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Lecture 20

PHY 2020 Lecture 20: Chapter 20
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Department
Physics
Course
PHY 2020
Professor
Fauerbach
Semester
Spring

Description
Chapter 20- Sound - Figure 20.1: (a) When the door is opened, compression travels across the room. (b) When the door is closed, a rarefaction travels across the room. - Figure 20.2: Compression and rarefactions travel (both at the same speed in the same direction) from the tuning fork through the air in the tube. - Figure 20.3: A Ping-Pong paddle vibrating in the midst of Ping-Pong balls produces vibration of the balls. o Low pressure behind, high pressure in front - Figure 20.4: Waves of compressed and rarefied air, produced by the vibrating cone of the loudspeaker, make up the pleasing sound of music. - Figure 20.5: (a) vibrations of the radio loudspeaker send compressions of air (red lines) and produce similar vibration in the microphone, which are displayed on an oscilloscope. (b) The shape of the waveform on the oscilloscope screen reveals information about the sound. o Microphone and speaker are the same with different properties. #50. Suppose a sound wave and an electromagnetic wave have the same frequency. What has the longer wavelength? - The light wave because it has a higher velocity. The speed of light is faster than sound. #3. What is the average range of a young person’s hearing? - 20 Hz-20, 000 Hz. o Older people vibrissae’s get stiffer thus the higher frequency range is lost. #57. Why is it so quiet after a snowfall? - Both carpet and snowfall have air within them. Sound reflects in a straight line, the carpet/snow absorbs more sound because the sound is disturbing though the air. - Figure 20.6: The angle of incident sound is equal to the angle of reflected sound. - Figure 20.8: The plastic plate above the orchestra reflects both light and sound. Adjusting them is quite simple: What you see is what you hear. o Concert halls can be tuned by adding curtains - Figure 20.9: sound waves are bent in air of uneven temperature o Sound travels faster in warm air - Why do we see lightning, but don’t hear thunder during distant thunderstorms o The difference in temperature causes this. #60. If the speed of sound were dependent on its frequency, would you enjoy a concert sitting in the second balcony? Explain. - No, the frequencies take longer to get to you. The further away will make it harder for frequencies of different pitches to be heard differently. #62. Why does sound travel slower in cold air than in warm air? - Particles move faster in warmer air. - What is ultrasound? Why jelly on belly? - Ultrasound is beyond human hearing (high frequencies). Jelly transmits the sound waves on the belly. - Infrasound- low frequencies that can be heard by the human ear. - Figure 20.11: A dolphin emits ultra-high-frequency sound to locate and identify objects in its environment. Distance is sensed by the time delay between sending sound and receiving its echo, and direction is sensed by differences in time for the echo to reach its two ears. A dolphin’s main diet is fish, and since hearing in fish is limited to fairly low frequencies, the fish are not altered to the fact that they are being hunted. - Figure 20.12: The natural frequency of the smaller bell is higher than that of the larger bell, and it rings at a higher pitch. - Figure 20.16: Manuel finds that he produces a large amplitude when he pumps in
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