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!"

... why don't I believe you? :)


Because it is seriously absurd.

It works on dark colored objects too. I just tried it on my dark blue check book cover and it does the same thing.

Late from an after party eh? :)

Sounds really weird. But there must be a rational explanation for your phenomenon.

I noticed some whine long ago on my C3 SS in med or low mode (regardless of color coded clothing XD). I then scheked the light and screw a 1/4 of the turn the body so i acustically dapmpened the whine. Never bothered to investigate the apparently called "coil" whine.

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..

Next project! An audio amplifier and a new topic: How does your flashlight sound like?

Reply with attachement:

My ultrafire 980 with Kd driver on Calvin Klein dark gree underwear hi-med-strobe.mp3

My Zebralight XY on Levi's black jeans stone washed hi-med-low.mp3

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My gucci t-shier actually produces an excellent mosquito repellant sound on med.

Wow....the Sky Ray 3 x T6 was pretty loud when you cover the black cloth/T-shirt over the head and move your ears next to it. Quite a bit of heat also.

My P60s did nothing.

Yeah, it's called photoacoustic effect.

In a spectroscopy lecture they told us an easy experiment: take a jar, blacken one side with soot and puncture the lid wit a small nail. Bring it close to a halogen lamp, and you can hear the frequency of the alternating current. Same thing here... but I didn't know you could do it with a flashlight. Cool find!

That effect is not completely new; if you hold a camera flash in front of a small gong, the flash will gong will sound a bit. The energy is partially absorbed in the topmost layer of the gong, heating it a little bit, and then heating up the air molecules colliding with it, giving them a higher momentum, and the gong itself gets a backwards momentum in return.

With PWM strobe flashes of quite some energy, a similar effect occurs; the black fabric abruptly heats up, that heats up the air around it, which then abruptly expands, like a nano-size explosion, giving a small, but audible shock wave each time. Occouring periodically at PWM frequency gives a (nasty) tone with that frequency.

Edit: Ah, Confusius was faster, I typed too slowly :)

Yep...if i put a piece of paper on my SB-900 and fire it off, there will be a pop. If I used a black piece of paper, there will be a POP. Sealed

Heck...............imagine the sound made by a mega output HID when it hits something matt black !!!

But the output of the HID is not modulated...so no sound. The ballast might be AC but you can't see the flicker, some metal halide causes white balance shifts on cameras that's all.

Yep it works on my dark blue sheets. I can also hear the high pitched sound from my flashlights when you select med or lo output. I guess my ears make up for my lack of eyesite, so it is strange to hear a high pitch sound from your flashlights as well. It only happened on low and med but never on hi. I also just tried something....now if I shine the light on low directly in my ear so that I can hear what noise it makes....the sound is the same as shining it in my dark sheets just not as loud. I think you are hearing the PWM doing its job more than anything because the tailcap has the exact same sound to my ears.

My post was in response to yours immediately above it, 2100. I don't think that your SB-900 is modulated either, is it... I was talking of a single POP just as you were. ....

But all my ballasts start off quite dimly, even 100W overdriving 55W bulbs. No real pop.

How the light starts off is immaterial. Its how bright it is the first time you point it at the matt black target. ......

pwm rules !!!

If you believe in telekinesis, raise my hand.

I have 2 x 100W HIDs here, and they focus quite nicely too. They do not make a pop when i quickly point it to my T-shirt, my matt black boxes etc. Its supposed to work only when it jumps from 0 lux to like 500K lux in a very short period of time.

Yes, it has to be very abruptly.

Yeah, I guess you're right. You'd need a very fast shutter in between I guess to get the effect. About as fast as the near instantaneous flash with you SB-900.