物理聲音 DSE、CE 選擇題

1.

In the figure, a firecracker at a position $80$ m away from a building explodes and produces a bang. A girl standing $90$ m away from the firecracker hears two bangs. What is the time interval between the two bangs ? Given : speed of sound in air $= 340\ \mathrm{m\,s^{-1}}$

Building, firecracker 80 m from the building and a girl 90 m from the firecracker

2.

Which of the following statements about sound waves is **not** correct ?

3.

A sound wave travelling in water towards the water-air boundary

The figure shows a sound wave travelling in water. It is known that the sound waves travel faster in water than in air. After refraction, which of the following statements is/are correct ?

(1)The wavelength of the sound wave increases.

(2)The frequency of the sound wave remains unchanged.

(3)The sound wave bends away from the normal.

4.

Two loudspeakers connected in parallel to a signal generator; points O, P, Q and a sound intensity level meter moved from P to Q

Two identical loudspeakers are connected in parallel to a signal generator, $O$ is the midpoint between the loudspeakers. When a sound intensity level meter is moved from $P$ to $Q$ , the graph below shows the variation of the sound intensity level received with position.

Graph of sound intensity level against position, decreasing from P to Q

Which of the following statements is/are correct ?

(1)The sound intensity level at

Q

is smaller because the amplitude of the sound wave decreases as it is further away from the loudspeakers.

(2)Constructive interference occurs at point

P

while destructive interference occurs at point

Q

.

(3)The result of the experiment shows that sound is a wave.

5.

Loudspeakers S1 and S2 connected to a signal generator, point X, O the mid-point of S1S2 and Y below X; not to scale

Sound waves of $660$ Hz are produced from two identical loudspeakers $S_1$ and $S_2$ which are connected in parallel to a signal generator. The distances $S_1X$ and $S_2X$ are $2$ m and $4$ m respectively. $O$ is the midpoint of $S_1S_2$ . What kinds of interference occur at $X$ and $Y$ respectively ? Given that the speed of sound in air is $330\ \mathrm{m\,s^{-1}}$

	$X$	$Y$
	constructive	constructive
	constructive	destructive
	destructive	constructive
	destructive	destructive

6.

A ship uses a sonar system to detect the depth of the sea. A sound wave signal of frequency $30000$ Hz is sent vertically downward from the sea surface and the reflected signal is received $5$ s later. Which of the following is correct ? (The speed of sound wave in sea water is $1400\ \mathrm{m\,s^{-1}}$ .)

	Wavelength of the sound wave in sea water (m)	Depth of the sea (m)
	$4.67\times10^{-2}$	3500
	$4.67\times10^{-2}$	7000
	$9.33\times10^{-2}$	3500
	$9.33\times10^{-2}$	7000

7.

In the figure, $S_1$ and $S_2$ are two identical loudspeakers connected in parallel to a signal generator. The circles represent the wavefronts of the sound wave produced. What are the changes to the loudness of the sound at $X$ and $Y$ if loudspeaker $S_1$ is turned off ?

Wavefront circles around loudspeakers S1 and S2 with points X and Y where the circles overlap

	$X$	$Y$
	increases	increases
	decreases	decreases
	increases	decreases
	decreases	increases

8.

Which of the following descriptions about ultrasonic waves must be correct ?

9.

Two identical loudspeakers $X$ and $Y$ are connected in parallel to a signal generator. A microphone connected to a CRO detects a maximum when it is $0.2$ m from $X$ and $0.4$ m from $Y$ . It detects a minimum when it is $0.9$ m from $X$ and $0.4$ m from $Y$ . What is the possible wavelength of the sound wave ?

10.

Noise barriers built along highways are used to block the noise generated by road traffic. Which of the following statements correctly explain how the noise barriers can block the noise ?

(1)The noise from vehicles is reflected.

(2)The noise is absorbed by the noise barriers.

(3)The noise is diffracted at the top edge of the noise barriers.