Why foam or thin panels won't work for low end absorption

Intro

For sufficient absorption of low frequencies (below 200Hz), we need at least 150-160mm absorptive material depth, in combination with the "corresponding" air gap behind the panel. The reason that this depth/air gap is the minimum to start with, can be seen in the two diagrams below.

Example & explanation

Typical low end room problems (caused by room modes), live at frequencies between 30Hz (and below sometimes) and up to 200Hz as show below in diag 1.

Even a 160mm deep rockwool panel with an 50mm air gap behind it, reaches only at 82Hz at 50% absorption. The room problem at 60Hz will be "treated" at about 30% in our specific example which isn' t enough.

An even worse case, is trying to treat these problems with foam absorbers which are usually mounted flush on the wall (0 cm air gap behind them).

In this case 2 things are happening in our example:

• The resonance problems of our room at 60Hz, 110Hz & 190Hz will be controlled at about 5%, 10-12%, and 30% respectively.

• This results to a very poor low end absorption in combination with the 90-100% absorption in frequencies above 1Khz. This situation creates an un-balanced frequency spectrum for the listener, leaving the low end practically un-treated, ending up with our (low end) problem being exaggerated instead of controlled.

Quick Note: The 0.5 absorption coefficient or 50% absorption of any material (most common are fiberglass or rockwool), is considered enough to begin to tackle the low end issues of any room.

Diag 1.

Diag 2.

Conclusion

Dealing with low end resonances, even when you begin to acoustically treat a room, requires:

• deep enough panels (150-160mm rockwool) with air-gap of 300mm*¹ behind them (or at least 200mm in acoustic ceiling panels for example)

• usually a larger quantity of acoustic panels is needed than you may think is enough, so you should be ready to sacrifice a significant amount of room space to see major improvement in your room acoustics. Some examples to keep in mind as a rule of thumb:

  • Small Room basic treatment (~10-15m²) requires about 8-10 panels to manage to control the low end down to 40-50Hz

  • Medium Room mid to "pro" treatment (~20-25m²) requires about 13-15 panels for the equivalent result (down to 35-40Hz)

  • Large Room Full treatment (~30m² and above) requires up to 30-35 panels for the equivalent result (down to 30-40Hz)

• the understanding that broadband absorbers are your main tool & the best value for your money. More complicated panels/designs will be way more expensive, usually target specific problems & not the full frequency spectrum.

• diffusion can (and should) be used afterwards (➡ DF-20), or combined with absorption (➡ L16-DF). It will help but it won't solve your problem on its own.

Foam absorbers*² won't solve your problems mainly because:

• they are not deep enough to absorb low frequencies

• cannot be mounted on their own (without a frame around them) & with an air gap behind them

• are way more expensive for the same surface coverage

More comparison examples between materials & positions, are shown in our Frequency Diagrams section.

Notes (*):

1. The 300mm gap behind the panels is created for a panel with dimensions 100cm x 60cm, when placed at 90° corners in vertical or horizontal nearby walls. Vertical wall-to-vertical wall & vertical wall-to-ceiling are the most usual positions as shown below.

2. The problem in this case is not the foam material itself.