Chapter Questions
Why are sound waves in air characterized as longitudinal?
Draw the sine curve that corresponds to the sound wave depicted in Figure $13-21$
What is the difference between frequency and pitch?
Why can a dog hear a sound produced by a dog whistle, while his owner cannot?
What are the differences between infrasonic, audible, and ultrasonic sound waves?
Explain why the speed of sound depends on the temperature of the medium. Why is this temperature dependence more noticeable in a gas than in a solid or a liguid?
The Doppler effect occurs whena. a source of sound moves toward a listener.b. a listener moves toward a source of sound.c. a listener and a source of sound move away from each other.d. a listener and a source of sound move toward each other.e. All of the above
You are at a street corner and hear an ambulance siren. Without looking, how can you tell when the ambulance passes by?
Ultrasound waves are often used to produce images of objects inside the body. Why are ultrasound waves effective for this purpose?
If the wavelength of a sound source is reduced by a factor of $2,$ what happens to the wave's frequency? What happens to its speed?
As a result of a distant explosion, an observer first senses a ground tremor, then hears the explosion. What accounts for this time lag?
By listening to a band or an orchestra, how can you determine that the speed of sound is the same for all frequencies?
A sound wave travels in air at a frequency of $500 \mathrm{Hz}$ If part of the wave travels from air into water, does its frequency change? Does its wavclength change? Note that the speed of sound in air is about $340 \mathrm{m} / \mathrm{s},$ whereas the speed of sound in water is about $1500 \mathrm{m} / \mathrm{s}$
A fire engine is moving at $40 \mathrm{m} / \mathrm{s}$ and sounding its horn. A car in front of the fire engine is moving at $30 \mathrm{m} / \mathrm{s},$ and a van in front of the car is stationary. Which observer hears the fire engine's horn at a higher pitch, the driver of the car or the driver of the van?
A bat flying toward a wall cmits a chirp at $40 \mathrm{kHz}$. Is the frequency of the echo received by the bat greater than, less than, or equal to $40 \mathrm{kHz}$ ?
If a sound seems to be getting louder, which of the following is probably increasing?a. intensityb. frequencyc. speed of soundd. wavelength
Using Table $13-2(\text { page } 490)$ as a guide, estimate the decibel levels of the following sounds: a cheering crowd at a football game, background noise in a church, the pages of this textbook being turned, and light traffic.
Why is the threshold of hearing represented as a curve in Figure $13-10$ (page 489 ) rather than as a single point?
Under what conditions does resonance occur?
If the distance from a point source of sound is tripled, by what factor does the sound intensity decrease? Assume there are no reflections from nearby objects to affect your results.
Why is the intensity of an echo less than that of the original sound?
The decibel level of an orchestra is $90 \mathrm{dB}$, and a single violin achieves a level of $70 \mathrm{dB}$. How do the intensity and volume of the sound of the full orchestra compare with those of the violin's sound?
A noisy machine in a factory produces a decibel rating of $80 \mathrm{dB}$. How many identical machines could you add to the factory without exceeding the $90 \mathrm{dB}$ limit set by federal regulations?
Why are pushes given to a playground swing more effective if they are given at certain, regular intervals than if they are given at random positions in the swing's cycle?
Although soldiers are usually required to march together in step, they must break their march when crossing a bridge. Explain the possible danger of crossing a rickety bridge without taking this precaution.
A baseball coach shouts loudly at an umpire standing 5.0 meters away. If the sound power produced by the coach is $3.1 \times 10^{-3} \mathrm{W}$, what is the decibel level of the sound when it reaches the umpire? (Hint: See Sample Problem $13 \mathrm{A}$, then use Table $13-2$ on page 490 .
A stereo speaker represented by $P$ in Figure $13-22$ emits sound waves with a power output of $100.0 \mathrm{W}$. What is the intensity of the sound waves at point $x$ when $r=10.0 \mathrm{m} ?$(See Sample Problem 13A.)
What is fundamental frequency? How are harmonics related to the fundamental frequency?
Figure $13-23$ shows a stretched string vibrating in several of its modes. If the length of the string is $2,0 \mathrm{m},$ what is the wavelength of the wave on the string in $(\mathbf{a}),(\mathbf{b}),(\mathbf{c}),$ and $(\mathbf{d}) ?$
Why does a pipe closed at one end have a different harmonic series than an open pipe?
Explain why a saxophone sounds different from a clarinet, even when they sound the same fundamental frequency at the same decibel level.
Why does a vibrating guitar string sound louder when it is on the instrument than it does when it is stretched on a work bench?
Two violin players tuning their instruments together hear six beats in 2 s. What is the frequency difference between the two violins?
What is the purpose of the slide on a trombone and the valves on a trumpet?
A student records the first 10 harmonics for a pipe. Is it possible to determine whether the pipe is open or closed by comparing the difference in frequencies between the adjacent harmonics with the fundamental frequency? Explain.
A flute is similar to a pipe open at both ends, while a clarinet is similar to a pipe closed at one end. Explain why the fundamental frequency of a flute is about twice that of the clarinet, even though the length of these two instruments is approximately the same.
The fundamental frequency of any note produced by a flute will vary slightly with temperature changes in the air. For any given note, will an increase in temperature produce a slightly higher fundamental frequency or a slightly lower one?
What are the first three harmonics of a note produced on a $31.0 \mathrm{cm}$ long violin string if waves on this string have a speed of $274.4 \mathrm{m} / \mathrm{s}$ ? (See Sample Problem $13 B$.)
The human ear canal is about $2.8 \mathrm{cm}$ long and can be regarded as a tube open at one end and closed at the eardrum. What is the fundamental frequency around which we would expect hearing to be best when the speed of sound in air is $340 \mathrm{m} / \mathrm{s}$ ? (See Sample Problem 13 B.)
A pipe that is open at both ends has a fundamental frequency of $320 \mathrm{Hz}$ when the speed of sound in air is $331 \mathrm{m} / \mathrm{s}$ a. What is the length of this pipe?b. What are the next two harmonics?c. What is the fundamental frequency of this pipe when the speed of sound in air is increased to $367 \mathrm{m} / \mathrm{s}$ due to a rise in the temperature of the air?
The area of a typical eardrum is approximately $5.0 \times 10^{-5} \mathrm{m}^{2}$. Calculate the sound power (the energy per second) incident on the eardrum ata. the threshold of hearing.b. the threshold of pain.
The frequency of a tuning fork can be found by the method shown in Figure 13-24. A long tube open at both ends is submerged in a beaker of water, and the vibrating tuning fork is placed near the top of the tube. The length of the air column, $L,$ is adjusted by moving the tube vertically. The sound waves generated by the fork are reinforced when the length of the air column corresponds to one of the resonant frequencies of the tube. The largest value for $L$ for which a peak occurs in sound intensity is $9.00 \mathrm{cm} .$ (Use $345 \mathrm{m} / \mathrm{s}$ as the speed of sound in air.) a. What is the frequency of the tuning fork?b. What is the value of $L$ for the next two harmonics?
When two tuning forks of $132 \mathrm{Hz}$ and $137 \mathrm{Hz}$, respectively, are sounded simultaneously, how many beats per second are heard?
The range of human hearing extends from ap mately $20 \mathrm{Hz}$ to $20000 \mathrm{Hz}$ Find the wavelengths of these extremes when the speed of sound in air is equal to $343 \mathrm{m} / \mathrm{s}$
A dolphin in $25^{\circ} \mathrm{C}$ sea water emits a sound directed toward the bottom of the occan $150 \mathrm{m}$ below. How much time passes before it hears an ccho? (Sce Table13-1 on page 482 for the speed of the sound.
An open organ pipe is $2.46 \mathrm{m}$ long, and the speed of the air in the pipe is $345 \mathrm{m} / \mathrm{s}$a. What is the fundamental frequency of this pipe?b. How many harmonics are possible in the normal hearing range, $20 \mathrm{Hz}$ to $20000 \mathrm{Hz}$ ?
The greatest value ever achieved for the speed of sound in air is about $1.0 \times 10^{4} \mathrm{m} / \mathrm{s}$, and the highest frequency ever produced is about $2.0 \times 10^{10} \mathrm{Hz}$ Find the wavelength of this wave.
If you blow across the open end of a soda bottle and produce a tone of $250 \mathrm{Hz}$, what will be the frequency of the next harmonic heard if you blow much harder?
A rock group is playing in a club. Sound emerging outdoors from an open door spreads uniformly in all directions. If the decibel level is $70 \mathrm{dB}$ at a distance of $1.0 \mathrm{m}$ from the door, at what distance is the music just barely audible to a person with a normal threshold of hearing? Disregard absorption.
The fundamental frequency of an open organ pipe corresponds to the note middle $C(f=261,6 \mathrm{Hz}$ on the chromatic musical scale). The third harmonic $\left(f_{3}\right)$ of another organ pipe that is closed at one end has the same frequency. Compare the lengths of these two pipes.
A typical decibel level for a buzzing mosquito is $40 \mathrm{dB}$, and normal conversation is approximately $50 \mathrm{dB}$ How many buzzing mosquitoes will produce a sound intensity equal to that of normal conversation?
Some studies indicate that the upper frequency limit of hearing is determined by the diameter of the eardrum. The wavelength of the sound wave and the diameter of the eardrum are approximately equal at this upper limit. If this is so, what is the diameter of the eardrum of a person capable of hearing $2.0 \times 10^{4} \mathrm{Hz}$ ? Assume $378 \mathrm{m} / \mathrm{s}$ is the speed of sound in the ear.
The decibel level of the noise from a jet aircraft is $130 \mathrm{dB}$ when measured $20.0 \mathrm{m}$ from the aircraft.a. How much sound power does the jet aircraft $\mathrm{emit}$b. How much sound power would strike the eardrum of an airport worker $20.0 \mathrm{m}$ from the aircraft? (Use the diameter found in item 53 to calculate the area of the eardrum.)