Deck 14: Sound

Question 1

What temperature change from 0 °C would increase the speed of sound by 1.0%

Answer 1

The speed of sound in air is expressed as follows:
Here,
is the temperature where the speed of the sound is to be found.
Given that the speed of sound at
is
. Thus,
The speed of sound at
is:
…… (1)
Given that the speed of sound increased by
.
Now, the new speed of sound is:
Thus, the new speed can be expressed as follows:
Replace
with
.
…… (2)
Divide equation (1) with equation (2).
Now, solve for
.
Thus, the required temperature is
.

Question 2

The intensity of a sound wave is directly proportional to the (a) amplitude, (b) frequency, (c) square of the amplitude, (d) square of the frequency.

Answer 2

Intensity of the sound wave is directly propositional to square of the amplitude.
So answer is (c).

Question 3

Can a jet pilot flying faster than the speed of sound hear sound Explain.

Answer 3

Even the pilot is flying faster than the speed of the sound, the air inside the cabin of air craft is stationary and it is not affected by the flight speed.so the pilot will hear same as he can hear when the flight is moving with normal speed.

Question 4

(a) If the distance from a point sound source triples, the sound intensity will be (1) 3, (2) 1/3, (3) 9, (4) 1/9 times the original value. Why (b) By how much must the distance from a point source be increased to reduce the sound intensity by half

Question 5

A compact speaker puts out 100 W of sound power. (a) Neglecting losses to the air, at what distance would the sound intensity be at the pain threshold (b) Neglecting losses to the air, at what distance would the sound intensity be that of normal speech Does your answer seem reasonable Explain.

Question 6

Two identical strings on different cellos are tuned to the 440-Hz A note. The peg holding one of the strings slips, so its tension is decreased by 1.5%. What is the beat frequency heard when the strings are then played together

Question 7

An open organ pipe is 0.50 m long. If the speed of sound is 340 m/s, what are the pipe's fundamental frequency and the frequencies of the first two overtones

Question 8

The speed of sound in air (a) is about 1/3 km/s, (b) is about 1/5 mi/s, (c) depends on temperature, (d) all of the preceding.

Question 9

Do interference beats have anything to do with the "beat" of music Explain.

Question 10

The speed of sound in human tissue is on the order of 1500 m/s. A 3.50-MHz probe is used for an ultrasonic procedure. (a) If the effective physical depth of the ultrasound is 250 wavelengths, what is the physical depth in meters (b) What is the time lapse for the ultrasound to make a round trip if reflected from an object at the effective depth (c) The smallest detail capable of being detected is on the order of one wavelength of the ultrasound. What would this be

Question 11

Why are there no even harmonics in a closed organ pipe

Question 12

What is the intensity level of a 23-dB sound after being amplified (a) ten thousand times, (b) a million times, (c) a billion times

Question 13

How fast, in kilometers per hour, must a sound source be moving toward you to make the observed frequency 5.0% greater than the true frequency (Assume that the speed of sound is 340 m/s.)

Question 14

An organ pipe that is closed at one end is 1.10 m long. It is oriented vertically and filled with carbon dioxide gas (which is denser than air and thus will stay in the pipe). A tuning fork with a frequency of 60.0 Hz can be used to set up a standing wave in the fundamental mode. What is the speed of sound in carbon dioxide

Question 15

What is the difference between ultrasonic and supersonic

Question 16

Brass is an alloy of copper and zinc. Does the addition of zinc to copper cause an increase or decrease in the speed of sound in brass rods compared to copper Explain.

Question 17

The human ear can hear tones best at (a) 1000 Hz, (b) 4000 Hz, (c) 6000 Hz, (d) all frequencies.

Question 18

Assuming that the diameter of your eardrum is 1 cm (see Exercise), what is the sound power received by the eardrum at the threshold of (a) hearing and (b) pain
Exercise
The size of your eardrum (the tympanum; see Fig. 1 in Insight 14.2, The Physiology and Physics of the Ear and Hearing) partially determines the upper frequency limit of your audible region, usually between 16 000 Hz and 20 000 Hz. If the wavelength is on the order of twice the diameter of the eardrum and the air temperature is 20 °C, how wide is your eardrum Is your answer reasonable

Question 19

In a neighborhood challenge to see who can climb a tree the fastest, you are ready to climb. Your friends have surrounded you in a circle as a cheering section; each individual alone would cause a sound intensity level of 80 dB at your location. If the actual sound level at your location is 87 dB, how many people are rooting for you

Question 20

You are driving east at 25.0 m/s as you notice an ambulance traveling west toward you at 35.0 m/s. The sound you detect from the sirens has a frequency of 300 Hz. (a) Is the true frequency of the sirens (1) greater than 300 Hz, (2) less than 300 Hz, or (3) exactly 300 Hz (b) Determine the true frequency of the sirens. Assume normal room temperature.

Question 21

An open organ pipe 0.750 m long has its first overtone at a frequency of 441 Hz. What is the temperature of the air in the pipe

Question 22

The wave speed in a liquid is given by
, where B is the bulk modulus of the liquid and is its density. Show that this equation is dimensionally correct. What about
for a solid ( Y is Young's modulus.)

Question 23

A sound with an intensity level of 30 dB is how many tunes more intense than the threshold of hearing: (a)10, (b) 100, (c) 1000, or (d) 3000

Question 24

(a) Why does it seem particularly quiet after a snowfall (b) Why do empty rooms sound hollow (c) Why do people's voices sound fuller or richer when they sing in the shower

Question 25

A middle C note (262 Hz) is sounded on a piano to help tune a violin string. When the string is sounded, nine beats are heard in 3.0 s. (a) How much is the violin string off tune (b) Should the string be tightened or loosened to sound middle C

Question 26

A dog's bark has a sound intensity level of 40 dB. (a) If two of the same dogs were barking, the intensity level is (1) less than 40 dB, (2) between 40 dB and 80 dB, (3) 80 dB. (b) What would be the intensity level

Question 27

The frequency of an ambulance siren is 700 Hz. What are the frequencies heard by a stationary pedestrian as the ambulance approaches and moves away from her at a speed of 90.0 km/h (Assume that the air temperature is 20 °C.)

Question 28

When all of its holes are closed, a flute is essentially a tube that is open at both ends, with the length measured from the mouthpiece to the far end (as in Fig. 14.16b). If a hole is open, then the length of the tube is effectively measured from the mouthpiece to the hole.(a) Is the position at the mouthpiece (1) a node, (2) an anti node, or (c) neither a node nor an antinode Why (b) If the lowest fundamental frequency on a flute is 262 Hz, what is the minimum length of the flute at 20 °C (c) if a note of frequency 440 Hz is to be played, which hole should be open Express your answer as a distance from the hole to the mouthpiece.

Question 29

The speed of sound in water is about 4.5 times that in air. A single-frequency sound source in air is f o. On penetrating water, the frequency of the sound will be (a) 4 f o , (b) f o /4, (c) f o.

Question 30

(a) Is there a Doppler effect if a sound source and an observer are moving with the same velocity (b) What would be the effect if a moving source accelerated toward a stationary observer

Question 31

The size of your eardrum (the tympanum; see Fig. 1 in Insight 14.2, The Physiology and Physics of the Ear and Hearing) partially determines the upper frequency limit of your audible region, usually between 16 000 Hz and 20 000 Hz. If the wavelength is on the order of twice the diameter of the eardrum and the air temperature is 20 °C, how wide is your eardrum Is your answer reasonable

Question 32

Calculate the intensity level for (a) the threshold of hearing and (b) the threshold of pain.

Question 33

At a rock concert, the average sound intensity level for a person in a front-row seat is 110 dB for a single band. If all the bands scheduled to play produce sound of that same intensity, how many of them would have to play simultaneously for the sound level to be at or above the threshold of pain

Question 34

A jet flies at a speed of Mach 2.0. What is the halfangle of the conical shock wave formed by the aircraft Can you tell the speed of the shock wave

Question 35

An organ pipe that is closed at one end is filled with helium. The pipe has a fundamental frequency of 660 Hz in air at 20 °C. What is the pipe's fundamental frequency with the helium in it

Question 36

What is the difference between sound intensity and sound intensity level

Question 37

The speed of sound in steel is about 4.50 km/s. A steel rail is struck with a hammer, and an observer 0.400 km away has one ear to the rail. (a) How much time will elapse from the time the sound is heard through the rail until the time it is heard through the air Assume that the air temperature is 20 °C and that no wind is blowing. (b) How much time would elapse if the wind were blowing toward the observer at 36.0 km/h from where the rail was struck

Question 38

Equal loudness curves vary with sound (a) quality, (b) harmonics, (c) waveform, (d) pitch.

Question 39

Find the intensity levels in decibels for sounds with intensities of (a) 10 2 W/m 2 , (b) 10 6 W/m 2 , and (b) 10 15 W/m 2.

Question 40

At a distance of 12.0 m from a point source, the intensity level is measured to be 70 dB. At what distance from the source will the intensity level be 40 dB

Question 41

A fighter jet flies at a speed of Mach 1.5. (a) If the jet were to fly faster than Mach 1.5, the half-angle of the conical shock wave would (1) increase, (2) remain the same, (3) decrease. Why (b) Wfriat is the half-angle of the conical shock wave formed by the jet plane at Mach 1.5

Question 42

An open organ pipe, in its fundamental mode, has a length of 50.0 cm. A second pipe, closed at one end, is also in its fundamental mode. A beat frequency of 2.00 Hz is heard. Determine the possible lengths of the closed pipe. Assume normal room temperature.

Question 43

Suggest a possible explanation of why some flying insects produce buzzing sounds and some do not.

Question 44

A 0.75-m-long metal rod is dropped vertically onto a marble floor. When the rod strikes the floor, it is determined electronically that the impact produces a 4-kHz tone. What is the speed of sound in the rod

Question 45

Constructive and destructive interference of sound waves depends on (a) the speed of sound, (b) diffraction, (c) phase difference, (d) all of the preceding.

Question 46

The frets on a guitar finger board are spaced closer together the farther they are from the neck. Wiry is this What would be the result if they were evenly spaced

Question 47

At Cape Canaveral, on blastoff a rocket produces an intensity level of 160 dB as measured 10 m from the rocket. What would be the intensity level at 100 m away (Assume no energy is lost due to reflections, etc.)

Question 48

At a Fourth of July celebration, a firecracker explodes (Fig.). Considering the firecracker to be a point source, what are the intensities heard by observers at points B, C, and D, relative to that heard by the observer at A

FIGURE A big bang See Exercise.

Question 49

The half-angle of the conical shock wave formed by a supersonic jet is 30°. What are (a) the Mach number of the aircraft and (b) the actual speed of the aircraft if the air temperature is 20 °C

Question 50

Bats typically give off an ultrahigh-frequency sound at about 50 000 Hz. If a bat is approaching a stationary object at 18.0 m/s, what will be the reflected frequency it detects Assume the air in the cave is at 5 °C. [ Hint : You will need to apply the Doppler equations twice. Why ]

Question 51

What is the speed of sound in air at (a) 10 °C and (b) 20 °C

Question 52

If the air temperature increases, would the sound intensity from a constant-output point source (a) increase, (b) decrease, or (c) remain unchanged

Question 53

As a person walks between a pair of loudspeakers that produce tones of the same amplitude and frequency, he hears a varying sound intensity. Explain.

Question 54

On hiking up a mountain that has several overhanging cliffs, a climber drops a stone at the first cliff to determine its height by measuring the time it takes to hear the stone hit the ground. (a) At a second cliff that is twice the height of the first, the measured time of the sound from the dropped stone is (1) less than double, (2) double, or (3) more than double that of the first. Why (b) If the measured time is 4.8 s for the stone dropping from the first cliff, and the air temperature is 20 °C, how high is the cliff (c) If the height of a third cliff is three times that of the first one, what would be the measured time for a stone dropped from that cliff to reach the ground

Question 55

(a) If the power of a sound source doubles, the intensity level at a certain distance from the source (1) increases, (2) exactly doubles, or (3) decreases. Why (b) What are the intensity levels at a distance of 10 m from a 5.0-W and a 10-W source, respectively

Question 56

An office in an e-commerce company has fifty computers, which generate a sound intensity level of 40 dB (from the keyboards). The office manager tries to cut the noise to half as loud by removing twenty-five computers. Does he achieve his goal What is the intensity level generated by twenty-five computers

Question 57

An observer is traveling between two identical sources of sound (frequency 100 Hz). His speed is 10.0 m/s as he approaches one and recedes from the other. (a) What frequency tone does he hear from each source (b) How many beats per second does he hear Assume normal room temperature.

Question 58

You hear sound from two organ pipes that are equidistant from you. Pipe A is open at one end and closed at the other, while pipe B is open at both ends. When both are oscillating in their first-overtone mode, you hear a beat frequency of 5.0 Hz. Assume normal room temperature. (a) If the length of pipe A is 1.00 m, calculate the possible lengths of pipe B. (b) Assuming your shortest length for pipe B, what would the beat frequency be (assuming both are still in their first-overtone modes) on a hot desert summer day with a temperature of 40 °C

Question 59

A sound wave with a frequency of 15 Hz is in what region of the sound spectrum: (a) audible, (b) infrasonic, (c) ultrasonic, or (d) supersonic

Question 60

Where is the intensity greater and by what factor: (1) at a point a distance R from a power source P , or (2) at a point a distance 2 R from a power source of 2 P Explain.

Question 61

A person holds a rifle horizontally and fires at a target. The bullet has a muzzle speed of 200 m/s, and the person hears the bullet strike the target 1.00 s after firing it. The air temperature is 72 °F. What is the distance to the target

Question 62

The quality of sound depends on its (a) waveform, (b) frequency, (c) speed, (d) intensity.

Question 63

The intensity levels of two people holding a conversation are 60 dB and 70 dB, respectively. What is the intensity of the combined sounds

Question 64

A 1000-Hz tone from a loudspeaker has an intensity level of 100 dB at a distance of 2.5 m. If the speaker is assumed to be a point source, how far from the speaker will the sound have intensity levels (a) of 60 dB and (b) barely high enough to be heard

Question 65

A bystander hears a siren vary in frequency from 476 Hz to 404 Hz as a fire truck approaches, passes by, and moves away on a straight street (Fig.). What is the speed of the truck (Take the speed of sound in air to be 343 m/s.)

FIGURE The siren's wail See Exercise.

Question 66

An open organ pipe with a length of 50.0 cm is oscillating in its second-overtone or third-harmonic mode. Assume the air to be at room tempera hue and the pipe to be at rest in still air. A person moves toward this pipe at 2.00 m/s and, at the same time, away from a highly reflective wall. (a) Will the observer hear beats: (1) yes, (2) no, or (3) can't tell from the data given (b) Calculate the frequency of sound emitted. (c) Calculate the beat frequency the observer would hear. [ Hint : There are two frequencies, one directly from the pipe and one from the wall.]

Question 67

Explain why sound travels faster in warmer air than in colder air.

Question 68

A tuning fork vibrates at a frequency of 256 Hz. (a) When the air temperature increases, the wavelength of the sound from the tuning fork (1) increases, (2) remains the same, (3) decreases. Why (b) If the temperature rises from 0 °C to 20 °C, what is the change in the wavelength

Question 69

Beats are the direct result of (a) interference, (b) refraction, (c) diffraction, (d) the Doppler effect.

Question 70

Is it possible for an open organ pipe and a closed organ pipe, each of the same length, to produce notes of the same frequency Justify your answer.

Question 71

A point source emits radiation in all directions at a rate of 7.5 kW. What is the intensity of the radiation 5.0 m from the source

Question 72

During practice in a huddle, a quarterback shouts the play in anticipation of crowd noise during the actual game. To a receiver 0.750 m away from the quarterback in the huddle, it seems as loud as the noise from a screaming child. When they get into practice formation, the quarterback yells at twice the output power, yet the instructions seem only about as loud as normal conversation. Use typical values in Table 14.2 to estimate how far from the quarterback the receiver is in the formation.

Question 73

Bats emit sounds of frequencies around 35.0 kHz and use echolocation to find their prey. If a bat is moving with a speed of 12.0 m/s toward a hovering, stationary insect, (a) what is the frequency received by the insect if the air temperature is 20 °C (b) What frequency of the reflected sound is heard by the bat (c) If the insect were initially moving directly away from the bat, would this affect the frequencies Explain.

Question 74

A whale is swimming at a steady speed either directly at or directly away from an underwater cliff (you don't know which). When the whale is 300 m from the cliff, it emits a sound and it hears the echo 0.399 s later. (a) Which way is the whale traveling: (i) toward the cliff or (ii) away from the cliff Explain your reasoning. (b) How fast is the whale traveling (c) If the emitted sound has a frequency of 12.1 kHz, by how much has the frequency changed by the time the whale hears the echo

Question 75

The speed of sound in air on a summer day is 350 m/s. What is the air temperature

Question 76

The decibel scale is referenced to a standard intensity of (a) 1.0 W/m 2 , (b) 10 12 W/m 2 , (c) normal conversation, (d) the threshold of pain.

Question 77

How can Doppler radar used in weather forecasting measure both the location and internal motions of a storm

Question 78

A bat moving at 15.0 m/s emits a high-frequency sound as it approaches a wall that is 25.0 m away. Assuming that the bat continues straight toward the wall, how far away is it when it receives the echo (Assume the air temperature in the cave to be 0 °C.)

Question 79

Two sound sources have intensities of 10 9 W/m 2 and 10 6 W/m 2 , respectively. Which source is more intense and by how many times more

Question 80

A bee produces a buzzing sound that is barely audible to a person 3.0 m away. How many bees would have to be buzzing at that distance to produce a sound with an intensity level of 50 dB