Cycle. A complete single excursion of a vibrating molecule.
Frequency. The number of cycles of vibration in a given unit of time, usually cycles per second (cps) or hertz (Hz).
Sound Wave. The portion of a sound between two successive compressions or rarefactions.
Wavelength. The distance between two successive rarefactions or compressions in a sound wave.
Amplitude. Maximum displacement, beyond its normal, or rest, position, of a vibrating element. In most audible sounds, these excursions are very small, although low-frequency sound may cause large excursions (as would be observed in the motion of a loudspeaker cone reproducing very low frequency sounds at audible level). Amplitude of motion is related to the increased pressure created in the medium, and to the intensity of the energy involved.
Velocity. Speed at which a sound impulse travels (not the speed of movement of any particular molecule). In a given medium, under fixed conditions, sound velocity is a constant. Therefore, the relationship between velocity, frequency, and wavelength can be expressed by the equation:
Velocity = frequency x wavelength (11.8)
Because velocity is constant in air, low-frequency sound has long wavelengths, and high-frequency sound has short wavelengths. This is important to remember in acoustical design.
In addition to the longitudinal (compression or squeeze) waves by which sound travels, there is another type of vibrational motion to which most building materials and construction systems are subjected, a transverse wave (Fig. 11.90). This is the familiar motion of a vibrating string or reed. This type of wave, too, transmits energy that can be felt or heard, or both.
Sheets or panels, studs and joists, hangers and rods, and similar slender and somewhat flexible members are particularly apt to vibrate in a transverse mode and to transmit sound energy along their length, as well as radiating it from their surfaces to the surrounding air.