Specs & measurements
Impedance
Also called ohms, nominal impedance
Impedance, given in ohms (Ω), is how much a headphone or speaker resists the current an amplifier sends it. High-impedance headphones need more voltage to play loud; low-impedance ones draw more current and are easier for phones to drive.
Headphones range from under 10 Ω for some multi-driver IEMs to 300 Ω or more for studio and audiophile designs. A Sennheiser HD 600 at 300 Ω wants an amplifier with voltage to spare, while a 32 Ω portable headphone plays loud from a phone. Impedance alone does not say how loud a product gets; Sensitivity does the rest.
Impedance also varies with frequency. Dynamic headphones often have a hump in the bass, and if the amplifier’s output impedance is high, that hump changes the tonal balance. The usual rule of thumb is an amplifier output impedance under an eighth of the headphone’s.
Speakers are typically rated at 4, 6 or 8 Ω. A 4 Ω speaker asks more current from an amplifier, which is why some receivers specify which loads they support. The figures in the catalog are what makers publish, not measurements.