How big does it have to be (so to speak) to make a difference? - Home Theater Forum and Systems - HomeTheaterShack.com
 
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post #1 of 8 Old 06-08-08, 09:45 PM
deadhead
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How big does it have to be (so to speak) to make a difference?

Bryan,

Thanks for all your help so far via email.

Ethan,

Your posts and FAQ make great reading.

Anyone,

My room has a lot of narrow spaces. For example, I can't get a Tri Trap in one corner of my room because of a window and an electrical outlet. On the wall directly opposite from my sub is a window and my DVD cabinet with about 14" in between. There's a foot of space below that window. I believe treating all those locations might show positive results, but I'm not clear on the theory. All I know bass waves are huge. How big does a treament have to be to make a worthwhile difference? Would a 12" X 12" X 36" stack of dense foam between the window and the cabinet have any effect? Or a 12" X 12" X someothernumber" X 36" triangle? What about a 12" wide 2" thick panel of some height?

Also, the center of the driver on my sub is 4 feet from the side wall of my wooden component cabinet and hits it at a 45 degree angle. Should I do anything or just stop worrying?

It's funny. I see all these funky shaped wall treatments, but I never see anything like diffusers for the floor. I'm tempted to buy a couple of rolls of R-13 insulation and put them under my coffee table.

deadhead
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post #2 of 8 Old 06-08-08, 11:45 PM
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Re: How big does it have to be (so to speak) to make a difference?

From a pure decay time standpoint, those kinds of things do all add up. Just understand that there are some things that need to be addressed purely for reflections/frequency related issues - and those thing tend to need to be in specific places.

If you have a 'challenging' room, you sometimes have to tolerate less than optimal results when you can't address specific areas. That said, getting some bass control in the room will always be better than none.

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post #3 of 8 Old 06-10-08, 03:17 PM
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Re: How big does it have to be (so to speak) to make a difference?

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deadhead wrote: View Post
It's funny. I see all these funky shaped wall treatments, but I never see anything like diffusers for the floor. I'm tempted to buy a couple of rolls of R-13 insulation and put them under my coffee table.
Carpet is recommended over a hard surface just for that reason. It would be hard to cover the floor at the reflection points with much of anything since you probably still want to walk there. It's also not recommended to have a flat coffee table in front of you because of the hard reflective surface. I think the WAF comes into play quite a bit here.

What would be great would be a sub floor that's raised about 2-6" and the floor (OSB/Plywood) would have small 1/2" holes drilled uniformly over it's entire surface. Underneath the sub floor would be some OC703 or similar.

Of course that would be pretty labor intensive and loosing inches from room height is not always desirable. Some people stuff their stages/seating platforms with insulation and drill holes in the sides so that it acts like a big bass trap. You could probbaly do the same with a sofa or coffee table.
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post #4 of 8 Old 06-10-08, 03:26 PM
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Re: How big does it have to be (so to speak) to make a difference?

Something like that can, and has been done. There are just 2 issues with it.

1. It's a very narrow range absorber as it's basically a Helmholz resonator that will work over maybe 1 octave.

2. Unless the floor underneath the carpeted one is also sealed and isolated, you've just lost your isolation from the outside world.

It can be very well used in a riser provided you can predict exactly where your problem is going to be and set the hole size and pattern spacing properly or you take your measurements and get everything else set prior to carpeting the room.

Bryan

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post #5 of 8 Old 06-11-08, 09:41 AM
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Re: How big does it have to be (so to speak) to make a difference?

MatrixDweller,

There's lots of space on my floor that isn't walked on--under my couch, under the armchair, under the equipment cabinet, under the coffee tables--that could be used to put some kind of treatment.

For example, could I make a pedastle coffee table that uses 705 for the pedastle material. It could be two feet square, but only a foot-and-a-half tall. How big is big enough?

deadhead
post #6 of 8 Old 06-11-08, 11:36 AM
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Re: How big does it have to be (so to speak) to make a difference?

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deadhead wrote: View Post
MatrixDweller,

There's lots of space on my floor that isn't walked on--under my couch, under the armchair, under the equipment cabinet, under the coffee tables--that could be used to put some kind of treatment.

For example, could I make a pedastle coffee table that uses 705 for the pedastle material. It could be two feet square, but only a foot-and-a-half tall. How big is big enough?

deadhead
That would help significantly if you shoved it into a corner.

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post #7 of 8 Old 07-14-08, 03:00 AM
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Re: How big does it have to be (so to speak) to make a difference?

Optimally speaking (meaning we ignore the acoustical impedance of the material itself), a material must be at least 1/4 wavelength of the lowest frequency it is used to diffuse or absorb.

Assuming a idyllic 100% absorptive material 1" thick and an incidence of 45 degrees, the lowest frequency that would be effected is 2387 Hz! Hardly a frequency at issue! At the normal of 90 degrees: 3376Hz

A carpet and pad, the total thickness of .5 inches with an angle of incidence or 45 degrees which is assumed to be 100% effective (hmmmm...) will not have an effect on frequencies less than 5915 Hz! At the normal of 90 degrees: 6752Hz

And, similarly, for a 100% absorptive material .25 inches thick at an angle of incidence of 45%, the lowest effective frequency is 9552 Hz!!! At the normal of 90 degrees: 13,503 Hz!

And thats assuming IDEAL materials with an acoustical impedance completely identical to the acoustic source rendering it TOTALLY absorptive! If you find any such material, please PM me!

From this, one should get an idea of just how ineffectual curtains are as acoustical treatment as well! But conversely, you should be able to understand why they are much better for 'blocking' light!

Except for room modes, where we simply become cognizant of the frequencies at issue having wavelengths typically larger than the dimensions encounters, but where we are really concerned with the persistence of the resonant peaks in time which tend to correspond to the frequency peaks, focusing on the frequency domain is not a productive way of focusing upon tuning the room.

The awareness of the frequency response of a material becomes of importance only in evaluating the suitability of the absorptive and diffusive material capabilities.

Once room modes are selectively addressed, we want to examine the response based upon the arrival times and the intensity of the specular reflections. This is provided by an envelope time curve (ETC) measurement. From there various absorptive and diffusive treatments can then be utilized and verified such that a sufficient initial signal delay gap (where the preponderance of the signal is the direct signal) and the later arriving signals can be adjusted such their intensity is well-behaved and the focused characteristics of the specular reflections that dominate a small acoustical space can be sufficiently diffused to create a semi-reverberant acoustical space.

Small acoustical spaces lack a statistically reverberant field where the reflective nature of any signal is equally likely form any direction - meaning that the reflections cannot be resolved to a particular reflective source. Thus general decay measurements/calculations such as RT60's, etc. literally become meaningless. Thus we rely upon the ETC to build a more greatly diffuse sound field with the remnants of the finite energies contained in the focused specular reflections.
post #8 of 8 Old 07-14-08, 07:11 AM
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Re: How big does it have to be (so to speak) to make a difference?

IF you can fill the full underside area of the coffee table (2'x2'x?) with layers of OC703 or equivalent, that would function nicely as a broadband absorber. Probably not enough given the room size but it's a start anyway. Even if you can't get it in the corner, it can still help to address decay time issues in the low end.

Bryan

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