Sound energy is the energy produced by the vibrating bodies. The sound energy produced progress in the form of mechanical waves through out the medium (such as air). As sound waves are mechanical waves they need a medium to propagate. So sound can not travel in vacuum. When a body is set to vibrate, it causes a disturbance in the medium. The participles in the medium collide with each other and transfer the energy. When the particles of the medium come closer to each other it is called compressions. When the participles of the medium move away from each other it is called rearfactions. This compressions and rearfactions cause energy transform. This compressions and rear factions will continue until they run out of the energy.
For example take the case of ringing of a bell. Once a bell is set to vibrate, it causes collisions in the air particles. These air partials farther collide with some more air particles. When our ear comes with in the range of the vibrations our eardrum starts vibrating causing a sensation of hearing.
Sound energy is mostly measured in units of energy, such as joules. Many measure it in terms of pressure and intensity using pascals and decibels. Sound intensity is defined as the sound energy flowing through a unit area in a unit time in given direction. It is normally presented in terms of dB. Sounds produced by all vibrating bodies are not audible. The frequency limits of audibility are from 20 Hz to 20,000 Hz. Sounds of frequencies less than 20 Hz are called infrasonic and greater than 20, 0000 Hz are called ultrasonics.
The speed of sound depends on the medium of propagation. Among the three media, sound travels fastest through solids and slowest through gases. This is due to the fact that solid media have compact particles, which make it easier and quicker collide with each other, compared to those in liquid and gas. The gas molecules, which are far from each other, have the lowest tendency of colliding with each. In water, sound travels quicker than through the air. Aside from the medium of propagation, temperature and humidity also affect the speed of sound. With increase in humidity, density of air increases, causing increase in velocity.
Acoustics is study of generation, propagation and reception of mechanical waves and vibrations i.e, sound energy. Acoustics is a science with innumerable applications that requires sound knowledge of sound energy. The best applications of acoustics can be seen in the theaters where sound energy produced is evenly distributed to maintain monotony. The generation of echo is common in big theaters, this problem is solved by providing sound traps and broken angles as well as by use of non reverberating materials.