What Is Sound? How Sound Waves, Frequency and Decibels Work

Music, a voice, a door slamming: we hear sound all day long without giving it a second thought. But what actually is sound? In short: a vibration that travels through the air as a wave. Your ear picks up that vibration and your brain turns it into sound.

In this article you will learn how sound is created, what frequency and decibels mean, how fast sound travels and what the difference is between infrasound and ultrasound.

What is sound?

According to the dictionary, sound is a vibration in the air that you perceive with your ears. Physicists take a broader view: to them, sound is any pressure wave that travels through a medium. That can be air, but also water or a solid, such as a wall. And it doesn’t have to be audible. Sound that is too low or too high for our ears counts too.

So what one creature hears, another may not. A dog or a bat hears tones that are inaudible to us. For those animals, it is simply sound.

How is sound created?

Sound always starts with something that vibrates: a guitar string, your vocal cords or the cone of a speaker. Every time the source moves forward, it pushes the air particles in front of it together. The air pressure there briefly rises, which is called a compression. When the source moves back, an area of lower pressure forms: a rarefaction.

The compressed particles pass their push on to the particles next to them, and those pass it on to the next ones. This is how the vibration travels as a wave, while the air itself largely stays in place. You can compare it to a pebble dropped in water: the ripples move away from the spot where the pebble fell, but the water doesn’t travel with them.

The further the wave spreads, the more air particles have to share the energy. That is why sound gets quieter the further you are from the source, until at some point you can’t hear it any more.

And because sound needs a medium to travel through, there is no sound in space. If two satellites collide in a vacuum, it happens in complete silence.

What does a sound wave look like?

You can’t see sound, but you can draw it. If you plot the air pressure at one point against time, you get a wave. The peaks are the compressions, the troughs the rarefactions.

Two properties of that wave determine what you hear: how often it vibrates, and how big the pressure differences are.

Frequency: high or low (Hz)

The number of vibrations per second is called the frequency, and you measure it in hertz (Hz). A wave that moves back and forth a thousand times per second has a frequency of 1,000 Hz. The higher the frequency, the higher the tone.

Frequency and wavelength are linked: the higher the tone, the shorter the wave. A low tone of 20 Hz has a wavelength in air of about 17 metres, a high tone of 20,000 Hz less than 2 centimetres.

A healthy human ear hears roughly from 20 to 20,000 Hz. That varies from person to person. With age, it is mainly the high tones that decline, and the same goes for noise damage. Your ear is most sensitive to mid-range tones, around 1,000 to 5,000 Hz. That is also where the sounds that make speech understandable are.

Loudness: loud or quiet (dB)

The size of the pressure differences in the wave is called the amplitude. The bigger the amplitude, the louder you hear the sound. You express that loudness in decibels (dB).

The decibel scale is logarithmic. Every 10 dB more is ten times as much sound energy, and sounds roughly twice as loud to your ear. Every 3 dB more already doubles the energy. A few examples:

From about 80 dB, sound can damage your hearing with long exposure, and above 100 dB that happens within minutes. Worth knowing: the pain threshold is much higher. So sound can damage your hearing long before it hurts. How long you can safely be around which sound is explained in how many decibels is harmful.

How fast does sound travel?

In air at 20 °C, sound travels about 343 metres per second, more than 1,200 kilometres per hour. You notice this during a thunderstorm: you see the lightning straight away, but the thunder comes later. Count the seconds and divide by three, and you know roughly how many kilometres away the storm is.

That speed is not a fixed value. Sound travels faster in warm air than in cold air: at -40 °C it is about 306 metres per second, at +40 °C around 355. In liquids and solids, sound travels much faster still, because the particles are closer together. In water it is about 1,480 metres per second, in steel around 5,900.

In other respects, sound waves behave just like water waves. They can bounce off a wall, which you hear as an echo or reverberation. They bend around corners, which is why you can hear someone talking around the corner. And two waves can reinforce or cancel each other out. Noise-cancelling headphones use that last effect: they create an opposing sound that cancels out the ambient noise.

Infrasound and ultrasound

Not all sound is audible to us. Below 20 Hz we speak of infrasound. You can’t hear it, but at high levels you can sometimes feel the vibrations, for example from heavy traffic or a big bass. Elephants use infrasound to communicate over long distances, because low tones carry far.

Above 20,000 Hz it is called ultrasound. Bats and dolphins use it to find their way and track down prey: they send out a sound and listen for the echo. People use ultrasound too, for example for an ultrasound scan in hospital and at the dentist to remove tartar. And with sonar, ships measure the distance to the seabed by sending out a sound pulse and timing how long the echo takes.

From sound wave to hearing

Everything we hear lies between 20 and 20,000 Hz: from a car horn to a symphony. How your ear picks up those vibrations and how your brain turns them into sound, speech and music is explained in our article on how hearing works.

Too much sound can damage that hearing for good. Want to know how to prevent it? Read our tips to prevent hearing damage, or see how custom earplugs reduce the sound while you still hear everything.