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A number of factors affect the sound quality of headphones, but the four most significant ones are described below.

Impedance

The headphones' impedance is short for their AC impedance, and their size is a function of the DC resistance of the coil and the inductive resistance of the coil. According to the settings of different environments, headphones impedance settings will generally vary, and mobile devices to match the impedance of the headphones are generally below 100Ω, and desktop devices to match the impedance of the headphones are generally above 150Ω, or even up to 600Ω. While headphones with high drive impedance require more power, low drive impedance headphones are relatively easier to drive, but current requirements are also higher.

Sensitivity

In reality, the sensitivity of the headphones refers to the sensitivity level of the headphones, which is based on headphones with a power rating of 1mW. A 1mW power rating is calculated on the basis of a standard impedance of the headphones at a frequency of 1000Hz, which is obtained by coupling the sound pressure level with the simulated ear (human head recording equipment). Another unit of sensitivity is dB/Vms, which is the voltage applied to the headphones when the sound pressure level generated is 1Vms. It is important to note that high sensitivity means that the headphones must be powered at a low level in order to achieve a certain sound pressure level. In general, moving coil headphones have a sensitivity greater than 90 dB/mW, while portable headphones generally have a sensitivity in excess of 100 dB/mW. For similar sensitivity values of headphones and speakers, headphones require power equivalent to 1/1000 of that of a speaker, but in fact, this value is even smaller since few listeners are at a distance of one meter from the speaker. Speaker sensitivity is generally defined as the input power of 1W in 1m at the sound pressure level.

Frequency response

There are different values of sensitivity at different frequencies, which is the frequency response. The dependence of sensitivity on frequency can be expressed as a curve, which is the frequency response curve. Human hearing is generally limited to a range of 20Hz to 20kHz; beyond this range, the vast majority of the human body is incapable of perceiving sounds. The frequency band that headphones are capable of reproducing is quite wide. Excellent headphones can play back frequencies between 5Hz and 45kHz. In general, headphones have a frequency response of about 10 decibels, while professional headphones and some high-end headphones have a frequency response of about 3 decibels. During normal use, headphones' sound performance is not excellent because the sound signal in the ear canal before entering the head, the outer area of the ear collision, and the peaks and valleys of sound waves, are all influenced by the frequency response curve. Several equalization methods are commonly used in headphones design to minimize this effect, resulting in a relatively flat frequency response curve received by the ear.

Distortion

When the maximum withstands power is reached, the total harmonic distortion (THD) of headphones is usually less than or equal to 1%. This distortion is inaudible to the human ear, as it is much smaller than the distortion of the speaker. In addition, the distortion of more advanced headphones is also much smaller. The distortion of electrostatic headphones is significantly reduced than the distortion of ordinary moving coil headphones.