Q: A sound has an intensity of $$1.0 \times 10$$^{-6}$$\ \mathrm{W/m}^2. $$What is its sound intensity level in decibels (dB)? (Assume $$I_0 = 1.0$$ \times $$10^{-12}$$\ \mathrm{W/m}^2$$)

$$\beta = (10\ \mathrm{dB}) \log_{10}\left(\frac{1.0 \times 10$$^{-6}$$}{1.0 \times 10$$^{-12}$$}\right) = 60\ \mathrm{dB}$$

Q: A Doppler ultrasound probe emits a frequency of 5.0 MHz and detects a maximum frequency shift of 400 Hz from blood moving in an artery. What is the maximum speed of the blood? (Speed of sound in tissue $$v = 1540\ \mathrm{m/s})$$

$$v_{blood} = \frac{\Delta f \cdot v}{2f_0} = \frac{400 \times 1540}{2 \times 5.0 \times 10^6$} = 0.062\ \mathrm{m/s}$$

Q: A sound wave has an intensity level of 90 dB. What is the corresponding intensity in $$\mathrm{W/m}^2$$? (Assume $$I_0 = 1.0$$ \times $$10^{-12}$$\ \mathrm{W/m}^2$$)

$$I = 1.0 \times 10$$^{-3}$$\ \mathrm{W/m}^2$$

Q: Which of the following is true about electromagnetic waves?

They do not require a medium and all travel at $$c = 3.00 \times 10^8$\ \mathrm{m/s} in a $$vacuum.

Question 1

A point source emits sound waves uniformly in all directions. If you move from 2 meters to 4 meters away from the source, by what factor does the intensity of the sound decrease?

Accepted Answer

It decreases by a factor of 4.

Question 2

A laser pointer emits 2.0 mW of light power into a beam with a diameter of 2.0 mm. What is the intensity of the laser beam?

Accepted Answer

$$I = \frac{P}{A} = \frac{0.0020\ \mathrm{W}}{\pi (0.0010\ \mathrm{m})^2$} = 640\ \mathrm{W/m}^2$$

Question 3

Which of the following best describes the difference between transverse and longitudinal waves?

Accepted Answer

Transverse waves have particle motion perpendicular to the direction of wave travel, while longitudinal waves have particle motion parallel to the direction of wave travel.

Question 4

A sound has an intensity of $$1.0 	imes 10$$^{-6}$$\ \mathrm{W/m}^2. $$What is its sound intensity level in decibels (dB)? (Assume $$I_0 = 1.0$$ 	imes $$10^{-12}$$\ \mathrm{W/m}^2$$)

Accepted Answer

$$\beta = (10\ \mathrm{dB}) \log_{10}\left(\frac{1.0 	imes 10$$^{-6}$$}{1.0 	imes 10$$^{-12}$$}ight) = 60\ \mathrm{dB}$$

Question 5

If two identical sound sources each produce 70 dB, what is the total sound level when both operate together?

Accepted Answer

74 dB

Question 6

A spacecraft is powered by a 1.0 $$\mathrm{m}^2$$ array of solar cells with 12% efficiency. If the intensity of sunlight above Earth's atmosphere is $$1300\ \mathrm{W/m}^2$$, what is the maximum power output of the solar cells?

Accepted Answer

$$P = 156\ \mathrm{W}$$

Question 7

A police siren emits a frequency of 550 Hz as it approaches and 450 Hz as it moves away from a stationary observer. If the speed of sound is $$340\ \mathrm{m/s}$$, what is the speed of the police car?

Accepted Answer

$$v_s = 34\ \mathrm{m/s}$$

Question 8

Which of the following statements about the Doppler effect is correct?

Accepted Answer

The Doppler effect causes the observed frequency to increase as the source approaches the observer.

Question 9

A Doppler ultrasound probe emits a frequency of 5.0 MHz and detects a maximum frequency shift of 400 Hz from blood moving in an artery. What is the maximum speed of the blood? (Speed of sound in tissue $$v = 1540\ \mathrm{m/s})$$

Accepted Answer

$$v_{blood} = \frac{\Delta f \cdot v}{2f_0} = \frac{400 	imes 1540}{2 	imes 5.0 	imes 10^6$} = 0.062\ \mathrm{m/s}$$

Question 10

Which of the following best explains why a shock wave (sonic boom) is produced?

Accepted Answer

When a source moves faster than the speed of sound, the waves overlap and add up to a large amplitude wave called a shock wave.

Question 11

A sound wave has an intensity level of 90 dB. What is the corresponding intensity in $$\mathrm{W/m}^2$$? (Assume $$I_0 = 1.0$$ 	imes $$10^{-12}$$\ \mathrm{W/m}^2$$)

Accepted Answer

$$I = 1.0 	imes 10$$^{-3}$$\ \mathrm{W/m}^2$$

Question 12

Which of the following is true about electromagnetic waves?

Accepted Answer

They do not require a medium and all travel at $$c = 3.00 	imes 10^8$\ \mathrm{m/s} in a $$vacuum.

Question 13

If the intensity of a sound increases from $$1.0 	imes 10$$^{-8}$$\ \mathrm{W/m}^2 to$$ $$1.0 	imes 10$$^{-7}$$\ \mathrm{W/m}^2$$, by how many decibels does the sound intensity level increase?

Accepted Answer

10 dB

Question 14

A stationary observer hears a frequency $$f_0$$ from a stationary source. If the observer moves toward the source at speed $$v_o$$, what frequency does the observer hear? (Speed of sound is $$v.)$$

Accepted Answer

$$f = f_0$ \left(\frac{v + v_o}{v}\right)$$

Question 15

Which of the following best describes the red shift observed in distant galaxies?

Accepted Answer

It is caused by galaxies moving away from us, lengthening the wavelength of light.

Question 16

A person shouts, emitting 1.0 W of sound energy as a spherical wave. What is the sound intensity at a distance of 2.0 m?

Accepted Answer

$$I = \frac{1.0}{4\pi (2.0)^2$} = 0.020\ \mathrm{W/m}^2$$

Question 17

Which of the following is a correct statement about the relationship between intensity and area for a wave?

Accepted Answer

Intensity decreases as the area increases for a given power.

Question 18

A sound wave reflects off a moving object and returns to the source. If the object is moving toward the source, what happens to the frequency of the reflected wave?

Accepted Answer

It is Doppler shifted to a higher frequency.

Waves: Energy, Intensity, Loudness, and the Doppler Effect

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