Damping material in scoops

JPF wrote: I am asking this because I have at the moment a couple of scoops that have poliurethan foam 120 kg/m3 density and 2cm thick as a damping material and I dont know if it brings any banefit. My concerns are: - Does this material reduce the volume of the chamber? - Does it affect the tonality and how the scoop perform in the low frecuencies? - Is it worth to have it in and leave it as it is? if not what would you do, replace it by other more suitable material or just remove it?

Scoops were some of the first bins I built myself in the early 90s. The only damping I ended up using was on the panel directly behind the driver, and it was just a few mms of bitumen from my car hifi installs called "brown bread". I didn't have any measurement capabilities back then, but to me it made the most sense to damp the panel exposed to the highest energy. Maybe it was just down to my poor construction abilities, but it sounded tighter and less resonant.

The function of damping materials in non sealed box sub designs is to control any out of band resonances that may result from the cabinet or driver, the audible difference between damping or no damping should be pretty subtle in most cases.

If the resonance is out of band, there is no signal to excite the resonance, which is why the difference between with and without damping is, as you say, subtle.

If you have ever noticed, a completely empty room echoes a lot, move even a small amount of soft furnishings into it and the sound changes substantially.

If you have ever noticed, even a very small room is much larger than a typical loudspeaker, which is why you can actually fit in it, with or without furniture. If you use Sabine’s formula to calculate the approximate reverb time of a 200 ltr enclosure it comes out at about 0.15 seconds. Whether it is accurate at such small volumes I don’t know, but I doubt that reverb is a problem. At low frequencies a typical enclosure is probably too small for even resonant modes to exist so damping material will have little effect.

For two way cabinets such as a 15/12 inch plus horn, mid-range resonances can have an impact on the response and damping material helps.

Never tried scoops with wadding myself but did a test with some ES18 a few years ago. The sound was noticeably better and tighter with the wadding.

Completely filling the enclosure with wadding can, depending on who you speak to, lower the box Q (less resonant, more damped), making the cabinet behave as if it was slightly larger, slightly changes the speed of sound, changes the compression from being adiabatic towards being isothermal. They are all sort of intertwined.

The only way to see for sure is to try it but I think you'll want to be trying to measure time domain (waterfall/impulse) type responses rather than the frequency response to see the biggest differences.

A waterfall plot is displaying the frequency domain at a succession of discrete time intervals. When using FFT to display the response, the time and frequency domains are directly related, which is how one can be calculated from the other. Maybe somewhat pedantic, but if a certain professional sound forum can have multi-page threads because someone mentioned phase when they should have said polarity….

Maybe it was just down to my poor construction abilities, but it sounded tighter and less resonant.

Possibly a placebo effect. We all convince ourselves that the latest modification has improved things.

Energy absorption is directly proportional to density of material ..so bitumen or roofing rubber sheet would probably be as good as it gets .....Has anyone tried Sorbothane ?

The internet tells me that bitumen has a density of 1.25 gm/cc, concrete 2.5 gm/cc and steel 7.8 gm/cc. If absorption is directly proportional to density, leaving a steel speaker chassis uncovered should be best. For loudspeaker completeness, fibre board has a density of medium.

To the OP the best way to determine the effectiveness of damping material is to experiment by measuring. It is a pain if you have to remove a drive unit every time you make a change, but one sometimes has to suffer for one’s art.

Personally, I use a cut-off of 150Hz. If the loudspeaker is only used below 150Hz I don’t bother with wadding. If it goes above, and into the mid range. I measure first and if the response is uneven I experiment wadding before fiddling with the eq.

Edited by levyte357- - 23 September 2025 at 12:51pm