The level of pressure variation (amplitude) determines the loudness; the number of pressure variations per second determines the frequency / (CPS. Hz)
Sound travels through air at a speed (speed of sound) dependent on temperature. The speed of sound at 70°F is approximately 1130(FPS) feet per second. Higher temperatures generate faster speeds of sound. The frequency (CPS, Hz) of sound can be measured. The human ear hearing range is 20 Hz to 20,000 Hz, and is most sensitive in the 1000 to 4000 Hz range.
Wavelength is an essential variable in describing the sound characteristics, and the resulting silencer design. For example, the wavelength of 20 Hz at 70°F is 56.5 feet, at 2000 Hz at 70°F it is 7 inches. Low frequencies (Hz) have long wavelengths; high frequencies have short wavelengths. Most sound sources in industry are referred to as broad band, a mixture of low and high frequency sounds.
The human ear is remarkably sensitive to low energy sound, as a consequence, exposure to excessive high levels of sound is a safety and comfort concern.
Sound is the sensation of hearing; hearing in the human ear is perceived when very small pressure variations occur in the air, the human ear can detect very small pressure variations. Pressure variations are produced by vibrating surfaces (tuning fork); these vibrations can be caused by:
Animations courtesy of Dr. Dan Russell, Kettering University.