Sound

Key points

Making sound

• Sound is made by vibrating objects. Examples: a guitar string, the cone of a loudspeaker, the skin of a drum, the vocal cords in your throat.
• The vibrating object pushes and pulls on the air next to it. This passes the vibration on through the air as a sound wave.
• Sound is a longitudinal wave. The air particles vibrate backwards and forwards, parallel to the direction in which the sound travels (see 3.1).
• The particles do not travel from the source to your ear. Each particle vibrates about a fixed position. The energy travels.
• A compression is a region where the particles are closer together than normal. The pressure there is higher. Extended
• A rarefaction is a region where the particles are further apart than normal. The pressure there is lower. Extended
• When a loudspeaker cone moves forwards, it squeezes the air in front of it and makes a compression. When the cone moves back, it makes a rarefaction. A sound wave is a series of compressions and rarefactions moving away from the source. Extended
• The wavelength of a sound wave is the distance from the centre of one compression to the centre of the next compression. Extended
• Humans can hear sounds with frequencies from about 20 Hz to 20 000 Hz (20 kHz). This is the audible range.
• Extra detail: the highest frequency a person can hear usually becomes lower as they get older.

Sound needs a medium

• A medium is a material (a solid, a liquid or a gas) that a wave travels through.
• Sound needs a medium. It is passed on by particles, so it cannot travel through a vacuum.
• Experiment: put a ringing electric bell inside a sealed glass jar. Pump the air out of the jar. The sound gets quieter and quieter. You can still see the hammer hitting the bell, because light can travel through a vacuum. Let the air back in, and the sound returns.
• The speed of sound in air is about 330–350 m/s.
• Light travels much faster than sound. So you see lightning before you hear the thunder.
• In general, sound travels faster in solids than in liquids, and faster in liquids than in gases. Extended
• Extra detail: the particles in solids and liquids are much closer together than in a gas, and they are held by strong forces. This helps vibrations to pass from particle to particle quickly.

Measuring the speed of sound in air

• The idea: measure a distance the sound travels and the time it takes. Then speed = distance ÷ time.
• Method 1: two people, a long distance apart.
  1. Measure a long distance (for example across a sports field) with a measuring tape or a trundle wheel.
  2. Person A, at one end, makes a loud sound that can also be seen. For example, A bangs two wooden blocks together.
  3. Person B, at the other end, starts a stopwatch when they see the blocks hit. Light arrives almost at once.
  4. B stops the stopwatch when they hear the bang.
  5. Speed of sound = distance ÷ time.
  6. Repeat several times and find the average time.
• Method 2: echo from a wall.
  1. Stand a measured distance d from a large flat wall, such as the side of a building.
  2. Clap. Then clap again so that each clap happens at the same moment as the echo of the clap before.
  3. Start a stopwatch on one clap. Count 20 more claps, and stop the stopwatch.
  4. Time for one echo, t = total time ÷ 20.
  5. In this time the sound goes to the wall and back, a distance of 2d. So speed = 2d ÷ t.
• Method 3: two microphones. Put two microphones a measured distance apart, in a line with the sound. Connect them to an electronic timer. Make a sharp sound, such as a clap, beyond one end. The timer starts when the sound reaches the first microphone and stops when it reaches the second. Speed = distance between microphones ÷ time.
• Ways to improve the result:
  • Use a large distance. The time is then longer, so your reaction time is a smaller part of it.
  • Repeat and take an average.
  • Timing many echoes, as in Method 2, also makes reaction time less important.

Loudness and pitch

• Loudness depends on amplitude. A larger amplitude gives a louder sound.
• Pitch depends on frequency. A higher frequency gives a higher-pitched sound.
• A microphone connected to an oscilloscope shows a sound wave as a trace on a screen:
  • a taller trace (larger amplitude) means a louder sound
  • more waves across the screen (higher frequency) means a higher pitch.
• All sounds travel at the same speed in air. So, from v = fλ, a higher frequency sound has a shorter wavelength.

Echoes

• An echo is a sound wave that has been reflected from a surface. Hard, flat surfaces such as walls and cliffs reflect sound well.
• In an echo, the sound travels to the surface and back. So the distance it travels is twice the distance to the surface.

Ultrasound

• Ultrasound is sound with a frequency higher than 20 kHz (20 000 Hz). It is too high for humans to hear.
• How ultrasound is used: a short pulse of ultrasound is sent out. Part of it is reflected at each boundary between two materials. The time taken for each echo to return shows how far away the boundary is. Extended
• Non-destructive testing: ultrasound pulses are sent into a metal part, such as a pipe, a rail or an aircraft part. An echo comes back from the far side. A crack inside the metal sends back an extra, earlier echo. The time of that echo shows how deep the crack is. The part is not damaged by the test, which is why it is called non-destructive. Extended
• Medical scanning of soft tissue: for example, scanning an unborn baby. Pulses reflect from the boundaries between different tissues. A computer uses the echo times to build up an image. Ultrasound is not ionising, so it is safer than X-rays for this kind of scan. Extended
• Sonar: a ship sends a pulse of ultrasound down through the water. The pulse reflects from the seabed (or from a shoal of fish) and returns. The ship measures the time t for the echo to return. Extended
• Depth of the water = speed of sound in water × t ÷ 2. Divide by 2 because the pulse travels down and back up. Extended
• Ferries use echo sounders to check the depth of water in a harbour. Fishing boats use sonar to find shoals of fish. Extended

Common mistakes

• Students forget that an echo travels there and back. / The mark scheme wants the distance doubled (or the time halved) in echo calculations.
• Students write that the air particles travel from the source to the listener. / The mark scheme wants: the particles vibrate backwards and forwards about fixed positions; the energy is transferred.
• Students link loudness to frequency, or pitch to amplitude. / The mark scheme wants: loudness ↔ amplitude; pitch ↔ frequency.
• Students write that sound can travel through a vacuum. / The mark scheme wants: sound needs a medium; it cannot travel through a vacuum.
• Students define ultrasound as “a very loud sound” or “sound above 20 Hz”. / The mark scheme wants: sound with a frequency higher than 20 kHz.
• Students write that sound travels fastest in gases. / The mark scheme wants: in general, sound is fastest in solids and slowest in gases. Extended
• Students describe a compression as the “top of the wave”. / The mark scheme wants: a region where the particles are closer together and the pressure is higher. Extended

Exam tips

• State the audible range with units: 20 Hz to 20 000 Hz.
• State the speed of sound in air as a range: 330–350 m/s.
• Describe a method for the speed of sound by giving: what you measure (a distance and a time), the instruments (measuring tape or trundle wheel, stopwatch), the equation (speed = distance ÷ time), and one improvement (large distance; repeat and average).
• Describe the effect of a change: “a larger amplitude gives a louder sound”; “a higher frequency gives a higher pitch”. Always link the wave property to what you hear.
• Calculate depth from an echo in three lines: write depth = v × t ÷ 2, substitute, give the answer in m. Extended
• Describe a use of ultrasound in steps: pulse sent → reflected at a boundary → time for the echo measured → distance calculated. Extended
• A typical 1-mark answer: “An echo is a reflected sound wave.”
• A typical 2-mark answer to “Why can sound not travel through space?”: “Sound needs a medium to travel through (1). Space is a vacuum, so there are no particles to pass on the vibrations (1).”
• A typical 3-mark answer to “Describe how ultrasound is used to find a crack in a metal rail”: “A pulse of ultrasound is sent into the rail (1). The crack reflects some of the pulse, so an extra echo returns earlier than the echo from the far side (1). The time of this echo is used to find how deep the crack is (1).” Extended