER use 1" at primary reflection points (3" is the minimum) though since you will end up with a nicely distorted reflection coming back at you.

How about 2 inch plus a 1 inch airspace between the panel and the surface?

Nyal Mellor · Feb 15, 2011

RBFC said:
Nyal,

So the virtually undamped reverberation below 100 Hz is not an issue? Or were you simply looking at the higher frequencies and expecting that some additional bass trapping would be employed?

Lee

Lee, with the reverberation time graph I posted, one is not using it to identify and correct long decay times due to room modes. The graph only shows one third octave bands, which is not enough resolution when dealing with low frequencies. For that you want to use a higher resolution frequency response and waterfall type diagram such as these ones.

So I was just using that graph to check the higher frequencies. That one graph is not the only thing that is used. Each measurement has its purpose.

Ethan Winer · Feb 15, 2011

audioguy said:
How about 2 inch plus a 1 inch airspace between the panel and the surface?

Yes, that would be better than one inch material and no space.

--Ethan

Nyal Mellor · Feb 15, 2011

microstrip said:
According to some experts you should not use fiberglass board absorbers at the first lateral reflection points - it spoils the "feeling of space" and excessively attenuates high frequencies.

I have tried fiberglass 40mm fiberglass boards and RPG abbfusors at these points, but had to remove them - they killed my system.

As usual, also on room acoustics I have found that one man's food in another man poison.

I generally agree. Lateral reflections, or those from the left and right side wall, can have some beneficial effects on soundstage width / envelopment. A pure absorber placed at the reflection points tends to result in reduced soundstage width / envelopment but increase focus. A combination absorber / diffusor absorbs just enough energy to lock in the vocalist and instruments within a soundstage but crucially also scatters enough energy back into the room to keep soundstage width and envelopment high.

IMO the lateral reflection points are ones best left either reflective or treated using a combination absorber / diffusor such as the abfussor you mentioned.

You can use choose which material to use at side walls reflection points as you prefer - they are imaging vs spaciousness controls. If you like orchestral music and never listen to singer songwriters you might want fully reflective. If you like only singer songwriters and don't listen to orchestral then maybe use absorbers. Those who prefer a balance, use combination absorber / diffusers.

All the research is there in Toole's book to review.

Nyal Mellor · Feb 15, 2011

Ethan Winer said:
Yes, that would be better than one inch material and no space.

--Ethan

We're basically looking for constant absorption down to around the room's transition frequency which is between 150 and 300Hz or so. The 3" requirement is generally industry accepted practice and is documented in CEA/CEDIA-CEB22 as well as Toole's book.

microstrip · Feb 15, 2011

RBFC said:
I don't want to give the impression that I have any idea how "strategic placement", etc. would be undertaken according to the graph. I'm simply trying to learn from ground zero about how measurements and graphs can guide us toward good decisions in sound reproduction!

Lee

Lee,

Please see point 4.3.5 Conventional Acoustical Measures in Small Listening Rooms of "Sound Reproduction" by F. Toole .

But the conclusion will disappoint you.

Toole states that although not irrelevant, the measurements are not direct answers to the important questions in small rooms. Careful experimentation is needed to understand the perceptual effects of the sound that arrives at the listener.

Nyal Mellor · Feb 15, 2011

microstrip said:
Lee,

Please see point 4.3.5 Conventional Acoustical Measures in Small Listening Rooms of "Sound Reproduction" by F. Toole .

But the conclusion will disappoint you. Toole states that although not irrelevant, the measurements are not direct answers to the important questions in small rooms. Careful experimentation is needed to understand the perceptual effects of the sound that arrives at the listener.

I would clarify that Toole is referring to traditional acoustic measurements, which is basically a single figure RT60 measurement.

The Bogg · Feb 15, 2011

Ethan Winer said:
In the mean time you could experiment with large pieces of reflective material placed on the ceiling (not at reflection points) and elsewhere.

--Ethan

I had thought about that, Adam had suggested using something like particle board which is readily available. Any other suggestions?

The Bogg · Feb 15, 2011

Nyal Mellor said:
What you need is someone to measure your room. If you can't find someone local and don't want to use a remote design service then try measuring it yourself using the guides stickied on Ethan's forums, post them here and you might get Ethan, Jeff and me giving you some advice! The basics of frequency response, modal decay, reverberation time and energy time curves can be measured and have a very strong correlation with sound quality.

Nyal,
I had Adam Pelz come up here and he did measure the room. I believe he's an HAA guy like yourself. He didn't do an RT60 b/c he didn't think it had that much relevance for small room acoustics. Instead he did an energy time curve - I don't recall what frequencies it covers but mine was short at 180ms.

The room response shows a significant dropoff in the bass below around 60hz which was surprising b/c my speakers are full range. When he did a nearfield measurement of the speakers it confirmed that they do go down to below 30hz. He was able to blend in the front 2 JL F113 subs I have for 2 channel (I use 4 for ht). I'll try to attach the graph but it shows basically only a couple of db deviation between 100hz and 20hz with 1/30th octave resolution which amazed both of us.

My first reflection points are reflectors. I initally thought they were diffusors but since they are flat, angled pieces they are just simple reflectors. Here is a pic of them, they are from floor to soffit:

mep · Feb 15, 2011

This thread should have been titled "Need qualified room acoustician to work for free."

Ethan Winer · Feb 16, 2011

^^^ LOL, good point.

Ethan Winer · Feb 16, 2011

The Bogg said:
Adam had suggested using something like particle board which is readily available. Any other suggestions?

I've used cardboard sheets, though those are not quite as reflective at high frequencies as stiffer materials. If you sprayed them with clear lacquer that would help a lot. Home Depot sells pre-cut 2x4 foot plywood sheets. 1/8 inch thick would be perfect, but I've never seen thinner than 1/4 inch. Thin "shiny" plastic sheets are probably your best bet if you can find some cheap.

--Ethan

Nyal Mellor · Feb 16, 2011

The Bogg said:
Nyal,
I had Adam Pelz come up here and he did measure the room. I believe he's an HAA guy like yourself. He didn't do an RT60 b/c he didn't think it had that much relevance for small room acoustics. Instead he did an energy time curve - I don't recall what frequencies it covers but mine was short at 180ms.

The room response shows a significant dropoff in the bass below around 60hz which was surprising b/c my speakers are full range. When he did a nearfield measurement of the speakers it confirmed that they do go down to below 30hz. He was able to blend in the front 2 JL F113 subs I have for 2 channel (I use 4 for ht). I'll try to attach the graph but it shows basically only a couple of db deviation between 100hz and 20hz with 1/30th octave resolution which amazed both of us.

My first reflection points are reflectors. I initally thought they were diffusors but since they are flat, angled pieces they are just simple reflectors. Here is a pic of them, they are from floor to soffit:

180ms is a very dead room! Like Ethan said, putting some particleboard over the existing absorption on the ceiling, but not at mirror points, is probably the best way to proceed. You going to need quite a few boards to have an effect and try and spread them out a little rather than covering one area totally. You would be doing yourself a favor by getting REQ Wizard or XTZ Room Analyzer or Fuzzmeasure. Start by aiming for a single figure RT60 of 0.4s.

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