Ch 8Sound — Class 11 Physics, concept-first.

A wave is a periodic, regular disturbance that travels through (or without) a medium, carrying energy and momentum from one point to another without causing any net flow of the medium's own material.

This section sets out the quantities that describe ANY wave -- illustrated here mainly for mechanical waves, though the definitions carry over to every wave type.

Progressive (travelling) waves come in two kinds, distinguished by how the medium's particles vibrate relative to the direction the wave itself is moving.

A progressive wave needs a single mathematical function that captures both HOW the wave's shape looks at any instant (a function of position x) and HOW each individual particle moves over time (a func…

How fast a mechanical wave travels is decided entirely by two properties of the medium it moves through: its elastic (restoring-force) properties and its density (inertial properties) -- never by the…

On a stretched string, a disturbance travels as a transverse wave. The restoring force that pulls a displaced bit of string back toward its rest line is supplied by the string's own tension T, and the…

A longitudinal wave propagates by alternately compressing and rarefying the medium as its particles oscillate back and forth along the direction of travel; this compression/rarefaction cycle advances…

Isaac Newton was the first to study how a longitudinal (sound) wave's speed relates to the medium's properties.

Laplace's key insight was that the compressions and rarefactions of a sound wave are NOT a slow process, as Newton had implicitly assumed, but a genuinely RAPID one -- for a sound of frequency 256 Hz,…

As sound travels through the open atmosphere, three environmental factors affect its speed: pressure, temperature, and humidity.

Waves are remarkably unselfish about sharing space: unlike solid objects, two (or more) wave patterns can pass through and overlap in the very same region without displacing or disturbing each other i…

Sound waves, being waves, obey the same laws of reflection that govern light: they bounce off a rigid surface with the angle of reflection equal to the angle of incidence, and this reflected sound is…

An echo is the repetition of an original sound, heard again a short time later, caused by that sound reflecting off some rigid surface at a distance from its source.

If the reflecting surface is CLOSER than roughly 15 m to the sound source, the geometry above flips: the returning reflected sound arrives back well within the ear's 0.1 s retention window, so instead…

Acoustics is the branch of physics dealing with the production, transmission and reception of sound. It matters enormously in the construction of theatres and auditoriums, where careless design leaves…

Whenever we describe a sound as 'audible', what actually matters is how a human listener PERCEIVES it -- a purely subjective response, unlike the wave's own objective, measurable properties.

Everyone has noticed a passing train's whistle sound distinctly different -- higher-pitched as it approaches, lower-pitched as it recedes -- even though the train's whistle itself is producing one unc…

Consider a listener L at rest, and a source S of sound moving AWAY from L (a 'relative recede') at speed ; let v (always taken positive) be the speed of sound waves through the medium, and suppose the…

Now instead let the source S stay fixed and let the listener move TOWARD it with speed . Let the source emit its first wave at , when the listener is at , an initial distance d from the source; suppos…

When BOTH the source and the listener are in motion (relative to the medium), the two separate results above combine into one general expression for the observed frequency,

Sound's Doppler effect and light's Doppler effect share several qualitative features. (A) Wherever there is genuine relative motion between a listener/observer and a source -- whether of sound or of l…

Despite their shared qualitative features, sound's and light's Doppler effects differ in several important, specific ways.