Sound is all around. It wakes us up in the morning, it accompanies us on our journeys through our headphones or on a car radio. It is sound that ultimately allows us to communicate with each other. But do you really know what a sound is? What physical phenomenon is hidden behind a Mozart opera or the percussive crash of a jackhammer?
Air vibrations
Sound is the vibration of molecules that propagates through a medium (air, water, concrete). Sound does not spread in the void (those the light saber swooshes from Star Wars... they weren't real!) In this article we are going to focus on sound in the air: the sound waves are actually slices of air molecules that vibrate around a position of equilibrium. The vibration of a molecule corresponds to a round trip, a continuous back and forth of the molecule (like the pendulum of a clock).
The maximum distance the molecule travels in its back-and-forth movement is called the amplitude of the sound. This physical measure corresponds to the volume of the sound. Consequence: if you turn the volume knob to put the music louder, the molecules vibrate by moving further away from their equilibrium position.
Did you know ?
The smallest sound amplitude a human ear can perceive is a displacement of the molecules of 0.1 ångström, a displacement smaller than the radius of an atom! That's to say if your ear is precise.
Another important characteristic of sound is the frequency with which the particles vibrate. It is simply the number of round trips per second made by the molecule of air. In terms of perception, the frequency of the sound corresponds to what is called its pitch: if the frequency is low, the sound will be low-pitched, if the frequency is high the sound will be high-pitched.
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The 440 Hz A corresponds to air molecules that make 440 round trips per second !
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The lowest sound that the human ear can hear has a frequency of 20 Hz, and the highest sound has a frequency of 20 000 Hz. These values vary for each individual (in particular according to their age).
The sound in equation
The simplest sound is the pure sound. A whistle, a tuning fork or a violin playing in the very high register are examples of sounds that are very similar to pure sound. In the case of the pure sound, the slice of the air molecule vibrates regularly over time, like the pendulum of a clock. We can represent its movement in time by a sinusoidal curve:
Mathematically we can write:
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f is the frequency of oscillation. It is a quantity directly related to the period (the duration of a round trip, in blue on the image).
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A is the sound amplitude.
Complex sounds
We have seen the example of pure sound, but in real life sounds are much more complicated. A held note played on the saxophone, for example, contains several superimposed frequencies, even if a defined pitch is heard. A complex sound is always the superposition of several pure sounds of different frequency and amplitude.
For this kind of sound, the lowest frequency is called the fundamental frequency. It is this frequency that determines the pitch of the note that is heard. The other frequencies are called the harmonics of the sound: they will not change the pitch of the sound but rather its timbre.
AND SOUNDS THAT ARE NOT NOTES?
Most sounds have no pitch: we can think of the different sounds of the drums, that of a car that goes on the street... These sounds actually contain all frequencies equally. When no frequency is stronger than the others, the sound is more appropriately described as noise. On the other hand, if one frequency emerges among the others, one will hear a pitch at that sound.
Summary:
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Sound is a physical phenomenon corresponding to the vibration of air molecules
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It is characterized mainly by its frequency (pitch of the note) and its amplitude (sound volume)
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Pure sounds like the tuning fork only have one frequency, but most sounds consist of the superposition of an infinity of pure sounds.
