Understanding the impedance and sensitivity of audio headphones (2019)

2
15630
  • 20
    Partages

If you wonder how to interpret the impedance and sensitivity of audio headphones so as to choose those capable of producing an adequate sound level when paired up with your smartphone or DAP, you are certainly not alone. Here are a few elements to help you interpret headphones’ specs.

The Focal Stellia headphones have an impedance of 35 ohms and a high sensitivity rating of 106 dB/mW. Therefore, they can be easily driven by most amps and portable devices.

Headphone and earphone sensitivity

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 with 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 meter away from the transducers, but very close to them, often at the end of an artificial ear canal. Sensitivity is thus commonly established at between approximately 90 and 120 dB.

The Sennheiser HD 820 headphones’ two transducers feature a high sensitivity rating of 103 dB/mW for an impedance of 300 ohms.

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, earphones and 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 and DAPs 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. This raises two questions: when and why should you choose such a powerful amplifier?

The Pioneer U-05-S headphone amplifier features a high power output of 2 x 300 mW at 32 ohms.

Headphone and earphone impedance

Just like speakers, headphones and earphones 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 excellent Meze 99 Classics headphones, with their sensitivity of 103 dB and impedance of 32 ohms, are very easy to power.

The output impedance of headphone amplifiers and the inevitable loss of power

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.

The Cayin IHA-6 headphone amplifier has a very high power output of 2 x 7000 mW at 32 ohms.

Understanding how an amplifier’s power output is determined

An amplifier’s power output is usually communicated in the form of milliWatts (mW) for a given impedance, but it can also be expressed as volts. In this case, you must 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).

To determine the power output of an amplifier, 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).

The iFi Audio iDSD Pro high-end amplifier’s voltage is rated at 11.2V at 600 ohms. It can therefore drive any pair of hi-fi headphones with ease.

How powerful should the amplifier be?

It is important to recall that the impedance of headphones and earphones varies according to frequency. For example, 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.

With their low impedance of 32 ohms and high sensitivity of 99.8 dB, the Grado SR-125e headphones can be easily paired with any amplifier or smartphone.

However, 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 1W, would be more suitable.

With a power output rated at 80 mW at 32 ohms, the Audioquest DragonFly Cobalt USB DAC is the ideal solution to improve the output of a computer or smartphone.


  • 20
    Partages

2 COMMENTS

  1. Output impedance of an amplifier is responsible for additional coloration of final frequency response of whole system. It is good to know, that impedance of headphones is not constant over full freq. spectr. For an instance lets assume ATH-MSR7 together with output impedance of an amp of 10 Ohms. You will get ~2.3dB (as I remember) coloration anywhere in frequency response of the system. You can easily listem this difference by A/B especially that mentioned HP are near the term “prone to sibilance” for someone and therefore you can make this property worsen.

Share your opinion!

This site uses Akismet to reduce spam. Learn how your comment data is processed.