If you wonder how to interpret the impedance and sensitivity of audio headphones so as to choose those most capable of producing an adequate sound level when paired up with your smartphone or portable amplifier, you are certainly not alone. Here are a few elements to help you interpret the specs of different headphones.
Manufacturers of both headphones and speakers indicate their products’ sensitivity ratings at 1 kHz (midrange) for 1 mW (milliWatt) of power. However, speaker impedance is measured at 1 Watt, and very few headphones can handle such a significant amount of power. Moreover, the proximity of the headphones’ transducers to the listener’s ears ensures a high sensitivity with only several hundredths of milliWatts of power. To measure the sensitivity of a pair of headphones, the microphone is evidently not placed 1 m away from the transducers, but very close to the them and often at the end of an artificial ear canal. Sensitivity is thus commonly established at between approximately 90 and 120 dB.
At what volume do we really listen?
Most often between 60 and 80 dB, and exceptionally at 90 dB for the most daring amongst us. On average, headphones have a sensitivity rating of 100 dB, which corresponds to the sound produced by a jackhammer. That is to say that one milliWatt of power is almost never required. However, most headphone amplifiers deliver between 50 and 200 mW, and even up to 1 W for the most powerful models. Headphone impedance is to blame… as well as that of amplifiers.
Just like speakers, headphones have a certain resistance to the electric current transmitted by the amplifier: this is impedance. Expressed in Ohms, it often ranges between 8 and 600 Ohms, depending on the model. As they do for speakers, manufacturers measure headphone impedance at 1 kHz (midrange), but in reality, impedance varies according to frequency (from lows to highs). It can double, triple or quadruple as the transducer’s mechanical resonance frequency is approached, which complicates the amplifier’s task by requiring it to produce more power.
The output impedance of headphone amplifiers and the inevitability of power loss
Headphone amplifiers feature an output impedance which, once added to the headphones’ input impedance, results in a more or less significant loss of power. If the impedance of each is equal, only 6 dB of power is lost. But as hi-fi quality depends on the headphone amplifier having a low impedance (0.1 to 3 Ohms), differences between the amplifier’s output impedance and the headphones’ impedance are often substantial. With 0.1 Ohm at one end (amplifier) and 32 Ohms at the other (headphones), 15 dB are lost. This value increases to nearly 30 dB with a pair of headphones with an impedance rating of 600 Ohms.
In other words, an amplifier’s power is divided by 10 or even 100. This is why many headphones struggle to reach their maximum potential with entry-level headphone amplifiers which can only deliver less than 100 mW of power.
In a few rare cases, output impedance is adjustable. Such is the case for the very high-end Audio-Technica AT-HA5050H amplifier, which proposes output impedance levels of 0,1, 30, 80 and 120 Ohms. This amplifier is therefore adapted to headphones with very high impedance and low sensitivity, such as the Sennheiser HD-80, as it reduces power loss, maximizes sound volume and provides a more balanced audio restitution.
Understanding how an amplifier’s power output is determined
An amplifier’s power output is sometimes communicated in the form of MilliWatts for a given impedance, but it can also be expressed as volts. In this case, you can calculate the power supplied based on the impedance of the headphones you’re planning to acquire.
The formula involves dividing the voltage squared by the impedance. For example, 2 Volts corresponds to 125 mW of power at 32 Ohms (2^²/32) and 13 mW at 300 Ohms (2^²/300).
How powerful should the amplifier be?
It is important to recall that impedance varies according to frequency and that 13 mW at 300 Ohms and 1 kHz can be reduced to 5 mW at certain frequencies. To this consideration must be added the power loss generated by the difference between the amplifier’s output impedance and the headphones’ impedance. If the headphones’ sensitivity rating is very high (> 100 dB), an amplifier capable of supplying 100 mW will suffice. If the headphones’ sensitivity rating is less than 100 dB, it is best to choose a headphone amplifier capable of delivering approximately 300 mW. If the headphones’ sensitivity rating is less than 100 dB and its impedance is above 100 Ohms, a more powerful amplifier, capable of supplying at least 1 W, would be more suitable.This post is also available in: French