I have discovered the most absurd thing ever.

After being evacuated from my apartment due to a false fire alarm, I found out that you can hear a flashlight with PWM by shining it on black cloth.



I was walking around out side while the fire fighters inspected the complex and noticed that when I shine the light on my black t-shirt, I could hear the PWM very loudly. I thought something was wrong with my light so I put it next to my ear. The noise stopped. Crap, I thought, something on the driver board is falling off. I shook the light, no difference. So, I held the light at my chest again and there was the sound!

Okay, this was really blowing my mind. Where was the sound coming from? I put my hand in front of the light, the noise stopped!

Experiment time! I grabbed the corner of my black sheets, held them up and pointed the light at the sheets. I could hear the PWM!

Now to try more lights. Both of the PWM lights I tried (Yezl Z1X and my 4Sevens Mini AA2) made the sound. The current controlled light, my Spark SL6, did not make the sound. I believe I am hearing the fabric vibrate as the black dye absorbs the photons.



Try it out! Just not around people, I feel awkward just telling you that I was walking around with a sheet in one hand and a light in the other, holding them up to my ear while proclaiming "I CAN HEAR THE PULSE WIDTH MODULATION!"

My PWM controled lights haven't this behavior. Perhaps it is a problem of spatial orientation! In few words, if you put your flashlight with the body vertically near your head, you can hear the PWM sound. But, if you put it horizontally, with the tailcap near your head, perhaps you can't hear anything!

So, IMO, it's only the relative position of your flashlight relative to your ear. Try with a white sheet in the same condition and let we know!

PS: the photon absorption doesn't make noise, because it involves only electrons and their bands, nothing moving...

Maybe you hear your flashlight say "help! the black hole sucking my power."
:laughing:

Wow, I can confirm this! Interesting find, it only seems to work with high frequency PWM though. It's much louder when shone on black or dark cloth surfaces. It sounds like it's the fabric itself making the noise..

April Fools???

I've never noticed this phenomenon, now I have to wear a black shirt and find some PWM lights to shine on me. *sigh* Thanks for making me look goofy today CPF.

I'm kind of afraid to comment. This is seriously strange. I'll bite and go give it a try in a bit.

Geoff

Flying Turtle said:

I'm kind of afraid to comment. This is seriously strange. I'll bite and go give it a try in a bit.

Geoff

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let us know Geoff......if anyone can debunk this- it would be you.


thx...Jeff

That's not PWM you are hearing. It's the screaming of 1,000 unhappy Chip Weevils. They don't like black.

You guys forgot to wear your :tinfoil:

I only have one light that emits a pitch, a DIY-MCE dropin in an ultrafire host. Walked around with it held next to my ear like a headlamp and pointed it at various colors. I could not get it to decrease or change amplitude when pointed at black objects.

It did change slightly as I moved around, but it does this all the time. the inductor coils inside are affected by gravity, so tipping/rolling the light around can get the noise to stop.

DM51 said:

That's not PWM you are hearing. It's the screaming of 1,000 unhappy Chip Weevils. They don't like black.

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I can confirm this. I had a bad case a few years back...


:sick2: lovecpf

carrot said:

April Fools???

I've never noticed this phenomenon, now I have to wear a black shirt and find some PWM lights to shine on me. *sigh* Thanks for making me look goofy today CPF.

Click to expand...

Perhaps revenge for a few of the challenges?

Well, I dusted off some old lights with PWM (Jetbeam C-LE, Rexlight, L0D Q4) plus a Maratac AAA and LF2XT that had some whine and gave it a try. No changes that I could detect by shining on black cloth. I didn't really expect anything, but it was worth the attempt, and it's always fun to have an excuse to play with the lights. Maybe these old ears were just not up to it, or I've been punked (is that the right term?). Since none of our electronic boffins have chimed in, I suppose the latter is the answer.

Geoff

Flying Turtle said:

Well, I dusted off some old lights with PWM (Jetbeam C-LE, Rexlight, L0D Q4) plus a Maratac AAA and LF2XT that had some whine and gave it a try. No changes that I could detect by shining on black cloth. I didn't really expect anything, but it was worth the attempt, and it's always fun to have an excuse to play with the lights. Maybe these old ears were just not up to it, or I've been punked (is that the right term?). Since none of our electronic boffins have chimed in, I suppose the latter is the answer.

Geoff

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Not punked, just hard to hear.

More info here: http://en.wikipedia.org/wiki/Photoacoustic_spectroscopy

might be the material. cotton or synthetic, nylon or polyester, silk. same as some material is much better at static electricity tricks.

That's very interesting, KLowD9x. Thanks for that link. Please pardon my false accusation.

Who would have thunk it? Think I'll have to try the test again.

Geoff

I was a bit skeptical about this, and tried my Quark mini X earlier on low, and got nothing - couldn't hear a thing.

About 10 minutes ago I decided to try again, this time with the mini X on medium. Pointed it at a black piece of fabric, and behold, I heard the light - a definite whine. No fabric, no whine.

So I did some more testing. ..

Seems to work with red cloth, black nylon, and black cardboard too. In fact on all colours of paper, it is audible, but more so on darker colours.

Also seems to work if I shine the light directly on my ear if pointed at the ear perpendicular to the head at close range, but not tangential - and no, its not inductor whine.

Does not appear to work on wood, metal or stone.

My ITP A3 does nothing at all, but its pwm frequency is a lot lower.

Hmmm . . . .


lovecpf

I tried it on my NB XML 3 level drop-in my only light that I have that whines. I took my light and held it vertically with the bezel pointing down next to my ear. The whinning did got louder when I pulled my black shirts sleeve in front of the bezel.

Lanque said:

no, its not inductor whine.

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So what does it say? :naughty:

In response to the OP. I wear black t-shirt almost everyday, sort of like my uniform, and I have yet to hear anything. Maybe I should give it a try tonight. Amazing topic.