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.

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?

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 $v = \lambda \times f$, how the wavelength of a sound changes when its frequency increases in the same medium.

A loudspeaker generates a pure tone of $200\ \text{Hz}$ in air.

1. a) Calculate the period of the wave.
2. b) Calculate its wavelength. (Sound travels at $340\ \text{m/s}$.)

Two people play the same key on a piano, one much harder than the other.

1. a) Which of the two sounds is heard louder?
2. b) The loudness of a sound (loud or quiet) is related to the property called intensity: which wave quantity does it depend on?
3. c) Does the pitch of the note change when it is played harder? Justify your answer.

A violin and a flute play the same note, an A at $440\ \text{Hz}$, at the same volume. The figure shows the waveform produced by each instrument.

1. a) Why can we still distinguish which instrument is playing?
2. b) What is this sound quality called? Explain how the waves produced by each instrument differ.

At an outdoor concert, the sound next to the loudspeakers is deafening, whereas $100\ \text{m}$ away it is barely bothersome. The figure represents how the wave energy spreads out as it moves away from the loudspeaker.

1. a) Which property of the sound wave decreases with distance from the source, and which remains constant?
2. b) The loudspeaker emits its energy, spreading it over increasingly large spherical surfaces as the wave travels away: using this idea, explain why the volume drops so much with distance.
3. c) Relate the above to the recommendation not to stand right next to loudspeakers for long periods.

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 $340\ \text{m/s}$.

During a thunderstorm, you see lightning and count $4\ \text{s}$ until you hear the thunder, as shown in the figure.

1. a) How far away from you is the thunderstorm? (Sound travels at $340\ \text{m/s}$.)
2. b) Why can you neglect the time it takes for the light from the lightning to reach you?

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 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?

A healthy young human ear perceives sounds between roughly $20\ \text{Hz}$ and $20\,000\ \text{Hz}$. State whether we can hear each of these sounds and name the range to which they belong:

1. a) A sound of $15\ \text{Hz}$.
2. b) A sound of $440\ \text{Hz}$.
3. c) A sound of $25\,000\ \text{Hz}$.

What are sounds with frequencies above $20\,000\ \text{Hz}$ called, and which animal uses them to navigate in the dark?

A whale sings at $20\ \text{Hz}$ and a bat emits calls at $68\,000\ \text{Hz}$.

1. a) Can the human ear perceive both sounds? Justify your answer using the hearing range, from $20\ \text{Hz}$ to $20\,000\ \text{Hz}$.
2. b) Calculate the wavelength of each sound in air and give it in a suitable unit. (Sound travels at $340\ \text{m/s}$.)
3. c) Relate the very small wavelength of the bat's calls to its ability to detect tiny insects.

A bat emits an ultrasound pulse and receives the echo produced by a moth $0.02\ \text{s}$ later. (Sound travels through air at $340\ \text{m/s}$.)

1. a) How far away is the moth?
2. b) Why can humans not hear the bat's calls?
3. c) Objects much smaller than the wavelength barely reflect the wave: explain why it is advantageous for the bat to use high frequencies with short wavelengths to hunt insects.

In a city, traffic, roadworks, outdoor bar terraces playing music and ambulance sirens coexist.

1. a) Explain what is meant by noise pollution.
2. b) State two effects of excessive noise on human health.
3. c) Propose one measure to reduce noise by acting at the source, and another by acting on the medium through which it travels.

A family lives in a flat next to a busy avenue and suffers from traffic noise at night as well.

1. a) Propose a measure to reduce noise by acting on the source, another by acting on the medium of propagation, and another on the receiver, justifying each physically.
2. b) Explain why a double-glazed window with a cavity insulating layer reduces noise better than single glazing.
3. c) Indoors, thick curtains, carpets and bookshelves filled with books reduce noise: what do these materials do to the sound instead of reflecting it?