Beats: An important occurrence of the interference of waves is in the phenomenon of beats. In the simplest case, beats result when two sinusoidal sound waves of equal amplitude and very nearly equal frequencies mix. The frequency of the resulting sound (F) would be the average of the two original frequencies (f1 and f2)
Compression: decrease in volume of any object or substance resulting from applied stress. Compression may be undergone by solids, liquids, and gases and by living systems. In the latter, compression is measured against the system’s volume at the standard pressure to which an organism is subjected—e.g., the pressure of the atmosphere at sea level is the standard, or reference, for most land animals, but the standard for deep-sea fishes and similar specialized forms is the normal pressure of their environment.
Forced vibration: Forced vibrations occur if a system is continuously driven by an external agency. A simple example is a child’s swing that is pushed on each downswing. Of special interest are systems undergoing SHM and driven by sinusoidal forcing. This leads to the important phenomenon of resonance. Resonance occurs when the driving frequency approaches the natural frequency of free vibrations
Infrasonic: vibrational or stress waves in elastic media, having a frequency below those of sound waves that can be detected by the human ear—i.e., below 20 hertz. The range of frequencies extends down to geologic vibrations that complete one cycle in 100 seconds or longer.
Natural frequency: The frequency at which a system oscillates when not subjected to a continuous or repeated external force.
Pitch: represents the perceived fundamental frequency of a sound. It is one of the three major auditory attributes of sounds along with loudness and timbre.
Rarefaction: in the physics of sound, segment of one cycle of a longitudinal wave during its travel or motion, the other segment being compression. If the prong of a tuning fork vibrates in the air, for example, the layer of air adjacent to the prong undergoes compression when the prong moves so as to squeeze the air molecules together. When the prong springs back in the opposite direction, however, it leaves an area of reduced air pressure. This is rarefaction. A succession of rarefactions and compressions makes up the longitudinal wave motion that emanates from an acoustic source.
Resonance: In sound applications, a resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object.
Ultrasonic: vibrations of frequencies greater than the upper limit of the audible range for humans—that is, greater than about 20 kilohertz