Membrane absorbers-Acoustics Forum-Acoustic Fields Forum

Dennis is this information supposed to be helpful or were you planning on following this up with a “technology” suggestion?

Redwood, you have good questions and clearly have a decent understanding of acoustics so please don’t let people like Dennis with his masterful vagueness deter you from asking.

The driver diameter is an unlikely concern at these frequencies unless you’re using 18inch drivers to produce your 400Hz where beaming may start forming, and you’re measuring off axis, but unlikely.

Although not ideal acoustically, this is actually a fairly generous room size for this application in comparison to the average that we see.

If I understand your design correctly, a noticeable dip around 400Hz could be caused by a coincidence coupling between the 2 compliant parallel surfaces, but it’s unlikely with a 10cm gap, more like 10mm. Thin panel membrane absorbers like yours are a good method for tackling low-frequency issues where space and budget is low because they work with pressure rather than velocity like fibrous materials, but their real-world behaviour is not always easy to predict. Trial and error is usually better here by adjusting the mass. I would use a generous amount of flexible adhesive rather than rigidly fixing the panels to the frame, increasing the absorption and predictability by reducing stiffness the stiffness of the system and reducing the modal behaviour as the energy travel outwards to the edges of the membrane to be partially absorbed.

The 400Hz dip could possibly be formed by the air spring – mass of the panel instead. Just give the panel a tap with the back of your knuckle and an RTA app on your phone with peak hold set. You can change this by adding masses to the centre of each panel to adjust the resonant frequency.

A few suggestions and questions,

– Do you have any small acoustic instruments in the room that may be causing this dip?

– Are you measuring the reverberation time in 1 position or in various positions throughout the room? If the former, reflected surfaces could cause this issue.

– Pre-construction reverberation measurements with the measurement method and locations and pictures of the room will really help in sorting this issue out. Happy for you to send them to me in a PM if you prefer.

– I would personally opt for a limp membrane absorber using mass loaded vinyl, giving you more predictable behaviour and a wide Q with insulation if you like, but this increases cost.

– The modes can also be controlled with a moderate budget using 4 or 2 small subwoofers placed in the centre of each wall in a cuboid room, instead of 1 large one or 2 stereo speakers placed in less than ideal locations. Have a look here, https://www.andyc.diy-audio-en…..fore_after. This site also provides links to some great studies on this topic. You can switch back to the 2 stereos speakers for mastering after mixing.