Principle of superposition

One characteristic feature of a linear system is the principle of superposition. For mechanical waves the principle means that the amplitude of the resultant wave pattern created by the interference of travelling waves opens in a new window, is the sum of the amplitudes of the travelling waves.

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diagram of two transverse waves

At all times the travelling waves act independently of each other (eg ripples pass through ripples).

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Constructive interference

When two wave pulses overlap and positively contribute to the displacement of particles of a medium (see diagram below), the effect is termed constructive interference.

two approaching crests or troughs add together when superimposed creating a larger crest or trough

Destructive interference

When two wave pulses overlap and negatively contribute to the displacement of particles of a medium (see diagram below), the effect is termed destructive interference.

superimposed crest and trough will tend to negate each other to some extent temporarily creating a smaller amplitude wave

If the two pulses had identical amplitudes (A = B) and widths then for a moment when they overlap, the particles of the medium are not displaced – the 'wave' appears 'flat'. In this situation the medium is said to be experiencing total destructive interference.