Chapter 12 Sound

1. Chapter 12Sound

2. 12.1 Sound Waves The Production of Sound Waves • The vibrating prong of a tuning fork sets air molecules near it in motion. • A region of high molecular density and high air pressure is called compression. • The molecules to the right spread apart and the density and pressure in this region becomes lower than normal, this is called rarefaction.

3. Sound Waves are longitudinal • With sound, air molecules are parallel to the direction of wave motion. • The crests are the compressions and the troughs are the rarefactions. Animation

4. Characteristics of Sound Waves • Frequency is defined as the number of cycles per unit of time. • The average human ear can hear “audible sound” waves. • These range from 20 Hz to 20,000Hz • Infrasonic waves are 20 Hz or less. • Ultrasonic waves are over 20,000 Hz. • Dogs can hear ultrasonic frequencies. Frequency Human Ear onlinetonegenerator.com

5. Frequency determines pitch • The frequency of a sound wave determines how high or low we perceive the sound to be. This is known as pitch. • The frequency of a wave is an objective(measured) quantity. • Pitch referrers to how different frequencies are perceived to the human ear. • Pitch not only depends on frequency but other factors like background noise and loudness. Pitch

7. Speed of sound depends on the medium • Sound waves can travel through solids, liquids and gases. • Waves consist of particle vibrations. • Speed depends on how quickly one particle transfer’s its motion to another particle. • Sound travels fastest in materials where the particles are closer together, like in a solid. • The speed of sound can travel faster in warmer air. This is because warmer air contains more energy. • Temperature differences in solids and liquids do not affect speed that much. *Note the speed of air at the different temperatures; 346 m/s = about 770 mph!

8. Sound waves propagate in three dimensions • Sound waves travel out from their source in all directions. • Spherical waves can be represented graphically. • The centers of compression are called wave fronts. • The distance between wave fronts = wave length. • The perpendicular to the waves are rays. • These become nearly parallel - the farther from the source.

9. The Doppler Effect • An ambulance can produce a sound at a certain frequency and we perceive this pitch a certain way. • Do moving sounds appear to change their pitch as they approach and move away? http://newton.umsl.edu/exhibit/doppler.html

10. Relative motion creates a change in frequency • The Doppler effect is an observed change in frequency when there is relative motion between the source of waves and an observer. • This changes the pitch. • Most familiar with sound but effects all waves, even light. Doppler

11. Practice • The the speed of sound in sea water is 1530 m/s. • If a dolphin is creating sound waves at a frequency of 2.00 x 105 Hz, what are the wavelengths of this energy? v = ƒλ Answer 7.65 x 10-3 m

12. Questions1. With sound, air molecules are _______ to the direction of wave motion.2. Infrasonic waves are 20 Hz or less. _________ waves are over 20,000 Hz.3. The frequency of a sound wave determines how high or low we perceive the sound to be. This is known as _____.4. Sound travels fastest in materials where the particles are closer together, like in a liquid or _____.5. The Doppler effect is an observed change in _________ when there is relative motion between the source of waves and an observer. parallel Ultrasonic pitch solid frequency

13. 12.3 Harmonics-Beats Beats • When two waves of slightly different frequencies interfere, the interference pattern varies in such a way that a listener hears an alternation between loudness and softness. • This variation from soft to loud and back to soft is called a beat.

14. Sound waves at slightly different frequencies produce beats • This example shows how the frequencies of two different tuning forks will cause constructive and destructive interference. • When the two waves are opposite each other, they are out of phase (t1) • At (t2) the crests of each wave match up and are said to be in phase causing constructive interference. Beats

15. The listener hears the sound getting softer and louder and then softer again. The number of beats per second corresponds to the difference between frequencies • If the time that lapses from t1 to t3 is one second, the blue wave completes one more cycle per second than the red.

16. Practice • A person strikes two tuning forks, one with a frequency of 60 Hz, and another with the frequency scratched off. • If the person hears two beats per second, what two possible frequencies could the other fork be? Answer 62 Hz and 58 Hz

17. Questions1. When two waves of slightly different frequencies interfere, the interference pattern varies in such a way that a listener hears ______ or an alternation between loudness and softness. 2. When the two frequencies are in phase, we have ( constructive or destructive ) interference. 3. T / F The number of beats per second corresponds to the difference between frequencies.4. If the frequency of one tuning fork is 220 Hz and another fork is 222 Hz, how many beats per second will the listener hear? beats _________ True 2

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