Hanging blankets tent vs semi-circle 703 panels , which is better?

A

aznwonderboy

New member
For the purpose of vocal recording, which technique will deaden / absorb reflections better?

1.) a tent or fort made from moving blankets and PVC pipes (pic "1": from Hi_D_Ho_Man)

2.) a semi-circle behind the mic made from 3 OC 703 panels (pic "2": from RealTraps.com)

3.) a free-standing corner made from a blanket and PVC (pic "3": from KidVybes)

I heard that blankets on the wall only absorb mid and high frequencies; lows are still reflected. Is it true that as long as the blankets are placed far from the wall, then I don't have to worry about the reflecting low frequencies?

What do you suggest to take care of the low frequencies with each method?
 

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Blankets aren't going to reflect anything low and absorb lots of highs. 703 doesn't reflect anything, a sudo audio black hole.
 
I just made some gobos from 3 24" doors and some auralex. It isn't reflective on one side and is on the other. I can get either or a combination of both.
 
Resistance absorbtion(703) only works if there is a BOUNDARY behind the material, such as a wall. At least at its rated coefficients. It MAY absorb very high frequencies at a very low coefficient when used in this manner...but who knows. No one would test material in this configuration because of the latter fact. What would be the point? As to low frequencies...forget it.
 
RICK FITZPATRICK said:
Resistance absorbtion(703) only works if there is a BOUNDARY behind the material, such as a wall. At least at its rated coefficients. It MAY absorb very high frequencies at a very low coefficient when used in this manner...but who knows. No one would test material in this configuration because of the latter fact. What would be the point? As to low frequencies...forget it.
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Rick,

I've always been a bit confused by that, is the idea that the boundary reflects the wave back into the absorbtive material so it gets two passes to grab it?

Thanks,
Daav
 
is the idea that the boundary reflects the wave back into the absorbtive material so it gets two passes to grab it?
Click to expand...
No. Soundwaves are at a MAXIMUM pressure and ZERO velocity when encountering a boundary, but at 1/4 wavelength from the boundary the velocity is maximum with zero pressure. It is this fact that determines the frequency at which absorption coefficients begin to roll off for a given thickness(more or less). As the velocity of the wave is maximum at a 1/4wavelength from the boundary, this means the wave is having to move within the interstices of the material, which means resistance to the air molecule movement. This resistance is what converts this energy to heat. Hence the term, "resistance absorption".

Look at it this way. A 100hz wavelength is 11.3ft long. The 1/4 wavelength is approx. 34". For a panel to absorb 100hz at MAXIMUM coeffecient it would have to be this thick. This is for soundwaves that are perpendicular to the face of the panel. For soundwaves with angles of incidence at less than 90 degrees absorption is not as effective.

So, to LOWER the frequency of absorption for a panel of 703, increase the distance from the face of the panel to the boundary by allowing an airgap between the panel and a boundary, which in essence is increasing the thickness. A 2" thick panel with a 1" airgap will have approximately the same absorption coefficients as a 3" panel. Although, there is a point of diminishing returns by increasing the aigap depth. I have read of people using 4" thick panels with airgaps as deep as 16", although I've never encountered Lab tests that confirm the effectiveness of such deep airgaps. Personally, I believe the airgap should be no more than the thickness of the panel as higher frequencys would pass through and have NO absorption material within the 1/4 wavelength when the soundwave encounters the boundary. Afterall, a 1000 hz 1/4 wavelength is only 3 3/8" long.

Now for my disclaimer. :D I'm certainly no expert on this subject and those that are may find fault with my explaination. If so, please enlighten me as I am all ears. :)

Oh, btw, since a wave will transmit through a resistance absorber till it reaches a boundary, this is why it does NOTHING for transmission loss. The wave will STILL excite a boundary.
fitZ
 
*i am aware that i am dragging up an old thread(damn.. shouldnt have used the search function :p). i thought it would be more useful to keep all the information in one place*

Following Rick's explanation and the link he provided, if i wanted freestanding panels like the ones suggested would i be best off making one side of them closed (with plywood) so that there is a boundary?
 
RICK FITZPATRICK said:
Oh, btw, since a wave will transmit through a resistance absorber till it reaches a boundary, this is why it does NOTHING for transmission loss. The wave will STILL excite a boundary.
fitZ
Click to expand...

That's not really true. An absorber will absorb sound without a hard boundary. The reason absorbers do very little for transmission loss is because of sound flanking the absorbers. If you built a room with 703 and allowed no air leeks you would indeed have reduced transmission of the higher frequencies. Enough bass and mids would get through to make it somewhat pointless as a soundproofing measure though.
 
I have no tests, but there is logic behind it. The reason why the absorbers work best with a boundary is that it forces there to be a maximum velocity point 1/4 of a wavelength away, as you said yourself. However sound just flowing through would still have some of its pressure wave be at maximum velocity while in the panel and if the panel is at least half of a wavelenth thick it would have to have a maximum velocity point inside the panel.

So, yes it would not be as effective (particularly at lower frequencies) as having a boundary but to say that it would be ineffective without the barrier is inaccurate.
 
So, yes it would not be as effective (particularly at lower frequencies) as having a boundary but to say that it would be ineffective without the barrier is inaccurate.
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I'll accept that to a certain extent, but testing Labs use Standarrds to rate the absorption coefficients using a boundary as required in the Standards. What would you guess the absorption coefficient table of 703 mightr look like? Say for a panel of 4" thick, with an "x" mounting standard such as hanging from wire or such, in the middle of the room, 24" from the ceiling, parallel to NO walls. All edges exposed, and both faces exposed. Care to guess? :)
 
So if you want to absorb as many frequencies as possible (say, to deaden an area of the room for vocals/guitar) it'd be best to make panels with plywood on the back? Or place them right near a wall? :confused:
 
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