Resonance vibration transfer and the ear When you push a person on a swing, a series...

Resonance vibration transfer and the ear When you push a person on a swing, a series of small pushes timed to match the swinger’s swinging frequency makes the person swing with larger amplitude. If timed differently, the pushing is ineffective. The board shown in Figure 19.20 (from the Exploratorium in San Francisco) is made of rods of different length with identical balls on the ends of each rod. Each rod vibrates at a different natural frequency, the long rod on the left at lower frequency and the short rod on the right at higher frequency. If you shake the board at the high frequency at which the short rod vibrates, the short rod swings with large amplitude while the others swing a little. If you shake the board at the middle frequency at which the two center rods vibrate, the center rods undergo large-amplitude vibrations and the rods on each end do not vibrate. Imagine now that you have a fancy board with 15,000 rods, each of slightly different length, the shortest on the left and the longest on the right. Shaking the board at a particular frequency causes the rods in one small region of the board to vibrate at this frequency and has little effect on the others. The inner ear (the cochlea) is a little like this fancy board. Sound reaching the tympanic membrane, or eardrum, is greatly amplified by three tiny bones in the middle ear—the hammer, anvil, and stirrup (Figure 19.21). These bones vibrate, pushing on the fluid in the inner ear and causing vibrations along its entire length. A basilar membrane with about 15,000 hair cells passes along the center of the inner ear. The basilar membrane is narrow and stiff near the entrance to the inner ear and wide and more flexible near the end. When a single-frequency vibration causes the fluid to vibrate, the membrane and the hair cells respond best at a single place—high frequencies near the oval window and low frequencies near the end of the basilar membrane. The bending of these hairs causes those

nerve cells to fire. Thus, we detect the frequency of the sound by the part of the membrane from which the nerve signal comes.

If you shake the board shown in Figure 19.20 at a frequency higher than the natural frequency of the rod on the right, then what happens?

(a) None of the rods vibrate.

(b) All of the rods vibrate.

(c) The shortest rod vibrates.

(d) The longest rod vibrates.

(e) The middle rods vibrate.