Q: A police siren emits a frequency of 800 Hz. If the police car moves toward a stationary observer at 30 m/s (speed of sound is 340 m/s), what frequency does the observer hear?

$$f' = \dfrac{v}{v - v_s} f = \dfrac{340}{340 - 30} \times 800 \approx 872 Hz$$

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

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

Question 1

Which of the following statements best describes the difference between mechanical and electromagnetic waves?

Accepted Answer

Mechanical waves require a medium to propagate, while electromagnetic waves do not require a medium.

Question 2

A string under tension $$F_T$$ and with linear mass density $$\mu$$ supports a transverse wave. What is the correct formula for the wave velocity $$v on $$the string?

Accepted Answer

$$v = \sqrt{\dfrac{F_T}{\mu}}$$

Question 3

A sound wave travels from air into water. If the frequency remains constant, what happens to the wavelength and speed of the wave?

Accepted Answer

Both the speed and wavelength increase.

Question 4

A point source emits sound uniformly in all directions with a power output $$P. $$What is the intensity $$I at a $$distance $$r$$ from the source?

Accepted Answer

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

Question 5

A 400 Hz tuning fork and a 404 Hz tuning fork are sounded together. What is the beat frequency heard?

Accepted Answer

4 Hz

Question 6

Which of the following best describes the principle of superposition?

Accepted Answer

The net displacement at any point is the sum of the individual wave displacements at that point.

Question 7

A police siren emits a frequency of 800 Hz. If the police car moves toward a stationary observer at 30 m/s (speed of sound is 340 m/s), what frequency does the observer hear?

Accepted Answer

$$f' = \dfrac{v}{v - v_s} f = \dfrac{340}{340 - 30} 	imes 800 \approx 872 Hz$$

Question 8

A string of length $$L$$ fixed at both ends supports standing waves. What is the wavelength $$\lambda_n of $$the $$nth $$harmonic?

Accepted Answer

$$\lambda_n = \dfrac{2L}{n}$$

Question 9

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

Accepted Answer

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

Question 10

Which of the following is true about the interference of two waves from two coherent sources?

Accepted Answer

Constructive interference occurs when the path difference is an integer multiple of the wavelength.

Question 11

A flute can be modeled as an open tube of length $$L. $$What is the fundamental frequency $$f_1 of $$the standing wave if the speed of sound is $$v$$?

Accepted Answer

$$f_1$$ = \dfrac{v}{2L}$$

Question 12

A wave passes from a medium with speed $$v_1$$ into another with speed $$v_2$$ \u003c $$v_1. If $$the frequency remains constant, what happens to the wavelength?

Accepted Answer

The wavelength decreases.

Question 13

Which of the following best describes diffraction?

Accepted Answer

The bending of waves around obstacles or through openings.

Question 14

A wave on a string is described by $$y(x,t) = 0.05 \sin(4\pi x - 200\pi t)$$, where $$x$$ and $$y$$ are in meters and $$t in $$seconds. What is the wave speed?

Accepted Answer

$$v = \dfrac{\omega}{k} = \dfrac{200\pi}{4\pi} = 50 m/s$$

Question 15

A sound wave travels from muscle (impedance $$Z_1) $$into bone (impedance $$Z_2). $$What fraction of the intensity is reflected at the interface?

Accepted Answer

$$R = \left( \dfrac{Z_2$ - Z_1$}{Z_2$ + Z_1$} \right )^2$

Question 16

Which of the following is true about standing waves on a string fixed at both ends?

Accepted Answer

Nodes are located at both ends of the string.

Question 17

A sound wave in air has a frequency of 500 Hz and a wavelength of 0.68 m. What is the speed of sound in air?

Accepted Answer

$$v = f \lambda = 500 	imes 0.68 = 340 m/s$$

Question 18

Which of the following is NOT a characteristic of a wave?

Accepted Answer

Mass

Question 19

A tube closed at one end and open at the other has length $$L. $$What is the wavelength of the fundamental mode?

Accepted Answer

$$\lambda = 4L$$

Wave Motion and Sound: Study Notes (Chapters 15 & 16)

Study Guide - Practice Questions

Study Guide - Flashcards