' 456 ENERGY MAY BE PROPAGATED BY VIBRATIONS speeds of the two cars. The same effect may be observed as the whistle of an approaching locomotive decreases in pitch as it passes and recedes from an observer. This apparent change in frequency, produced by the relative motion of a sound source and an observer, is called the Doppler effect. The Doppler eflfect results from the fact that when the distance between the sound source and the observer is decreasing, the number of waves entering the ear per second is increased, and vice versa. The same eflfect produced in light rays was discussed in Unit II, Section 3. The Ear Includes a Device to Convert Compressional Waves into Mechanical Vibrations. The perplexing problem of how we hear has been the object of a great How does the ear differentiate between the buzz of a deal of research. mosquito, the shrill shriek of a siren, the high tones of a violin, and the Ear Drum J / / 7^ Middle Ear^// Oval Window — / / Round Window-/ , Eustachian Canal— Inner Ear Tynfipanic Membrane- FiG. 219. Diagram of the ear. deep rumble of the bass drum? How can all of these things be heard at once and be recognized? How can the student hear a speaker when his neighbors are carrying on a competitive conversation? What is the advantage of two ears? One hypothesis that helps to answer some of these questions is that parts of the membrane of the ear are tuned to a certain frequency and vibrate when compressional waves of that frequency are received. A nerve fiber attached to that part of the membrane would then conduct the impulse to the brain. A study of the ear will show the basis for this hypothesis.
SOUND PRODUCED BY VIBRATIONS IN MATTER 457 Our hearing apparatus is divided into three chambers, shown in Fig. 219. The first chamber, the outer ear, consists of a short narrow tube, which, connecting with the ordinary, visible portion of the ear, acts on the principle of the speaking-tube, megaphone, or stethoscope to concentrate sound in one direction by means of reflection. The stethoscope is simply a small funnel-shaped tube which is placed in contact with the patient's body and from which sounds are conducted through small tubes to the physician's ear. This short, narrow tube of the outer ear is closed at its inner end by an elastic diaphragm, the eardrum. The middle ear consists of a system of three little bones, one end of which is attached to the eardrum and the other to another diaphragm, the ovaj window. The middle ear acts as a sort of lever to transmit vibrations into the inner ear. The vibrations are thus transmitted with decreased amplitude and increased force. The inner ear is filled with liquid and contains a coiled spiral enclosure with bony walls in the shape of a snail's shell. The length of it, except at the tip, is divided by the basilar membrane, where the auditory nerve endings are to be found. There is another flexible window between the middle ear and the inner ear. Its purpose seems to be to provide for the vibrations in the fluid of the inner ear. Something has to give way as the oval window vibrates, and the liquid in the inner ear is practically incompressible, while the walls are rigid. The vibrations are thus transmitted up one side of the basilar membrane from the oval window and down the other side to the round window. The higher-frequency vibrations appear to be transmitted directly through the lower part of the basilar membrane. Low-frequency vibrations are transmitted by the liquid around the top of the membrane, while vibrations of intermediate frequency are transmitted by both methods. Oval Window- Round Window 15.000 10,000 6,000 3,000 2,000 1.500 1,000 500 300 200 100 1 2 Fig. 220. The inner ear straightened out. (After Fletcher.) Figure 220 shows the spiral of the inner ear straightened and much The frequencies to which given portions of this membrane simplified. are believed to respond are indicated.