Low frequency sound can be absorbed actively or passively. LENRDs, Venus, MAX-Wall, Sunbursts and other broadband (i.e., they absorb sound across the frequency spectrum) foam products from Auralex are passive absorbers. They absorb sound mostly through their size and placement in the room.
Active absorbers are devices that resonate at certain frequencies. Through proper construction and placement of these narrowband (i.e., they are tuned to a small range of frequencies) devices, low frequencies can be controlled. Panels do this by resonating at certain frequencies with the energy being absorbed by absorbent material placed in an air cavity behind the panel. “Helmholtz resonators” do this by having a cavity tuned to a frequency and – again – using absorbent material to absorb and dissipate the energy.
We would never claim that passives are better than actives, or vice versa. It’s all dependent on the specific application. Here are what we believe to be the pros and cons of each:
Passive/Broadband (LENRDs, Venus, etc.) Advantages:
- One type of product can treat many problem frequencies.
- Easy to install in corners, on walls, or on stands in the room.
- It’s difficult to have “too many” passive absorbers.
- Many varieties available.
- For deep bass control, larger devices are needed.
Active/Narrowband (panel, diaphragm, Helmholtz, etc.) Advantages:
- Very specific problems can be addressed in the room.
- With panels, not necessarily as much space required for deep bass control. (“Helmholtz resonators” are still large.)
- Design tolerances within ±1/128” (roughly ±0.008”) required for maximum control.
- If mounted incorrectly, they are ineffective.
- If placed in the wrong part of the room, they are ineffective.
- They can introduce the possibility of higher order reflections and “ringing” into the room.
- Prefabricated devices are expensive.
Auralex does not manufacture panel or Helmholtz devices. The sheer number of different products this would require to cover all ranges of frequencies would be difficult to manage.
However, we would be very happy to help you design devices of your own. We have a wealth of resources available whereby we can help you inexpensively build your own active devices. Please don’t misunderstand – active devices can and do work for controlling bass problems. We’ve helped quite a few customers design them and use them successfully.
Bottom line: You should be wary of any prefabricated devices out there. They are not likely to be specific enough to be more effective in your room than – for example – LENRDs or Venus.
If you would like more help with this, please let us know!