# Physics chapter 20

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 How does sound relate to energy? Sound waves transport energy from one place to another. What is the relationship between frequency and pitch? Pitch is the human perception of sound frequency. What is the average range of a young person’s hearing? 20 to 20,000 hertz Distinguish between infrasonic and ultrasonic sound waves. Infrasonic is too low in frequency (below 20 Hz) for humans to hear, whereas ultrasonic is too high (above 20,000 Hz). Relative to solids and liquids, how does air rank as a conductor of sound? Relative to solids and liquids, air is a poor conductor of sound. Why won’t sound travel in a vacuum? There is no material in a vacuum to vibrate and carry the sound. Distinguish between a compression and a rarefaction. In a compression, the molecules are closer together than average; in a rarefaction, they are further apart. Do compressions and rarefactions travel in the same direction, or in opposite directions, in a wave. They travel in the same direction at the same speed. What factors does the speed of sound depend upon? What are some factors that it does not depend upon? It depends on temperature and humidity, not on loudness and frequency What is the speed of sound in dry air at 20°C? 340 m/s Does sound travel faster in warm air or in cold air? In humid air or dry air? Faster in warm air, faster in humid air Which is normally greater, the energy in ordinary sound or the energy in ordinary light? How does the speed of sound compare to the speed of light? The energy in ordinary light is greater than the energy in ordinary sound. The speed of sound is a million times less than the speed of light. What ultimately happens to the energy of sound in the air? It becomes thermal energy. The pitch of sound is most closely related to _________. frequency What is an echo? Reflection of sound is an echo. What is a reverberation? A sound that persists after many reflections Sound travels from one place to another mainly in a series of _________. compressions and rarefactions What is the difference in the speed of sound on a warm day versus on a cold day? It is faster on a warm day and slower on a cold day. Why did the campers in Dr. Hewitt’s story hear the sound of the campers across the lake more clearly at night than during the day? The temperature change from cold at the surface to warm at the top changed the shape of the sound waves. How does the direction of sound travel compare to the shapes of the sound waves? The sound is at right angles to the waves. How does the cooler air above the lake affect the movement of sound at night? Because it is colder just above the surface and warmer higher above, the sound wave is refracted toward the ground. How do the waves of sound travel on the night that produces the temperature inversion, and why? The waves bend toward the ground because it is warmer above and colder lower below, and waves travel more slowly through the colder air. What is the cause of refraction? Different speeds of sound in a medium Does sound tend to bend upward or downward when its speed is less near the ground? Downward Why does sound sometimes refract under water? Changing temperature changes the speed of sound in water. What is ultrasound? Sound above the frequency of human hearing Why will a struck tuning fork sound louder when it is held against a table? The table is forced to vibrate, and its large surface vibrates more air than the tuning fork. Which two factors determine the natural frequency of an object? Elasticity and shape The speed of sound in air varies with _________. air temperature How do forced vibrations relate to resonance? When forced vibrations match an object’s natural frequency, resonant amplification occurs. When you listen to a radio, why do you hear only one station at a time rather than all stations at once? The natural frequency of the electronics is tuned to resonate with the frequency of one station. How did wind-generated resonance affect the Tacoma Narrows Bridge in the state of Washington in 1940? The resonant amplification of bridge motion destroyed the bridge. A reverberation occurs when sound undergoes _________. multiple echoes Refraction of sound occurs with changes in wave _________. speed What physical phenomenon underlies the production of beats? interference What beat frequency will occur when a 370-Hz and a 374-Hz sound source are sounded together? 4 Hz How does a radio wave differ from a sound wave? A radio wave is an electromagnetic wave that travels at the speed of light, whereas a sound wave travels at the speed of sound through air or another elastic medium. Forced vibrations mainly occur in _________. sounding boards The natural frequency of a small bell is _________. higher than for a larger bell Resonance occurs when forced vibration _________. matches natural frequency Interference is a property of _________. sound, light, and water waves The phenomenon of beats is most closely related to _________. interference

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