What makes a strobe or flash pop?

electric arc ... makes noise ... just like the thunder from lightning

But it's a vacuum.

Lightning occurs in air, thunder is the sound waves in air.

Sound does not travel in a vacuum.

It's not a vacuum. The tube is filled with gas at pressure that's lower than atmospheric pressure, but it's still got some pressure.

Anyway, the gas in the tube very rapidly heats up tremendously. That heating increases the pressure. A sudden change in pressure is pretty much the definition of a sound.

Handy Wikipedia reference to how a flash tube is constructed http://en.wikipedia.org/wiki/Flashtube

Xenon gas--set off by the electrical charge from the capacitor.

The flash tube doesn't contain a vacuum -- if it were, you wouldn't get the flash. In an ordinary flash, the tube is filled with Xenon gas (or at least that's the primary element). You have a plate at each end charged to ~300-500 volts. Then you have an electrode wrapped around the outside of (part of) the flash tube. When you want the flash to fire, you feed ~10,000-15,000 volts onto that electrode. That ionizes the Xenon, making it conductive. The ~300-500 volt charge then flows through the ionized Xenon, causing the flash.

Although a flash tube converts the energy to light more efficiently than an incandescent bulb, there's still a fair amount of heat released. A decent sized battery operated flash is about 10 watt seconds, but that 10 watt seconds is released in roughly 1/1000th of a second, so during the discharge you have ~10,000 watts of power flowing. Especially given the relatively small size of the tube, even a small percentage of that being converted to heat causes the gas in the tube to heat up quite abruptly. That makes the gas expand quickly, which makes the glass of the tube vibrate, in turn vibrating the air around the tube, which we hear as a sound.

The previous responses are correct, but there are also several other sources of the sound that comes from a flashgun that has just been triggered:

a) Whenever you discharge a capacitor, the attractive forces between the plates go from a large value (when the capacitor is fully charged) to a much smaller value (when the capacitor is discharged). They are held apart by materials which serve as a dielectric and mechanical spacer. When the forces suddenly drop, the plates try to spring back to their non-charged separation. Depending on the exact construction of the cap, this can make a significant sound.

b) Whenever you send a large current pulse through a wire, it generates a rapidly changing pulsed magnetic field. The B field from one part of the wire can interact with the field produced by the wires in another part of the circuit (...say, the ground return...). Just like two magnets, depending on their orientation, the two pieces of wire may either momentarily attract or repel each other. This leads to the wires moving slightly, but in a very short period of time, so very rapidly. For circuits of this size, this usually leads to a weak "click" sound.

c) The glass or quartz envelope of the flash tube gets a burst of UV energy deposited in it by radiation from the plasma. This leads to it quickly (ie, on the time scale of ms) expanding radially because of the coefficient of thermal expansion of the material of the envelope. Any rapidly moving surface in air (ie, the exterior of the flash tube) produces sound.

d) I have seen claims that the air around the flashtube is rapidly heated by the pulse of broadband optical energy from the plasma. I'm not so sure of this mechanism as air has almost no optical absorption in the portion of the spectrum passed by the envelope of the flash tube, but perhaps particles or gaseous impurities in the air do have absorption in the visible and near UV and so could also contribute to sound production.

Which of the above mechanisms of sound production is the most significant depends dramatically on details of construction of the flash unit.

Tom M

Thanks for the replies

There were flashes discharging at high speed with operating voltages of 2000 VDC.
Some tubes instead of polarizing electrode wrapped outside, have a narrow strip of conductive electricity paint at the back of the tube.