6dB. that the dBs are of voltage comparison rather than of power. (See Table 8.3.) Hearing Table 8.3 Power and voltage ratios at different decibels 8.3.1 The Ear Decibels Power ratio Voltage ratio Figure 8.2 shows a simplified section through the human ear. The visible part, the outer ear or pinna, is generally of little use although it may help in the location of high frequency sounds where the wavelengths are comparable with the dimensions of folds and ridges. The ear drum, or tympanum, is a delicate membrane which is caused to move by sound wave pressures and attached to it is a chain of bones called the ossicles. These serve as an impedance transforming set of levers to match the impedance of the air to the impedance of the fluid in the inner ear, thus ensuring that as much sound energy as possible is conveyed to the actual receptor system inside the cochlea. The inner ear is represented in Figure 8.3 where the cochlea is shown straightened out. The final bone in the ossicles operates a small membrane, the oval window or fenestre ovalis. The section through the cochlea shows the basilar membrane which runs the length of the cochlea. This membrane varies in width, being widest at the far end, near the 100.0 helicotrema which is a small aperture allowing vibrations to pass into the lower section below the basilar membrane. The round 1000.0 window simply gives compliance to the system; without it very little 100.0 wave energy would enter the cochlea because of the relative incom- 316.0 pressibility of fluids. Along the basilar membrane is a complex structure called Corti's Organ. When sound waves enter the ear, because of the shape and other physical characteristics of the cochlea, regions of maximum Although the decibel is a unit of comparison there are many cases vibration occur depending on the sound frequency. The lowest where it is useful to have an agreed reference as a base, SO that frequencies reach a maximum near the helicotrema, the highest powers, voltages or, as we shall see, sound pressures may be c