What is sound?

Sound is a vibration that travels through the air, transmitting energy, but never matter. The speed of sound changes as the medium in which the wave propagates changes.

The figure above shows the intensity of a sound wave with respect to time.

Sound is a wave capable of propagating through the air and other media from the vibration of its molecules. Sounds are perceived by us when they affect our hearing aid, which are translated into electrical stimuli and directed to our brain, which interprets them.

Humans are capable of hearing a range of sound frequencies, called the audible spectrum , which extends between approximately 20 Hz and 20,000 Hz. Sounds of frequencies less than 20 Hz are called infrasound , while sounds of frequencies greater than 20,000 Hz are called ultrasound. Other animals such as dogs, cats and bats are able to hear much wider ranges of frequencies.

The speed with which sound waves propagate depends exclusively on the characteristics of the medium in which they travel, in air, the speed of sound is approximately 340 m/s.

As sound has wave properties , it can undergo several phenomena, such as reflection, refraction, diffraction and also interference. In the latter, two or more sound waves can either be canceled or added, according to their position.

sound characteristics

The main characteristics that distinguish one sound from another sound are three: pitch, intensity and timbre.

  • Height: The pitch of the sound refers to its frequency. Loud sounds are those that have high frequencies, also called high-pitched sounds. Low sounds, on the other hand, are those that have low frequencies, being, therefore, bass sounds.
  • Intensity: The intensity of sound refers to the amount of energy that the sound wave transmits. This intensity is related to the amplitude of the sound wave: the greater its amplitude, the greater its intensity. This property of sound is measured in decibels: loud sounds are called loud sounds, while low-intensity sounds are called soft sounds.
  • Timbre: The timbre of sound is what allows us to distinguish the nature of its source. When we hear two sounds of the same frequency and intensity, but which were produced by different instruments, we can easily differentiate them. The timbre is the mode of vibration of the sound wave, and each sound source has its characteristic timbre.

Phenomena suffered by sound

As sound is a wave, it is subject to several wave phenomena, check them out:

  • Reflection : Reflection happens when sound is emitted towards some elastic screen. Sound reflection gives rise to sound echo, for example.
  • Absorption: Some media are capable of absorbing sound waves, thus functioning as good sound dampers. Anechoic chambers are practical examples of sound absorption, almost no external sound is able to enter these chambers.
  • Refraction: Refraction occurs when the sound changes medium and undergoes changes in velocity. This phenomenon is especially useful for performing ultrasound examinations.
  • Diffraction : If the sound passes through an obstacle or slit with dimensions similar to its wavelength, it will be diffracted. The diffraction of sound causes it to pass through cracks, under doors, and be heard.
  • Interference : Interference concerns the superposition of sound waves, at some points in space, the sound produced by one or more sources will overlap their crests and waves, producing regions of constructive and destructive interference. In theaters and cinemas, the sound system is designed so that there are minimal regions of destructive interference.

Despite being a wave, sound is a longitudinal wave and, therefore, is not capable of being polarized.

sound in physics

For Physics , sound is a longitudinal and mechanical wave and , therefore, needs a physical medium to propagate. We can understand the sound as a vibration that propagates in the air and in other media forming regions of compression and rarefaction, that is, regions of high and low pressures.

How does sound propagate?

Sound is produced by vibrations transmitted into the air. These vibrations generate regions of compression and rarefaction of atmospheric gases that are periodically intercalated, according to the frequency of the source that produces the vibrations.

Because it is a wave, sound is not capable of transporting matter, such as small particles, but only energy. Look at the figure below, it is possible to observe how the sound is able to propagate itself:

Sound waves produce regions of compression and rarefaction.

The speed of sound propagation depends directly on factors such as the elasticity of the medium . The more elastic a medium is, the greater the speed of propagation of sound waves within it. We say that a medium is elastic when it is able to vary its volume greatly if it is subjected to pressure.

waves and the sound

Sound is a longitudinal wave, as it propagates in the same direction as the vibration responsible for producing it. Furthermore, sound is a mechanical wave , as it is only able to propagate itself in physical media such as air, water, metal, etc. Being a wave , sound has properties such as propagation speed wavelength , frequency and amplitude :

  • The speed (v) of sound depends on the medium in which it is propagated, physical media with greater elasticity tend to propagate sound more easily, due to the proximity between their molecules. For comparison, while sound propagates in air at a speed close to 340 m/s, its propagation speed can exceed 5000 m/s when it is propagated in an iron rod.
  • The frequency (f) of a sound wave is measured in Hz, this frequency defines its pitch, that is, the higher the frequency of the sound, the more acute, or louder, that sound is. On the contrary, low frequency sounds are called bass sounds, or lows. Humans are only able to perceive sounds between 20 Hz and 20,000 Hz.
  • The wavelength (λ) of sound is the space required for the sound wave to produce a complete oscillation, it can also be understood as the distance between two crests or two valleys of a wave Half a wavelength is the equivalent of the distance between a crest and a valley.
  • The amplitude of the sound wave defines its intensity, or the amount of energy that this wave carries with it, which can also be understood as the “volume of the sound”. The amplitude of the sound wave is shown in the wave profile shown in the figure below:

The figure above relates the intensity of the sound wave to the distance it travels.

Speed ​​of sound

The speed of sound is measured in relation to the medium in which it is propagated. There is no relative velocity between sound and its observer, and this behavior gives rise to the Doppler effect : the change in the apparent frequency of sound due to the relative motion between a sound source and an observer.

We can calculate the speed of sound if we know its frequency and wavelength. To do this, we just do the following calculation:

v – speed of sound

λ – wavelength

f – frequency

The table below shows the speed of sound propagation in some known media, check it out:

Quite Speed ​​of sound
Air (21°C) 344 m/s
Water 1480 m/s
Concrete 3400 m/s
Aluminum 5150 m/s
Glass 5200 m/s

In addition to its dependence on the medium, the speed of sound also depends on temperature. For example, at a temperature of 30°C, the speed of sound in air is approximately 350 m/s, while at a temperature of 21°C, its speed is 344 m/s. The empirical law that defines the dependence of the speed of sound on the temperature of the air is shown below, check it out:

T – air temperature, in degrees Celsius

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