MadSci Network: Anatomy

Re: (how)do we know does a sound come from front or rear?

Date: Mon Jul 3 08:47:46 2006
Posted By: Sam Reyes, Otolaryngolgogy Resident
Area of science: Anatomy
ID: 1145542954.An

The brain has several strategies for determining where a sound comes from. Binaural cues are differences in the sound heard between the two ears. These can be either amplitude (loudness) or temporal (time) differences. for example. If a bag of marbles is dumped on the floor to your right, the sound will be louder in your right ear than your left because your head blocks some of the sound from reaching your left ear. There will also be a slight delay between the time the sound arrives at each ear because of the time it takes for the sound to travel the distance across your head. Binaural cues work very well and have been applied widely to "virtual reality" and sound effects in videogames.

While binaural cues work well for many situations, there are some areas where they break down. One is in determining if a sound if coming from a source directly in front of, above, or behind your head (aka sound on the mid sagital plane or MSP). Since the sound is equidistant from both ears determining the above strategies will not work. Instead your brain uses cues based on the spectrum of the sound. Your head, shoulders, neck and outer ear (or pina) act as a directional filter which changes the frequency spectrum of the sound depending on where the sound is coming from. Each person has a head related transfer function (HRTF) that depends on the shape and density of these structures. Below 4000 Hz there are some characteristics common to most peoples' HRTF. Sounds from behind are generally louder at 1000 Hz while those from in front are louder at 3000 Hz. However above 6000 Hz, the HRTF is highly individualistic. This limits the applicability of HRTFs in virtual reality since an individual HRTF would have to be created for each person.

An excellent review of the above topic with more detailed information can be found at:
Hartmann. How We Localize Sound. Physics Today. 1999

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