cies in the region 2 to 5 kHz, when the intensity required to produce a just-detect- 8.3.3 Loudness able sensation is less than -Superscript(2) (Taking the opening of the ear canal as being of the order of 1cm2, 2 or 10 2 the power An important point about the curves in Figure 8.4 is that they are needed to produce a sensation is roughly 10-10W or uW!) equal loudness curves. The lowest of all is usually referred to as the At low frequencies, about 20Hz, the ear is relatively insensitive Threshold of Hearing (or Threshold of Audibility). and requires a sound intensity of 0~5W/m² to cause an audible The use of the decibel for comparison of sound pressures is sensation. fortunate in that the ear has a generally logarithmic response to 3. The sensitivity also decreases at high frequencies. This is in sound pressure, and the decibel, being logarithmic by definition, practice less obvious as the most important speech and music thus has a roughly proportional relationship with loudness. For frequencies are below 5 to 6kHz. example it is found that, for most sounds, a 1dB change in level is just detectable with steady, pure tones; a 3dB change is just detect- able with speech or music, while a change of 10dB is perceived as Oval window approximately a doubling (or halving) of loudness. Helicotrema It will be noticed that the curves in Figure 8.4 are labelled Phons. The Phon is an unit of loudness, and it must be remembered that Basilar membrane loudness is a subjective effect and cannot therefore be measured by external means, although there are, as we shall see, methods of making measurements which correlate sufficiently well with sub- Round window jective assessments to be useful. The Phon is defined in the following way: a sound is said to have a loudness of n phons if it has the same loudness as a pure tone at a level of n dB above 0.00002Pa at a frequency of 1kHz. It will be Figure 8.3 The cochlea (unwound) noted that, for example, the 60 phon curve passes through the 60dB line at 1kHz, and similarly for the other c