Our interest in acoustics is mainly due to our ability to perceive sound directly with our hearing organ, that is, without any artificial aids. In this function our auditory system proves to be of amazing sensitivity – in fact, were it just slightly more sensitive we could hear the molecules of the air beating down on our eardrum, which would not be of any use. Another fascinating point is the wide range of pitch and loudness which can be processed by our hearing. Concerning the former it should be recalled that the frequency range of our eye is much more limited in that it spans just the ratio 1 : 2, that is, one octave. In contrast, the range of audible frequencies
is from about 16 Hz to more than 20 000 Hz, which corresponds to more than three powers of ten or about ten octaves. Equally phenomenal is the wide range of sound intensities which can be processed by our hearing without being overloaded; it comprises more than 12 powers of ten – without any switching between different ranges as is usual with electrical measuring instruments. Furthermore, our hearing can detect slight differences in pitch and timbre. This admirable performance is made possible by the delicate anatomical – not to say, mechanical – construction of our hearing organ, by its non-linear properties and by the complicated way in which our brain
processes the electrical nerve impulses into which the acoustical signal is converted by the inner ear. In order to dwell a little more on the comparison of our auditory system with the visual sense it may be mentioned that the spatial range of auditory perception is extended over all directions in contrast to the field of vision which is limited to a relatively small solid angle. However, there is also a restriction of auditory performance: although our hearing can detect with relatively high accuracy the direction from which sound arrives if there is just one sound source, it is more difficult to distinguish between several different and spatially distributed sound sources. The reason for this limited resolution are the long acoustical wavelengths and the fact that only two
acoustical ‘sensors’ are at our disposal each of them being small compared to the wavelength. In contrast, in our eye the extension of the sensitive organ – the pupil or the retina – covers many wavelengths. This is the reason why we cannot receive a true acoustical ‘image’ of our environment in the same sense as the image portrayed by our eyes. The knowledge of the auditory performance and of the way it is brought about is interesting in its own right. Beyond this, however, studying the properties of our hearing has important practical aspects. Thus the quality
of any natural or electroacoustic sound transmission, or the efficiency of some measure of noise reduction, cannot be assessed quantitatively if there is no criterion to do this. This holds not only for the judgement of actual situations but also for acoustical planning, for instance, of industrial
sites or of buildings of any kind, for designing hearing aids or with respect to electroacoustical installations. Likewise, in modern techniques of sound recording certain properties of human hearing such as masking are deliberately exploited. For this reason we shall deal particularly with the performance of auditory perception which after all yields the yardstick for tasks of the sort mentioned earlier.
Acoustics
An introduction
Heinrich Kuttruff
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