Headphone Imaging
If you’ve ever listened to headphones critically, or for an extended period of time, you’ve probably found that there are some things very wrong with the sound. Most people perceive the audio image produced by headphones to be a blob on the left, a blob on the right, and a blob in the middle. In addition, after an hour or two of listening, you may have felt that the headphones became annoying and that you were tired of listening.
These problems are very real and can be explained technically. Imagine again, as in the “How We Hear” section, that you are listening to a pair of speakers. If you turn off the left speaker, both ears continue to hear the right speaker, but the left ear hears the sound after a short time delay (ITD) and with an equalization difference (IAD). Now think about listening to a pair of headphones. If you turn off the left channel, only the right ear hears the sound. This is unnatural; in a speaker-based listening environment both ears hear both speakers. In everyday life, sounds are generally heard by both ears. Your mind doesn’t really know what to do with sound that it only hears in one ear so, for most people, the sound ends up being localized at the ear.
There are some types of recordings (called binaural recordings) that are designed specifically for headphone listening and do not have these problems. Special microphone setups that look like a human head with microphone elements in the ears are used for these recordings. Unfortunately, since binaural recordings are best heard on headphones, they are not as desirable for speaker listening, so few companies actually produce them. There are also a few other types of microphone techniques that can be used with both headphones and speakers, but don’t do either particularly well.
The bottom line is that when the recording engineer placed the microphones and mixed the sound, he or she was listening to two speakers. Thus, it is in a speaker-based acoustic environment that the desired audio image is recreated. Because headphones are a significantly different acoustic configuration, audio designed for speaker listening just doesn’t sound right when played back on headphones.
