Hi there! In this issue we will talk about the sound and its propagation in the air.

Let's analyze all the sound propagation features:

Every sound, once generated by a vibrating elastic body, is transmitted through the surrounding medium, creating a series of waves where particles the medium is composed of are subject to condensation and rarefaction. It could seem pretty obvious, but it's better to point out that a mean of conduction is essential for sound transmission. No transmission is possible in vacuum, differently to what happens for light transmission.

Here we are with some frequency values and relevant wavelengths (for waves propagating through air):

It's worth to remind that the wavelength can be determined by considering the distance between its crests (i.e. between two consequent maxima or two consequent minima).

The maximum amount of particle displacement is called vibration amplitude. As soon as we go away from the perturbation source, then vibration amplitude decreases, because of the damping factor, therefore the particles gradually get back to their original static equilibrium position (due to the g-force).

The physic propagation can occur in different ways, for instance:

• transversal: where propagation starts from the source on a plane, following all the half lines departing from it
• longitudinal: oscillations transmitted by the particles go in the propagation direction, that is, from the source following the relative half line

Transversal and longitudinal waves are two simplified limit cases of mechanical waves, which are in general more complex phenomena. Now, let's talk about our main interest, that is, the wave propagation through spherical waves.

• with spherical waves, propagation starts from a given point in the space along all the half lines generated by the source.

As far as waves propagation is concerned, there are also some particular features to take into account:

1. the sound intensity as perceived is inversely proportional to the square root of the distance from the source

2. the sound intensity is modified if turbulences are present

3. both intensity and speed of sound are determined by the conductive medium density

4. the speed of sound does not depend on its frequency nor its intensity

Let's see how the speed of sound changes according to the conducting media.

In dry air, the speed of sound is about 344 meters per second (at 20°C and sea level)

The speed is then 1480m/s in water, 3350m/s in wood, 3400m/s in cement, 5050m/s in steel and 5200m/s in glass

As we can easily notice from the different speed details, the stronger the conducting medium is, the faster the propagating speed is.

In the next issue we will analyze the connection between environment and waves.

Bye!