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Author Topic: Small rooms  (Read 3665 times)

jimmyjazz

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Small rooms
« on: August 24, 2006, 11:02:36 am »

Francis, thank you so much for taking time to speak acoustics with us!  Many of us on these boards own or often work in smaller studios, and as you know, the acoustics issues presented by those spaces can be "tricky" at the very least.  Could you give us your thoughts on some of the problems that arise in small spaces?

-- low frequency modal distribution and control; i.e., heavy absorption, bass traps, Helmholtz resonators, optimal placement of products, etc.

-- below what room size do you consider an RT60 calculation "useless"?

-- for a small mix room, do you tend to prefer LEDE or other design styles?


Any other thoughts related to small room acoustics would of course be welcome.  Thanks!
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franman

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Re: Small rooms
« Reply #1 on: August 24, 2006, 12:03:58 pm »

Small Rooms:

1. Modal calculations and simulations (RPG Room Optimizer and Room Sizer are good) are absolutely imperative. This is the single biggest oops that many folks make. They go to all kinds of efforts to make rooms with non parallel walls and end up with some impossible to predict, and very odd room modes. Spend some time on this while you layout rooms. It's the FIRST thing we do!
2. Bass Traps: Can't have enough!! Regardless of room size. Add broadband bass trapping. Tuned traps are generally used once problems are identified. We use Helmholtz tuned resonators and some membrane absorbers for small room problem areas.
3. Speaker Placement: this is the single biggest thing YOU CAN DO to improve bass response in your setup. Experiment with speaker placement. The boundary interference and interaction with the woofers (and ports sometimes) in your loudspeakers is a big variable. Move your speakers around (within reason) to find the best and smoothest bass response, by ear if necessary. Then getting good stereo imaging and depth is a matter of making very small adjustments and listening! LISTEN LISTEN LISTEN!!!
4. RT-60 is generally not even a consideration in our control rooms. For large performance rooms and recording rooms, we will sometimes calculate RT-60 but rooms less than 5000 CSF it's pretty much a waste of time (*IMHO).
5. Small control room, mid sized control room, large room watch out for early first reflections! These will destroy the imaging, time response AND frequency response by causing audible comb filters in the midrange all the way up!! Reflection Free Zone is our general approach (modified LEDE).
6. Any other thoughts... yep, hire a professional if at all possible!! We do this every day and have a feel for what is best to spend your money on. Thanks!! Laughing
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Francis Manzella - President, FM Design Ltd.
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Consul

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Re: Small rooms
« Reply #2 on: August 26, 2006, 01:05:10 am »

Fran, first I'd like to say thank you for giving your time to us on this board. I'm not one of the big-time posters around here, just a small-fry musician who wants a home studio to record in. Only I understand the importance of acoustics.

I have a question about small rooms. Specifically, what do you consider to be a small room. There are some people on this board who consider 50 x 75 x 12 feet to be a small room, and I'm just curious as to where you define the boundary yourself.

As for me, I have a approximately a 9.5 x 11.5 x 8 high room, with a drop ceiling, wood floor, and a cutout in one corner, which seriously limits my options. Small, to be sure. Oh, and I want to shoehorn some synths and a small drum set in here, too. Go, me! Smile

I've thought a lot about my treatment options (the drop ceiling is actually useful in that regard), and can only conclude that killing the whole thing dead than a doornail with a few bales of OC705 is about the only way to go.

Again, thank you for your time.
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Darren Landrum

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franman

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Re: Small rooms
« Reply #3 on: August 26, 2006, 09:47:19 pm »

to me small rooms are those that are not large enough to behave as "statistically reverberant environments"... In other words rooms where Ering and SAbine classic RT-60 calculations aren't valid...

How small??? depends on the surface materials but most of the rooms we design, including most of the live rooms are "small rooms"...

I would say rooms less than 5000-6000 Cubic Sq Ft are definitely small rooms (acoustically)...
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Francis Manzella - President, FM Design Ltd.
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Consul

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Re: Small rooms
« Reply #4 on: August 26, 2006, 10:29:38 pm »

Yeah, my approx. 874 cubic feet has nothing on that. Smile

Thank you for the answer.
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Darren Landrum

"Never be afraid to try something new. Remember that a lone amateur built the Ark. A large group of professionals built the Titanic." - Dave Barry

Sin x/x

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Re: Small rooms
« Reply #5 on: February 04, 2007, 05:15:07 am »

franman wrote on Thu, 24 August 2006 11:03

Small Rooms:

5. Small control room, mid sized control room, large room watch out for early first reflections! These will destroy the imaging, time response AND frequency response by causing audible comb filters in the midrange all the way up!! Reflection Free Zone is our general approach (modified LEDE).


I'm going to make a (even bigger) fool out of myself:
Comb filtering only appears when there's only one or a few early first reflections.
If there are lots of frequency independent early first reflections, no comb filtering will occur.
So an other option would be to create a diffuse room.

True or false?
And if false where does my reasoning go off line?
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franman

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Re: Small rooms
« Reply #6 on: February 04, 2007, 12:41:39 pm »

Yes, LOTS of early reflections are LESS intrusive than just one or two, BUT!!.. we still feel that control of early reflections is the way to go. There are some rooms out there that concentrate diffusion to 'bombard' the listener with a lot of time scattered early return information.

I generally feel this is more applicable for a home theater or living room listening environment... In these applications the "enhancement" of the playback is more acceptable than in a control room or mastering room where you want as accurate a representation of the recording as possible.... You want to "hear the production, not the 'reproduction'"...

We have to maintain a clear understanding that sound is captured and shaped in the production and the sound reproduction is supposed to be as neutral (re accurate) as we can make it. The reproduction is not supposed to color or affect the production...
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Francis Manzella - President, FM Design Ltd.
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Ethan Winer

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Re: Small rooms
« Reply #7 on: February 05, 2007, 05:42:35 pm »

Sin x/x wrote on Sun, 04 February 2007 05:15

If there are lots of frequency independent early first reflections, no comb filtering will occur.


I think I know where you're going with that line of thinking, but it would be more accurate to say that each reflection creates its own comb filtering pattern. So when combined, the peaks from some reflections tend to fill in the nulls from others.

Also, comb filtering can occur even with no reflections at all. If you have the same mono source coming from two or more speakers at once, the different arrival times will create peaks and nulls. Now, if you were to measure with one microphone at the precise center, the filtering will theoretically not occur. But if you sit in the center, your left ear is a few ms closer to the left speaker and vice versa for your right ear and speaker.

--Ethan

jfrigo

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Re: Small rooms
« Reply #8 on: February 11, 2007, 03:16:32 am »

Ethan Winer wrote on Mon, 05 February 2007 17:42

But if you sit in the center, your left ear is a few ms closer to the left speaker and vice versa for your right ear and speaker.


Which helps a lot with the whole stereo hearing thing... directional cues for the signal processor between our ears come from these very small differences in arrival times.

By the way, to revive a bit of the older posts in the thread, one good way to think about large room vs. small room is where the cutoff frequency or Schroeder frequency gets low enough that all bands of interest have a modal density that prevents marked peaks and nulls. Of course there is disagreement about how exactly to calculate the cutoff frequency, and how much modal density is enough, but you can get in the ballpark of the accepted range and call it a day.

As Fran observes, most all rooms that we deal with in recording studios are small rooms, acoustically speaking. As touched on above, the higher the frequency band, the greater the modal density, which is to say that with increasing frequency, the spacing between modal frequencies gets smaller and smaller This has the effect of reducing the obtrusiveness of the peaks and valleys and makes a generally pretty even room response (certainly not ruler flat, but subjectively pretty even from a modal point of view). The larger the room, the lower the frequency where the modal density is great enough to reduce the obtrusive inconsistencies.

Once the cutoff frequency gets down to 20 Hz, you really have no worries about modes lower than that for our purposes. This is a definitely large room by anybody's standard, and is probably a small to moderate-sized auditorium. The determination depends on the RT60, total surface area, desired modal density, and a little bit of math. Depending on the criteria, different acousticians will make the call from small to large in different palces, though nothing below Fran's example of 6000 cu. ft. would ever be called large. In fact, I'd personally probably make the call somewhat larger on purely technical grounds, depending on the conditions present, but I agree that a certain something happens at around 6000 cu. ft. that is good for recording spaces.
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Ethan Winer

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Re: Small rooms
« Reply #9 on: February 12, 2007, 03:41:19 pm »

Jay,

Excellent post. Exactly. Very Happy

--Ethan
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