Sensitivity vs impedance: what actually makes headphones hard to drive

Two numbers on a spec sheet decide whether your source can drive a headphone: sensitivity and impedance. Sensitivity says how loud the headphone plays for a given input, quoted either as dB SPL per milliwatt (dB/mW) or per volt (dB/Vrms). Impedance (ohms) says how the headphone converts your source's voltage into current and power.

The two units are convertible once you know the impedance: dB/Vrms = dB/mW + 10·log10(1000/Z). This matters because the two units can make the same headphone look easy or hard to drive. A 300 Ω headphone at 97 dB/mW is about 102 dB/Vrms, and one volt (a typical dongle) already gets it past 100 dB peaks.

'Hard to drive' therefore comes in two distinct flavors. High-impedance dynamics (250-600 Ω) need voltage swing. Many phone outputs run out of volts before the headphone runs out of ability. Low-sensitivity planars (often 80-90 dB/mW at low impedance) need current. A weak source clips or sags even though the impedance looks friendly. The fix differs: the first wants a higher-voltage output, the second wants a genuinely powerful amp.

Very sensitive IEMs (105-125 dB/mW) have the opposite problem: everything drives them, but they expose source hiss and are more affected by output impedance. For those, a clean, low-Zout output matters more than watts.

Manufacturers' sensitivity specs are approximate and measured under varying conventions, so calculator outputs are best read with a few dB of slack. The verdict tiers on our power calculator already build that in.