Sound Flashcards

All 19 cards in this deck

What are the steps by which a vibrating object produces a sound wave in air?
e.g. a struck tuning fork

1. The prongs vibrate and make nearby air particles vibrate.
2. The air particles vibrate parallel to the direction the wave travels (longitudinal).
3. This creates compressions and rarefactions that travel out through the air, transferring energy.

In which direction do air particles vibrate in a sound wave?

Parallel to the direction the wave travels — sound is a longitudinal wave.

In a sound wave travelling through air, what is a compression and what is a rarefaction?

A compression is a region where the particles are pushed closer together (higher pressure); a rarefaction is a region where they are spread further apart (lower pressure).

What happens when a sound wave in air reaches a solid surface?
e.g. an ear drum or a microphone diaphragm

The compressions and rarefactions push on the surface, making it vibrate at the same frequency as the sound wave. (A solid vibrating in air does the reverse and produces a sound wave.)

Why do processes that convert between sound waves and vibrations in solids only work over a limited frequency range?

A solid part only vibrates well at frequencies close to its own natural range, which depends on its size, stiffness and mass.

What are the steps by which the human ear detects a sound?
e.g. hearing a 500 Hz note

1. The sound wave makes the ear drum vibrate at the frequency of the sound.
2. Small bones amplify the vibration and pass it to the cochlea.
3. The cochlea converts the vibrations into electrical nerve signals sent to the brain.

What is ultrasound?

Sound with a frequency greater than 20 000 Hz (20 kHz).

What is the approximate range of frequencies the human ear can detect?

About 20 Hz to 20 000 Hz (20 kHz).

What is infrasound?

Sound with a frequency less than 20 Hz.

True or false? Ultrasound is sound that is too quiet for humans to hear.

False. Ultrasound is defined by its frequency (above 20 000 Hz), not its loudness or amplitude.

How is ultrasound used to make an image of a foetus?

Ultrasound pulses are partly reflected at each boundary between different tissues, and the reflected pulses are processed into an image. It is used because it is non-ionising and safe.

How is infrasound used to explore the Earth's core?

Very low frequency waves from earthquakes or explosions pass through the Earth and are detected at the surface; how they reflect and refract reveals the structure of the core.

What are the steps to find the depth of water using sonar?
e.g. an ultrasound pulse returns after 0.02 s, speed of sound in water 1500 m/s

1. Send an ultrasound pulse down and time the echo: $t = 0.02$ s.
2. Total distance $= v \times t = 1500 \times 0.02 = 30$ m.
3. The pulse goes down and back, so depth $= 30 \div 2 = 15$ m.

What happens to the frequency of a sound wave when it passes from one medium into another?

It stays the same — the frequency is set by the source.

What happens to the speed and wavelength of a sound wave when it passes into a medium in which sound travels faster?
e.g. from air into water

The speed increases and, because the frequency is unchanged, the wavelength increases ($v = f\lambda$).

True or false? When a sound wave passes from air into water its frequency increases because it travels faster.

False. The frequency stays the same; the speed and wavelength both increase.

Why does a sound wave change direction (refract) when it crosses a boundary between two media at an angle?

Because the speed of sound is different in the new medium, so the wavelength changes and the wave bends.

True or false? Sound can travel through a vacuum.

False. Sound needs particles of a medium to vibrate, so it cannot travel through a vacuum.

True or false? Ultrasound can be used to detect cracks inside a solid metal object.

True. A crack reflects the ultrasound pulse back sooner than the far side of the object, revealing the fault.