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 :
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?