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I have a led headlight which seems to use pwm modulation to ajust brightness (led lenser xeo 19R). With an intermediate light power (i.e. not 0% or 100%), the light produce a really small noise, which is almost inaudible. This is probably some kind of harmonic of the pwm modulation.

When I point the light beam to a square of foam (similar to the picture below) the foam produce the same kind of noise, but this time this is easily audible :

Foam

Firstly, I was thiking of sort kind of resonance of the foam (like a guitar) but I made 2 test which tend to prove that this is really the light which make the foam "noisy":

  • If I place a transparent filter between the light and the foam : noise is still present. If I use a marker to make the same filter opaque : noise is reduced
  • I place the headlight and the foam one next to the other but the beam is facing a wall : no noise. If I place a mirror on the wall so the beam go to the foam : noise start.

Have you any physical explanation of such a phenomenon?

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  • $\begingroup$ Could you try hear the noise from one piece of foam placed near the illuminated one? I mean to rule out once more resonance $\endgroup$ – Alchimista Oct 18 '17 at 19:46
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There isn't really enough information, but if the light is both bright (and bike LED headlights are pretty bright in my experience) and being switched at audio frequencies, then an obvious candidate is heating: the foam is black, and so absorbs plenty of energy from the light, and also has a lot of trapped or partly-trapped air which it can heat. So the light heats the foam, and this heating is periodic at some audio frequency.

A good test would be to try black paper, which ought also to show the effect, although probably much less strongly (less trapped air). You could also try spray-panting the foam white or silver which should reduce the effect, if I am right.

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  • $\begingroup$ With Black paper : no noise. I don't have spray available yet but i'll test soon But you're probably right. With full power, the foam temperature increase by 10°C $\endgroup$ – Golgot Oct 18 '17 at 20:21
  • $\begingroup$ Oh that smells convincing then: there's obviously a lot of energy getting dumped into it (or, probably not very much but it's foam and has tiny thermal capacity). $\endgroup$ – tfb Oct 18 '17 at 20:46

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