Sound is a form of energy which travel as waves. Sound waves are produced by objects that vibrate in a medium. The medium can be a gas, liquid or solid. A vibrating object produces sound waves by the shifting layers of air particles. The vibrating object pushes and pulls on the air which send the air vibrations in waves of compressions and rarefactions. These waves are longitudinal because the air particles vibrate along the direction in which the waves travel energy.
Compressions and rarefactions are changes in air pressure. Compressions are areas where the air pressure is slightly higher than the surrounding air; rarefactions are areas where the air pressure is slightly lower than the surrounding air. The wavelength λ, of a sound wave is the distance between two consecutive rarefactions (or compressions). The maximum pressure change gives the amplitude A, of a sound wave.
Transmission of sound
Sound waves are mechanical because they require a medium to travel and cannot travel in a vacuum. This can be tested by listening to a bell in a bell jar. As the air is pumped out of the bell jar the ringing sound fades away. Any medium with vibrating particles will transmit sound, but the nature of the medium will affect the speed of the waves. Sound waves travel fastest in solids and slowest in gases. Altering the physical conditions also in air also affect the speed of sound.
Audibility
The range of frequencies audible to the human ear is approx. 20 Hz at the lower limit to 20 000 Hz at the upper limit. The ranges of audibility differ from person to person e.g. older people have a lower range of audibility than younger people.
Reflection of sound
An echo is the reflection of sound heard when sound waves bounce back from hard, flat surfaces. Multiple echoes of sound can be heard in an empty hall because of the many reflection of the walls surfaces. The prolonged sound due to the many echoes is called reverberation.
Uses : Ships and submarines transmit signals to detect shoals of fish and measure the depth. Time interval is noted between signal emission and the arrival of the echo. Bats use echosounding to detect obstacle sin their flight.