Information on Horizontal-Plane Localisation Demonstration

Our ability to determine the location of a sound source in the horizontal plane relies on having two ears placed either side of the head.  In this demonstration, a pure tone is presented from one of twenty loudspeakers placed in a circle around a subject at intervals of 18°.  The movie illustrates the waveforms measured at the two ears.  By selecting different loudspeakers and different tonal frequencies, you can begin to explore the acoustical cues available to the auditory system for horizontal-plane localisation.

Interaural Cues

When the sound source is in front of the listener (e.g. the uppermost speaker in the demo), the sound arrives at both ears at the same time and with the same amplitude.  However, when the sound is played from other loudspeakers, the sound may arrive at the ears at slightly different times and with different amplitudes.  The time delay between the ears is referred to as the Interaural Time Difference (ITD) and the amplitude disparity between the ears is referred to as the Interaural Level Difference (ILD).

This demonstration displays the magnitude of the ITD and ILD for each loudspeaker location and frequency, which can be used to investigate the usefulness and limitations of the interaural cues.  Careful examination of the movies reveals further important complexities regarding the relative phase of the waveforms, particularly with some frequencies.

The Duplex Theory of sound localisation, attributed to Lord Rayleigh (1907), was developed on the basis of the acoustical considerations illustrated in this demonstration (see Hafter, 1984). 

Assumptions of the Demonstration

It was assumed that measurements were made in anechoic conditions.  The ITD was calculated from Woodworth’s (1938) simple geometric model and assuming a typical diameter of an adult head, the predictions of which are close to the data reported by Fedderson et al. (1957) using clicks.  The ILD was calculated using an expression that roughly captures the data reported by Fedderson et al. (1957).

Further Issues to Consider

This demo shows that the ability of ITD and ILD to cue different sound source locations is limited in many ways beyond those considered in the context of the Duplex Theory.  What are these limitations and how might the auditory system resolve them?

This demo does not capture some of the more complex but important aspects of the interaural cues.  What acoustical effects of changing loudspeaker position and frequency are not captured by the demo?

There are a number of other limitations with the Duplex Theory that you could explore.  For example, are there aspects of spatial hearing that the theory can not account for?  Does the theory apply to more complex stimuli?

Further Reading and References

For further background reading, see Moore (2003) and Hafter and Trahiotis (1997).  For more detailed considerations of the acoustics of localisation, see Kuhn (1987), Wightman and Kistler (1996) and Blauert (1997). 

Blauert J. Spatial hearing: the psychophysics of human sound localisation. Cambridge (MA): MIT Press; 1997.

Fedderson WE, Sandel TT, Teas DC, Jeffress LA. Localisation of high–frequency tones. J Acoust Soc Am 1957;29:988–91.

Hafter ER. Spatial hearing and the duplex theory: how viable is the model? In: Edelman GM, Gall EW, Cowan WM, editors. Dynamic aspects of neocortical function. New York: John Wiley and Sons; 1984. p. 425–48.

Hafter ER, Trahiotis C. Functions of the binaural system. In: Crocker MJ. Encyclopedia of Acoustics. Vol 3. New York: John Wiley and Sons; 1997. p. 1461–79.

Kuhn GF. Physical acoustics and measurements pertaining to directional hearing. In: Yost WA, Gourevitch G, editors. Directional hearing. New York: Springer–Verlag; 1987. p. 3–25.

Moore BCJ. An introduction to the psychology of hearing. 5th ed. San Diego: Academic Press; 2003.

Rayleigh Lord (Strutt JW, 3rd Baron of Rayleigh). On our perception of sound direction. Phil Mag 1907;13:214–32.

Wightman FL, Kistler DJ. Factors affecting the relative salience of sound localization cues. In: Gilkey RH, Anderson TR, editors. Binaural and spatial hearing in real and virtual environments. Mahwah, New Jersey: Lawrence Erlbaum Associates; 1996. p. 1–23.

Woodworth RS. Experimental psychology. New York: Holt, Rinehart and Winston; 1938.

This demonstration was developed by Damion Theobald and Daniel Rowan.  Please send feedback to dr@isvr.soton.ac.uk