What Is Sound? How It Works and Why We Hear It

When we put on a catchy tune, we tend to start enjoying it almost instantly. That has everything to do with the way the human ear works, converting the so-called mechanical energy of sound waves into electrochemical signals. Through the sense of hearing, the cerebral cortex turns the sound it receives into something we can perceive. Yet that still isn’t the full story. To truly understand music, we need to know what sound actually is, even before our body processes it.

A definition of sound

The Grote Van Dale dictionary describes sound as vibrations in the air that are perceived by the organ of hearing. Various other dictionaries support that definition and therefore establish a link between vibrations, or a change in air pressure, and the perception of them. According to that view, anything we cannot hear is not sound. Logically, a dolphin will experience sound differently than a human: what counts as sound for a dolphin doesn’t have to be sound for a human. That, at least, is how we can describe sound in its narrowest form.

From a scientific point of view, however, we can describe sound far more broadly. Here we leave behind the self-centred perspective of the human ear and speak, in the wider sense, of the sound waves, or the vibrations, that move through the air and can be picked up by an organ of hearing. In an even broader interpretation, we don’t limit ourselves to changes in air pressure, because it can just as easily involve another medium. On top of that, the term sound can also be used when it no longer concerns an audible change. This is why we also speak of “ultrasound”.

How do those sound waves come about?

Sound waves arise from air vibrations. This happens because those subtle tremors locally compress the air particles. As a result, the air pressure rises in that spot, after which the compressed particles come into contact with the particles around them. This creates a chain reaction in which the energy is continuously passed along: the sound travels until it finally comes to rest again. Incidentally, this is precisely why sound is an earthly phenomenon. If two satellites in space were to collide and shatter into thousands of pieces, no sound at all would be perceptible. After all, there is no air or other medium there to carry the waves, let alone for sound waves to form in the first place…

This also explains how a speaker works. A speaker is really just a component that is made to vibrate, albeit in a clever way. Those movements cause the air around the speaker to be set in motion as well. Perhaps the best comparison is a pebble dropped into water: ripples form and move away from the source. As those circles grow larger, more and more air particles have to share the energy of the previous circle, which is why the sound eventually fades into silence. It does so at a speed of roughly 300 m/s, but it stands to reason that the sound becomes weaker the further you move away from the source. If you stand far enough from the sound source, the sound will ultimately no longer be audible. No longer audible to humans, that is, because some animals simply hear far better than we do.

What does a sound wave look like?

We cannot see sound. At least, not without technical aids. But, as the word itself already suggests, a sound wave consists of a wave. Such a wave, in turn, has a wavelength and an amplitude. How often a wave completes a cycle is what we then call the frequency. That frequency is no small matter: the human ear, for example, can hear a minimum frequency of twenty and a maximum frequency of 20,000 hertz. The wavelength, by the way, is inversely proportional to this: as the frequency rises, the wavelength becomes shorter (a higher tone), and vice versa.

The amplitude, in turn, is the result of the difference between the average value of the pressure and the maximum value of the pressure. We define the resulting intensity using the more familiar term decibel. That term, too, is far from trivial: from 120 decibels onward, even short-term exposure will cause hearing damage. The pain threshold, however, lies at 134 decibels, which explains why hearing damage can also occur unnoticed.

Finally, we mentioned earlier that sound waves behave much like water waves. They can bend around an object, bounce off, change direction, or come into contact with other sound waves, after which they either reinforce or cancel each other out. That last effect is also how specialised headphones work: they emit a sound to cancel out other sound waves.

The speed of sound is not a constant

Earlier we mentioned that the speed of sound is about 300 m/s. In fact, that isn’t entirely accurate. It is more or less correct for travel through air, but sound can also move through solids and liquids. The speed of sound in aluminium, for instance, is 6,260 m/s at 293 kelvin (roughly room temperature), and 1,120 m/s in methanol at a similar temperature. In carbon dioxide, by contrast, sound is quite a bit slower, with a speed of 259 m/s at 273 kelvin (roughly freezing point).

Even in air, the claim that the speed of sound is about 300 m/s isn’t entirely true. Here, a complex formula is used to determine the speed of sound.

Formula for the speed of sound:**√[γ(RT/M)]

We’ll leave a detailed discussion aside, but the main thing to remember is that temperature and humidity play an important role here. At a temperature of 233 kelvin (about −40 °C) the sound will indeed be roughly 307 m/s, but at a warmer 313 kelvin (about +40 °C) that already climbs to a much higher 354 m/s. In air, then, sound travels faster when the temperature is warmer. In practice, though, sound travels so fast that, after an avalanche, it certainly won’t be your biggest concern.

Finally, when it comes to the speed of sound in water, the formula becomes a good deal more complex still. Here we have to take into account not only the temperature, but also the salinity and the water depth.

Sound for humans and animals

For human hearing, sound vibrations are noticeable within a range of roughly 20 to 20,000 hertz (20 kHz). However, this varies from person to person. Older people in particular will have more difficulty detecting sound vibrations at high frequencies.

Curious about the highest frequency you can perceive? Play the video below and find out.

The importance of frequency for humans and animals

We know the lower hearing limit of 20 Hz mainly as the boundary with infrasound. We cannot hear such infrasound, but sometimes we can feel the vibrations. Infrasound is, however, used by a number of animals. After all, it helps them communicate over long distances. Elephants, rhinos and giraffes, among others, use infrasound for this reason. Yet it can serve another purpose too. Some species of whale use infrasound to stun their prey, mainly squid.

When the upper hearing limit (20 kHz) is exceeded, we speak of ultrasound. Some animal species use ultrasound to orient themselves (e.g. bats) or to track down prey (e.g. dolphins). Humans, however, have also devised a number of applications. One important application of ultrasound is the ultrasound scan (using ultrasound to make differences between soft and hard tissue visible). Another example can be found in dentistry (cleaning dental instruments and removing tartar). For yet another application of ultrasound, though, we have to look back to the past: in the 1970s, for example, it was used to operate television sets.

Finally, there is one last category of sound: hypersound. This involves sound with frequencies from 800 MHz onward. The concept really came into being because, for a long time, humans were unable to generate such sound frequencies. Thanks to the piezoelectric effect, that is no longer an obstacle today. Among other things, it is used to study solids.

The importance of sound level for humans and animals

Not only sound frequency plays a role, but also the sound level (dB). In principle, a human can hear a sound level between 0 and 130 dB, but that isn’t always the case. When it isn’t, we speak of noise-induced deafness. With a hearing loss of thirty to sixty dB, a hearing aid is usually used. For comparison: the rustling of leaves has a sound level of about 10 dB, while a television at living-room volume (about 1 metre away) has a sound level of roughly 60 dB.

Alongside this, we also have to consider the unpleasant nature of loud sounds. From 90 dB onward, hearing damage already occurs with prolonged exposure. That is roughly the sound you experience when standing next to a motorway. A nightclub, in turn, has sound levels climbing up to 100 dB, comparable to the sound of a jackhammer one metre away. Here we exceed the maximum decibel level for prolonged exposure, so be sure to be careful with festival weekends where you are exposed to high sound levels non-stop.

From a maximum of 120 dB onward, hearing damage occurs even with short-term exposure. That is roughly the sound of a jet engine about 100 metres away. The pain threshold, finally, lies at 134 dB, lower than the sound we experience from a rifle shot one metre away. In such cases, without protective equipment, hearing damage will almost always occur.

Various applications of sound

Humans don’t only use sound to experience music. It is also extremely important for communicating with one another, for instance. Yet the number of (potential) applications is far more extensive than that. The ultrasound scan mentioned earlier demonstrates this, among other things. If we know the speed of sound or other properties, we can determine a great deal. Earlier, for example, we noted that we can calculate the speed of sound in water. By sending out a sound pulse underwater and measuring the time until the pulse reflects, we can calculate the distance to an object or to the seabed. In other words, sound is more than a musical gift, a means of communication or a warning signal: it forms an integral part of our present-day technical knowledge.

The audible nature of sound

All of this finally brings us back to those two magical limits: 20 Hz and 20 kHz. The sound between these two limits is, after all, the most ‘tangible’ form of sound for us. It is the honking of a car, but it is also the Ninth Symphony in D minor. It is sound in its narrowest form: sound that is simply perceptible to humans. That ultimately leaves the question of how we are able to perceive that sound.

It is in fact the auricle that captures the sound vibrations and carries them through the ear canal to the eardrum. At the eardrum, the sound is transferred to the ossicles (hammer, anvil and stirrup), which act as an amplifier. The fluid in the cochlea will then move, prompting the hair cells to follow suit. Those movements finally stimulate the nerve endings that are connected to the brainstem. The brain then “translates” the incoming stimuli and sends them to the thalamus. This part of the brain also processes the information from what we see or feel and interprets the “translated information”. This interpretation is finally passed on to the cerebral cortex.

In the cerebral cortex, the audible sound arrives at the end of its journey. Here it is given meaning. It links the sound to our memories. It helps us recognise sound: a mother’s voice or the sound of an approaching train. Those memories can also evoke feelings. Hearing the voice of someone who has passed away, for example, can be a source of sadness or happiness, even if we don’t recognise the voice or aren’t consciously aware of it. Just as sound is wondrous, so too is our brain. And that is precisely why we love sound so much, and why famous musicians can lead such lavish lives.

Giving more meaning to sound is part of what makes humans human.