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Physics for all



How do humans determine the source of a sound, and how accurately can they do this?

The human brain uses several methods to determine the source of sound. A key method, common for human and many mammals, is based on the difference in arrival time of the sound to the two ears. It is important in the localization of sounds, as it provides a clue to the direction or angle of the sound source from the head. If a signal arrives at the head from one side, the signal has further to travel to reach the far ear than the near ear. This path length difference results in a time difference between the sound's arrivals at the ears, which is detected and aids the process of identifying the direction of sound source.

A second method is based on the level difference (amplitude, or intensity) of sound that reaches the two ears: if a source of sound is, say, to our left, the sound will be heard stronger in the left ear than in the right ear. This is because the head acts like a barrier, and shadows the signal. This effect depends on the frequency of the sound waves.

A third method is based on minute differences in the signal that arrives to our ears, caused by sound reflection from different parts of our body, such as the shoulders and torso.

The method(s) used by our brain depends on the frequency of the sound. As sound is a wave, its frequency is related to its wavelength. Thus, sounds at lower frequencies, have larger wavelength. The smaller the frequency of a wave is, the larger its wavelength. For example, the wavelength of a sound wave at 800Hz is approx. 42 cm, which is the typical twice the distance between the ears. This means that at frequencies lower than that (the human ear can typically hear sound at frequencies 20 Hz - 20,000 Hz), the distance between the ears is less than 1/2 the wavelength. It is therefore easy for the brain to determine the time (equivalent to phase) difference between the signals that reach the two ears. On the other hand, the "shadow" produced by the head is not effective. At very low frequencies, below 80 Hz or so, the wave length of the sound becomes so big that also the time difference method is difficult to implement by the brain, and most people will not be able to determine the lateral source of the sound.

At high frequencies, above ~1600Hz, the time difference method is not effective, as the dimension of the head is bigger than the length of a sound wave. However, lateral positioning based on the level difference is effective. Another effective method is the delay between the original sound and the reflected sound from nearby objects; again, small differences between the arrival times of the echo to the two ears are used by our brain to determine the direction of the sound source.

Overall, most people can localize a sound source to an accuracy of about 1 degree for sources in front of a listener, and 15 degrees for sources to the side.

As far as determining the distance to a sound source, people are typically not very good at that. The human brain uses various methods, including:
- Ratio of direct to reflected signal
- Loudness
- Spectrum of the sound: high frequencies damped quicker in the air than lower frequencies. Therefore a distant sound source sounds more muffled than a close one, because the high frequencies are attenuated. For sound with a known spectrum (e.g. speech) the distance can be estimated roughly with the help of the perceived sound.
- Movement: Similar to the visual system there is also the phenomenon of motion parallax in acoustical perception. For a moving listener nearby sound sources are passing faster than distant sound sources.
- Level Difference: Very closely sound sources cause a different level between the ears.

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