Sound is an energy made by vibrations or caused due to vibration of particles. When an object vibrates, it causes movement in surrounding air molecules. These molecules bump into the nearby molecules, causing them to vibrate as well. This is how the sound is produced and it propagates in a given medium.
The waves which are produced due to distortion of medium are called as sound waves. These are also the longitudinal waves, which travel in the form of compression and rarefaction. Compression is the high density region, which is formed due to particles coming close to each other. On the other hand rarefaction is the low density region formed when the particles go far away from each other. One compression and one rarefaction is one wavelength of the longitudinal wave.
Number of vibrations per second is called as the time period. The reciprocal of time period is the frequency of the wave. Audible sound that is the sound which humans can hear has a frequency between 20 Hz to 20,000 Hz. The sound whose frequency is higher than 20,000 Hz is called as ultrasonic. It is heard by dogs, used by bats for navigation. It is used in SONAR, the Sound Navigation and Ranging, a device used to find the depth of sea. Infrasonic is that part of longitudinal waves, whose frequency is less than 20 Hz. Elephants and dolphins are sensitive to these waves.
The maximum displacement of particles from their mean position when the sound is produced is called as the amplitude of the sound wave. The medium through which the sound propagates oscillates, while the sound wave propagates forward.
Sound travels due to distortion of the medium in which it propagates. Hence in a more elastic medium the velocity of the sound wave will be more. Thus sound travels faster in solids as compared to liquids or gases. It cannot travel through the vacuum.
Sound travels with a speed of 340 m/s in air. Its velocity is smaller than the velocity of light. This is the reason why the thunder clouds are heard after the lightning is seen.
There are different types of musical instruments, based on how they produce sound. The ones which produce sound by plucking of strings are called string instruments. For eg. guitar and sitar. The instruments which produce sound by vibration of thin membrane are called as percussion instruments. The examples are drum and table. There are some instruments which produce sound by the vibrating air column like flute and shehnai while there are some which use vibrating water column like jal tarang. There are a few instruments which produce sound due to vibrating reeds like harmonium and mouth organ.