Question 1

A boat sounds a fog horn on a day when both the sea water and the air temperature are 25.0° C. The speed of sound in sea water is 1 533 m/s. How much earlier (in s) does a dolphin 1 000 m from the source hear the sound than a person in a boat that is also 1 000 m distant? (Ignore the time it takes the sound to reach the water surface.)
A) 0.652
B) 2.12
C) 2.24
D) 2.77
E) 2.90

Accepted Answer

The speed of sound in air at 25°C is approximately 346 m/s. The time for sound to travel 1,000 m in water is 1000/1533 = 0.652 s. The time for sound to travel 1,000 m in air is 1000/346 = 2.89 s. The difference is 2.89 - 0.652 = 2.24 s.

Question 2

A speaker is producing 30 W of sound. At which of the following frequencies would it sound the loudest?
A) 50 Hz
B) 100 Hz
C) 500 Hz
D) 1 000 Hz
E) 10 000 Hz

Accepted Answer

The human ear is most sensitive to sounds in the range of 1,000 to 5,000 Hz, which means that a speaker producing sound at 1,000 Hz would sound the loudest. Additionally, 30 W is not a lot of power for a speaker, so it is unlikely that it can produce very loud sound at lower frequencies like 50 or 100 Hz.

Question 3

A source of sound waves is placed at the center of a very large sound-reflecting wall. The source emits 0.500 W of power. Two meters from the source the intensity in W/m² is A) 9.95 × 10−³. B) 1.99 × 10−². C) 0.313. D) 0.399. E) 0.625.

Accepted Answer

The formula relating intensity and power is:

$I=\frac{P}{4\pi r^2}$

where $r$ is the distance from the source. Plugging in the given values:

$I=\frac{0.500	ext{ W}}{4\pi (2	ext{ m})^2}=1.99	imes10^{-12}	ext{ W/m}^2$

This is choice (B).

Question 4

When a workman strikes a steel pipeline with a hammer, he generates both longitudinal and transverse waves. The two types of reflected waves return 2.4 s apart. How far away is the reflection point? (For steel, v_L = 6.2 km/s, v_T = 3.2 km/s)

Accepted Answer

The answer of When a workman strikes a steel pipeline...

Question 5

An airplane traveling at half the speed of sound emits sound at a frequency of 5 000 Hz. At what frequency does a stationary listener hear the sound as the plane approaches, and after it passes by? Assume the airplane is not flying very high.

Accepted Answer

The answer of An airplane traveling at half the speed...

Question 6

A bat, flying at 5.00 m/s, emits a chirp at 40.0 kHz. If this sound pulse is reflected by a wall, what is the frequency of the echo received by the bat? (v_sound = 340 m/s.)

Accepted Answer

The answer of A bat, flying at 5.00 m/s, emits...

Question 7

If a plane is flying at a speed so that the apex half-angle of its shockwave is 30°, what is its speed if the speed of sound is 320 m/s?
A) 960 m/s
B) 640 m/s
C) 320 m/s
D) 160 m/s
E) Choose this answer if a shock wave cannot form in this situation.

Accepted Answer

The relation between the speed of sound (a), the velocity of the object (v), and the apex half-angle of its shockwave (θ) is given by:
sinθ = a/v
Substituting the given values, we get:
sin 30° = 320/v
v = 640 m/s (option B)

Question 8

If a supersonic plane is flying at increasing speed as the temperature decreases, what happens to the Mach number in this instance?
A) It stays the same.
B) It increases.
C) It decreases.
D) It can do any of the above.
E) It can do either (a) or (b) but not (c).

Accepted Answer

The Mach number, which is the ratio of the speed of the object to the speed of sound in the medium, increases as the speed of the object increases or as the speed of sound decreases. Since the speed of sound decreases with a decrease in temperature, and the plane's speed is increasing, both factors contribute to an increase in the Mach number.

Question 9

On a day when the speed of sound in the upper air is 320 m/s, you fly coast to coast in the United States, a distance of about 4 850 km, in about one hour, if the Mach number for the speed of your airplane is about
A) 1.
B) 2
C) 3
D) 4
E) 5.

Accepted Answer

The answer of On a day when the speed of...

Question 10

The intensity level of an orchestra is 85 dB. A single violin achieves a level of 70 dB. How does the intensity of the sound of the full orchestra compare with that of the violin?

Accepted Answer

The answer of The intensity level of an orchestra is...

Question 11

A microphone in the ocean is sensitive to the sounds emitted by porpoises. To produce a usable signal, sound waves striking the microphone must have an intensity of 1.02 × 10−¹¹ W/m². If porpoises emit sounds with a power of 0.049 9 W, how far away can a porpoise be and still be heard? Disregard absorption of sound waves by the water.

Accepted Answer

The answer of A microphone in the ocean is sensitive...

Question 12

The fundamental frequency of a above middle C on the piano is 440 Hz. This is the tenor high A, but a convenient note in the mid-range of women's voices. When we calculate the wavelength, we find that it is
A) much shorter than the length of either a man's or woman's lips.
B) shorter than the length of a man's lips, but about the length of a woman's lips.
C) longer than a woman's lips, but about the length of a man's lips.
D) much longer than the length of either a man's or a woman's lips.
E) about the same length as either a man's or woman's lips.

Accepted Answer

The wavelength of a 440 Hz sound wave in air (assuming a speed of sound of approximately 343 m/s) is about 78 cm, which is much longer than the length of human lips.

Quiz 18: Superposition and Standing Waves

A speaker is producing 30 W of sound. At which of the following frequencies would it sound the loudest?

A source of sound waves is placed at the center of a very large sound-reflecting wall. The source emits 0.500 W of power. Two meters from the source the intensity in W/m2 is

When a workman strikes a steel pipeline with a hammer, he generates both longitudinal and transverse waves. The two types of reflected waves return 2.4 s apart. How far away is the reflection point? (For steel, vL = 6.2 km/s, vT = 3.2 km/s)

An airplane traveling at half the speed of sound emits sound at a frequency of 5 000 Hz. At what frequency does a stationary listener hear the sound as the plane approaches, and after it passes by? Assume the airplane is not flying very high.

A bat, flying at 5.00 m/s, emits a chirp at 40.0 kHz. If this sound pulse is reflected by a wall, what is the frequency of the echo received by the bat? (vsound = 340 m/s.)

If a plane is flying at a speed so that the apex half-angle of its shockwave is 30°, what is its speed if the speed of sound is 320 m/s?

If a supersonic plane is flying at increasing speed as the temperature decreases, what happens to the Mach number in this instance?

On a day when the speed of sound in the upper air is 320 m/s, you fly coast to coast in the United States, a distance of about 4 850 km, in about one hour, if the Mach number for the speed of your airplane is about

The intensity level of an orchestra is 85 dB. A single violin achieves a level of 70 dB. How does the intensity of the sound of the full orchestra compare with that of the violin?

The fundamental frequency of a above middle C on the piano is 440 Hz. This is the tenor high A, but a convenient note in the mid-range of women's voices. When we calculate the wavelength, we find that it is

A source of sound waves is placed at the center of a very large sound-reflecting wall. The source emits 0.500 W of power. Two meters from the source the intensity in W/m² is

When a workman strikes a steel pipeline with a hammer, he generates both longitudinal and transverse waves. The two types of reflected waves return 2.4 s apart. How far away is the reflection point? (For steel, v_L = 6.2 km/s, v_T = 3.2 km/s)

A bat, flying at 5.00 m/s, emits a chirp at 40.0 kHz. If this sound pulse is reflected by a wall, what is the frequency of the echo received by the bat? (v_sound = 340 m/s.)