Ohms, Volts & Drivers

Spec sheets for headphones and speakers are full of numbers that look important and mostly aren't. Three of them do matter: impedance, the driver, and the voltage a source can deliver. Not one of them is a quality score. They describe how a transducer and an amplifier will get along, and a mismatch there is one of the few ways to make good equipment sound bad.

The good news is that the whole subject fits inside one analogy, two interactive toys, and a short beat you can listen to. Let's start with the analogy.

Every electrical idea here has a water equivalent that is close enough to be useful. Voltage is pressure. Current is flow. Impedance is how narrow the pipe is. Power, the thing that actually moves the speaker cone and makes sound, is pressure times flow.

That is really the whole of Ohm's law. For a given pressure, a wide pipe passes more water than a narrow one. For a given voltage, a low-impedance headphone draws more current, and therefore more power, than a high-impedance one. Power is voltage squared divided by impedance, which is why halving the impedance quadruples nothing and doubles the power, and why the voltage number matters so much more than people think.

Impedance by itself tells you nothing about sound quality. A 300 Ω headphone is not better than a 16 Ω one. It tells you what kind of source the headphone wants: high-impedance designs are voltage-hungry and sound quiet from a phone; low-impedance designs are current-hungry, get loud from anything, and, as we'll see, are far pickier about the source in a different way.

Impedance is only half of the loudness story. The other half is sensitivity: how loud the driver gets for a given input. Two headphones with the same impedance can differ by 10 dB in sensitivity, a bigger gap than most impedance differences produce. Combine the two and you get the only question that matters: how many volts does this headphone need, and how many can my source give?

Music has peaks 15–20 dB above its average level, so a comfortable target is about 110 dB of peak capability. Pick a source and a headphone below and watch the needle.

Notice what the toy shows. The in-ear monitor reaches painful levels from a fraction of a volt; nothing you own is too weak for it. The 300 Ω studio headphone needs roughly 2.5 V for the same peaks, so a phone leaves it flat and polite while an audio interface gets it there comfortably. And the planar, despite its friendly-looking 32 Ω, is the hungriest of all, because low sensitivity costs more than high impedance does.

Headroom matters more than average power. An amplifier that sits happily at average loudness but clips on the peaks sounds harsh and squashed long before the volume knob runs out. That, and not "power," is what people are hearing when a pairing sounds strained.

Here is the counter-intuitive part. The easy-to-drive headphone is often the fussy one.

Every amplifier has an internal resistance of its own, called output impedance. It sits in series with the headphone, so the two share the voltage between them like two pipes in a row. The rule of thumb is that the source's output impedance should be no more than one-eighth of the headphone's; the ratio is called the damping factor.

Break that rule and two things go wrong. A headphone's impedance is not constant: a dynamic driver's impedance rises around its resonance, typically somewhere between 50 and 150 Hz. Where the headphone's impedance is high it takes a larger share of the voltage, where it is low it takes less, so the frequency response tilts, usually as a bass bump. And the amplifier loses its grip on the diaphragm: a low output impedance electrically brakes the cone after each beat; a high one lets it wobble on. That reads as loose, bloated bass.

Drag the slider and swap headphones. The 16 Ω in-ear monitor plugged into a 10 Ω source has a damping factor under 2, and its tuning drifts audibly from what its designer intended. The same source driving the 300 Ω headphone has a damping factor of 30 and behaves almost perfectly. The "hard to drive" headphone is the forgiving one, and the effortless one is the one you must be careful with.

This is the single most common matching problem in modern audio, and it is why an expensive in-ear monitor can sound worse from a laptop's headphone jack than a cheap pair does: the cheap pair is probably higher-impedance or flatter, and the laptop's few ohms of output impedance simply don't bother it. Real multi-driver earphones with crossovers have even wilder impedance curves than the smooth bump modelled here, so their treble moves too.

Numbers are one thing; ears are another. The player below runs a short synthesized loop (kick, bass, hats) through a model of a source-and-headphone pairing. Pick a pairing and listen to what changes. Everything is level-matched, so what you hear is the character changing, not the volume.

Turn your volume down before pressing play, then bring it up. It's a demonstration of distortion, so it deliberately gets ugly in the last setting.

Ideal match is the reference: a low-output-impedance source with headroom to spare. Laptop → IEM models a 16 Ω earphone on a jack with about 10 Ω of output impedance: the bass around the driver's resonance swells by a few decibels and rings on slightly, so the kick gets rounder and the bass line smears into the next note. Phone → 300 Ω models a voltage-starved pairing turned up to compensate: the source runs out of swing on the peaks, so the kick's transient flattens and a gritty edge appears on the bass, especially where kick and bass land together. In the waveform you'll see the peaks squared off in red.

Neither flaw is subtle once you know what to listen for, and both disappear the moment the pairing is fixed. That is the whole argument of this page in four bars.

The driver is the motor that turns electricity into moving air. Its type shapes what a designer can do; it never guarantees good sound. In particular, the number of drivers on the box is nearly meaningless. A single well-engineered driver with a light, stiff diaphragm and careful acoustic damping can out-resolve a design with seven drivers stitched together with a crossover.

Hybrids mix types to cover each one's weaknesses, most often a dynamic driver for bass under balanced armatures for the top. Done well they combine the best of both; done badly they sound like two earphones glued together, with a seam you can hear where the crossover hands off.

The bars above are tendencies, not verdicts. Every one of these has been used to make a superb product and a dreadful one.

Too much power is not a problem. A big amplifier driving a sensitive earphone at low volume is fine, with one catch: every amplifier has a faint noise floor, and a 123 dB/V in-ear monitor turns a hiss that a 300 Ω headphone would bury into an audible hush between tracks. Hiss is a source problem, not an earphone problem. The other nuisance of pairing a sensitive load with a powerful source is that analog volume knobs often have a channel imbalance in the first few degrees of travel, exactly where you'll be using them.

Voltage is not tone. None of these numbers describe tuning, which is the thing you actually hear. Two headphones perfectly matched to the same amplifier can still sound completely different, and that difference is the designer's choice, not the electronics'.

A worked example ties it together. A 300 Ω open-back studio headphone rated near 97 dB/mW needs about 2.5 V for loud peaks, more than a phone gives but well within an audio interface's headphone output, and its high impedance makes it nearly immune to output-impedance effects. A 16 Ω in-ear monitor rated at 123 dB/V hits the same level from a fraction of a volt and can be driven by anything, but any output impedance above about 2 Ω begins to alter its bass and treble. Neither is harder to use than the other. They simply make their demands on different parts of the source.