Sound wave. An easy way to understand what a sound wave is

A sound wave can be considered from two points of view. The physical characteristics of sound refer to its nature, and the psychoacoustic characteristics refer to how sound waves are perceived. And we want to emphasize that sound is sensed differently depending on the specie listening. In this post, we are going to talk about some basic concepts related to sound, like the definition of sound, sound wave, pressure wave, compression and rarefaction. We are going to describe them from the physical characteristic point of view. We will also explain what type of wave is a sound wave.

What is sound?

A sound is essentially a form of energy, acoustic energy. This type of energy differs from others, such as electricity or light, in that the vibration of materials produces this energy. Therefore, every time the membrane of a drum is struck, an object falls to the floor, or a person speaks, vibrations are produced in these materials. In the case of people, in the oral cords. These vibrations are transmitted through the air to our ears.

How are sound wave vibrations transmitted?

For sound vibrations to propagate, in other words, to travel from one point to all directions, a medium (liquid, gaseous or solid) is required. This means that sound cannot travel through a vacuum, such as outer space. Waves that require a medium to propagate are known as mechanical waves; those that can propagate in a vacuum like light are called electromagnetic waves.

The reason why sound requires a medium to travel is that the acoustic energy is transmitted to the particles that are next to the vibrating object. When the object vibrates, it transmits the energy which pushes the particles around it. These particles transmit the energy and push the following particles and return to their place. And the following particles transmit the energy and push the next ones and return to their place; and so on.

What is vibration?

Vibrate is when a body has small and rapid movements around its natural point of equilibrium. In other words, it moves quickly and with little amplitude.

Therefore, the sound vibrations push the surrounding particles again and again and again. These internal movements of the particles in a medium (liquid, gaseous or solid), generate pressure changes in that medium. We are talking about pressure changes in a small area where the particle moves and returns to its equilibrium point. Therefore, what is sound?

Sound definition

Sound is a type of energy created by a vibrating object that propagates as a pressure wave, which means that the acoustic energy travels by variating the pressure in a material medium.

What is a sound wave?

Sound waves transmit energy. They do not transfer matter

In a wave, energy is propagated, not matter. The energy that displaces matter does not fall into the wave classification. For example, we call impact energy the energy that causes the bullet to move from one place to another when a gun is fired. In physics, the word wave refers to events in which energy is transmitted from one place to another without displacement of matter. In the case of sound, as previously explained, when you talk to a friend, pressure changes are generated within the material or medium in which the sound travels; in this example, the environment is air. But when you talk, there is no particle displacement from your mouth to your friend’s ear; only energy, and vibrations, are displaced.

That is why physicists usually define a wave as a disturbance that travels from one point to another through a medium without the transport of matter; because the wave is something that breaks the state of equilibrium in which the medium was, but once it passes through that medium, it leaves it in its initial state.

Sound wave definition

In conclusion, a sound wave is a type of wave generated by a vibrating force that travels through a medium.

What type of wave is a sound wave?

That depends on how you want to classify the wave. As we said before, a sound wave is a mechanical wave. But from another point of view, it is generally considered a longitudinal wave.

Longitudinal wave

Typically, sound travels as a longitudinal wave, which are waves where the particles vibrate in the same direction the wave is moving. When sound travels in liquids such as the ocean, in a gas such as air or plasma such as the Earth’s ionosphere, it travels as a longitudinal wave, also known as a compressional wave.

However, in December 2021, a team of researchers at the City University of Hong Kong (CityU) discovered what they call the “sound wave of air,” which, to the amazement of many, vibrates transversely and carries both spin and orbital angular momentum as light does. They hope to use this technology to encode data in the future using transverse sound.

Transverse wave

A transverse wave is one that oscillates perpendicular to the direction in which the wave travels.

Sound pressure

A pressure wave, also called a compression wave or longitudinal wave is one in which the disturbance force is a variation of pressure in a material medium. It is a repeating pattern of high-pressure and low-pressure zones called compression and rarefaction.

Let’s look at how sound waves are created when an object vibrates. It initially pushes nearby particles causing them to collide as energy is transferred from one particle to another.

As you may have noticed, the sound source, the blow on the drum’s membrane, caused the air particles to move in a back-and-forth motion that stops the instant the sound source stops vibrating.

When the drum stops vibrating, the air particles around it return to their state of rest or initial position.

What is compression?

We speak of compression when a movement displaces particles that are at rest and pushes them together with the following particles. When this occurs, the density of the medium in that zone is temporarily increased. In other words, the mass of particles in that volumetric space increases.

Compression is the increase of density in a medium generated when the sound wave generates a force that causes the particles of the medium to move toward each other, creating a high-pressure zone.

What is rarefaction?

We know that when an object vibrates, it moves back and forth. As we have just seen, when the object moves in the direction of the surrounding particles, it generates the compression of the medium. When it moves in the reverse direction, it decreases the pressure of the zone and reduces the particle density in that zone.

Rarefaction is the decrease of density in a medium generated when the force of the sound wave withdraws from the medium. When the wave retreats, it causes some particles to separate from others producing a low-pressure zone.

In the following graph, you can see the relationship between the motion of a longitudinal sound wave and its graphical representation.

Consequently, in a compression zone, the pressure is higher than the normal atmospheric pressure of the medium in a resting state. And in a rarefaction zone, the pressure is less than the normal atmospheric pressure in a state of rest.

Combining sound waves

Finally, we want to talk about a popular definition of sound. The word sound is so generic that when a person says to you, “do you hear that sound?” The “sound” they are referring to can be a piece of music, which is, in fact, a combination of multiple sound waves.

A very important characteristic of the waves is that they can pass through each other undisturbed or, when they collide, they can add up to a wave of greater amplitude or lesser amplitude. Therefore, what you identify as a sound can be a combination of one or more sound waves, generating a more complex sound wave with its unique characteristics.

This phenomenon is so common that it is considered unlikely that you will ever hear an isolated sound wave. You will always listen to combinations of sound waves. In order to hear an isolated sound wave, you would have to create it electronically or digitally and isolate yourself acoustically so that no sound combines with the isolated sound wave you created.