What is a decibel?
How sound level is measured: how level in dB is related to sound pressure, intensity and power.

 Links to related material and teachers' resources Pitch, loudness and timbre Frequency and pitch. Amplitude and loudness. Timbre examples, with envelope and spectrum. Sound pressure and density; Transverse vs longitudinal waves Transverse vs longitudinal waves. y(x') in a longitudinal wave. Density variations. A travelling longitudinal wave. Variations in pressure give rise to accelerations. Sound transmission through air. Bell jar experiment. If sound diffracts, why doesn't light? Speed of sound Time-of-flight measurements of the speed of sound. Clap-echo measurement. Clap-board (image vs sound) measurement. Experiments using two microphones and a long cable. Adiabatic expansion and compression The relationship between pressure, volume and temperature for a rapid (adiabatic) change in volume of a gas. Physics of the sound wave. Wave equation. Acoustic impedance Displacement, compression and pressure. Newton's second law and acceleration. The wave equation for sound. Speed of sound. Acoustic impedance. Acoustic compliance, inertance and impedance Acoustically compact regions in which inertia dominates (inertance) or compressibility dominates (compliance). The characteristic impedance of a duct. Acoustic impedance, intensity and power Acoustical intensity and power are related to pressure and acoustic flow. Again, the acoustic impedance is involved. What is a decibel? Decibels, logarithmic scales, sound files and examples. Psychophysical measures: phons and sones. A and C weighting filters (dBA, dBC, dBlin). Recording levels. Intensity radiation and impedance. Examples. The Mexican Wave The mexican wave (or stadium wave) is an example of a transverse, polarised, travelling wave. An interesting idea for teachers: the wave speed can be faster than the particle speed.