Waves, Light Waves, Sound Waves and Ultrasound (The Physics of)

Abstract

Mechanical waves (sound waves, waves on water) are a mechanism for transferring energy through a medium (the air or water) without transferring matter. Another definition is a periodic disturbance in some property of the medium, the medium itself remaining relatively at rest. Waves have the following measurable properties:

• 1. Wavelength (symbol λ) is the distance between two successive crests (in metres, m). A typical value is ~500 nm for light and ~20 cm for sound.

• 2. Frequency (f) is the number of λ that passes by in 1 s (in hertz, Hz). Typical values are 500 THz for light and 500 Hz for sound. Frequency is related to pitch (for sound) and colour (for visible light).

• 3. Period (T) is the time it takes for one λ to pass by (in seconds, s).

• 4. Speed (v) is how fast a wave is moving in the direction of propagation (in metres per second, m/s). The speed of light travelling through air is 3 × 10⁸ m/s, while for sound, speed in air is about 330 m/s. In tissue, sound moves faster, at about 1560 m/s.

• 5. Amplitude (A) is the maximum displacement from the mean (or rest) position. For example, the vertical distance between a trough and a crest of a wave in water is two times the amplitude. Amplitude (or intensity) is related to loudness of sound and brightness of light and to the amount of energy being carried by the wave.

• 6. Phase refers to how far out of step the oscillation of one part of a wave is when compared with another part. A phase of 0° or 360° means that the two parts are in step, while a phase difference of 180° means that the two points are completely out of step. Differences in phase between the sounds entering each ear allow us to localise the source of a sound.