Waves Revision Notes

What is Infrasound?

Infrasound is defined as sound with frequencies below the threshold of human hearing, specifically less than 20 hertz (Hz). Unlike audible sound, it cannot be heard by humans.

Characteristics and Sources

These low-frequency sound waves can travel long distances and penetrate obstacles more effectively than higher-frequency sounds. Natural sources include earthquakes, volcanoes, avalanches, and severe weather. Some animals use infrasound for communication.

Transverse Waves

In transverse waves, the oscillations (vibrations) of the medium's particles are perpendicular (at right angles) to the direction of energy transfer. Key characteristics include crests (highest points) and troughs (lowest points). Examples: electromagnetic waves (radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma ray), seismic S-waves (secondary waves), and ripples on the surface of water.

Longitudinal Waves

In longitudinal waves, the oscillations (vibrations) of the medium's particles are parallel (in the same direction) to the direction of energy transfer. Key characteristics include compressions (regions of high pressure and density) and rarefactions (regions of low pressure and density). Examples: sound waves and seismic P-waves (primary waves).

Wave Types and Examples

Sound waves are longitudinal. Electromagnetic waves are always transverse. Seismic waves include both types: P-waves are longitudinal, and S-waves are transverse. Water waves are complex; while surface waves are often modelled as transverse, the particle motion is actually circular/elliptical, combining both transverse and longitudinal components, with the most common approximation for surface ripples being transverse.

Definition of Ultrasound

Ultrasound is defined as sound with frequencies greater than 20,000 hertz (Hz). This frequency is beyond the typical range of human hearing.

Human Hearing Range

The average human ear can detect sound frequencies between approximately 20 Hz and 20,000 Hz. Frequencies below 20 Hz are infrasound and above 20,000 Hz are ultrasound.

Wavelength (λ)

Wavelength is the spatial period of a wave – the distance over which the wave's shape repeats. It's the distance between consecutive corresponding points of the same phase, such as two adjacent crests, troughs, or compressions. Wavelength is measured in metres (m).

Frequency (f)

Frequency is the number of complete wave cycles or oscillations that pass a point in one second. It quantifies how often a repetitive event occurs. Frequency is measured in Hertz (Hz), where 1 Hz represents one cycle per second. A higher frequency implies more oscillations per unit time.

Relationship between Wavelength and Frequency

For a wave travelling at a constant speed, its frequency and wavelength are inversely proportional. This means if one increases, the other decreases. This relationship is given by the wave equation: wave speed (v) = frequency (f) × wavelength (λ). Understanding this relationship is crucial for analysing different types of waves.