Longitudinal wave and seismic P-waves (created by earthquakes and explosions). In longitudinal waves, the displacement of the medium is parallel to the propagation of the wave, and waves can be either straight or round. A wave along the length of a stretched Slinky toy, where the distance between coils increases and decreases, is a good visualization. In the case of longitudinal harmonic sound waves, the frequency and wavelength can be described by the formula where: The quantity "x"/"c" is the time that the wave takes to travel the distance "x". The ordinary frequency ("f") of the wave is given by The wavelength
Acoustic wave wave becomes a travelling wave moving rightwards, for formula_14 the wave becomes a travelling wave moving leftwards. A standing wave can be obtained by formula_15. In a travelling wave pressure and particle velocity are in phase, which means the phase angle between the two quantities is zero. This can be easily proven using the ideal gas law where Consider a volume formula_18. As an acoustic wave propagates through the volume, adiabatic compression and decompression occurs. For adiabatic change the following relation between volume formula_18 of a parcel of fluid and pressure formula_2 holds where As a sound wave propagates through