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This lesson covers the properties and behaviour of sound waves — as required by the Edexcel GCSE Physics specification (1PH0), Topic 4: Waves. You need to understand how sound is produced and transmitted, the factors affecting pitch and loudness, the human hearing range, and applications of ultrasound.
Sound is produced by vibrations. When an object vibrates (e.g. a guitar string, a tuning fork, a loudspeaker cone), it causes the particles of the surrounding medium (usually air) to vibrate back and forth.
Exam Tip: The classic demonstration is the bell jar experiment: a ringing bell is placed inside a bell jar and the air is gradually pumped out. As the air is removed, the sound gets quieter and eventually cannot be heard — proving sound needs a medium. The bell can still be seen vibrating (light can travel through a vacuum), but no sound is heard.
The speed of sound depends on the medium it is travelling through:
| Medium | Approximate Speed of Sound |
|---|---|
| Air (at room temperature) | ~330 m/s |
| Water | ~1500 m/s |
| Steel / Iron | ~5000–6000 m/s |
| Vacuum | Cannot travel (no medium) |
Exam Tip: A common misconception is that sound travels faster in gases because the particles "move faster." This is wrong — the speed of sound depends on how quickly vibrations are passed between particles, which happens fastest when particles are close together and strongly bonded (i.e. in solids).
The pitch of a sound is determined by its frequency:
Frequency is measured in hertz (Hz), where 1 Hz = 1 vibration per second.
On an oscilloscope (or time-base graph):
The loudness of a sound is determined by its amplitude:
On an oscilloscope trace:
| Property | High Pitch | Low Pitch | Loud | Quiet |
|---|---|---|---|---|
| Frequency | High | Low | — | — |
| Wavelength | Short | Long | — | — |
| Amplitude | — | — | Large | Small |
| On oscilloscope | Waves close together | Waves spread apart | Tall waves | Short waves |
Exam Tip: Pitch and loudness are independent properties. A sound can be high-pitched and quiet (small amplitude, high frequency) or low-pitched and loud (large amplitude, low frequency). Do not confuse them.
The human ear can detect sounds with frequencies between approximately:
20 Hz (very low pitch) to 20,000 Hz (20 kHz) (very high pitch)
Ultrasound is sound with a frequency above 20,000 Hz (20 kHz) — above the upper limit of human hearing.
| Application | How It Works |
|---|---|
| Medical imaging (pregnancy scans) | Ultrasound pulses are sent into the body. They reflect off boundaries between tissues (e.g. between fluid and bone). A computer calculates the distance to each boundary using the time taken for the echo to return, and builds up an image. |
| Sonar (echo location) | Ships send ultrasound pulses towards the seabed. The time for the echo to return is measured. Distance = speed × time / 2. Used to measure ocean depth and detect objects (fish, submarines). |
| Industrial cleaning | Ultrasound vibrations are used to clean delicate items (e.g. jewellery, surgical instruments) by causing tiny bubbles to form and collapse, dislodging dirt. |
| Quality control / flaw detection | Ultrasound is passed through materials. If there is a crack or internal defect, the wave reflects back early, revealing the location of the flaw. |
Exam Tip: In calculations involving sonar/echo location, remember the sound travels to the object and back, so the total distance is 2 × distance. If you are given the total time and speed, divide the result by 2 to get the distance to the object.
An echo is a reflection of sound. When sound waves hit a hard, flat surface (e.g. a wall, cliff, or building), they reflect back and can be heard as a repeat of the original sound.
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