Headphone Sensitivity: dB/mW, dB/V, and What They Tell You

The two units, and why the difference matters

Sensitivity answers one question: how much sound do you get out for what you put in. The confusion is entirely about what "what you put in" means.

dB/mW is sound pressure per milliwatt of power. It describes the driver as an energy converter, and it is the figure most headphone manufacturers publish.

dB/V is sound pressure per volt. It describes what you get from a source at a given output voltage, which is closer to what actually happens when you plug something in, because amplifiers are voltage sources.

These are different measurements of the same headphone and they produce different numbers. You cannot compare a dB/mW figure from one manufacturer against a dB/V figure from another and conclude anything.

Converting between them

The two are linked by impedance, because power, voltage and resistance are linked by Ohm's law. For a headphone of impedance Z ohms:

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

Work an example. A 32 ohm headphone rated 100 dB/mW:

10 x log10(1000 / 32) = 10 x log10(31.25) = 14.9
dB/V = 100 + 14.9 = 114.9 dB/V

And a 300 ohm headphone also rated 100 dB/mW:

10 x log10(1000 / 300) = 10 x log10(3.33) = 5.2
dB/V = 100 + 5.2 = 105.2 dB/V

Same power sensitivity, nearly 10 dB apart in voltage sensitivity. That gap is the whole reason two headphones with identical dB/mW figures can behave completely differently on the same amplifier, and it is why a spec sheet quoting only one unit is telling you half the story.

What counts as high or low

There is no standard, but the working bands are roughly:

• Above 100 dB/mW is efficient. Most of these run acceptably from a phone or a laptop.
• 90 to 100 dB/mW is the mainstream. A competent portable or any desktop amplifier handles them.
• Below 90 dB/mW is genuinely demanding, and this is where amplifier choice stops being optional.

The band matters more than the exact figure. A 2 dB difference is small; a 15 dB difference is the difference between working from a phone and needing a desktop chain.

Why you need both numbers

Sensitivity tells you how loud, impedance tells you how that input has to be delivered. Neither is sufficient alone, and the combinations behave differently:

• High sensitivity, high impedance. Easy. Needs voltage, but not much of it.
• High sensitivity, low impedance. Easy, and the most forgiving of weak sources. Watch for hiss: an efficient low-impedance headphone amplifies whatever noise floor the source has.
• Low sensitivity, high impedance. Needs voltage swing. This is where portable sources clip.
• Low sensitivity, low impedance. The hardest case, because it demands current, and current is what small sources run out of first. Many planar magnetic headphones sit here.

A caution about published figures

Sensitivity is not measured to a single mandated standard across the industry. Coupler type, measurement bandwidth and whether the figure is an average or a peak all vary between manufacturers, so figures from different brands are approximate at best when compared directly.

Treat a published sensitivity as a band rather than a precise value, and give a headphone that lands close to your source's limit some margin. Headroom costs nothing; running an amplifier at its ceiling costs you the peaks, which is where the music is.