om- peaking near the oval window. Corti's organ is thus set into vibra- pared with it. A particularly common instance in audio engineering tion at places corresponding to the frequency. Part of Corti's Organ is to refer powers to 1 milliwatt (1mW) and this is indicated by dBm. consists of an array of haircells, small hairs which are attached to 8/6 Sound, speech and hearing Pinna Middle Inner Ossicles semicircular Meatus canals Cochlea Round Oval window Tympanum window Eustachian Figure 8.2 Section through the ear nerve endings SO that as the hairs are set into vibration the nerves The curvature of the lines becomes less pronounced at higher are stimulated and signals are sent to the brain. levels. 8.3.2 The response of the ear Although not shown explicitly in Figure 8.4 it can be stated that the average adult ear has a hearing range from about 16 to 16 000Hz. A curiosity about the human ear is that not only does it not have a The lower figure is difficult to measure accurately and the upper flat frequency response but that this response varies with sound figure varies very much with age and individual persons. Young level; in other words its sensitivity depends on both frequency and people in their teens and early twenties may have an upper limit of level. This is best illustrated by the curves in Figure 8.4, sometimes over 20kHz, falling with age. There is, however a great deal of known as the Fletcher-Munsen curves because of the original work variation. 16kHz may be taken as a useful value for the upper limit by these men in the 1930s, but also, and more correctly, referred to of audibility of the average adult. as the NPL curves, from work done at the National Physical Labor- Figure 8.4 also shows a decibel scale. The reference (0dB) is atory in the UK in the 1960s. standardised as a sound pressure level of 0.00002Pa. This is near to It will be seen from the curves in Figure 8.4 that: the pressure required to produce a sensation in the average ear at about 3kHz. 1. The normal ear is most sensitive at frequen