Sound is a physical phenomenon resulting from the vibration of matter and propagating in elastic media. The form of sound known in acoustics is that of an alternating change in air pressure that propagates in space in waves, spatially and temporally. It is generated by vibrations in musical instruments or actuators, such as the loudspeaker.
The pressure changes are transmitted via the medium of air as a pressure change to the ear, where they are converted into an acoustic sensation. The human ear reacts to the sound pressure by a corresponding sensory impression. The sound frequencies audible to humans are between 20 Hz and 20 kHz. Sound frequencies above this are referred to as ultrasound, while those below are referred to as infrasound. Sound frequencies above those of ultrasound are referred to as hypersonic.
Sound can only be transmitted in matter, in solid, liquid, and gaseous media, because in the transmission of sound, the molecules of the media oscillate about their rest position in their inertial and elastic forces. Sound waves are subject to absorption, attenuation, transmission, diffusion and diffraction.
The change in air pressure triggered by sound pressure is expressed in bar and pascals. Since sound propagates spherically from the sound source, the sound power is distributed over an increasingly larger area as the distance from the sound source increases. The sound pressure of the sound source thus decreases with distance. In addition, there is also frequency attenuation. Higher frequencies are attenuated more than lower frequencies with increasing distance.
The speed of sound, the speed of sound, propagates in the air at 20 °C at 343 m/s.