November 06, 2011

Perception and hearing explained by new research (1)

The human hearing mechanism is a biological transducer and psych-acousticians have studied it extensively in an attempt to model its behavior mathematically. The frequency analysis of the ear is carried out by the basilar membrane which is enclosed within the spiral cochlea or inner ear. The basilar membrane resonates to continuous tones at different frequencies along its length, determining the frequency limits of human hearing. After resonance has begun, the physical position or place of the resonance along the membrane allows the listener to determine the pitch. In case of a timbral sound the harmonics will excite a specific pattern of spaced resonances which will be unique for each type of instrument allowing the listener to recognize its type.

The human hearing mechanism does not just detect the existence of sound, it also estimates the direction of the source as well as analyzing the content of the sound to determine the most likely cause. In musical sounds, the pitch will also be determined. Josef Manger has been studying these mechanisms for over 20 years. He has found that each mechanism takes a different time to operate following an initial transient. The location and nature of the sound source are completely discerned before the pitch is recognized.

Pitch and timbral recognition is described by the well-established place theory, in which different parts of the basilar membrane resonate according to the frequencies in the sound. However, various authorities, such as Keidel, Spreng, Klinke and Zenner, have suggested that there is another, faster acting, mechanism which works in the time domain.
The theory could not be tested with conventional loudspeakers. Confirmation of the theory was not possible until Josef Manger used his newly developed transducer as the sound source.

Fig.3 illustrates this principle of transient analysis and shows an idealized transient pressure waveform following an acoustic event.
There are three important points made in the figure:

1/ A complete cycle is quite unnecessary for the recognition of the sound source. Only the initial transient pressure change A-B is required. The time of arrival of the transient at the two ears will be different and will locate cause, i.e. the source laterally within around a millisecond.
2/ Following the event which generated the transient, the air pressure equalizes itself along the line B-F. The period of time between B and F varies and allows the listener to establish the likely size of the sound source.
3/ Only after the recognition of the source from the transient is the pitch recognized according to the place theory of the basilar membrane from the part of the wave-form beyond F.
The information in the initial transient pressure waveform goes beyond locating the source.

Fig. 4 illustrates how the size of a sound source affects the pressure equalization time.

Pressure waveforms from a hand gun, a rifle and a cannon are shown. It will be seen that the larger the source, the longer the pressure equalization time.

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