Sound

In a laboratory, an electric bell is suspended inside a glass bell jar, and the air is gradually removed. The figure shows the experimental setup.

1. a) Describe what is heard as the air leaves.
2. b) What does this experiment demonstrate about what sound needs in order to travel?
3. c) Why do two astronauts on the surface of the Moon talk to each other by radio even when standing next to each other?

Sound travels through air as a series of compressed and rarefied regions that move from the source to the ear. The figure shows these regions of compression and rarefaction.

1. a) Does the air travel from the loudspeaker to your ear, or does it only oscillate around its position?
2. b) Considering the direction in which the air oscillates relative to the direction in which the wave travels, is sound a transverse or longitudinal wave? Justify your answer.
3. c) Describe how you would model it using a stretched spring on a table.

The loudspeaker at the sports centre is $68\ \text{m}$ away from where you are, as shown in the figure.

How long does it take for the sound to reach your ears? Given data: sound travels through air at $340\ \text{m/s}$.

On a guitar, the first string (the thinnest) vibrates open at $330\ \text{Hz}$ and the sixth (the thickest) at $82,5\ \text{Hz}$.

1. a) Which of the two strings sounds higher-pitched?
2. b) Which wave quantity determines the pitch (high or low) of a sound?
3. c) In what unit is this quantity measured?

You shout in front of a court wall and hear the echo $0,5\ \text{s}$ later, as shown in the figure.

1. a) Explain the path travelled by the sound from when it left your mouth until you heard it.
2. b) Calculate how far away the wall is. (Sound travels at $340\ \text{m/s}$.)

Fireworks explode $1700\ \text{m}$ from your house. How long does it take for you to hear the bang after seeing the flash? (Sound travels at $340\ \text{m/s}$.)

A whistle emits a sound of $340\ \text{Hz}$ in air, where sound travels at $340\ \text{m/s}$.

1. a) Calculate its wavelength.
2. b) Another whistle emits at double the frequency, $680\ \text{Hz}$: calculate its wavelength.
3. c) Explain, using $v = \lambda \cdot f$, how the wavelength of a sound changes when its frequency increases in the same medium.

A hiker shouts in front of a distant rock face and hears the echo $3\ \text{s}$ later.

1. a) How far away is the wall? (Sound travels at $340\ \text{m/s}$.)
2. b) They walk $170\ \text{m}$ towards the rock face and shout again: how long does the echo take now?
3. c) Explain why the time has decreased in the same proportion as the distance.

A ship's sonar sends an ultrasound pulse towards the seabed and the echo returns $0,4\ \text{s}$ later, as shown in the figure. Sound travels through water at $1500\ \text{m/s}$.

1. a) What is the depth under the ship?
2. b) Why does sonar use ultrasound instead of audible sound?
3. c) If the ship sails to an area where the seabed is at $600\ \text{m}$, how long will it take for the echo to return?

There is a rule of thumb for thunderstorms: "every $3\ \text{s}$ between the lightning and the thunder means $1\ \text{km}$ of distance".

1. a) Check the rule by calculating what distance sound travels in $3\ \text{s}$ ($v_{\text{sound}} = 340\ \text{m/s}$).
2. b) What error does the rule make, in metres and as a percentage? Do you find this acceptable for everyday use?
3. c) The rule assumes that light arrives instantaneously: calculate how long light from the lightning actually takes to travel $1\ \text{km}$ ($c = 3\cdot10^{8}\ \text{m/s}$) and justify whether this assumption is reasonable.