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5 feet of absorption w/ 5 foot air gap on all sides = perfectly flat room?
Old 13th August 2019
  #1
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5 feet of absorption w/ 5 foot air gap on all sides = perfectly flat room?

Hello, everyone.

1. Would a room have flat frequency response from 30Hz to 20kHz if all reflected frequencies above 30Hz were absorbed?

2. Would (5 feet of absorption + 5 foot air gap) on all 4 sides and ceiling absorb all reflections above 30Hz?



A 30Hz wave is 37.5 feet and absorption is most effective at quarter wavelengths, so 37.5/4=9.375, meaning the absorption would have to be 9.375 feet deep to fully absorb 30Hz.

But from what I understand absorption with an air gap is effective for a given frequency as long as absortpion is present 1/4 wavelength from the wall for that given frequency, so instead of 9.375 solid feet of absorption half of it could be air gap (4.6875 feet of absorption with a 4.6875 foot air gap to absorb everything above 30Hz).

So in theory, if a room had 5 feet of absorption and a 5 foot air gap behind the absorption on all 4 sides plus the ceiling, then the room would have zero reflections from 30Hz upward other than reflections of the floor, door, and gear, and thus a flat frequency response (even regardless of other factors like size/shape/arrangement).

This thought has been bouncing around in my head and I'm wondering if it's correct.
Old 13th August 2019
  #2
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It would be a quasi anechoic chamber.
For a anechoic chamber you need a mesh floor.

And yes, it would have a nice frequency response but these rooms are not comfortably to be in for longer time.
Old 13th August 2019
  #3
Yep thats an anechoic chamber.
You don´t want to mix in a room like that.
Most people feel very uncomfortable after a short period of time and the results won´t be be very natural sounding.
You can hear your blood flow through your veins
Old 13th August 2019
  #4
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Synthpark's Avatar
Quote:
Originally Posted by In Nashville View Post
So in theory, if a room had 5 feet of absorption and a 5 foot air gap behind the absorption on all 4 sides plus the ceiling, then the room would have zero reflections from 30Hz upward other than reflections of the floor, door, and gear, and thus a flat frequency response (even regardless of other factors like size/shape/arrangement).

This thought has been bouncing around in my head and I'm wondering if it's correct.
not really. You would need absorption coefficients of > 0.9 (apart from the room being way to dry).
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Old 13th August 2019
  #5
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Quote:
Originally Posted by Synthpark View Post
not really. You would need absorption coefficients of > 0.9 (apart from the room being way to dry).
Your chart says the absorbers have a "rigid backing", which seems like it would defeat the purpose of the air gap. Is that why it isn't above 0.9 absorption coefficient for the lowest frequencies?
Old 13th August 2019
  #6
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Synthpark's Avatar
Quote:
Originally Posted by In Nashville View Post
Your chart says the absorbers have a "rigid backing", which seems like it would defeat the purpose of the air gap. Is that why it isn't above 0.9 absorption coefficient for the lowest frequencies?
The rigid backing is simply an acoustically perfectly reflecting wall, which is assumed to come after the air gap, it is not something attached to the absorber.

But good that you asked: in reality, if the wall is not concrete, but some plasterboard wall, some bass may disappear through the wall, increasing the effective absorption coefficient.
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