by ʇolɐǝz ǝɥʇ qoq

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When recycling, some flashes make a whistling sound, going from low to high frequency for a few seconds. After a more intensive flash, the sound is more intense as well.

What is the source of this whistling?

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The whistling noise always reminds me of "Doh... Ray... Egon..." from Ghostbusters as they power on their protopacks, and thus always makes me smile :-) – Andrew Heath Jan 28 '12 at 5:17
up vote 37 down vote accepted

There's an oscillator that creates AC current from the DC supplied by the batteries. The AC is needed to step the voltage up to the 300 or so volts that the flash tube needs, and that voltage is rectified back to DC and used to charge the capacitor (which can deliver a lot of DC current in a very short time). The sound you hear is mechanical vibrations caused in the transformer as the voltage is stepped up -- the transformer converts the low-voltage AC from the oscillator to changing magnetic fields, which, in turn, induce higher-voltage AC current in a second coil of wires on the transformer. The changing magnetic fields distort the transformer very slightly, and those mechanical distortions are what you are hearing. As for the changing frequency, that has to do with the amount of voltage (think of it as "electrical pressure") the oscillator is working against, which changes the operating conditions of the oscillator circuit.

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rogers: I think your answer is basically correct, but the details of the circuitry are not quite accurate. Modern cameras charge flash capacitors using what's called a "boost DC-DC converter." The circuit consists of an inductor, a diode, and a transistor switching on and off around 5-15 kHz. The whining you hear is the mechanical vibration of circuit elements, but there are no transformers involved any more, because they're big and heavy. More info on boost converters: – pingswept Dec 19 '10 at 16:59
Its basically doing this : . Unintentional in your case, intentional in the video. – Tejas Kale Jul 11 at 15:24

If you get a new high-end flash, such as the Canon EX 550 II, there will no longer be that whistling sound.

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The sound is not a problem for me. I was just wondering what is the source of this sound, and the answer by Stan Rogers explains it very well. – MainMa Dec 19 '10 at 15:23
I was surprised when I got a nice new Metz flash, and no longer had that sound to indicate when the flash was done recycling. Now I have to look for the "ready" light. (Oh, the humanity.) – mattdm Dec 19 '10 at 15:41
I think the sound is cool! – Zabba Dec 19 '10 at 20:41
@Zabba - you can always buy an CP-E4 if you want to hear the whistle again. It has a nice comfortingly loud one. I too enjoy the sound of it charging.:) – AJ Henderson Apr 1 '13 at 19:30

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