Question 1

Two cars,one in front of the other,are traveling down the highway at 44 m/s.The car behind sounds its horn,which has a frequency of 530 Hz.What is the frequency heard by the driver of the lead car? (v_sound = 340 m/s)

Accepted Answer

Since both cars are moving at the same speed and in the same direction, there is no relative motion between them. Therefore, the frequency heard by the driver of the lead car is the same as the frequency of the horn, which is 530 Hz.

Question 2

A sound source of frequency 1,090 Hz moves at 40 m/s toward a listener who is at rest.What is the apparent frequency heard by the listener? (speed of sound = 340 m/s)

Accepted Answer

The answer of A sound source of frequency 1,090 Hz...

Question 3

What sound level change corresponds to a factor of five change in intensity?

Accepted Answer

According to the formula for sound intensity level, a factor of five change in intensity corresponds to a difference of approximately 7 dB. This is because the formula is IL = 10 log (I/Io), where I is the intensity being measured and Io is the reference intensity (usually taken to be the threshold of human hearing). If I is increased by a factor of 5, then IL will increase by 10 log (5) = 7 dB. Therefore, the correct answer is (C) 7 dB.

Question 4

A train station bell gives off a fundamental tone of 470 Hz as the train approaches the station at a speed of 28 m/s.If the speed of sound in air is 349 m/s,what will be the apparent frequency of the bell to an observer riding the train?

Accepted Answer

The Doppler effect formula for frequency of a sound source moving towards a stationary observer is given by:
f' = (v + v_observer) / (v + v_source) * f
where v is the speed of sound in air, v_observer is the speed of the observer, v_source is the speed of the source, and f is the actual frequency of the source.

In this case, the source (the bell) is moving towards the observer (the person riding the train) with a speed of 28 m/s. The speed of sound in air is given as 349 m/s. The actual frequency of the bell is 470 Hz.

Plugging these values into the Doppler effect formula, we get:
f' = (349 + 28) / (349 + 0) * 470
f' = 1.080 * 470
f' = 508 Hz

Therefore, the apparent frequency of the bell to an observer riding the train is 508 Hz.

Question 5

You stand by the railroad tracks as a train passes by.You hear a 1,100-Hz frequency when the train approaches,which changes to 870 Hz as it goes away.How fast is the train moving? The speed of sound in air is 340 m/s.

Accepted Answer

We can use the formula for the Doppler effect:

f' = f(vs ± vd) / (vs ± vo)

Where:
f = frequency emitted
f' = frequency received
vs = speed of sound 
vd = speed of the detector (train)
vo = speed of the observer (you)

Let's solve for vd:

f1 = 1100 Hz
f2 = 870 Hz
vs = 340 m/s

Using f1:

1100 = f(vs + vd) / (vs + vo)

Using f2:

870 = f(vs - vd) / (vs + vo)

Dividing the equations:

1100/870 = (vs + vd)/(vs - vd)

Simplifying:

vd = vs ((f1 - f2)/(f1 + f2))

Substituting the given values:

vd = 340 ((1100-870)/(1100+870)) = 39.7 m/s

Therefore, the answer is D) 39.7 m/s.

Question 6

As a train starts from rest and then accelerates down the track,coming toward me faster and faster,the speed of the sound waves coming toward me will be:

Accepted Answer

The speed of sound waves in air is determined by the properties of the air (temperature, humidity, etc.) and not by the speed of the source of the sound. Therefore, even if the train accelerates towards you, the speed of the sound waves coming towards you remains equal to the normal speed of sound in air.

Question 7

By what amount does the sound intensity decrease when the distance to the source triples?

Accepted Answer

The answer of By what amount does the sound intensity...

Question 8

A 520-Hz whistle is moved toward a listener at a speed of 50 m/s.At the same time,the listener moves at a speed of 70 m/s in a direction away from the whistle.What is the apparent frequency heard by the listener? (The speed of sound is 340 m/s. )

Accepted Answer

The formula for Doppler effect when the source is moving and the observer is stationary is:

f' = f * (v + v₀) / (v + vᵢ)

where:
f' is the apparent frequency heard by the listener
f is the actual frequency of the whistle
v is the speed of sound
v₀ is the speed of the source (in this case the whistle)
vᵢ is the speed of the observer (in this case the listener)

Plugging in the given values:

f' = 520 * (340 + 50) / (340 - 70)
f' = 484 Hz

Therefore, the apparent frequency heard by the listener is 484 Hz, which corresponds to choice B.

Question 9

The Doppler shift of ultrasonic waves can measure the speed of blood in an artery.If the frequency of the stationary source is 95 kHz and the reflected sound has a Doppler shift of 150 Hz,what is the blood flow speed? (The speed of sound inside the body is 1 500 m/s. )

Accepted Answer

The answer of The Doppler shift of ultrasonic waves can...

Question 10

While standing at a crosswalk,you hear a frequency of 590 Hz from an approaching police car.After the police car passes,its frequency is 440Hz.What is the speed of the police car? (speed of sound = 340 m/s)

Accepted Answer

The answer of While standing at a crosswalk,you hear a...

Question 11

If the distance between a point sound source and a dB detector is increased by a factor of 5,what will be the reduction in intensity level?

Accepted Answer

The intensity level in decibels decreases by 20*log10(5) ≈ 14 dB when the distance is increased by a factor of 5.

Question 12

An airplane flying with a constant speed flies from a warm air mass into a cold air mass.The Mach number will:

Accepted Answer

The Mach number, which is the ratio of the speed of the airplane to the speed of sound in the surrounding air, will increase when the airplane flies from a warm air mass into a cold air mass. This is because the speed of sound decreases in colder air, assuming the airplane's speed remains constant.

Question 13

In the afternoon,the decibel level of a busy freeway is 85 dB with 90 cars passing a given point every minute.Late at night,the traffic flow is only 9 cars per minute.What is the late-night decibel level?

Accepted Answer

The decibel level is not directly affected by the number of cars passing by, only by their noise level. Therefore, the late-night decibel level will be the same as the afternoon level, which is 85 dB. The number of cars passing by is irrelevant.

Question 14

Quadrupling the distance between sound source and a listener will change the intensity,as detected by the listener,by what factor?

Accepted Answer

According to the inverse square law, the intensity of sound decreases by 1/4 (or 0.25) for every doubling of the distance between the sound source and the listener. Therefore, quadrupling the distance will result in a decrease of (0.25)^2 or 1/16 in intensity. So, the answer is A) 1/16.

Question 15

If the intensity of a sound is increased by a factor of 100,how is the decibel level changed? The new decibel level will be:

Accepted Answer

The decibel (dB) scale is logarithmic, meaning that a 10-fold increase in intensity corresponds to an increase of 10 dB. Therefore, a 100-fold increase in intensity (i.e., 10^2) corresponds to an increase of 20 dB (i.e., 10 x 2). Thus, the new decibel level will be 20 units greater. Therefore, the correct answer is option D.

Question 16

What is the intensity of sound from a band with a sound level of 123 dB? (I₀ = 10−¹² W/m²)

Accepted Answer

The intensity of sound is given by the formula:$$I = I_0 	imes 10^{dB/10}$$where:* I is the intensity of sound in W/m^2* I0 is the reference intensity of sound (10^-12 W/m^2)* dB is the sound level in decibelsSubstituting the given values into the formula, we get:$$I = 10^{-12} W/m^2 	imes 10^{123 dB/10}$$$$I = 2 W/m^2$$Therefore, the intensity of sound from the band is 2 W/m^2.

Question 17

A plane is traveling at Mach 0.93 through air at a temperature of 0°C.What is the plane's speed? (Speed of sound at 0°C is 331 m/s. )

Accepted Answer

The speed of the plane is calculated by multiplying the Mach number by the speed of sound at the given temperature: 0.93 * 331 m/s = 308.43 m/s, which is approximately 308 m/s.

Question 18

A bat,flying at 6 m/s toward a wall,emits a chirp at 48 kHz.If the wall reflects this sound pulse,what is the frequency of the echo received by the bat? (v_sound = 340 m/s)

Accepted Answer

The Doppler effect formula for sound frequency received by a moving source and observer is used. The bat is both the source and observer. First, calculate the frequency heard by the wall: f' = f(v_sound + v_bat) / v_sound. Then, calculate the frequency of the echo received by the bat: f'' = f'(v_sound) / (v_sound - v_bat). This results in approximately 49.7 kHz.

Question 19

A very loud train whistle has an acoustic power output of 94 W.If the sound energy spreads out spherically,what is the intensity level in dB at a distance of 95 meters from the train? (I₀ = 10−¹² W/m²)

Accepted Answer

The formula relating power and intensity is:
P = I*A
where P is power, I is intensity, and A is the area through which the sound is passing. Since the sound energy spreads out spherically, the area through which it is passing increases as distance from the source increases. Specifically, the area of a sphere is given by:
A = 4*pi*r^2
where r is the distance from the source. 
Combining these two equations, we can solve for intensity:
I = P/(4*pi*r^2)
Plugging in the given values, we get:
I = 94/(4*pi*95^2) = 1.28 x 10^-5 W/m^2
Next, we use the formula for sound level in decibels:
L = 10*log(I/I0)
where I0 is the reference intensity, which is defined as:
I0 = 10^-12 W/m^2
Plugging in the values, we get:
L = 10*log(1.28 x 10^-5/10^-12) = 89 dB.

Question 20

The intensity level of sound 18 m from a jet airliner is 120 dB.At what distance from the airplane will the sound intensity level be a tolerable 100 dB? (Assume spherical spreading of sound. )

Accepted Answer

Use the inverse square law to solve for the new distance.
I1/I2 = (r2/r1)^2
I1 = 120 dB, I2 = 100 dB
r1 = 18 m
r2 = ? 
Substitute the values and solve for r2.
100/120 = (r2/18)^2
r2 = 232 m (approx)

Quiz 14: Sound

Two cars,one in front of the other,are traveling down the highway at 44 m/s.The car behind sounds its horn,which has a frequency of 530 Hz.What is the frequency heard by the driver of the lead car? (vsound = 340 m/s)