The necessity of headphone equalization

Some of the examples of music and speech on this web page can be played through high quality monitor loudspeakers, placed close to the listener. Localization and intelligibility can be accurately determined by this method if the loudspeakers preserve the relative loudness of low and high frequencies. Unfortunately, the examples that involve spaciousness or envelopment require headphones for accurate reproduction.

One of the most common problems of acoustic spaces is lack of clarity, muddiness, or poor intelligibility. These perceptions depend strongly on the balance between the low and the high frequencies. For reasons that are beyond the scope of this note, most headphones severely emphasize high frequencies. This includes the most highly touted and expensive models. The extra high frequency emphasis may increase sales of the product, but it gives a false impression of acoustic sound quality by increasing both clarity and intelligibility.

To accurately play the examples here a headphone equalizer needs to be used. There are many programs on the market for playing sound files that include a graphic equalizer, such as Sony “Sonic Stage” or many versions of Winamp. The graphic equalizer can be used to flatten the headphone response – if the response of the headphone when attached to a particular listener can be determined. Alas, the coupling of high frequencies to the eardrum varies greatly among individuals. It is influenced by the volume of the concha, the diameter and geometry of the ear canal, the eardrum impedance and other factors. Lacking probe microphone measurements at the eardrum, the best way to equalize a headphone is by listening.

The process of equalizing a headphone requires some patience, and perhaps some practice. The most convenient way is to play a series of noise signals of equal sound power and different frequencies, while adjusting an equalizer so they all have the same loudness.

I have included sound files on this site that alternate a noise band at 1kHz with noise bands at other frequencies. There are three files. The one with 1k, 5k, and 3k works at high frequencies with a simple equalizer such as Winamp. There are two files with frequencies in octave bands. These might be more appropriate for Sonic Stage, or any of the Creative Labs products. All files are in .mp3 form. Early versions of Winamp had a bug which made the sliders work incorrectly when playing .mp3. Winamp Version 5.32 appears to be OK.

www.theworld.com/~griesngr/1k_5k_3k.mp3

www.theworld.com/~griesngr/1k_2k_4k_8k.mp3

www.theworld.com/~griesngr/1k_500_250_125.mp3

Download the files, and play them one at a time as a continuous loop, using your favorite sound player. Adjust the filters until all signals are perceived with equal loudness. You might be surprised at the amount of adjustment that is typically needed!

You then need to load the sound examples from the papers into the same player to take advantage of the equalization. This is painful but necessary.