The sound is a vibration that propagates as an audible wave of pressure through a medium such as air. In other words, the sound waves are pressure waves.
There are two essential concepts that form sound waves: Source and medium.
Source of Sound Waves
The source is the first and the most important thing of a sound wave. Because it creates the disturbance that forms the sound waves. The source of a sound wave is a vibrating object. This vibrating object can be anything from the vocal cords of a human to the strings of a violin. It generates a disturbance that forms the wave. For an acoustic guitar, bass, violin, and piano, the vibrating objects are their strings. The reeds in the mouthpieces of flutes and saxophones vibrate. For trumpet and trombone, the lips of the players vibrate. The vibrating objects of drums are their membranes. It is the solid surface for cymbals, cowbells, etc. For electronic instruments, the vibrating object is their loudspeakers. As I mentioned before, the vocal cords of a human also vibrate.
Medium is the substance or environment that carries a wave. For sound waves, the medium can be solid, liquid, or gas. The air is the most common medium for sound waves. If there is no medium, we cannot talk about sound waves. For example, there is a vacuum in outer space. Therefore, there is no sound in outer space. Because sound waves cannot travel through a vacuum.
Production of Sound Waves
Let’s say we have an object that vibrates back and forth and the air is the medium around it. The air has a certain density. We call it the ambient air pressure. It is the pressure of the medium. Now let’s see how we produce sound waves with a tuning fork.
Here is a tuning fork which is a two-pronged steel device we use to give a note of specific pitch. Usually, we use this device for tuning musical instruments. It vibrates at 440 Hz, a tone we know as concert A. It is the A above the middle C. Now this tuning fork is at its rest position. So it doesn’t vibrate and give any tones.
When we hit this tuning fork against something firm, it starts to vibrate. As the tuning fork vibrates forth, it displaces the air molecules around it and pushes them together. This movement changes the air pressure and forms an area of high air pressure. We call it a compression.
As it moves back, it pulls the air molecules apart. This movement changes the air pressure again and forms an area of low air pressure. We call it a rarefaction.
Of course, this vibration doesn’t happen once. The tuning fork will keep on vibrating for some time. Over time, this change propagates out and it affects all other molecules near these areas and forms other low and high-pressure areas. These alternating pressure areas form what we know as a sound wave.
In this figure, you can see the compression and rarefaction areas represented on a sine wave.
One thing to remember here is, when this vibration produces a sound wave, the air molecules don’t move from one place to another. That would be a wind. Instead, you should think of it as a Mexican wave in a stadium. People don’t run around, they stay in their place while forming the wave. Only the alternating pressure variations travel, not the air molecules.
What Kind of Wave Is a Sound Wave?
Because it needs a source and a medium, a sound wave is a mechanical wave. The energy in a sound wave spreads in the same direction as the wave. A sound wave is also a longitudinal wave where the disturbance moves parallel to the direction of the wave.
When we want to visualize sound waves, we usually draw them like sine waves. However, in reality, sound waves can take any form. It makes them difficult to analyze. It is difficult to see where one longitudinal wave ends and the next one begins. Also, the amplitude of a longitudinal wave is not obvious. It is a contrast between different pressure areas. It is why we often use sine curves (transverse) to represent sound waves.
If we want to record the pressure variations of a basic sound wave we can use an instrument called oscillograph.
In this figure, you can see the basic principle of an oscillograph. When the sound wave hits the diaphragm on the left, the arm draws a graph. It keeps on moving and drawing as long as the diaphragm vibrates. Notice that compression areas cause the arm to move upward, while the rarefaction areas cause it to move downward. While the diaphragm is at rest position, the arm doesn’t move.