LED Flashlight makes weird buzzing noise when pointed at black surfaces

[joshk]

A hoodie and a cast iron door shouldn't vibrate. I would say either it's directly emitting sound, or there's some interference being emitted that reflects back into the flashlight and does that LED-as-a-solar-panel trick to feed back into the circuit. Kinda like how a microphone and a speaker create a feedback loop and a new sound.

 

[staticx57]

I think you misunderstand
he is using the strobe
strobe is pwm
thats why there are black bands in the video
when power turns off and on, it can generate sound when the pulses of light are causing a target to resonate, as in the black cast iron and the black sweatshirt

or also this effect can be captured by a solar cell, and fed to a speaker, to create god awful noises, that I dont know why anyone would want to hear..

Sorry this is not correct. PWM has a specific definition and is a reference to regulating power delivered to a device. It is not a term that's interchangeable with strobe/strobing.

 

[jon_slider]

Sorry this is not correct. PWM has a specific definition and is a reference to regulating power delivered to a device. It is not a term that's interchangeable with strobe/strobing.

pwm turns the light off and on
so does strobe

the OP is using strobe, which is turning off and on
things that can detect pwm, can also respond to strobe, which turns on and off, like pwm

anyway, what is your take on the cause of the audio created by strobing, as posted by the OP?

 

[thermal guy]

Found this "Photoacoustic spectroscopy". Big words folks. Big words.

 

[staticx57]

pwm turns the light off and on
so does strobe

the OP is using strobe, which is turning off and on
things that can detect pwm, can also respond to strobe, which turns on and off, like pwm

anyway, what is your take on the cause of the audio created by strobing, as posted by the OP?

I think thermal guy hit on the effect below. PWM is a specific technique to reduce or manipulate the average power in a system either through current or voltage. It is not interchangeable with strobe just because they both "flash"

Found this "Photoacoustic spectroscopy". Big words folks. Big words.

This sounds about right, if it is the light and its driver it should be coming from the light not from the surface as said in original post.

 

[jon_slider]

Found this "Photoacoustic spectroscopy". Big words folks. Big words.

thank you, google says:

"Alexander Graham Bell showed that thin discs emitted sound when exposed to a beam of sunlight that was rapidly interrupted with a rotating slotted disk"

when I point my light at rough black surfaces in strobe mode I can hear the surface pop/buzz along with strobe frequency. Its not the camera or the light because I can hear it coming from the surface.

I agree the sound is coming from the black surfaces, not from the light, nor its driver.