LED Flashlight makes weird buzzing noise when pointed at black surfaces

[fire-stick]

Sorry if this isn't the right section for this. Like I said, 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. When I point it away from surface it stops. I got it to make the sound on my cast iron stove door and a black hoodie. http://https://youtu.be/A89SphplMe4
Regular audio https://youtu.be/XBEthiHh1Ns
Audio 2x https://youtu.be/7kdO6j_l3r0
This is a link to the video of it happening.

 

[thermal guy]

WHAT THE!!!!!!

 

[BigBen]

WOW!
Never saw/heard THAT before.

 

[LRJ88]

Almost sounds like there's something heating up there right as the light hits them, do you get a similar effect if you have the light on high and just quickly move over other the black surfaces?

 

[bigburly912]

Wasn't there someone on here a long time ago that had a video similar to this with a black t shirt?

 

[thermal guy]

Sounds like inductor whining.

It's like your hearing the PWM

 

[jon_slider]

when I point my light at rough black surfaces in strobe mode I can hear the surface pop/buzz along with strobe frequency.

congratulations, you have discovered another way to convert pulses of light, to audible pulses

your cast iron stove, and black hoodie, are vibrating like an audio speaker

I dont recommend listening to PWM (strobe is a form of PWM), but if you want to, there are soundclips, and directions in this post:

Detecting PWM by Ear

Most flashlight PWM takes place in the audio frequencies.
A novel way to detect (some) PWM is to hook a PV cell (Solar cell) to a speaker.

 

[fire-stick]

Almost sounds like there's something heating up there right as the light hits them, do you get a similar effect if you have the light on high and just quickly move over other the black surfaces?

no it just does it in the super fast strobe like way faster than 16hz default probably like 40ish.

 

[thermal guy]

So you can actually hear PWM?

 

[Lynx_Arc]

It is possible that the noise is coming from the front of the light itself and pointing it lets it bounce off surfaces back to you.

 

[WalkIntoTheLight]

So you can actually hear PWM?

I'm not sure you can hear PWM, but you can hear inductor whine (as mentioned earlier). This happens when the component isn't well secured, and it can vibrate causing a high-pitched whine. I have an old 4sevens light that does this. Quite annoying.

 

[PhotonWrangler]

Try rotating the flashlight while this is happening and see if the whine changes. If it does then it's probably transformer whine in the buck/boost driver.

 

[bigburly912]

I can hear pwm coming off of LED fixtures with dimmers. Could be something else but on the lowest modes you can always hear it. Would make sense you could hear it in a flashlight. I could be wrong though

 

[thermal guy]

If it was just inductor whine you would be able to hear it all the time. This is only happening when pointing at a black surface. The Color must be absorbing the light or something. See if it works pointed at a flat screen TV or Monitor.

 

[staticx57]

The Noctigon in the video does not use PWM to regulate the output. It is also a linear driver so no inductor.

 

[PhotonWrangler]

The Noctigon in the video does not use PWM to regulate the output. It is also a linear driver so no inductor.

Hmm... this deepens the mystery.

 

[WalkIntoTheLight]

The Noctigon in the video does not use PWM to regulate the output. It is also a linear driver so no inductor.

I think lights with a linear driver still use PWM if they're not running at max output.

 

[alpg88]

pwm is not bad if the frequency is high enough, and parts are of a good quality that do not whine, i have several drivers that whine, most of them cheap 7135 with no inductors, I also have an old radio shack chameleon remote control, that also whines when it's lit

 

[staticx57]

I think lights with a linear driver still use PWM if they're not running at max output.

The Noctigon K1 is specifically a constant current + FET design. The FET is only used for Turbo so it is also not using PWM.

Linear drivers regulate voltage by "burning" off the excess voltage in the driver. It is not a reference to how current is regulated.

 

[jon_slider]

The Noctigon in the video does not use PWM to regulate the output. It is also a linear driver so no inductor.

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..