properties
-2 inch thickness
-2lbs/cu ft. density
open cell design
freq------NRC
125hz----0.18
250hz----0.32
500hz----0.92
1000hz---1.06
2000hz---0.93
4000hz---0.92
NRC rating 0.80
My question is if this product is good for a cabinet lining housing 2-12"AT for 20-200hz and a Volt 12 for 200-800hz
thanks john
-2 inch thickness
-2lbs/cu ft. density
open cell design
freq------NRC
125hz----0.18
250hz----0.32
500hz----0.92
1000hz---1.06
2000hz---0.93
4000hz---0.92
NRC rating 0.80
My question is if this product is good for a cabinet lining housing 2-12"AT for 20-200hz and a Volt 12 for 200-800hz
thanks john
NRC = Noise Reduction Coefficient.
simplified:
0 is "completely" reflective, no absorption.
1 is "completely" absorptive, no reflection.
testing methodology can result in >1 numbers.
The overall NRC is an average over a limited frequency range (250 to 2000 I think, essentially covering speech frequencies only).
If the sound is subject to bouncing off the material several times, such as within an air duct, or within a speaker cabinet, or in a small room, then the effective absorption is magnified due to the multiple opportunities to absorb the sound energy.
simplified:
0 is "completely" reflective, no absorption.
1 is "completely" absorptive, no reflection.
testing methodology can result in >1 numbers.
The overall NRC is an average over a limited frequency range (250 to 2000 I think, essentially covering speech frequencies only).
If the sound is subject to bouncing off the material several times, such as within an air duct, or within a speaker cabinet, or in a small room, then the effective absorption is magnified due to the multiple opportunities to absorb the sound energy.
The only thing clearly dodgy here is the uniformed opinions being expressed. (that said I can't vouch for the specs of this material, but there is nothing here to suggest anything dodgy about them).
The NRC specs for many materials are >1 at certain frequencies. This is due to the way the test is done, and is considered an acceptable result. This is because the number is not based on a single reflection, but rather, the attenuation after several reflections. The test is done in a special reverberation room, and the NRC number is related to the time it takes for the tone at the specified frequency to die down. Obviously any frequency will decay to nil after some time, but if the sound decays to a certain (small) level after a specified (small) amount of time, it can score a 1.00. If it decays even faster than that, then it can score better than 1. If it takes longer, it scores less than 1. Look up ASTM C423 if you want the gory details.
The NRC specs for many materials are >1 at certain frequencies. This is due to the way the test is done, and is considered an acceptable result. This is because the number is not based on a single reflection, but rather, the attenuation after several reflections. The test is done in a special reverberation room, and the NRC number is related to the time it takes for the tone at the specified frequency to die down. Obviously any frequency will decay to nil after some time, but if the sound decays to a certain (small) level after a specified (small) amount of time, it can score a 1.00. If it decays even faster than that, then it can score better than 1. If it takes longer, it scores less than 1. Look up ASTM C423 if you want the gory details.
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The answer is: not so good, at least for the 20 to 200 range. You would want a higer absorption at 125 and 250. It is hard to beat good old fiberglass, but you need some thickness and the efficiency of the material will increase if you can get it off the surface.
David S.
It will absorb more mids and highs. By your specs the material is a good absorber from 500 Hz up (.9 or higher).
Low frequencies are always difficult to absorb. The lower you want to absorb the thicker the material needs to be. In general you need material of a depth of about 1/6 the wavelength for good absorption.
David S.
Low frequencies are always difficult to absorb. The lower you want to absorb the thicker the material needs to be. In general you need material of a depth of about 1/6 the wavelength for good absorption.
David S.
i consider myself told
but seriously then, is this not the same coeffiecient used in architechural acoustics? forgive me i might be totally misinformed/confused about something, but is there another absorption standard? it just im sure i recall stumbling on a page with various materials as a comparison but perhaps a different metric?
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thats not the page lol but its very comprehensive
this is what i mean...maybe an old method or something? i duno if its the same, with a different name...
Sound Absorption Coefficients for some common Materials
according to this much shorter list, the best thing to stuff your speakers with, would be a dead body or two.....
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You mentioned two different woofers and 2 different frequency ranges. The material you describe will be pretty good for the 200 to 800 range but not very good for the 20 to 200 range. For the lower range then a thicker layer of better material (fiberglass) will do a better job.
The ideal goal is to absorb the resonance frequencies of the box. The resonance frequencies are determined by the internal dimensions and are the 1/2 wave dimensions and their harmonics. (Much like the resonances of a room but at higher frequencies from the much smaller dimensions.)
David S.
i consider myself told
but seriously then, is this not the same coeffiecient used in architechural acoustics? forgive me i might be totally misinformed/confused about something, but is there another absorption standard? it just im sure i recall stumbling on a page with various materials as a comparison but perhaps a different metric?
Yes, this is alpha or absorption coefficient exactly as used in architectural acoustics. Logically it would max out as 1.0 but it turns out that the formula and standard testing frequently gives numbers a little over 1.0. My understanding is that, in part, this is because materials are usually defined as their top surface area, but may have appreciable side surface also. The side surface isn't normally counted. Beranek says you really need to test materials laid out much as they will be used in the architectural situation, if you want the numbers to be totally relevant.
David S.
could you explain that formula further? say inside dimensions were 8"w x 10"d x 20"h for example.
david
Waveguide and Cavity Modes (Sound)
Look at the equation with the (52) in front of it.
q, n and m are integers (0, 1, 2, 3...)
d, a abd b are length with and height.
c is the speed of sound in the same units you specify length, width and height
for inches, resonances will be at:
f=13572/2*sqrt((q/8)^2+(n/10)^2+(m/20)^2)
20" resonates at multiples of 13572/(2*20)=339Hz
10" at multiples of 678 Hz and 8" at multiples of 848Hz, etc...
There are many, many resonant modes in an enclosure.... Perhaps worse than the modal resonances of the volume are modal resonances of the panels....
This is all very academic, honestly, just put some foam in them, stop obsessing and build them. The devil is always in the details.
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