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dB/mW vs. dB/V: Why Headphone Sensitivity Is Quoted Two Ways

Headphone sensitivity states how loud a headphone plays for a given amount of drive. It is published two ways: as sound pressure level per milliwatt of input power (dB SPL/mW) or per volt of input signal (dB SPL/V). For the same headphone the two figures are different numbers, and the gap between them depends on the headphone's impedance — so a sensitivity figure is only meaningful when the reference is stated. The NIST Guide to the SI sets out the general rule behind that: a quantity in decibels is a level relative to a reference, and the reference has to be identified.

The two references

  • dB SPL/mW (sometimes written dB/mW, or "dB at 1 mW") answers: how loud does this headphone get when one milliwatt of electrical power is delivered to it? This is a power reference.
  • dB SPL/V (dB/V, or "dB at 1 V") answers: how loud does it get when one volt of signal is applied across it? This is a voltage reference.

A third style appears in consumer listings: a bare number such as "102 dB" with no reference at all. That figure cannot be compared with anything until you know which of the two it is.

How the two are related

Power, voltage, and impedance are linked by P = V² / Z. One volt across an impedance of Z ohms therefore delivers 1000/Z milliwatts. Converting a power-referenced figure to a voltage-referenced one is a matter of accounting for that difference in decibels:

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

Two worked cases, using round numbers rather than any specific product:

  • A 32-ohm headphone rated 100 dB SPL/mW: 10 × log₁₀(31.25) ≈ 15 dB, so it is about 115 dB SPL/V.
  • A 300-ohm headphone rated 100 dB SPL/mW: 10 × log₁₀(3.33) ≈ 5 dB, so it is about 105 dB SPL/V.

Same power sensitivity, 10 dB apart on a voltage basis. That is the whole reason high-impedance models look insensitive next to low-impedance ones even when their efficiency per milliwatt is identical. The decibel arithmetic itself — ratios, not absolute amounts — is covered in what a decibel measures, and the suffix conventions in dBu, dBV, dBm, dBFS, dB SPL.

Why manufacturers pick one or the other

Power-referenced figures pair naturally with amplifier specifications, which are usually quoted as milliwatts into a stated impedance. Voltage-referenced figures pair with sources whose real limit is output voltage — a phone, a laptop jack, a small USB adapter. Voltage-referenced sensitivity is the easier number to use when you are comparing a headphone against a source's maximum output voltage swing; power-referenced sensitivity is the easier one when the source is specified in milliwatts.

Conditions that change the published number

Sensitivity is a measured quantity, and the measurement setup moves the result:

  • The acoustic fixture. Over-ear headphones are measured on an ear simulator or flat-plate coupler; in-ear models on a sealed ear simulator. Different fixtures give different pressures.
  • The test signal. A 1 kHz tone and a broadband or weighted noise signal do not yield the same figure on a headphone with an uneven response.
  • Tolerance. Unit-to-unit spread of a few decibels is normal, and some data sheets name a tolerance explicitly.

The practical consequence: comparing sensitivity figures from two different manufacturers is approximate at best, and comparing a power-referenced figure with a voltage-referenced one without converting is simply wrong.

Hearing-health context

Sensitivity figures describe potential output level, not a recommended one. Public-health bodies publish the actual guidance on listening levels and exposure time — see the NIDCD on noise-induced hearing loss and the WHO's Make Listening Safe program, which is also where the WHO–ITU safe-listening standards for personal audio devices sit.

Where to look next

Sources

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