The voice and the noise that we hear in our ears is known as sound. In terms of physics sound is a longitudinal wave which propagates because of the vibration of the particles of the medium in the longitudinal direction.
As can be seen in the picture there is compression of the particles as they transfer the pressure towards right and rarefaction as they return to their normal positions. Pressure is created by the transmitter of the sound wave. The particles in fact only vibrate at their position and only transfer energy in the direction of propagation (in this case right direction).
The sound wave is a mechanical wave which means it require a medium (air, water, solid) for transfer. Which means there is no sound waves in space or vacuum as there are no medium for it to propagate. Astronauts use radio waves (electromagnetic wave) for communication in the space and on the moon.
Producing a sound wave
Whether it conveys the shrill whine of a jet engine or soft melodies of a guitar, all the sound waves have its source in a vibrating object. Musical instrument produce sounds in variety of ways. The sound of clarinet is produced by a vibrating reed, the sound of a drum by vibration of the taut drumhead, the sound of guitar and piano by vibrating strings.
Characteristics of Sound Waves
The general motion of elements of air near a vibrating object is back and forth between regions of compression and rarefaction. This back and forth motion of elements of the medium in the direction of the disturbance is characteristics of a longitudinal wave. The motion of the elements of the medium in a longitudinal sound wave is back and forth along the direction in which the wave travels.
Categories of sound waves
Sound waves fall in three categories covering different ranges of frequencies.
Audible waves are longitudinal waves that lie within the range of sensitivity of the human ear, approximately 20 to 20 000 Hz.
Infrasonic waves are longitudinal waves with frequencies below the audible range. Earthquake waves are an example.
Ultrasonic waves are longitudinal waves with frequencies above the audible range for humans and are produced by certain type of whistles. Animals such as dogs can hear the ultrasonic waves.