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. When I first wrote this note I was hoping that by equalizing headphones so that all frequencies had similar loudness would be sufficient to match phones to different listeners. Recent experiments have shown that this is NOT the case.
It turns out that if you ask a subject to adjust a loudspeaker so that all frequencies have equal loudness different subjects come up with quite different equalization curves. And these curves are repeatable. What we need to do is match the loudness curve for a reference loudspeaker to the loudness curve we get from a particular headphone. When the headphone curve matches the loudspeaker curve, the headphone equalization is correct. – and my binaural examples can be heard as intended. I have written a Windows program that makes this process easy – and provides a dual 27 band graphic to actually do the needed correction. This program may eventually become a plug-in, or be offered here.
In the mean time, a patient listener can perhaps make use of the following files. Use a sound card with an equalizer as explained below to find a useable equalization for a high quality monitor loudspeaker, placed in front of a listener in the near field. Then do the same for a high quality headphone. Now subtract the equalization for the loudspeaker from the equalization obtained for the headphone, and set your equalizer for the difference. The headphone should now have the same timbre and frequency balance as the loudspeaker. This is what you want.
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.
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! Do this for your loudspeaker with the flattest on-axis response first, then for your headphone. Subtract the curves, set the equalizer to the difference, and see how it sounds.
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.