Have you ever wondered why a flash unit makes noise? No, not the kind of noise you get from low light or long exposure star photography. The audible kind. The loud kind. My speedlights make a popping sound when fired and a high-pitched whine when recharging. Why is that? In a digital age, when we can put cameras into a silent mode, why is it that my flash is still making noises?
OK, this isn’t contemplating the meaning of life or solving world hunger, and my aging body makes its share of pops, crackles and other mysterious noises, but the question did pique my curiosity. What are the shadowy forces behind these noises?
The Invention of Flash
Back in the early days of photography, there was no flash or artificial light. The camera shutter had to be open for a long time. So long that portrait photographers used braces behind their subjects to help them stay still during an exposure that could last two or three minutes. The introduction of flash powder as a way to generate portable, short-duration light, was a game changer. Like gunpowder, it burned so rapidly that there was an almost instantaneous flash of light. And, like gunpowder, it was dangerous, with a number of photographers suffering burns from flash powder accidents!
Everything changed when single-use flash bulbs were invented around the 1930s. The bulb, a glass element containing two electrodes connected by a filament or foil made of magnesium, zirconium or aluminum, was filled with oxygen and sealed at the base. One of the early flash bulbs, the Vacu-blitz, was actually filled with thin, crumpled-up aluminum foil!
Application of an electric charge caused the filament or foil to rapidly heat up and burn, generating heat and light. This occurred so quickly that the gas inside the bulb expanded and created a small pressure shockwave that caused a popping sound. A single shot would burn up the filament or foil, rendering the bulb useless. The rapid expansion of gas and resultant shockwave would also occasionally shatter the more fragile early glass bulbs. Photography wasn’t an especially safe profession.
Those of us who have reached a certain age can remember those old flash bulbs and how surprisingly hot they got from that incredibly brief flash. The bulbs were so hot you could burn yourself. Some cameras even had an ejection system that would pop the used bulb out automatically.
How Modern Flash Works
Now we have self-contained, multi-use flash units, built into the camera or separate units, either speedlights or studio lights. The blast of light that these modern flashes generate isn’t from a substance burning, but it does involve heat and gas. It’s the result of a series of actions that take place once you press the trigger and which involve several components:
- A battery, either the camera battery for built-in flash, or the batteries on your speedlight or studio light.
- An electrical circuit and boost converter
- A tube full of xenon gas
- A metal plate
- A reflector to direct the light
The tube is what emits the flash. Like a neon or fluorescent light, there’s an electrode at each end of the tube, which conduct an electrical current through the gas. The current excites the gas atoms, causing them to emit light. That’s a simplified explanation, but works for our purposes. Unlike the low and steady output of a fluorescent or neon lamp, a camera flash requires a much more powerful burst of light that lasts a tiny fraction of a second.
In their normal state, none of these gasses emit light. Otherwise, they’d be on even if the electricity was off. Instead, they need power. When an electric current moves from one electrode (negative) through the gas to the other electrode (positive), it moves free electrons. But there’s a problem. Xenon has few free electrons and applying a small current won’t do squat. So, your flash also has a metal plate. When the flash unit sends a brief, but strong positive voltage to the plate, that applies a strong pull on electrons (which are negatively charged). With enough voltage, electrons are pulled out of xenon atoms (becoming ions) and move from the negatively charged electrode at one end of the tube and towards the positively charged electrode at the other. In their journey, these free electrons collide with other xenon atoms, breaking them up and creating more free electrons. Eventually, enough ions are created and collide with enough xenon atoms that the atoms become energized and emit photons of light. That requires a lot of voltage.
A typical flash may use 1,000 or more volts. That’s a boatload of power, applied in an instant! The agitation and collision of atoms caused by this massive hit of voltage, creates a burst of heat and an instantaneous expansion of the xenon gas in the tube. That brief pulse of heat and expanding gas creates a small pressure shockwave, similar to thunder during a storm or a jet breaking the sound barrier (but, of course, much less dramatic). There’s also a bit of sound generated by the expanding gas slamming up against the constraint of the tube itself and from mechanical elements of the flash, but the majority of that flash pop is from the shockwave.
I’ve seen conversations in internet forums that indicate you can get a sense of the shockwave by holding a piece of paper up close to your speedlight when you fire the flash. The paper will move slightly from the shockwave, like it would from a puff of air. Unfortunately, there’s so much air moving in my apartment that a piece of paper would never be still enough to do this test. Another supposed way to experience the shockwave is to place the palm of your hand near the flash as it fires. You should feel a quick puff of air. I feel mostly heat. I’ll just choose to believe the experts.
OK, but what’s that high-pitched whine after I fire the flash?
We learned that a flash requires more than a thousand volts, but batteries are only 1.5 to 9 volts. Your on-camera flash or speedlight has to boost the voltage from almost nothing to thousands of volts.
Most modern speedlights use boost converters, sometimes called step-up converters, that take the power from a battery and increase the voltage. These are used in many applications, from electric vehicles, to portable lighting systems, to off-camera flash. They allow us to use much less powerful batteries or fewer batteries to accomplish a given task.
Boost converters work by cycling a current back and forth, each time increasing the voltage. That annoying (or satisfying) whine you hear from your speedlight after you take a shot? That’s this voltage-boosting process powering up your flash so it’s ready for the next shot.
Once the voltage is high enough, the current goes through a diode, which changes it into Direct Current (DC) and sends it to a capacitor, which stores the charge. This all happens fairly fast. If you have speedlights, you’ve likely heard the high-pitched whine and know about how long it takes to charge.
A few new speedlights don’t have that whine and some photographers miss it. They used to know the speedlight was ready when the whine stopped. Now they have to adjust their work flow to look at the ready light on the flash. Sounds similar to those with mirrorless cameras who miss the click of a mechanical shutter and aren’t sure if they actually took a photo until they see it on their LCD screen. Some manufacturers have even added a mechanical shutter sound effect to compensate. Will flash manufacturers now have to add an artificial whine?
So, there you have it. The “pop” of a flash is basically caused by the pressure shockwave generated when the gas in the flash tube gets jolted by a massive hit of voltage and instantaneously expands, heats up and generates light. The whine of many speedlights is caused when the boost converters are ramping up the voltage from your batteries for the next shot. All other noises, at least with this photographer, are from his aging joints doing a snap, crackle and pop and him whining about it.