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Propagation of sound.

Characterictics of sound waves.

Wavefront , acoustic beam.

Period and wavelength.

Frequency of the sound.

Longitudinal and transverse waves.

Speed of sound.



As we know, the sound is the sensation caused by the vibration of the flexible body in agile environment.

Parts of the agile environment transmit the vibrations through their mutual connection through intermolecular forces. The oscillation of every part of environment spread throughout the agile environment.

  • Distribution of the periodic pressure variation on agile environment called a sound wave..


  • The area of the space, in which the sound waves propagate, called a sound field.


Sound wave has several elements. These are:

  • Wavefront. As such we can determine the surface of agile environment that connects points located in the same phase of oscillation.


  • Acoustic beam – this is the direction of propagation of the sound wave. Can be described as a line perpendicular to the wavefront and pointing in the direction of wave propagation.



  • Sound waves as a distribution center are spherical and flat.

This definition is conditional because whenever we broadcast sound from one single point, a spherical wave is generated in all directions.However, when the distance of the wave from the sound source is significant, it can be assumed with a certain averaging, that we have a flat wavefront. Then the lines of compression and rarefaction are parallel.

  • What we call wavelength is the distance between two points of the wave, which have the same phase.

It passes this length for time, defined as a period of the wave.

  • The wavelength is measured in meters, and the period T – in seconds or fractions of a second.


  • The number of periods of the wave for a second we call frequency of the sound.


The frequency, wave length and speed are related by the formula:

, from which it follows that at the same speed of the wave we have inverse correlation of its frequency and length.

I.e. long waves have low frequency, and the short – a high.

The sound waves are of two types according to the axis of movement of the particles of the medium relative to the direction of travel of the wavefront.

  • Waves are longitudinal and transverse.


If the longitudinal oscillation of the particle is directed along the axis of propagation of the wave, we talk about longitudinal waves.

Such are the sound waves in the wind musical instruments which are described as so-called. “air columns”

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The transverse waves have a particle motion perpendicular to the movement of the wave. They are most clearly represented in the movement of the strings and in the string instruments.









In general we can conclude that while in the gas we have distribution only of longitudinal waves, in solids both longitudinal and transverse waves may occur .




Speed  of sound


The speed of sound in a medium is determined by the formula:

,  where K is the elastic coefficient of the body, and P – density.

As we can see from the formulation, the speed is independent of the frequency in the general case. Otherwise, the sound of different frequencies would arrive at the listener at different times.

However, it depends on the temperature. A hot medium has a greater mechanical energy of the particles and they better transmit the sound.

The speed of sound in air is 343 m / s or 1235 km / h at an air temperature of 20 degrees.

In different materials the speed of propagation of sound is different – in the water it is 1430 m / s, in iron – 5170 in stone – 4000 etc. This is why when the sound is distributed in two ways at once – in the air and in other media, we hear two sounds with a different arrival time.



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Filed under: ESPOL, Sound theory

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