Chapter 26: Sound

1. Chapter 26: Sound

2. The Origin of Sound • All sounds are produced by the vibrations of material objects • Pitch – our subjective impression of sound • A young person can normally hear pitches with frequencies from 20 to 20,000 Hz; as we grow older, this range shrinks • Infrasonic – sound waves with frequencies below 20 Hz • Ultrasonic – sound waves with frequencies above 20,000 Hz

3. Sound Waves and Frequency

4. Sound in Air • Compression – a pulse of compressed air • Rarefaction – pulses of lower pressure • Most sound waves are combinations of compressions and rarefactions which create a longitudinal wave

5. Sound in Air

6. Media That Transmits Sound • Most sounds you hear are transmitted through the air • Sound also travels through solids and liquids • Solids and liquids are generally better conductors of sound than air, sound waves travel faster in solids and liquids • Sound cannot travel through a vacuum (there is nothing to compress!)

7. Bell in a Vacuum

8. Speed of Sound • You hear thunder after you see lightning (evidence that sound is much slower than light) • The speed of sound in dry air at 0ºC is ~330 m/s (1/100000000 the speed of light) • For each degree increase in temperature, the speed of sound increases by 0.60 m/s (at normal room temperature of 20ºC, sound travels at 340 m/s) • The speed of sound in a material depends not on the density, but on its elasticity (ability to change shape in response to an applied force)

9. Loudness • The intensity of sound is proportional to the square of the amplitude of a sound wave • Sound intensity is objective and can be measured by an oscilloscope • Loudness is physiological sensation sensed in the brain • The unit of intensity for sound is the decibel (dB), after Alexander Graham Bell • Starting with zero at the threshold of hearing for a normal hear, an increase of each 10 dB means that sound intensity increases by a factor of 10 • Human hearing is approximately logarithmic

11. Forced Vibration • Forced Vibration – the vibration of an object which is made to vibrate by another vibrating object that is nearby • The mechanism in a music box is mounted on a sounding board (the object being forced to vibrate); without the sounding board, the sound would be almost inaudible • The vibration of guitar strings in an acoustic guitar follow the same principle

12. Forced Vibration

13. Natural Frequency • Natural Frequency – an objects own special set of frequencies, which together form its special sound • The natural frequency depend on factors such as the elasticity and shape of the object • Bells and tuning forks vibrate at their own characteristic frequencies • Even planets and atoms vibrate at one or more natural frequencies

14. Natural Frequency

15. Resonance • Resonance – occurs when the frequency of a forced vibration on an object matches the object’s natural frequency, a dramatic increase in amplitude occurs • In order for something to resonate, it needs force to pull it back to its starting position and enough energy to keep it vibrating

16. Interference • Sound waves, like any waves, can be made to interfere • Interference affects the loudness of sounds • Destructive interference of sound waves is usually not a problem, because there is enough reflection of sound to fill in canceled spots; although, “dead spots” are often evident in poorly designed arenas • Destructive sound interference is used in anti-noise technology

17. Interference Patterns

18. Beats • Beats – the periodic variation in the loudness of sound • Beats can be heard when two slightly mismatched tuning forks are sounded together; when the forks are in step, the sound is at a maximum, when the forks are out of step, the sound is at a minimum

19. Beats

20. Assignment • Read Chapter 26 (pg. 390-400) • Chapter 26 Assessment # 21-39 (pg. 402-403)