The process of hearing a sound and being able to locate it in space is called “psychoacoustic localization.” Just like your vision system, your hearing system evaluates and compares two inputs (left and right ears) to determine where a sound is coming from. Whereas your eyes evaluate angle and focus to determine distance, your ears evaluate time delay and frequency response.
Imagine you are sitting in front of a pair of speakers and you turn off the left speaker. Both ears continue to hear the right speaker, but the left ear, because it’s farther away, hears the sound slightly after the right ear. This difference in arrival time for sound sources that are off-center is called the “interaural time difference” (ITD). In addition, because the left ear is somewhat in the shadow of the head, it hears sound from the right speaker at a slightly lower volume and with a slightly altered frequency response. This is called the “interaural amplitude difference” (IAD). These two cues work well from about 400 Hz to about 2 kHz and are the most important psychoacoustic cues for left-to-right placement of sound sources (lateralization).
Once a sound is higher than 2 kHz, it becomes more difficult to tell where it’s coming from using ITD because the wavelength is shorter than the distance between the ears. High frequency localization is instead performed by listening to the effects of the short reflections that happen in the outer ear. If you look at your ear, around the entrance to the ear canal is a small basin, created by a ridge that takes up about half the outer ear. This is called the “concha ridge” and acts to focus sound into the ear canal.
As a sound approaches the ear, some of it goes directly into the ear canal and some reflects off the concha ridge before entering the ear canal. The direct signal is mixed with the reflected signal after a short time delay, causing a frequency response phenomenon called a “comb filter” to occur. As the source of the sound moves, or, more importantly, as the head moves, the artifacts in the comb filter shift, allowing your brain to decode changes that your perceptual system understands as sound source position. This information is used to fine-tune information from ITD to detect front/behind and elevation positions.
It is important to understand that it is mainly changes in perceived sound that the brain uses to localize, not the static cues themselves. Scientists have observed that people use a small left-to-right shaking of the head movement to localize sound. This subconscious act is believed to allow people to listen for the subtle changes in the psychoacoustic cues needed for accurate sound localization.
In addition to the primary psychoacoustic cues discussed above, your listening system also uses sound reflections off of your shoulders and torso and correlations between visual cues and reflections off walls and objects in the room to not only determine locations of sound sources but also to determine your physical orientation in the room. The listening system is used primarily as an alert and support system for the visual sense, and acts primarily at the subconscious level. This is one of the reasons that acute and accurate listening skills are difficult to develop.
A Final Word About Hearing & Listening Safely From Your Friends at Headphone.com
People have a natural tendency to listen to music on headphones at much louder levels than they would on speakers. If you hope to avoid permanent hearing damage, it’s important to be careful not to listen at extremely loud levels or to listen for too long at moderately loud levels. Generally speaking, when listening to headphones, you should only turn up the volume to the point at which the sound isn’t "too quiet."
The most common hearing damage caused by prolonged or excessively loud sound is called tinnitus. It manifests itself as a sustained ringing in the ears and can become a permanent condition. If you find that your ears are ringing or that there is a sensation of pressure or fatigue, give them a rest for at least a few days.
These symptoms are your body telling you that your ears need a break. Should you choose to ignore these symptoms, you are risking permanent hearing damage. As a general rule, sound pressure levels under 80 decibels (dB) will not damage hearing, even if listened to continually. On the other hand, anything over 100 decibels may cause permanent damage fast. Sound pressure levels anywhere in between can also be damaging. The louder the sound, the shorter the exposure time required to cause permanent damage.
Now, don’t fool yourself into thinking that you either have full-blown tinnitus or you don’t have it at all—you can indeed get a slight case. For example, you might only notice your ears ringing in bed at night. Just like frostbite, once you have a slight case of tinnitus, your ears are much more susceptible to further damage. So, if you get tinnitus, it’s important to be much more careful about exposure to loud sounds.
Now that we’ve told you to be careful, don’t blame us if you blow it. If you have any more questions about hearing damage, please call a doctor right away. Sorry to sound so damn sobering, but a lifetime of musical enjoyment requires great ears in tip-top shape!
International Electro-Technical Commission 268-7 IEC: 1996 Official Statement:
“To prevent possible hearing damage, do not listen to any headphone at high volume levels for long periods. Do not use the headphone while driving or cycling or in any situation where you should be able to hear other sounds. As a guide to setting the volume level, check that you can still hear your own voice when speaking normally while listening with the headphones.”
Additional Tinnitus Links:
National Institute of Health:
Mayo Clinic: Tinnitus Overview
American Academy of Otolarynology: About Tinnitus
American Speech-Language-Hearing Association: Managing Tinnitus