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WHAT IS SOUND?​

Writer's picture: Fernando Núñez IzuFernando Núñez Izu


Often times we try to postpone the learning of certain theoretical notions because we believe them to be far too removed from our daily practices. But if we are seriously interested in music, sooner or later we will find ourselves perplexed when staring at a mixer, a recording software, the knobs on an amplifier, etc. That is why in this article we will cover the basic concepts in a clear and simple manner, emphasizing its relevance to the everyday practice.


Definition



Sound is a vibration, that is, a form of enery that proppagates in waves travelling through an elastic medium. What does this mean? It means sound is not a thing that moves, but something that moves in things. In general, that which moves is the air, but it could just as well travel in water and other liquids, or other solid bodies or gases.

Sound waves travel through the air (or any other medium) by making its particles vibrate. The vibration squeezes them and then spreads them apart in the movements called compression and expansion, respectively. That's why we talk about an elastic medium, because it stretches and compresses with every ondulation.

Onda sonora representada en un oscilograma  ​
Sound wave as represented in an oscillogram.

Each wave then consists of a moment of compression followed by one of expansion. The visualization we can see in every recording software is called an oscillogram or oscilloscope. It represent the oscillations in the pressure level in the aire. On one side of the central axis it represents the pressure as it increases (compression), and on the other side it shows pressure diminishing (expansion). It is the graphic representation of a wave.

Any sound that lasts more than an instant comprises more than just a single wave, but a great succession of them, some louder, some softer, some faster, and some slower. The amplitude of the movement contributes to the perception of volume, the speed of the vibration determines the perception of pitch (the musical note).

When we play an instrument, what we are doing is generate a vibration in the air at a certain speed, which produces a musical note. The two videos (above and on the side) show the vibration of a guitar's strings and the skin of a drum.



Wavelength, Period and Cycle

Ciclos de un tono de 40 Hz
Cycles of a 40 Hz tone.

The cycle of a wave is the smalles part that repeats. It consists of a semi-cycle that's positive and another that's negative. It's sometimes referred to as the waveshape. All cycles begin and end in zero (represented as the center position); that is, with the medium's particles in its resting, original position. The wavelength is the distance that a wave travels until it completes a single cycle. The higher the frequency the shorter the wavelength. The period consists of the duration of each cycle, meaning the time it takes to form a complete wave. It is measured in miliseconds (ms).


Frecuency & Tone


Just like in everyday language, when we talk about frequency we refer to how often something happens. In this case, we refer to how often a soundwave oscillates within the span of a second. The concept of times per second is expressed in Hertz (Hz). A 100 Hz tone contains a hundred cycles per second.

the frequency of a tone is directly related to the perception of pitch. The faster a wave vibrates, or, in other words, the higher the frequency, the higher the pitch. That is why in the video that showed a guitar strings vibrating, the lower notes could be seen vibrating more slowly. In the video on the right we can appreciate the correlation between frequency and pitch.



Simple & Complex Waves



As we were saying, the frequency of a wave is related to the perceived pitch. A pure tone, as it is often called, is a simple wave, that is, a soundwave containing a single frequency. Its technical name is sine wave or sinusoidal wave.

Sumatoria de 3 senoidales en una onda compleja
Summing of 3 sine waves into a complex wave.

Sine waves (as seen in the first three examples on the right) are perfectly "smooth" in their movement. They constitute the "atoms" of sounds. Every other sound is born from the addition of sine wave. These combine into a complex wave, which then contains those multiple frequencies. For example, an A major chord is a complex wave formed by the summing of 440 Hz (the equivalent of an A note), 554 Hz (C#), and 659 Hz (E). In the image above we can observed each frequency's waveform separately and then their combination.

Ondas complejas correspondientes a una canción completa en estéreo ("Zig Zag Wanderer" por Captain Beefheart)
Complex waves corresponding to the left and right channels of a stereo song ("Zig Zag Wanderer" by Captain Beefheart)

Timbre & Complex Waves



By the timbre of a sound we refer to that quality which differentiates it from other sounds. It is that which makes the same musical note sound different when coming out of a guitar instead of a piano. But if both instruments are emiting the same frequecy, then, what sets them apart? The difference lays in the harmonics. The harmonics are the set of frequencies that accompany that other one we call the fundamental (in the previous example, the fundamental was an A at 440Hz). They are proportionate to the fundamental (the first harmonic being its double, the second its triple, and so forth). It is due to the presence of harmonics that instruments emit complex waves instead of pure tones.

Fundamental (220Hz) y armónicos de una guitarra eléctrica.
Fundamental (220Hz) and harmonics of an electric guitar playing an A note.

By using a spectrum analyzer, we can visualize the harmonics of an electric guitar (above) and a piano (below) playing the same A note (fundamental = 220 Hz). Each peak represents a sine wave at a specific frequency.

Fundamental (220Hz) y armónicos de un piano.
Fundamental (220Hz) y armónicos de un piano.

But not every frequency that constitutes a sound is harmonically related to the fundamental. While harmonics appear at different frequencies (since they follow the changes in pitch as an instrument plays different notes), other non-fundamental frequencies remain fixed. This is because they do not have their origin in the note in question, but in any other part of the instrument that remains unchanging. For example: the timbre of a piano comes in part from the sound of the hammers; in a guitar it comes from the sound of the pick or the guitarist's fingers, etc.


Final comments


While we still have not touched on the more practical and musical aspects in which these considerations come in handy, their importance can not be overestimated. Understanding the nature of sound is key to understanding how the different tools we employ work. In a future article we will pick up on this and analyze other fundamental concepts such as envelope, phase, psychoacoustics, analog and digital audio, transduction, etc.

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