Acoustics is the branch of physics devoted to the study of sound.

## Sound waves

Sound waves are longitudinal mechanical waves, capable of stimulating the human ears, which can perceive sounds in the range of 20 Hz to approximately 20,000 Hz (20 kHz). Other animals have different boundaries, listening to more bass or treble sounds.

The speed of sound depends on the medium in which it propagates. The table below gives some examples of material resources and the speed of sound in these media.
Speed ​​of sound
Media Speed ​​(m/s)
Air 0 $$^oC$$ 331
Air 15 $$^oC$$ 340
Water 20 $$^oC$$ 1482
Aluminum 6420
Steel 5941
Granite 6000

Some phenomena that occur with sound waves are:

Echo
It is the phenomenon that occurs when the sound is reflected by an obstacle to a distance greater than 17 meters. At this distance, there is a time interval greater than $$0,1 s$$ (tenth of a second) between the arrival of the sound from the source and the reflected sound. In this case, we can distinguish between the two sounds and hear the sound with an echo.
Reverberation
It occurs when there is prolongation of the auditory sensation by virtue of the reflection of the sound. It occurs when between the arrival of direct sound and that of reflected sound there is a time interval less than $$0.1s$$.

Some characteristics and nomenclatures of sound waves are presented below:

Height
Quality which distinguishes a bass (low frequency) sound of a treble (high frequency). Do not confuse the height of a sound with the volume.
Bass sound $$\rightarrow$$ low frequency.
Treble sound $$\rightarrow$$ high frequency.
Intensity
Quality that allows to differentiate a strong sound of a weak one (volume).
Timbre (tone color in music jargon)
Quality that allows you to sort the sounds of the same height and same intensity emitted by different sources. The timbre is related to the shape of the wave.
Interval ( $$i$$ )
The interval between two tones is the ratio of their frequencies: $$i = \frac{f_2}{f_1}.$$
Sound level ( $$\beta$$ )
As an observer moves away/approaches a sound source, the sound level decreases/increases rapidly, and it is more convenient to describe this intensity as: $$\beta = 10 \cdot log ( \frac{I}{I_o} ),$$ where $$I_0$$ is the audibility threshold ( $$10^{-12} W / m^2$$ ) and $$I$$ the physical sound intensity of interest.
Decibel ( $$dB$$ )
It is the most common unit of sound level. $$1 dB = 10^{-1} B$$ (Bel).
Note: The device that measures the sound level is the decibel meter. This device is used to check if cars are within the noise emission standards.