What is sound and how do we hear things? Hey Engineering Lovers, whether with your headphones , cell phone speaker or computer, you must be hearing me well, right? To consume content on social networks, we mainly use two senses, our vision and our hearing, and sound is linked to the second.
To hear anything, our ears are capable of decoding sounds, but what exactly is sound? In physics, sound is a mechanical wave of pressure and displacement, which we know as vibration, that propagates through a medium, such as air, water or a solid and these are what we call sound waves. These waves can vibrate at any frequency, but humans can only hear sound waves with frequencies between 20 Hz and 20 kHz.
Frequencies above 20 kHz are called ultrasound and below 20 Hz infrasound. But let's understand these sound waves better. Sound waves are disturbances or pressure variations that propagate in a medium.
They are produced when a sound source, such as a speaker, a person speaking or a musical instrument, causes particles in the surrounding medium to vibrate. To understand it better, think that you threw a stone into a calm lake. When the stone hits the water, it creates concentric waves that spread across the water's surface.
In a similar way, when a sound source emits sound, it creates waves in the same way. But sound waves are characterized by some properties. The frequency of a sound wave is related to the number of complete cycles that the wave completes in one second and as we already know, this number of cycles is what we call Hertz (Hz).
This frequency determines the tone of the sound. For example, high-frequency sounds are higher pitched and low-frequency sounds are lower pitched. Another information we have is the amplitude of a sound wave, and this is related to its height or intensity.
The greater the amplitude, the louder the sound and the smaller, the quieter. In other words, when we increase the volume of a song, we are increasing the amplitude, and when we reduce the volume, we decrease this amplitude. While frequency is measured in herts, the amplitude of a sound wave is measured in decibels (dB), which is a logarithmic unit that describes the relationship between the pressure of the sound wave and a standard reference level.
The common reference level for sound is generally defined as the limit of human hearing, which is approximately 20 microPascals (μPa) in sound pressure level for audible frequencies. Another information we also have is the wavelength, which is the distance between two consecutive peaks or valleys in a sound wave. It is inversely proportional to frequency, where high-frequency sound waves have short wavelengths, while low-frequency sound waves have long wavelengths.
And we can see this in a speaker for example. If we place an ultrasound with a short wavelength in a speaker, we can barely perceive the movement of its cone, which is exactly this main part of the sound box, because it is moving so quickly that we cannot see it. Now if we use infrasound with a longer wavelength, we can already see the movement of the cone, because it is moving slower.
However, the speed at which sound waves propagate depends on the medium in which they are traveling. For example, in air at an ambient temperature of 20 degrees Celsius, sound waves propagate at a speed of approximately 343 meters per second, something we call the speed of sound. However, this speed is not fixed, because if we increase the air temperature, the speed of sound also increases, and if we decrease the temperature, the air speed decreases.
But as a base, we use the speed of 343 meters per second as a standard, something we also usually call mach. The mach number is equal to once the speed of sound, so if a jet reaches the speed of sound, it reaches mach 1. Now if the jet reaches a much higher speed and reaches, for example, 3 times the speed of sound, it reaches mach 3.
When sound waves reach our ear, within its structure, they are converted into electrical signals which are then interpreted by the brain as sound. This is a complex process that involves the functioning of the outer, middle, and inner ear, as well as neural processing in the brain. So does this mean that when we make a sound, that sound causes air to move towards our ears?
As if they were being carried? In truth no. When sound waves propagate in a medium, such as air, the individual particles in the medium do not move with the wave in the same way as waves in an ocean move water.
Despite having given the example of a stone in a pond, it only serves to exemplify the waves, but the sound does not move the particles in the medium. Instead, the particles in the medium move back and forth in the direction of propagation of the sound wave. For example, when a sound wave moves through the air, air molecules alternately compress and expand in the direction of the wave's propagation.
This compression and expansion of air molecules is what constitutes the variation in air pressure that characterizes sound waves. If you notice on some speakers, you will see a “hole” like this. It serves to allow air to enter and exit the speaker, because the speaker cone, once propagating a certain wave, creates zones of air compression and expansion.
In this way, the movement of the cone moves the air particles around the air, and this movement ends up moving the air particles, but the sound wave itself is not caused by this displacement, but by the compression and expansion of the medium. If you turned up the volume and liked this topic, take the opportunity to subscribe if you are not subscribed, leave your like, and activate the notification bell and if you found our content useful, consider becoming a member and help us continue producing content here on the platform and be remembered in our videos. This topic about sound is very extensive and interesting.
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