15 Traveling Waves and Sound Slide 15-2

Energy and Intensity Slide 15-35

The Decibel Scale Sound intensity level is measured in decibels. Slide 15-36

Example Problem Your ears are, in principle, sensitive to sound down to 0 dB. In practice, though, background noise limits your threshold of hearing to about 20 dB. Suppose that someone is playing a stereo with the volume cranked up all the way, giving a sound intensity level of 110 dB at a distance of 1.0 m. How far away could you be and still hear the music? That is, at what distance from the stereo would the sound intensity level be 20 dB? (This example is a bit artificial because loss mechanisms work at these great distances and any practical situation would involve reflections, but it is instructive.) Slide 15-39

16 Superposition and Standing Waves Slide 16-2

Slide 16-3

Slide 16-4

Reading Quiz When two waves overlap, the displacement of the medium is the sum of the displacements of the two individual waves. This is the principle of __________. constructive interference destructive interference standing waves superposition Answer: D Slide 16-5

Principle of Superposition Slide 16-11

Principle of Superposition Slide 16-11

Principle of Superposition Slide 16-11

Constructive and Destructive Interference Displacements add Destructive: Displacements cancel Slide 16-12

How does the string appear at t = 2 s? Checking Understanding Two waves on a string are moving toward each other. A picture at t = 0 s appears as follows: How does the string appear at t = 2 s? Answer: A Slide 16-13

How does the string appear at t = 2 s? Answer Two waves on a string are moving toward each other. A picture at t = 0 s appears as follows: How does the string appear at t = 2 s? (a) Answer: A Slide 16-14

Standing Wave Modes Slide 16-15

Example Problems A particular species of spider spins a web with silk threads of density 1300 kg/m3 and diameter 3.0 µm. A passing insect brushes a 12-cm-long strand of the web, which has a tension of 1.0 mN, and excites the lowest frequency standing wave. With what frequency will the strand vibrate? Two strings with linear densities of 5.0 g/m are stretched over pulleys, adjusted to have vibrating lengths of 50 cm, and attached to hanging blocks. The block attached to string 1 has a mass of 20 kg and the block attached to wire 2 has mass M. When driven at the same frequency, the two strings support the standing waves shown. What is the driving frequency? What is the mass of the block suspended from String 2? Slide 16-16

Standing Sound Waves Slide 16-17

Example Problem: Physics of Music The Egyptian tomb of Tutankhamun contained many treasures, including two trumpets. These trumpets were simple instruments, consisting of straight tubes with a mouthpiece at one end and a bell at the other. The smaller of the two was 58 cm long, made of silver. This trumpet, like other similar musical instruments, couldn’t make a reasonable sound at the fundamental frequency, but it could produce all of the harmonics. What frequencies could the instrument have produced? Slide 16-18