Q: Beats are heard when two sound waves of slightly different frequencies are played together. What is the beat frequency?

$$f_{\text{beat}} = |f_1$ - f_2$|$$

Question 1

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

Accepted Answer

Transverse waves oscillate perpendicular to the direction of energy transfer, while longitudinal waves oscillate parallel to the direction of energy transfer.

Question 2

A wave traveling along a string is described by $$y(x, t) = A \cos \left( \frac{2\pi}{\lambda} x - \frac{2\pi}{T} t \right). If $$you want to 'surf' the wave (stay at a fixed $$y$$), what must happen as time increases?

Accepted Answer

The position $$x$$ must increase to keep the phase constant.

Question 3

When a wave pulse traveling on a string meets a boundary with a fixed end, what happens to the reflected wave?

Accepted Answer

It is reflected and inverted.

Question 4

If two waves of equal amplitude and frequency meet in phase at a point, what type of interference occurs?

Accepted Answer

Constructive interference

Question 5

For two sound sources emitting waves of the same frequency, constructive interference will occur at a point where the path difference is:

Accepted Answer

$$n \lambda$$, where $$n = 0, 1, 2, ...$$

Question 6

A standing wave is formed on a string fixed at both ends. What is true about the harmonics?

Accepted Answer

Each successive harmonic adds one extra half-wavelength loop.

Question 7

A church pipe is 25 cm long. If both ends are closed and the speed of sound is $$343\ \mathrm{m/s}$$, what is the frequency of the first harmonic?

Accepted Answer

$$f_1$$ = \frac{v}{2L} = \frac{343}{0.5} = 686\ \mathrm{Hz}$$

Question 8

Beats are heard when two sound waves of slightly different frequencies are played together. What is the beat frequency?

Accepted Answer

$$f_{	ext{beat}} = |f_1$ - f_2$|$$

Question 9

The intensity of sound follows an inverse square law. If you double your distance from a sound source, what happens to the intensity?

Accepted Answer

It becomes one-fourth as great.

Question 10

The decibel level $$B of a $$sound is given by $$B = 10 \log \left( \frac{I}{I_0$} \right)$$, where $$I_0$$ = $$10^{-12}$$\ \mathrm{$$W/m^2$$}$$. If the intensity increases by a factor of 100, how does the decibel level change?

Accepted Answer

It increases by 20 dB.

Question 11

A stationary observer hears a higher pitch as an ambulance approaches. Which effect explains this phenomenon?

Accepted Answer

Doppler effect

Question 12

Which equation correctly describes the observed frequency $$f'$$ when the source is stationary and the observer is moving toward the source at speed $$u ($$sound speed $$v$$)?

Accepted Answer

$$f' = (1 + \frac{u}{v})f$$

Question 13

A moving source emits sound of frequency $$f. If $$the source moves toward a stationary observer at speed $$u$$, what is the observed frequency $$f'$$?

Accepted Answer

$$f' = \frac{f}{1 - \frac{u}{v}}$$

Question 14

Which of the following is a real-world application of the Doppler effect?

Accepted Answer

Detecting tornadoes using Doppler radar

Question 15

When a wave crosses from a less dense to a more dense medium, how is the boundary treated and what happens to the reflected wave?

Accepted Answer

The boundary is treated as a fixed end and the wave is inverted.

Question 16

If two waves of the same amplitude and frequency are out of phase by $$\pi$$ radians (180 degrees), what type of interference occurs?

Accepted Answer

Destructive interference

Question 17

A medical device uses the Doppler effect to measure blood flow. If blood is moving toward the sensor, what happens to the observed frequency?

Accepted Answer

It increases.

Question 18

A guitar string is fixed at both ends and vibrates at its second harmonic. What is the wavelength of the standing wave if the string length is $$L$$?

Accepted Answer

$$\lambda = L$$

Question 19

Which of the following best explains why galaxies moving away from us appear redder?

Accepted Answer

The Doppler effect causes the observed wavelength to increase (redshift).

Question 20

If the path difference between two waves is $$n \lambda / 2$$ with $$n = 1, 3, 5, ...$$, what type of interference occurs?

Accepted Answer

Destructive interference

Waves and Sound II: Study Notes for PHYS 111

Study Guide - Practice Questions

Study Guide - Flashcards