What is Sound? - The Analogue Domain
In order to effectively work with sound, understanding how it is generated and how it behaves in space will greatly help artists to help make informed decisions during recording or doing basic post-production. We’ll talk more about audio capture and processing in upcoming blog posts. Today, let’s look at what is sound and how it behaves in nature — the analogue domain.
Generation and Propagation of Sound in Air
Sound waves are generated by objects that are in vibration. These vibrations cause disturbances in the medium in which they travel, at a molecular level. For the purpose of this study, the medium will be limited to air. Similar principles shall hold good for other mediums as well.
Sound waves can travel through any medium solid, liqiud or gas but not in vacuum.
The ‘Generators’ can be any of the following types.
Thin membrane — like the skin of drums, vocal chords, speaker domes.
A solid body that can resonate — like a xylophone, a marimba, a tuning fork.
String — like those on guitars, violins
A chamber which resonates when air passes through — like a flute, a Church organ.
When these sources vibrate, they trigger a disturbance in the air molecules around them, causing them to vibrate as well. This energy imparted, then travels as waves through the air in all directions and reaches our ears, enabling us to hear them.
When you hear your own voice, you not only hear the sound traveling through the air to and your ears but also hear the vibrations in your head.
That's why your voice always sounds different when you hear it back in a recording.
Sound waves in air can also be considered as pressure waves and they are omni-directional (travel in all directions) from the source. Even a tiny air gap is enough for sound to travel into a room. This is why you can hear someone from the other room, even when the door is closed. And that’s exactly how noise enters a room too.
You might have noticed that sometimes you can ‘feel’ sound through the floor or the walls. These are sound waves that travel through solid medium. Usually only low-frequency sounds travel as such. For those unfamiliar with ‘Frequency’, let’s look at what that means and also some other parameters that are characteristic of sound waves. You’ll come across these terms often, so here is a gist of what you should know.
Sound travels faster in solids than in liquids and faster in liquids than gases.
Frequency, Wavelength and Amplitude
‘Frequency’ is the number of vibrations that occur per second measured in Hertz (Hz). ‘Wavelength’ is the distance between two consecutive waves at an given instant and ‘Amplitude’ is the amount of displacement of the molecules caused by the wave.
Frequency is what we perceive as ‘Pitch’. We don’t need to worry about wavelength much, for the time being you can just understand that frequency(f) and wavelength(λ) are inversely proportional and are related to each other by the formula [ λ = v/f ], where ‘v’ is the speed of sound in that medium. Thus a sound wave of high frequency will have short wavelength while that of a low frequency will have long wavelength. The amplitude is what determines the perceived loudness of sound — higher the amplitude, louder the sound.
The speed of sound in air is roughly around 343m/s at room temerature and standard atmosphereic temperature.
The speed of sound varies with the pressure and temperature of the medium.
In the next blog, we’ll look at how sound is captured by a microphone, and further on we’ll see how it is converted into digital signals.
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