SoundCambridge IGCSE Physics: Revision notes

Sound is produced by vibrating sources. When an object vibrates, it creates disturbances in the medium around it, generating sound waves.

Sound waves are longitudinal waves, meaning the particles of the medium vibrate parallel to the direction of wave propagation. As the source vibrates, it creates regions of compression (where particles are pushed close together) and rarefaction (where particles are spread further apart). These compressions and rarefactions travel outward through the medium as the sound wave.

Unlike transverse waves, there is no up-and-down motion perpendicular to the direction of travel—only back-and-forth motion along the same line as the wave is moving.

A medium is needed to transmit sound waves. Sound cannot travel through a vacuum because there are no particles to vibrate and carry the disturbance.

Sound requires a material substance—gas, liquid, or solid—to propagate. The particles in the medium must be able to vibrate and transfer the compression and rarefaction regions from one point to another. In the absence of particles (such as in space), sound waves cannot form or travel.

This is a key difference between sound and electromagnetic waves (like light), which can travel through a vacuum. The speed at which sound travels depends on the medium it is passing through.

The approximate range of frequencies audible to humans is 20 Hz to 20,000 Hz (20 kHz). Frequencies below 20 Hz are called infrasound, and frequencies above 20 kHz are called ultrasound.

The speed of sound in air is approximately 330–350 m/s. This speed varies slightly depending on temperature and the exact composition of the air.

Speed of sound in different media:

In general, sound travels faster in solids than in liquids and faster in liquids than in gases. This is because particles are more densely packed in solids, allowing vibrations to transfer more efficiently. Use v = f × λ to relate speed, frequency, and wavelength.

A practical method to determine the speed of sound in air involves measuring distance and time.

Method:

1. Use two people or observers at a measured distance apart (at least 100 m for accuracy).
2. One person makes a loud sound (e.g., clapping, firing a cap gun, or hitting two wooden blocks together).
3. The other person measures the time taken for the sound to travel the known distance using a stopwatch or timer.
4. Calculate speed using: v = distance / time

Improvements to accuracy:

• Repeat the experiment multiple times and calculate the average time.
• Use a longer distance to reduce timing errors.
• Ensure the person timing can clearly hear the sound at the moment it is made.
• Alternatively, use an echo method: time how long a sound takes to echo back from a hard surface at a known distance, then divide the total distance (there and back) by the time.

This method is most practical in outdoor spaces with minimal background noise and hard reflecting surfaces nearby.

Amplitude and frequency are two independent properties of sound waves that affect different perceptual qualities:

Loudness and Amplitude:

• A larger amplitude means particles vibrate further from their equilibrium position.
• This creates higher-intensity compressions and rarefactions.
• The result is a louder sound.
• Amplitude can be increased by making the vibrating source oscillate with greater energy.

Pitch and Frequency:

• A higher frequency means more oscillations occur per second.
• The human ear perceives this as a higher pitch (like a high note on a musical instrument).
• A lower frequency produces a lower pitch (like a deep bass sound).
• Frequency is determined by the source; for example, a small tuning fork vibrates faster than a large one.

Important: Changing the amplitude does not change the frequency, and changing the frequency does not change the amplitude. They are independent properties.

Echoes are reflections of sound waves from a hard surface. When sound travels to a barrier (such as a cliff, wall, or water surface) and bounces back to the observer, we hear an echo. The time delay between the original sound and the echo depends on the distance to the reflecting surface.

Calculating distance or depth from echo:

If a sound takes time t to return as an echo and the speed of sound is v:

• Total distance travelled = v × t (there and back)
• Distance to the surface = (v × t) ÷ 2

Ultrasound (frequencies above 20 kHz) has practical applications because it travels through materials and can be reflected or absorbed in controlled ways:

Uses of ultrasound:

1. Non-destructive testing: Ultrasound can detect cracks, flaws, or discontinuities inside materials (metals, concrete, composites) without damaging them. The reflected waves reveal internal structure.

2. Medical scanning of soft tissue: Ultrasound is safe and non-ionising. It is used to image internal organs, blood vessels, and foetuses. Different tissues reflect ultrasound differently, creating detailed images.

3. Sonar: Ultrasound is transmitted into water; reflections from objects (fish, submarines, the sea floor) are detected. The time delay indicates distance. Using the formula: distance = (speed × time) ÷ 2, the depth or position of objects can be calculated.

Advantages of ultrasound:

• Does not ionise atoms (unlike X-rays), so is safer for living tissue.
• Can penetrate many materials opaque to visible light.
• Provides real-time imaging with good resolution.