Question 1

Two musical instruments are playing notes at frequencies $$f_1 = 440$$\ \mathrm{Hz}$$ and f_2$ = 444\ \mathrm{Hz}. $$What is the beat frequency heard by a listener?

Accepted Answer

$$4\ \mathrm{Hz}$$

Question 2

A loudspeaker emits sound uniformly in all directions with a power output of $$P = 10\ \mathrm{W}. $$What is the intensity $$I of $$the sound at a distance $$r = 5\ \mathrm{m}$$ from the source?

Accepted Answer

$$I = \dfrac{10}{4\pi (5)^2$}\ \mathrm{W/m^2$}$$

Question 3

Which of the following best describes a wavefront in the context of spherical waves?

Accepted Answer

A surface over which all points have the same phase

Question 4

The sound intensity level in decibels is given by $$\beta = (10\ \mathrm{dB}) \log \left( \dfrac{I}{I_0$} \right). If $$the intensity $$I is$$ $$10^{-8}$$\ \mathrm{$$W/m^2$$}$$, what is $$\beta$$?

Accepted Answer

$$\beta = 40\ \mathrm{dB}$$

Question 5

If the sound intensity at a rock concert is $$I = 1\ \mathrm{W/m^2$}$$, what is the sound level in decibels, given $$I_0$$ = $$10^{-12}$$\ \mathrm{$$W/m^2$$}$$?

Accepted Answer

$$\beta = 120\ \mathrm{dB}$$

Question 6

A sound source emits waves at $$f = 1000\ \mathrm{Hz}. If $$the source moves towards a stationary observer at $$u_s = 34.3\ \mathrm{m/s}$$ and the speed of sound is $$v = 343\ \mathrm{m/s}$$, what is the observed frequency?

Accepted Answer

$$f' = \dfrac{1000}{1 - \dfrac{34.3}{343}}$$

Question 7

A stationary observer hears a fire truck's siren at $$1600\ \mathrm{Hz} as it $$approaches. The siren's actual frequency is $$1400\ \mathrm{Hz}$$ and the speed of sound is $$343\ \mathrm{m/s}. $$What is the speed of the truck?

Accepted Answer

$$v_s = 43\ \mathrm{m/s}$$

Question 8

If you move away from a stationary sound source at speed $$u_d$$, what happens to the frequency you observe?

Accepted Answer

It decreases, given by $$f' = f \left( 1 - \dfrac{u_d}{v} \right)$$

Question 9

A jet travels faster than the speed of sound and produces a sonic boom. What is the physical cause of the sonic boom?

Accepted Answer

The sound waves accumulate in a cone behind the jet and are heard all at once as the shock wave passes

Question 10

If the sound intensity level drops from $$75\ \mathrm{dB} to$$ $$65\ \mathrm{dB}$$, by what factor does the intensity decrease?

Accepted Answer

By a factor of 10

Question 11

Which of the following is a correct property of logarithms used in sound level calculations?

Accepted Answer

$$\log(A) + \log(B) = \log(AB)$$

Question 12

A police car's siren has a frequency of $$1000\ \mathrm{Hz}. If $$you and the police car are moving towards each other, both at $$50\ \mathrm{km/h}$$, what is the observed frequency? (Speed of sound $$v = 343\ \mathrm{m/s})$$

Accepted Answer

$$f' = 1084\ \mathrm{Hz}$$

Question 13

A sound source emits $$5.0\ \mathrm{kHz}$$ waves. A plate moves towards the source at $$50\ \mathrm{m/s} ($$speed of sound $$343\ \mathrm{m/s}). $$What is the frequency at the plate?

Accepted Answer

$$f' = 5.7\ \mathrm{kHz}$$

Question 14

Which of the following is NOT a practical application of the Doppler effect?

Accepted Answer

Measuring the temperature of a gas

Question 15

If the intensity of a sound at $$2\ \mathrm{m}$$ from the source is $$3.2 	imes 10$$^{-5}$$\ \mathrm{W/m^2$}$$, what is the power emitted by the source?

Accepted Answer

$$P = 1.6\ \mathrm{mW}$$

Question 16

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

Accepted Answer

$$f' = f \left( \dfrac{v \pm u_d}{v \mp u_s} \right)$$

Question 17

If the intensity of a sound is doubled, by how many decibels does the sound level increase?

Accepted Answer

$$3\ \mathrm{dB}$$

Question 18

Which statement about the speed of sound in air is correct?

Accepted Answer

It is constant for a given temperature and pressure, regardless of the motion of the source or observer

Study Notes: Sound Waves, Intensity, and the Doppler Effect

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