Sound Waves and Ultrasound

This lesson covers sound waves, how they are produced and detected, the limits of human hearing, and the uses of ultrasound, as required by the AQA GCSE Physics specification (4.6.1). Sound is a longitudinal wave that requires a medium to travel through and has many practical applications in medicine and industry.

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How Sound Is Produced

Sound is produced when an object vibrates. The vibrations cause the particles of the surrounding medium (usually air) to vibrate, creating a series of compressions and rarefactions that travel outward as a longitudinal wave.

Examples of vibrating objects that produce sound:

Source	Vibrating Part
Guitar	Strings vibrate
Drum	Drum skin vibrates
Tuning fork	Prongs vibrate
Loudspeaker	Cone vibrates
Human voice	Vocal cords vibrate

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Properties of Sound Waves

Sound waves are longitudinal waves:

• The particles of the medium vibrate parallel to the direction of energy transfer.
• Sound waves consist of alternating compressions (high pressure) and rarefactions (low pressure).
• Sound waves require a medium to travel through — they cannot travel through a vacuum.

graph LR
    subgraph "Sound Wave in Air"
        C1["Compression (high pressure)"] --- R1["Rarefaction (low pressure)"] --- C2["Compression"] --- R2["Rarefaction"] --- C3["Compression"]
    end
    style C1 fill:#3498db,color:#fff
    style C2 fill:#3498db,color:#fff
    style C3 fill:#3498db,color:#fff
    style R1 fill:#ecf0f1,color:#333
    style R2 fill:#ecf0f1,color:#333

The Bell Jar Experiment

The classic demonstration that sound cannot travel through a vacuum uses a bell jar:

1. Place an electric bell inside a glass bell jar.
2. Switch on the bell — you can hear it ringing.
3. Use a vacuum pump to gradually remove the air from the jar.
4. As the air is removed, the sound gets quieter and quieter.
5. When (nearly) all the air is removed, you can see the bell vibrating but can no longer hear it.
6. When air is let back in, the sound returns.

This proves that sound requires a medium (air) to travel. In a vacuum, there are no particles to vibrate, so sound cannot be transmitted.

Exam Tip: The bell jar experiment does not create a perfect vacuum, so you may still hear a faint sound. In an exam, say that the sound "decreases significantly" or "becomes very quiet" rather than saying it disappears completely. However, you should state the conclusion: sound cannot travel through a vacuum.

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Speed of Sound

The speed of sound depends on the medium it travels through:

Medium	Speed of Sound (approximate)
Air (at 20 degrees C)	340 m/s
Water	1 500 m/s
Steel	5 000 m/s
Concrete	3 400 m/s
Wood	3 800 m/s

Key points:

• Sound travels fastest in solids and slowest in gases.
• This is because particles in solids are closer together and more tightly bonded, so vibrations are transmitted more quickly.
• The speed of sound in air increases with temperature — at higher temperatures, air particles have more kinetic energy and vibrate faster.

Measuring the Speed of Sound in Air

You can measure the speed of sound using: v = distance / time

Method 1 — Using two observers and a starting pistol:

1. Two people stand a known distance apart (e.g. 400 m).
2. One fires a starting pistol.
3. The other starts a stopwatch when they see the flash and stops it when they hear the bang.
4. Speed = distance / time.

Method 2 — Using echoes:

1. Stand a known distance from a large flat wall (e.g. 50 m).
2. Clap your hands and listen for the echo.
3. Time how long it takes for 10 echoes and divide by 10 to find the time for one echo.
4. The sound travels there and back, so the total distance is 2 x the distance to the wall.
5. Speed = total distance / time for one echo.

Exam Tip: When calculating the speed of sound using echoes, remember the sound travels twice the distance to the wall (there and back). A very common mistake is to forget to double the distance. Also, timing multiple echoes and dividing reduces the error.

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Pitch and Frequency

The pitch of a sound is related to its frequency:

• A high-pitched sound has a high frequency (many vibrations per second).
• A low-pitched sound has a low frequency (few vibrations per second).

Sound	Pitch	Frequency
Whistle	High	High
Bass drum	Low	Low
Bird song	High	High
Thunder	Low	Low

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Loudness and Amplitude

The loudness of a sound is related to its amplitude:

• A loud sound has a large amplitude (particles vibrate with greater displacement).
• A quiet sound has a small amplitude (particles vibrate with smaller displacement).
• Loudness is measured in decibels (dB).

Sound Level	Approximate Level (dB)
Threshold of hearing	0
Whisper	20
Normal conversation	60
Busy traffic	70
Rock concert	110
Threshold of pain	130
Jet engine (close)	140

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Human Hearing Range

The human ear can detect sounds with frequencies between approximately 20 Hz and 20 000 Hz (20 kHz).

• Sounds below 20 Hz are called infrasound — humans cannot hear them, but elephants and some other animals can.
• Sounds above 20 000 Hz are called ultrasound — humans cannot hear them, but dogs, bats, and dolphins can.

graph LR
    subgraph "Frequency Spectrum"
        I["Infrasound (below 20 Hz)"] --> H["Human hearing range (20 Hz - 20 000 Hz)"] --> U["Ultrasound (above 20 000 Hz)"]
    end
    style I fill:#9b59b6,color:#fff
    style H fill:#27ae60,color:#fff
    style U fill:#e74c3c,color:#fff

Exam Tip: The limits of human hearing are 20 Hz to 20 000 Hz. These exact numbers are frequently asked in exams. As people age, the upper limit decreases — older adults may only hear up to 15 000 Hz or less. This is why some shops use high-frequency sounds to deter teenagers that adults cannot hear.

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Ultrasound

Ultrasound is sound with a frequency above 20 000 Hz (above the upper limit of human hearing). Ultrasound has many important applications in medicine and industry.

How Ultrasound Works

Ultrasound waves are produced by electronic devices that vibrate at very high frequencies. When ultrasound waves meet a boundary between two different media (e.g. between tissue and bone, or between metal and a crack), some of the wave is reflected and some is transmitted. By detecting the reflected waves, information about the internal structure can be obtained.

Uses of Ultrasound

Application	How It Works
Medical imaging (prenatal scans)	Ultrasound waves are directed into the body; reflections from boundaries between tissues are detected and used to build an image of the foetus
Industrial flaw detection	Ultrasound is sent through metal castings; reflections from internal cracks indicate defects
Distance measurement (sonar)	Ultrasound pulses are sent towards the seabed or underwater objects; the time for the echo to return is used to calculate distance
Cleaning	High-frequency vibrations cause tiny bubbles to form and collapse, dislodging dirt from surfaces (e.g. jewellery, surgical instruments)
Physiotherapy	Ultrasound waves are used to treat soft tissue injuries by promoting blood flow

Calculating Distance Using Ultrasound (Echo Method)

When an ultrasound pulse is sent out and its echo is detected:

distance = (speed x time) / 2

The division by 2 is necessary because the sound travels to the object and back.