1. Primitive people made sounds not only with their voices, but also with drums, rattles, and whistles. Stringed instruments are at least 3000 years old. Some animals use for survival sound with frequencies too high for humans to hear. Bats, in particular, hunt flying insects by emitting pulses of very high-frequency sound. They learn how far away the insect is, how large it is, where it is, and what its relative velocity is. Sound and music are important components of the human experience.

2. The velocity of the sound wave in air depends on the temperature of the air. Sound waves move through air at sea level at a velocity of 343 m/s at room temperature (20°C). Sound can also travel through liquids and solids. In general, the velocity of sound is greater in solids and liquids than in gases. Sound cannot travel through a vacuum because there are no particles to move and collide. The speed in air increases 0.6 m/s for each °C increase.

3. FIGURE 15-2. In an anechoic chamber, used for acoustical research, sound is almost completely absorbed by the soft materials covering all the surfaces

4. Relationship between velocity, wavelength and frequency

5. A moving source “compresses” the waves on the side it is moving towards A wave tank demonstration Doppler Effect Applet A real Shock Wave

6. The physical characteristics of sound waves are measured by frequency, wavelength, and amplitude. In humans, sound is detected by the ear and interpreted by the brain. Sound characteristics are defined in terms describing what we perceive. Pitch is essentially the frequency of the wave. Loudness depends on the amplitude of the pressure variation wave.

7. The human ear is extremely sensitive to the variations in air pressure in sound. It can detect wave amplitudes of less than one billionth of an atmosphere (or 2 x 10 -5 N/m 2 ). At the other end of the audible range, the pressure variations that cause pain are one million times greater, (20 N/m 2 ). Notice that this is still less than one one- thousandths of an atmosphere.

8. Because of this wide range in pressure variation, sound pressures are measured by a quantity called sound level. Sound level is measured in decibels (dB). The level depends on the ratio of the pressure of a given sound wave to the pressure in the most faintly heard sound, 2 x 10 -5 N/m 2. Such an amplitude has a sound level of zero decibels (0 dB). A sound with 10 times larger pressure amplitude (2 x 10 -4 N/ m 2 ) is 20 dB. A pressure amplitude 10 times larger than this is 40 dB. Figure 15—5 shows the sound level in decibels for a variety of sounds.

9. Marin Mersenne (1588—1648) and Robert Hooke (1635— 1703) first connected pitch with the frequency of vibration. Pitch can also be given the name of a note on a musical scale. Musical scales are based on the work of Pythagoras, a Greek mathematician who lived in the sixth century B.C. He noted that when two strings had lengths in the ratio of small whole numbers, for example 2:1, 3:2, or 4:3, pleasing sounds resulted.

11. The shapes of the mouthpieces of a brass instrument (a) and a reed instrument (b) help determine the characteristics of the sound each instrument produces

12. In all the instruments illustrated, changes in pitch are brought about by changing the length of the resonating column of air.

13. Resonance -- waves which add together to be louder

14. Standing pressure waves resonating in closed pipes. The first three harmonics are shown.

17. The human ear is a complex sense organ that translates sound vibrations into nerve impulses that are then sent to the brain for interpretation.

18. Continuous exposure to loud sounds can cause serious hearing loss. In many occupations, workers such as aircraft line personnel and rock musicians wear ear protection.

19. Beats occur as a result of the superposition of two sound waves of slightly different frequencies.

21. Musical instruments sound very different from one another, even when playing the same note. This is true because most sounds are made up of a number of frequencies. The quality of a sound depends on the relative intensities of these frequencies. In physical terms, it depends on the spectrum of the sound. In musical terms, sound quality is called timbre (TOM bur) or sometimes "tone color."

23. The shape of the vocal tract determines the resonant wave forms for a as in sat (a) and u as in suit (b).