Duplex theory

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The duplex theory of localization was introduced in 1907 by John William Strutt , also known as Sir Rayleigh or 3rd Baron Rayleigh . This theory contributes significantly to the understanding of the natural hearing process .


The duplex theory ( duplex : Latin for "double") states that we use two probing phenomena at the same time for directional localization, depending on the frequency range :

However, this does not necessarily have to be the case when listening to the artificial loudspeaker stereophony .

Strutt observed that in the case of sound waves with a smaller wavelength compared to the distance between the ears, the head causes a shadowing of the sound , so that the sound pressure level on the ear facing the sound source (= ipsilateral ) is higher than on the opposite (= contralateral) ear. This means that there is an interaural level difference at high frequencies.

In addition, the different distance between the sound source and the ipsilateral or contralateral ear causes an interaural transit time difference. Strutt was able to demonstrate that these ITDs are particularly effective at low frequencies, where the ILDs are negligible.

The range between 800 Hz and 1600 Hz - i.e. between the two duplex ranges - lies exactly in Blauert's rear band , which results in a diffuse , distant and spatial sound when the frequencies close to 1000 Hz (the opposite of a bathtub filter ) are increased .

Limits of Theory

The duplex theory does not help in distinguishing whether a sound source is directly in front of or behind a person, which is known as the front-back exchange , since the values ​​for ITD and ILD are ideally zero. A sound source located on the surface of a cone on the central interaural axis (known as a cone of confusion ) has identical ITD values.

People can locate a sound source even if they have one-sided hearing impairment . The breakthrough in the explanation of this human ear localization was only achieved by knowledge about the special filter effects that are brought about by the auricles . The filter effects have evolved into the head-related transfer functions ( HRTF or ear signals) in the literature and include various sound localization features including ITD , ILD and changes in the spectral composition of the sound reaching a listener.

In a study in 1992, Wightman and Kistler were able to prove that the ITD dominates the localized direction for broadband sound, with the dominance depending on the lower limit frequency of the sound. 2002 Macpherson and Middlebrooks confirmed the Duplex theory: it could show that the ILD as opposed to the ITD on low-pass filtered noise hardly on high-pass filtered but clearly dominant worked. In lateralization experiments (i.e. with headphones ) in which the auditory event is perceived in the head, on the other hand, the ILD induce shifts in direction for all frequencies. This is an indication that with the transition from in-head localization to externalization of the directional image, not only the perceived location of the source changes, but also the localization process.

See also


  • Wightman, FL and Kistler, DJ: The Dominant Role of Low-Frequency Interaural Time Differences in Sound Localization. In: Journal of the Acoustical Society of America. 91 (3), 1992, pp. 1648-1661
  • Macpherson, EA and Middlebrooks, JC: Listener weighting of cues for lateral angle: The duplex theory of sound localization revisited. In: Journal of the Acoustical Society of America. 111 (5), 2002, 2219-2236

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