Let us find out the physical nature of sound phenomena.

As you know, to produce a clean sound enjoy tuning fork. When the tuning fork produces sound, the ball bounces off his feet, because it varies (rice. 25.1). Experience shows, that is always the sound of any oscillating body, During its fluctuations creates environmental Wednesday mechanical waves (rice. 25.2). When these waves reach the human ear, they lead to forced vibrations inside the ear eardrum, and the person feels the sound. Mechanical waves, that evoke a sense of human sound, call sound.

Sound waves in air consist of condensations and vacuum, t. e. are the longitudinal. Clear, that sense of sound (a person can get only, When between the sound source and the human ear has a Wednesday, which can spread sound waves. Through the airless space sound cannot be transferred. This is confirmed by the following experiences. EElectric Bell suspended on wires malouprugih, include network and a glass cover (rice. 25.3). Then the sound of a running call, still clearly audible. Then activate the pump, vykačivaûŝij air from under the hood. As dilution air under the hood sound becomes weak and with a large enough negative pressure completely disappears.

Study of sound phenomena revealed, that not all sorts of mechanical waves can cause the sensation of sound in human. It turns out, that only waves, Oscillation frequency which ranges from 16 to 20000 Hz, are sound (upper and lower limits of the frequencies of these fluctuations among different people may differ slightly from shown).

So, the person feels the sound, If the following conditions are true::

• There is a sound source, generating oscillation with a frequency range from 16 to 20 000 Hz;
• There is an elastic Wednesday between the ear and the sound source ’,
• power of sound waves is sufficient to obtain the feeling of sound in human.