The world that surrounds us can be confidently called the world of sounds, because around us constantly voices, music, twitter of birds, the sound of the wind. Sound waves help people communicate, receive information about the world around them. For animals, sounds are no less important. From the point of view of physics, sounds are mechanical vibrations propagating in an elastic medium: water, air, solid, and so on. Human ears are capable of hearing sound when sound frequencies are in the range of 16 to 20,000 Hz. Vibrations with higher or lower frequencies are not audible to humans.
The science of acoustics deals with a variety of issues, including issues related to the features and properties of hearing. The subject of the study of physiological acoustics is directly the organ of hearing, its structure, action and device. Architectural acoustics is studying how sound waves propagate in a room, examines the influence of the shape and size of a room on sound, and studies the properties of materials from the viewpoints of the propagation and suppression of sounds. Musical acoustics is engaged in the study of musical instruments, explores the conditions for the best sounding of a particular instrument.
Physical acoustics studies sound vibrations themselves , sound waves, recently it has also begun to cover vibrations that lie beyond the capabilities of the human auditory system.
Basic Acoustics
The appearance of sound is due to mechanical vibrations of elastic bodies and media. Air is a conductor for sound. This was proved by the experience of Robert Boyle. If you put any sounding body under the bell of the air pump, then as the air is pumped out from under the bell, the sound will become weaker. When the air under the bell ends, the sound will stop altogether.
During oscillations, the body alternately creates a vacuum in a layer of air adjacent to its surface, then compresses this layer. As a result, the propagation of waves in airspace begins with vibrations of the air layer at the surface of the body.
As sound waves propagate in space, sound attenuation is observed, which is associated with certain irreversible processes. The point is that part of the energy carried by the sound wave is absorbed by the medium.
The absorption coefficient is a quantity that is equal to the ratio of the sound energy absorbed by the medium to the energy that has entered the medium. The absorption coefficient is influenced by the internal friction or viscosity of the medium, its thermal conductivity, the density of the medium and the speed of wave propagation.
Propagating in the medium, a wave will ever reach its boundary. After this boundary, another medium begins, which consists of other particles and in which the different speed of sound. Sound is reflected at this boundary. In this case, the rarefaction of the particles turns into a thickening, and the thickening into a vacuum.
This effect occurs because the vibrations that the wave brings to the boundary of the medium are transmitted to the particles of another medium and become the source of a new wave. The secondary wave will propagate not only in the second medium, but also in the one from where it originally came. This will be the reflected sound wave.
At the boundary of the media there is a partial passage of sound into the second medium and partial absorption of sound. The fraction of reflected energy will depend on the ratio of the densities of the media, as well as on the state of the interface. For example, the reflection of a sound wave propagating in air from a liquid surface or solid occurs almost completely. Sound waves propagating in a solid will almost completely be reflected at the boundary with air.
The occurrence of echo is directly related to the phenomenon of reflection. The essence of this phenomenon is that the sound comes from the source to a certain obstacle, which will become the boundary of the media, and is reflected from it, returning to the place where the wave appeared.