Interaural time delays alone cannot fully determine sound source direction

Evidence

Such a sound source could in fact be anywhere on a cone in space that goes out from the ear at which the sound arrives earliest, since a sound source placed at any point on that cone’s surface will have the same time delay for times of arrival at the ears. This cone is known as the “cone of confusion.” There is one exception: if the sound from a source arrives at both ears at the same time (there is no delay between the times of arrival), then the cone has become flattened into a vertical “circle of confusion.” We can resolve the confusion by moving the head so that the change in amplitude and phase of the received sounds at the ears enables the direction to be resolved; indeed, we may even attempt to place it in front of us in the normal direction, where all the delays and intensities will be the same. The act of moving our head will change the direction of the sound arrival, and this change of direction will depend on the sound source position relative to us. Thus a sound from the rear will move in a different direction compared with a sound in front of or above the listener. This movement cue is one of the reasons we perceive the sound from headphones as being “in the head.” Because the sound source tracks our head movement, it cannot be outside and hence must be in the head. There is also an effect due to the fact that the headphones also do not model the effect of the head. Experiments with headphone listening that correctly model the head and keep the source direction constant as the head moves give a much more convincing illusion. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 735

This is a very small delay but a variation from this to zero determines the direction of sounds at low frequencies. Figure 2.25 shows how this delay varies as a function of angle, where positive delay corresponds to a source at the right of the median plane and negative delay corresponds to a source on the left. Note that there is no difference in the delay between front and back positions at the same angle. This means that we must use different mechanisms and strategies to differentiate between front and back sounds. … There is also a frequency limit to the way in which sound direc­ tion can be resolved by the ear in this way. This is due to the fact that the ear appears to use the phase shift in the wave caused by the interaural time difference to resolve the direction; that is, the ear measures the phase shift given by: where the phase difference between the ears in radia ITD Φ  ( ns and the frequency in Hz When this phase shift is greater than  radians (180°) there will be an unre­ solvable ambiguity in the direction because there are two possible angles- one to the left and one to the right-that could cause such a phase shift. — David Howard, Jamie Angus, coustics and Psychoacoustics, 712

Categorization

• DEFINE: Defines the cone of confusion and circle of confusion as the geometric ambiguity of interaural time-delay localization.
• SHARE: Nerdy spatial-audio tidbit worth sending to anyone designing headphone-based 3D sound: head tracking is what sells the illusion.
• XREF: The explanation of why headphones sound ‘in the head’ connects to earlier reading on silence and how we locate sounds in a soundscape (The Soundscape). — connects to: Directional hearing rests on interaural time and intensity differences. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 704; Phase delays beyond 180 degrees make direction judgments ambiguous — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 720
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 735; David Howard, Jamie Angus, coustics and Psychoacoustics, 712