Soundwaves are produced whenever any material vibrates. The vibration of the material causes the surrounding air, water, or other media to vibrate as well. Sound is the propagation of that vibration through a medium like air.
A sound is not a thing that moves through space. Instead, sound waves are a ripple effect of vibration that is transferred from one vibrating object to the objects nearby until it reaches your ears. When the wave is finally transferred to the bones in your middle ear, your brain interprets it as a sound.
How do Soundwaves Work?
Transverse and Longitudinal Waves
Before diving into the details of soundwaves, let’s take a look at the fundamentals of waves in general. There are two types of waves that are distinguished by how the vibrations of the wave propagate through space.
A transverse wave is one where the vibration of the particles involved moves out perpendicularly from the line along which the wave is traveling. Electromagnetic waves are examples of transverse waves.
A standard demonstration of a transverse wave is for two people to hold a rope between them, each person holding on to one end of the rope. When one of the people moves their end, the motion propagates down the rope from one end to the other. This movement propagates along the length of the rope, but the motion is perpendicular to the centerline.
Sound is a different kind of wave called a longitudinal wave. In a longitudinal wave, displacement of the medium (the stuff through which the wave travels) is parallel to the wave’s propagation. This means that, as the wave moves along through whatever material is carrying it, its vibrations push the material out along the same line.
To visualize a longitudinal wave, think of a line of bowling balls. If you give the first bowling ball a little push, it rolls and bumps into the next ball, which in turn goes and crashes into the ball after it. The wave of motion moves along the line of bowling balls, and the direction that balls move when they’re touched by the wave is along that same line. Soundwaves work in the same way.
The source of the soundwave vibrates and displaces the surrounding molecules of air (or whatever) away from it in a straight line. Each of those molecules, in turn, bumps into the molecule next to it and causes it to move away in a straight line just like tiny bowling balls. The soundwave moves out from the source in a straight line, and the vibrations move out from the wavefront in the same direction.
What is the Wavelength of a Soundwave?
Any wave, whether it’s a soundwave, lightwave, or an ocean wave, has a wavelength. The wavelength is the measurement between one point in the wave, and the next adjacent position in the wave where the intensity or magnitude is the same.
The most common way to represent a wave visually is to draw a curving graph that slopes up and down in a repeating pattern. The highest point in the curve represents the highest intensity of the wave, and the lowest point represents the lowest intensity. On this kind of graph, the wavelength is usually measured between the peaks of adjacent curves.
What is the Frequency of a Soundwave?
Soundwaves work by physically moving the material of whatever medium they travel through. This motion is set up by the vibration of the source. It propagates out from the source as areas of higher and lower density in the medium. Molecules in regions of the wave with a higher density are pushed together and create a higher-than-normal pressure area in the medium. The particles in the part of the wave with lower density are pulled apart and create an area of lower-than-normal pressure.
As the sound’s source material vibrates, the particles in the surrounding air, water, or whatever medium are pushed back and forth repeatedly. The soundwave is the areas of high and low pressure created by the back and forth movement of particles in the medium as they’re pushed and pulled by the source.
To make things simpler, let’s just assume our medium is air, and the source of the soundwave is a speaker.
So our speaker vibrates and pushes the surrounding air first forward and then back. Each time this vibration takes place, it creates an area of dense particles and an area of less-dense particles that propagates out from the speaker.
The frequency of a soundwave is a measurement of how quickly the cycle between dense and less-dense air takes place. Or, in other words, the number of dense and less-dense areas that are created in a given amount of time.
The count of vibrations that occur in a given time is a measurement of the frequency of the soundwave.
How to Find the Frequency of a Soundwave
As the source of the sound vibrates, it moves particles of air back and forth. A soundwave’s frequency is measured as the number of times in a second that the particles moved by the soundwave vibrate back and forth. This number is measured in Hertz (Hz), with 1Hz equal to 1 vibration in a second.
The frequency of a soundwave is uniform across the entire length of the wave. To understand why you can think back to our bowling ball example.
When you push the first bowling ball in the line a certain distance, let’s say 1 centimeter, it slams into the ball next to it and pushes that ball the same distance. No matter how far you push the ball, all the other balls will have to move the same distance to get out of its way.
This is the same thing that happens in a sound wave. When the source material pushes, the surrounding molecules are moved away and push the adjacent particles the same distance. When you measure the distance between any two high or low-density areas in the wave, it will be the same as between any other two regions of the same density.
How is Pitch Different from Frequency?
The pitch of a sound is a description of the sensation of the soundwave’s frequency. Where frequency is a physical measurement of the wave itself, pitch refers to our brain’s interpretation of that measurement as information.
The higher the frequency of a wave, the higher the pitch of the sound you’ll hear. The average human is capable of detecting frequencies between 20Hz (20 vibrations per second) and 20,000Hz (20,000 vibrations per second). The human voice vibrates at a frequency of about 85Hz to 255Hz.
Why are Some Sounds Louder than Others?
Where frequency and pitch are measurements of the rate of vibration in a soundwave, amplitude refers to how much energy goes into creating that vibration. The higher the amplitude of a soundwave, the louder your perception of the sound will be when the wave reaches your ear.
Amplitude can be measured by looking at the amount of energy going into each cycle of the vibration of a soundwave. The farther a molecule is displaced from its original position, the higher the wave’s amplitude. The higher the wave’s amplitude, the louder the sound.
What is the Speed of Sound?
You’ve probably heard people talk a lot about the speed of sound in the context of airplanes, superheroes, and other fast-moving objects.
“Faster than the speed of sound” sounds like an impressive velocity. The problem is, you need to know a lot about the material the sound is traveling through to determine its speed accurately.
If you match your velocity to the speed of sound as it travels in a chunk of metal that’s been cooled to absolute zero, you would be getting nowhere fast. That’s because, at absolute zero, the speed of sound is 0 meters per second.
To get a clearer understanding of the speed of sound, let’s take a look at how a soundwave reacts to different material properties.
What is Speed?
Before we can explore the speed of sound, we first need to be clear about what we mean by “speed.” Physicists define speed as the distance an object travels divided by the time it takes to travel that distance.
Speed = Distance/Time
When you measure the speed of a soundwave, you’re looking at how far the vibration of the sound’s source propagates through a medium in a given amount of time.
Sound Moves at Different Speeds Through Different Materials
The speed of sound is the rate at which the vibration of the sound’s source propagates through the medium it’s traveling through. Since different media have different properties that cause waves to propagate at varying rates, it’s impossible to state a universal “speed of sound.” Instead, we can see that the speed that a sound moves depends on the material through which it’s moving.
The two factors that most affect how sound travels through a given material are the properties of elasticity and density.
Soundwave Propagation and Elasticity
Elasticity at the molecular level is the measure of a material’s ability to recover its original shape after a force is applied. A material with low elasticity will be permanently deformed when a powerful force is applied. A material with very high elasticity might not change shape at all under that same force.
A soundwave will move through a highly elastic material much more quickly than through one that is inelastic.
Soundwave Propagation and Density
The density of a material is essentially a measure of how much energy it takes to move it. The denser the medium, the more energy it takes to move the particles in the material.
In a dense material, soundwaves will have a harder time propagating because it takes more force to push the molecules around. For this reason, sound moves more slowly through a dense material than it does through a light material.
The density of a medium can be affected by factors other than the material its made from. For example, the temperature of the material will affect its density with hotter temperatures causing a decrease in density and a higher speed of sound propagation.
How to Determine the Speed of Sound in a Material
Materials science is a vast and fascinating discipline, but it’s way beyond the scope of this article. Suffice it to say that understanding the properties of the medium through which a soundwave is passing is complicated stuff that takes a lot of training and research. The basic principle behind determining how fast a soundwave will propagate through a given material looks something like this:
Speed of Sound = (Elasticity / Density)
This simple formula expresses the relationship between the molecular density of the medium, its elasticity, and how easy it is for vibrations to propagate through it.
What is the Speed of Sound Traveling Through the Air?
Most people, when they talk about the speed of sound, probably mean the speed of sound in air. It’s rare for anyone but engineers and physicists to worry too much about how fast sound will move through lead, for example.
A rough estimate for the speed of sound in air is 343 meters per second or 767 miles per hour. However, just like with any other medium, the properties of the air determine the speed that waves move through it. These properties can change depending on environmental conditions like temperature and atmospheric pressure.
How Sound Bounces Off or Passes Through an Object
When a soundwave reaches the boundary of a medium, some portion of the wave’s energy is transmitted into the new medium. The energy that isn’t transmitted reflects back into the current medium.
How a sound wave reacts to the boundary between two mediums is determined by the properties of the materials involved. The more similar the two mediums, the more energy will be reflected, and the less will be transmitted.
The closer the density and elasticity of the new medium are to those of the current medium, the more energy will be transmitted and continue to propagate forward into the new medium. A significant difference in elasticity and/or density of the two mediums will cause more energy to be reflected and travel back through the original medium.
This principle is used in making materials for soundproofing. When you want to stop the transmission of soundwaves at the border between two materials, your best bet is to make the materials as different from one another as possible.
The diffraction of sound waves allows them to travel around and through obstacles.
When a low-frequency soundwave encounters an obstacle that’s smaller than its wavelength, it can bend around the obstacle and still be detected on the other side.
Lower-frequencies (longer wavelengths) of sound are more capable of surviving encounters with obstacles. This is because the long wavelengths can overcome the relatively minor interrupting presence of small hindrances.
Diffraction also occurs when a soundwave reaches a partial boundary with a small opening. The wave diffracts through the hole in the barrier and propagates on the other side.
This bending effect is part of what makes it possible to hear a sound source from around a corner. When a soundwave passes by an object, the wave bends around the object and continues to propagate. The combination of diffraction and the waves reflecting off of surfaces allows sounds to travel through varied landscapes.
When a sound wave passes from one medium into another, or when the properties of the medium it’s passing through change, the wave will undergo refraction. Refraction can alter the frequency as well as the direction of a soundwave.
When a wave propagating through a medium encounters another medium with different properties, the part of the wavefront that hits first will either speed up or slow down depending on the properties of the new medium.
The frequency changes because, when a soundwave moves from a low-density to a high-density medium, the wavefront will slow down as it struggles to move the more substantial material. The wavefront slows, and the parts of the wave behind the front catch up a bit. As the different areas in the wave pile up, the frequency of the wave increases.
The direction is changed because different parts of the wavefront reach the new material at different times.
Imagine the leading edge of the wave as a line of people walking hand-in-hand. Now imagine that the person on one end of the line suddenly encounters tougher terrain. The person on the right slows down while the rest of the line continues walking at the same pace for a moment until they, too, come to the new terrain. The effect is that the line will be distorted and will turn in a different direction. The same thing will happen when different parts of the soundwave encounter obstacles.