AIR GAP BEHIND ABSORPTION PANELS

 



I don't know why people misunderstand this. Perhaps it is me and my replies to specific questions. (If the teacher is good, even children will understand.) - I owe you an apology.


What's with the 1" (25 mm) gap? The gap I'm talking about HERE is NOT the air gap. It is the opening through which the pressure can escape.


Ex. Let's take a two-foot-square porous absorber: it is mounted either directly on the wall or ceiling, or spaced away from the wall or ceiling by a maximum of 1 inch. Up to this spacing, the pressure is allowed to build up and compress against the wall or ceiling at the frequencies relative to this absorber. This will shift the knee of the high-pass part of the absorption slightly lower in frequency.


This is the part that is probably misunderstood. Again, I apologize.


Here, I am only talking about the open gap that can allow pressure to escape.


Now, let's get into the actual air space behind absorption:


Let's use the same 2 ft by 2 ft treatment example.


Say it's 4" (10 cm) of porous absorption. A wooden frame covers the sides completely.


This frame can be 4" and up to 6".


**I use the half-thickness of the actual porous absorber as the maximum air space behind the absorber, since that is where we achieved the highest efficiency.


Efficiency is essential here. This will efficiently extend the LF knee of the absorption curve. Again, never allow anything to relieve the pressure if you want this benefit.


So, now we mount THIS 6" thick panel (with 2" of air space behind the absorption) directly to the wall or ceiling, with no gap between the frame and the wall or ceiling. This system provides the optimal absorption.

*Sometimes a small gap is unavoidable due to the mounting constraints.*

But never allow an open gap larger than 1" to maintain the pressure trapping. (I have reams of testing data that confirm this)

You will hear or read things like "you can make an air gap up to 2.5 times the thickness of the absorption"...


This is wrong, since we THEN enter resonance. For example, the 1/4 wave resonance of 12" is 281 Hz. The full-wave resonance is 1126 Hz (reference level); the half-wave (563 Hz) efficiency is 50% (-10 dB); the quarter-wave (281 Hz) efficiency is down again by 50% from the half-wave, for a total of 25% of the reference.

This means the best you can get from the quarter-wave resonance is around 1.5-2 dB of attenuation. Which, in my honest opinion, is not NEAR enough to be worthwhile.


Conclusion:


The Maximum air space behind a porous absorber should never exceed half the actual absorber thickness. The FRAME should extend to the partition, sealing the pressure behind the absorber to realize the benefits of pressure trapping.

Please ask questions if this isn't understood clearly.

*Note this is the same thing that happens at the bottom of the wells of the Schroeder diffuser, creating undesirable absorption.


The same thing happens when one of my 510 or 511 units (4" deep) are placed directly on the walls. We do this on purpose in many tracking rooms. (ask me more on this subject) *



Best Regards,

John Brandt

Email: john@jhbrandt.net

jhbrandt.net

If you are planning a studio, mixing room, or upgrading an existing space, don’t rely on assumptions. Proper acoustic performance can only be achieved through detailed analysis and 3D modeling.
Contact us to evaluate your room and get expert guidance tailored to your exact requirements.

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