Headphone sensitivity: dB/mW, dB/V and what they tell you

What a sensitivity figure measures

A maker feeds the headphone a fixed input, often a 1 kHz tone, and measures the sound pressure level it produces on an ear simulator or coupler. The result is quoted in decibels of SPL for that input. Couplers, test frequencies and averaging differ from one maker to another, so treat a published figure as accurate to a few decibels when you compare brands.

Decibels make the arithmetic simple once you know three rules. Doubling the power adds 3 dB. Doubling the voltage adds 6 dB. Ten times the power adds 10 dB. A headphone 10 dB less sensitive than another needs ten times the power to play as loud.

dB/mW, dB/V and converting between them

The input can be stated two ways. dB/mW is the level from 1 milliwatt of power, and it describes the headphone as a converter of energy. dB/V is the level from 1 volt, which is closer to how a source works, since amplifiers deliver a voltage and the headphone draws whatever current its impedance allows.

Power, voltage and impedance are linked, so one unit converts to the other through the headphone's impedance Z in ohms:

dB/V = dB/mW + 10 x log10(1000 / Z)

The Simgot EM6L is a worked example. Its spec is 119 dB/V at 26 ohms, and 10 x log10(1000 / 26) is 15.9, so it converts to about 103 dB/mW. At 32 ohms the gap between the two units is 14.9 dB, and at 300 ohms it is 5.2 dB. Two headphones with the same dB/mW figure can therefore sit far apart in dB/V, which is why a single number without its unit is half a specification.

dB/V is also the quicker unit when you know your source's output voltage. From 1 V the EM6L reaches 119 dB, and every halving of the voltage takes 6 dB off, so 0.5 V gives 113 dB and 0.25 V gives 107.

The conversion needs both figures. The KBEAR KB02 is rated 108 dB/mW, which its review reads as easy to drive from any phone, but no impedance is stated, so its dB/V figure can't be worked out.

When the unit is missing

Many spec sheets print a bare figure such as "112 dB". The Meze 105 Silva review gives exactly that, at 42 ohms, with no reference, so the figure is recorded as it was given. Read as dB/mW, 112 dB is very efficient. Read as dB/V, it would be 13.8 dB lower in dB/mW terms, which is about 24 times the power for the same level. Either way the review found a phone gives it plenty of volume, and Meze includes a USB-C dongle.

This site records a figure with its unit only when the unit is known: stated at the source, or settled because the other reading would make the headphone less sensitive than any in production. The table further down this page shows the headphones whose figure has a unit, and the reviews print the bare ones as the makers gave them.

Turning sensitivity into amplifier power

Pick a target level and the rest is subtraction. The power calculations on this site size for 110 dB peaks, which covers loud listening with the transients intact. A headphone rated 96 dB/mW needs 14 dB more than its 1 mW figure, which is 10 to the power 1.4, about 25 mW. At 86 dB/mW the same peaks take about 250 mW, ten times as much, and at 106 dB/mW about 2.5 mW.

The HIFIMAN HE400se sits at the low end of the headphones reviewed here whose figure has a unit, 91 dB/mW at 32 ohms, and its review says it needs an external amp to perform, because from a phone it sounds thin and quiet. Power is only half the check. The source also has to supply the voltage or current that power means at the headphone's impedance, which the impedance explainer covers, and the headphone power calculator works out the milliwatts, RMS volts and peak voltage swing for any pair and level.

Very sensitive IEMs and hiss

High sensitivity cuts both ways. An IEM that plays loud from a trickle of power also plays the source's own noise loud, so a hiss that would be inaudible on a full-size headphone can be plain on a sensitive in-ear. The Kiwi Ears Orchestra Lite, at 16 ohms and 112 dB, is easy to drive from anything, and its review warns that cheap, low-quality amps can let hiss through. A source with a low noise floor matters more here than extra power, and the best headphone amplifiers guide covers the amps reviewed here.