April 29, 2011

Do small foam traps actually work?

The short answer is NO! Small foam "bass traps" achieve very little. Foam traps must be oversized in order to be effective, because foam isn't the ideal material. If you put four foam single bed mattresses in four vertical corners, you would get an acceptable result, but the 1x1ft foam wedges sold as bass traps are well short of being enough. Shown below is what I call a "minitrap" which is about 0.3 x 0.3 x 2m in size, placed in a corner.

Here you can see the response is virtually identical. When bass trapping is adequate, we should at least see the Q of the peaks smoothed out. 

The waterfall shows a more complete picture. Again it confirms no real improvement. A great deal more trapping is required. Grey is the empty room, green is with the minitrap.


  1. Open cellular foam is characterised by its flow resistivity, i.e. how much energy does the foam absorb by hindering the flow of air through it. This definition reveals that if the flow is zero, no energy absorption can occur. This placing foam near a reflective boundary where the particle velocity (i.e. the flow) is zero by definition will not result in any absorption.

    If foam is to be used for low frequency absorption, it needs to be placed at the room modes' nodal lines, not at the walls. That's why acoustic engineers never use foam for low frequency absorption.

  2. Hi Svenr,

    I have used foam traps and shown meausurements elsewhere on my blog. The small ones fail due to their size. Large foam traps can be very effective.

  3. A saw the measurements. The large foam traps did something - but not what one actually wants. You achieved absorption above 120 Hz over the entire bandwidth, but none at low frequencies. So your reverberation time curve simply shifted down in time with an even more pronounced exaggeration at low frequencies. I'm sure you heard significant differences, but you didn't get a better controlled reverberation time.

    Absorption at low frequencies needs to act frequency selective. Foam, due to its broadband absorption, is not suitable for that. Feasible solutions include plate absorbers, Helmholtz absorbers or stud walls of very low flexural rigidity backed by absorptive material.

  4. svenr, I can't agree with any of your points.

    The traps have excellent measured performance and are effective down to 25 Hz; more recent versions have very little room for improvement.

    That some absorbers are narrow band is certainly not a desirable feature. Their strong point is their low profile. Their narrow bandwidth means that more of them are required to cover a wider bandwidth. Typical rooms already have pressure based absorbers built in.

  5. I may not have seen the measurements you meant. What's the name of the entry?

  6. This one was take later:

    However there are more recent measurements that I had a look at when considering adding membrane traps. These aren't on the blog, but they suggest no real benefit.

  7. When comparing the effects with / without the corner foam traps, did you also measure the reverberation time over the entire bandwidth?

  8. I use REW which by default also includes reverb time, but it's not something that I pay a lot of attention to. I'm more interested in the decay plots because they show how well modal ringing has been damped. In some of my more recent measurements, the in-room waterfall plots start to look like nearfield measurements.

  9. I suggest calculating the RT60 from the impulse responses you measured. Surely, increasing the decay rate of individual modes is important. But by using broadband absorptive devices you also increase th effective absorption coefficient, thus lower the RT60 above the 200Hz limit of your plots.
    Referring to Toole's latest book; their is a lower limit of RT60 below which the listener envelopment and apparent source width decrease and adversely affect the subjective quality of reproduction.


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