Measuring headphone impedance and sensitivity
Two numbers on the spec sheet tell you almost everything about whether an amplifier will drive a headphone well: impedance (in ohms) and sensitivity (in dB per milliwatt or dB per volt). Most pairing mistakes come from reading only one.
Impedance is a curve, not a number
The rated impedance (e.g. "32Ω") is a nominal value, usually quoted at 1kHz. The actual impedance varies with frequency — dynamic drivers show a peak at the fundamental resonance (often 80–120Hz). For amp matching, the nominal value is what matters.
dB/mW vs dB/V
dB/mW (sensitivity) tells you how loud the headphone gets per milliwatt of input power — useful when comparing across different impedances. dB/V (efficiency) tells you how loud it gets per volt of input — useful when comparing different headphones on the same voltage-output amplifier.
Conversion formula
dB/V = dB/mW + 10·log10(1000/Z). A 32Ω headphone at 100dB/mW sensitivity = 100 + 10·log10(31.25) ≈ 115dB/V. A 300Ω headphone at 100dB/mW = 100 + 10·log10(3.33) ≈ 105dB/V — 10dB less efficient at the same voltage despite identical mW sensitivity.
Required amp power
Target peak SPL: 110dB. Required mW = 10^((110 − sensitivity_dB_mW) / 10). For a 90dB/mW headphone: 10^2 = 100mW. For an 86dB/mW planar: 10^2.4 ≈ 250mW. Reserve 6dB of headroom above your average listening volume.