Understanding Headphone Impedance
Impedance, measured in ohms (Ω), describes how much a headphone resists electrical current flow. Higher impedance headphones demand more voltage to achieve the same sound pressure level as lower impedance models. This relationship, governed by Ohm's law, explains why a 600 Ω professional headphone requires a different amplifier than a 32 Ω portable model.
Impedance varies significantly across headphone types. Budget consumer headphones typically range from 16–32 Ω, mid-range models sit around 50–150 Ω, and studio-grade headphones often exceed 250 Ω. Your amplifier's output impedance should ideally be less than one-eighth of your headphone impedance to avoid frequency response colouration and power loss.
Headphone Sensitivity: Efficiency Matters
Sensitivity measures how efficiently headphones convert electrical power into sound. Expressed as dB SPL at 1 milliwatt (dB SPL/mW) or dB SPL at 1 volt (dB SPL/V), sensitivity tells you the loudness you'll achieve with a fixed input signal.
A headphone rated at 100 dB SPL/mW is considerably more efficient than one rated at 90 dB SPL/mW—a difference that means you need roughly 10 times less power to reach the same volume. When comparing headphones, always check which sensitivity standard the manufacturer uses, as the two metrics can differ by 10 dB or more depending on impedance. The calculator automatically converts between both standards if you provide one.
Power, Voltage, and Current Relationships
Three quantities govern headphone operation. Power determines how hard your amplifier must work; voltage represents the electrical potential driving current through the headphones; and current is the actual flow of electricity. These relate through impedance and the sensitivity rating.
Power (W) = 10^[(Loudness − Sensitivity) ÷ 10] ÷ 1000
Voltage (V) = √(Power × Impedance ÷ 1000)
Current (A) = Voltage ÷ Impedance
Sensitivity @ 1V = Sensitivity @ 1mW + 10 × log₁₀(Impedance ÷ 1000)
Power— Electrical power delivered to the headphones in watts or milliwattsLoudness— Target sound pressure level in dB SPLSensitivity— Headphone efficiency rating at 1 mW or 1 V RMSVoltage— RMS voltage supplied by the amplifierCurrent— RMS current flowing through the headphonesImpedance— Headphone resistance in ohms
Common Pitfalls and Real-World Considerations
Several practical factors affect whether your headphone–amplifier pairing will sound good and remain safe.
- Not all amplifiers state their output impedance — Many portable DACs and phone audio jacks lack published impedance specifications. When in doubt, assume output impedance under 5 Ω. If your headphones measure 32 Ω, the 1/8 rule means your amp's impedance should stay below 4 Ω for minimal colouration.
- Sensitivity ratings assume a flat test signal — In real music, dynamic range varies. A headphone rated at 95 dB SPL/mW may feel quieter than its rating suggests when playing soft classical pieces, but loud rock tracks can approach or exceed the calculated peak level quickly.
- Impedance changes with frequency — Headphone impedance varies across the audio spectrum. Manufacturers publish a nominal value, usually the DC or low-frequency impedance. Bass-heavy headphones may show higher impedance at 20 Hz than at 1 kHz, affecting actual power delivery at different frequencies.
- Overdriving damages headphones and hearing — Never assume an amplifier's maximum power output is safe for headphones. Always verify that your amp can limit output to prevent clipping, and keep your listening level below 120 dB SPL to protect your hearing and the headphone drivers.
Choosing an Amplifier for Your Headphones
Once you know your headphones' impedance and sensitivity, select an amplifier rated for that power output at the specified impedance. Most reputable amplifier manufacturers list continuous power into standard loads: 32 Ω, 300 Ω, and 600 Ω are common reference points.
For portable headphones under 50 Ω, most smartphones, tablets, and DAPs deliver adequate power without a dedicated amplifier. However, professional studio headphones exceeding 250 Ω almost always benefit from a dedicated headphone amplifier—typically a desktop unit rated for 100–500 mW at your headphone impedance. High-impedance models (above 400 Ω) may require specialised amplifiers designed specifically for that range to achieve clean, undistorted sound at listening levels.