Q: A sound wave has a frequency of 500 Hz and travels through air at 343 m/s. What is its wavelength?

$$\lambda = \frac{343}{500} = 0.686\ \text{m}$$

Q: Which formula is used to calculate the sound intensity level in decibels ($$\beta) $$given intensity $$I$$ and reference intensity $$I_0$$?

$$\beta = 10\ \text{dB} \cdot \log_{10}\left(\frac{I}{I_0$}\right)$$

Q: A train is approaching a stationary observer at 44.7 m/s. The horn frequency is 415 Hz and the speed of sound is 343 m/s. What is the observed frequency?

$$f_0 = 415$$\ \text{Hz} \times \frac{1}{1 - \frac{44.7}{343}} = 477\ \text{Hz}$$

Question 1

Which of the following best describes the nature of sound waves in air?

Accepted Answer

Sound waves in air are longitudinal waves that propagate by oscillating parallel to the direction of travel.

Question 2

A loudspeaker produces sound by rapidly oscillating its diaphragm. What is the physical effect on air molecules immediately in front of the speaker?

Accepted Answer

The air molecules vibrate back and forth, creating alternating regions of condensation and rarefaction.

Question 3

Which equation correctly expresses the speed of sound in a liquid, given the adiabatic bulk modulus $$B_{ad}$$ and density $$\rho$$?

Accepted Answer

$$v = \sqrt{\frac{B_{ad}}{\rho}}$$

Question 4

A sound wave has a frequency of 500 Hz and travels through air at 343 m/s. What is its wavelength?

Accepted Answer

$$\lambda = \frac{343}{500} = 0.686\ 	ext{m}$$

Question 5

Which of the following statements about the speed of sound is correct?

Accepted Answer

Sound travels fastest in solids, slower in liquids, and slowest in gases.

Question 6

The intensity of sound from a point source decreases as you move away from the source. Which equation describes this relationship?

Accepted Answer

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

Question 7

A conversation produces a pressure amplitude of $$3 	imes 10$$^{-2}$$\ 	ext{Pa}. $$How does this compare to atmospheric pressure ($$1.01 	imes 10^5$\ 	ext{Pa}$$)?

Accepted Answer

The pressure amplitude is much smaller than atmospheric pressure.

Question 8

Which formula is used to calculate the sound intensity level in decibels ($$\beta) $$given intensity $$I$$ and reference intensity $$I_0$$?

Accepted Answer

$$\beta = 10\ 	ext{dB} \cdot \log_{10}\left(\frac{I}{I_0$}ight)$$

Question 9

A train is approaching a stationary observer at 44.7 m/s. The horn frequency is 415 Hz and the speed of sound is 343 m/s. What is the observed frequency?

Accepted Answer

$$f_0 = 415$$\ 	ext{Hz} 	imes \frac{1}{1 - \frac{44.7}{343}} = 477\ 	ext{Hz}$$

Question 10

Which phenomenon explains why the frequency of a siren sounds higher as an ambulance approaches and lower as it moves away?

Accepted Answer

The Doppler effect

Question 11

If a sound source emits a pure tone of 1000 Hz and moves away from a stationary observer, which of the following will occur?

Accepted Answer

The observer will perceive a frequency lower than 1000 Hz.

Question 12

Which of the following best describes the difference between condensation and rarefaction in a sound wave?

Accepted Answer

Condensation is a region of high pressure, rarefaction is a region of low pressure.

Question 13

A sound wave travels through steel at 5960 m/s. If the frequency is 200 Hz, what is the wavelength?

Accepted Answer

$$\lambda = \frac{5960}{200} = 29.8\ 	ext{m}$$

Question 14

Which of the following is NOT a requirement for sound propagation?

Accepted Answer

A vacuum

Question 15

Which term describes sound waves with frequencies above 20 kHz?

Accepted Answer

Ultrasonic

Question 16

A sound source emits 10 W of power uniformly in all directions. What is the intensity at a distance of 2 m?

Accepted Answer

$$I = \frac{10}{4\pi (2)^2$} = 0.199\ 	ext{W/m}^2$$

Question 17

Which equation gives the observed frequency $$f_0$$ when both the source and observer are moving?

Accepted Answer

$$f_0 = f_s$$ \frac{1 \pm \frac{$$v_0$$}{v}}{1 \mp \frac{v_s}{v}}$$

Question 18

Which real-world phenomenon is explained by the Doppler effect for light?

Accepted Answer

Redshift observed in distant galaxies moving away from Earth

Question 19

If the intensity of a sound at a certain distance is $$0.1\ 	ext{W/m}^2$$, what is the intensity level in decibels?

Accepted Answer

$$\beta = 10\ 	ext{dB} \cdot \log_{10}\left(\frac{0.1}{1.00 	imes 10$$^{-12}$$}ight) = 110\ 	ext{dB}$$

Waves & Sound: Properties, Propagation, and the Doppler Effect- lecture 26

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