Index Signal & AmplificationHeadphone Power

Headphone Power

Voltage, current and power needed to reach a listening level.

Voltage2.45V rms

Power20mW

Current8.16mA

Sensitivity per volt102.2dB/V

Loudness against drive voltage

Dashed lines: a phone dongle (1 V), a desktop DAC (2 V) and a desktop headphone amplifier (8 V) at full volume.

Input

Typical headphones

SensitivitydB/mW

Sensitivity is quoted per

Impedance

Z

Peak level wanteddB SPL

Music at 85 dB average needs about 105 to 110 dB for clean peaks.

How it works

Two ways to quote sensitivity

Makers state how loud a headphone plays either for 1 mW of power or for 1 V of drive. The two are linked by the impedance, since 1 V into $Z$ ohms delivers $1000/Z$ milliwatts:

S_{V} = S_{mW} + 10\log_{10}\!\left(\frac{1000}{Z}\right)

At 32 Ω the difference is about 15 dB; at 300 Ω it is about 5 dB. Mixing the two up is the most common mistake in headphone amplifier maths.

What the amplifier must deliver

V = 10^{(L - S_V)/20}

P = \frac{V^2}{Z}

I = \frac{V}{Z}

High-impedance headphones ask for voltage and very little current. Low-impedance planars ask for current. An amplifier has to satisfy whichever limit comes first.

Choosing a target

Recordings with wide dynamics peak 20 dB or more above their average. Sizing for 110 dB peaks does not mean listening at 110 dB; it means the peaks will not clip when the average sits at a comfortable 85 to 90 dB.