Impedance in ohms represents the resistance of the headphones.
The impedance indicates how easily a headphone can be driven by an amplifier. Conclusions about the volume can only be drawn at a given (amplifier) power and considering the sensitivity of the headphones. Therefore, a headphone with low impedance (e.g. 32 ohms) achieves the same volume with less voltage than a headphone with high impedance (e.g. 600 ohms).
Example:
We connect two headphones with different impedances to an Apple iPad via minijack. This tablet can (fictitiously) supply a level of 1 volt.
- Headphone I has an impedance of 32 ohms.
- Headphone II has an impedance of 600 ohms.
We calculate the power:
-
- Headphone I:
PI = (1V)2/32Ω = 31mW - Headphone II:
PII = (1V)2/600Ω = 1,6mW
- Headphone I:
What impedance headphones should I buy?
The advantage of high impedance headphones is better resolution and sound, but for mobile use (with smartphones, MP3 players, laptops) headphones with low impedances of about 16 to 80 ohms are recommended. Unless you plan to purchase an external DAC that can connect to smartphones (among others) and also drive high-impedance headphones.
Why is that? Batteries and rechargeable batteries of these devices provide quite little voltage and thus also limit the maximum output voltage at the headphone output. Thus, a low impedance headphone (such as those included with the Apple iPhone or Samsung Galaxy) will be louder than a high impedance one when used on the same mobile device.
If, on the other hand, you want to listen to music exclusively at home in the living room with your HiFi system (amplifier) or have the headphones in use in the studio, then you should buy high-impedance headphones. The amplifier can and may then output a higher maximum voltage and can “operate” the headphones with this higher resolution than a low impedance.
Headphones with higher resistances can also be connected in parallel through a Y-split cable to the same headphone output – if you use headphones with low resistance, you soon run the risk of exceeding their power handling capacity when the amplifier is turned up loud, which can lead to damage (see ohmic resistances in parallel circuits and ohm’s law).
