Wave per unit distance (typically per meter), can be associated with the wavelength by the relation The period formula_33 is the time for one complete cycle of an oscillation of a wave. The frequency formula_34 is the number of periods per unit time (per second) and is typically measured in hertz denoted as Hz. These are related by: In other words, the frequency and period of a wave are reciprocals. The angular frequency formula_28 represents the frequency in radians per second. It is related to the frequency or period by The wavelength formula_30 of a sinusoidal waveform traveling at constant speed
Sound pressure Since formula_18, the amplitude of the specific acoustic impedance is given by Consequently, the amplitude of the particle displacement is related to that of the acoustic velocity and the sound pressure by When measuring the sound pressure created by a sound source, it is important to measure the distance from the object as well, since the sound pressure of a "spherical" sound wave decreases as 1/"r" from the centre of the sphere (and not as 1/"r", like the sound intensity): This relationship is an "inverse-proportional law". If the sound pressure "p" is measured at a distance "r" from the centre