Originally Posted by spike9876
I keep coming back to this question... I had previously contacted audimutesoundproofing.com and today I spoke with a sales person and they are recommending placing 2" acoustic panel (mainly for mid & high frequencies) behind flat wall against my couch and told me that bass trapping are more effective on corners and not much effective against flat wall...
you haven't mentioned the type of insulation you're using (the density, the gas flow resistivity)...etc. so it's difficult to have this discussion when we could be talking about porous insulation that clearly will have different characteristics. why don't you ask the vendor selling the product for measurements to backup their claims that their 2" panel will absorb all broadband specular reflection energy - or enough to at least diminish the reflection by -20 to -25dB - if your requirement is to create a reflection free zone? if they're selling a product, then they should have lab tested results for said product.
also, maybe you could ask them to explain how a 2" panel flush against the wall will be able to provide enough absorption of lower frequencies to help mitigate nulls off the real wall (where the distance between the listening position and the rear wall is 1/4wavelength of a particular freq)?
Originally Posted by spike9876
Again, I would like to confirm whether I actually need a 4" bass traping panel against my couch which is flush against flat wall.
ok - you seem to have become confused. just because a panel is 4" thick of rigid fiberglass does not make it a bass trap.
there are two types of velocity based absorbers that we focus on regarding porous material (eg rigid fiberglass). one is a broadband absorber used to absorb a wide range of frequencies ... usually placed at first reflection points to absorb specular reflections so that they do not arrive within 20ms or so of the original signal (you can google reflection-free-zone for more info). the other is a "bass trap" which is located in the corners where the large wavelengths cannot effectively 'diffract' around the panel, to absorb low freq energy sound that functions as a wave.
as a sound wave approaches a boundary, pressure increases as velocity goes to zero. porous absorption works by converting kinetic energy into heat, and thus is most effective at 1/4wavelength where particle velocity is highest. thus, placing your panel directly against the wall where pressure is highest but velocity goes to zero, minimizes the effectiveness of the absorber.
for absorbing 5000hz, the wavelength is 2.7" long...thus 1/4wavelength is 0.675 so if a 2" panel was flush with the wall then this would be ok. however, a 600hz wavelength is 22 inches long .. thus the absorption material is most effective at 1/4wavelength which is 5.5" spaced from the boundary. if you are only putting 2" of material flush against the wall (whose performance is then decreased further as incident angle is not likely to be 0* (more likely 45*)) then you will likely absorb the highs but not necessarily the mids and certainly not the lower end specular reflections. a common issue i think is people lining their entire walls with 1" or 2" of absorption material which can do a great job of absorbing high freqs, but if you're not treating the mids and lows then you're left with a dead and muddy sounding room.
bass traps need to be very deep to be effective (as the wavelengths in that region are much, much longer - and thus, the fiberglass needs to be placed further from the boundary to be effective). by straddling a 4" panel in the corner, you are technically spacing it from the boundary and thus increasing the effectiveness. bass traps usually have a reflective material on the outside to help reflect some mid and high freq energies back into the room. if your corner bass traps were broadband, then they may cause too much mid/high absorption and make the room sound dead. also, dont delude yourself into thinking one or two "bass traps" will make a noticeable difference. you need lots of coverage in the 2D and 3D corners to be truly effective.
it is imperative that you understand how the material works in the first place in order to properly apply treatments. anyone selling treatments should be providing lab measurements of their products, or at least be able to explain how their product works and help you with a solution for your room. that is pretty obvious imho.
ive tried to keep this as high level as possible without going into too many details, but it's recommended to understand how the material works before blindly spending your money and possibly applying treatments improperly. this is why measurements are always a good idea. you don't want to blindly apply absorption unless you have a specific issue you're looking to treat.
if anyone else has a better way to explain, please do.