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What is sound?

Photo to illustrate What is sound

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When we put on a hip tune, we quickly appear to enjoy it. This has everything to do with the functioning of the human ear, which converts the so-called mechanical energy of sound effects into electrochemical signals. Via hearing, the cerebral cortex converts the received sound into something we can perceive. Yet that is not yet the full explanation of the story. To really understand music, we need to know what sound is and this even before it is processed by our bodies.

Description of sound

The Great Van Dale defines sound as vibrations in the air perceived by the hearing organ. Several other dictionaries subscribe to that definition and thus establish a link between vibrations or a change in air pressure and its perception. According to them, therefore, what we cannot hear is not sound. Logically, say a dolphin will experience sound differently from a human: what is sound to a dolphin need not be sound to a human. At least that is how we can describe sound in its most narrow form.

From a scientific point of view, then, we can define sound much more broadly. Here we leave the egocentric slope of the human auditory organ and talk in the broader sense about the sound effects or vibrations that move through the air and can be picked up by a hearing organ. Finally, in an even broader view, we are not limited to the changes in air pressure, but can also refer to any other medium. Moreover, one can also use the term sound if it no longer refers to an audible change. In this way we also speak of "ultrasound".

How do these noise effects occur?

Sound waves are created by air vibrations. This is so because those subtle quivers locally compress air particles. This will increase the air pressure there, after which the compressed particles come into contact with particles in the environment. This creates a chain reaction in which the energy is continuously transmitted: the sound moves until it finally comes to rest again. Incidentally, this is immediately the reason why sound is an earthly phenomenon. If two satellites collide in space and allow themselves to be pulverized into thousands of pieces, no sound at all will be perceptible. After all, here there is no air or any other medium that can move the waves, let alone create sound waves....

This also explains the operation of a speaker, which is really just a component that is vibrated, or smartly so. These movements will cause the air around the speaker to move as well. You might best compare it to a pebble in water: it creates waves that move away from the source. As these circles get bigger, more and more air particles have to share the energy of the previous circle, so the sound eventually dies a silent death. It does so at a speed of about 300 m/s, but it is not illogical that the sound becomes weaker the farther you move from the sound source. If you move far enough away from the sound source, eventually the sound will no longer be audible. No longer audible to humans, because some animals simply hear much better than us.

What does a sound wave look like?

Seeing sound, we cannot. At least, not without technical aids. But, as the word actually indicates, a sound wave thus consists of a wave. Such a wave, in turn, has a wavelength and an amplitude. How often such a wave is passed through is then called the frequency. This frequency is not insignificant: for example, the human ear can hear a minimum frequency of twenty and a maximum frequency of 20,000 Hertz. With this, by the way, the wavelength is inversely proportional: with an increase in frequency, the wavelength will become shorter (higher tone) and vice versa.

The amplitude, in turn, results from the difference between the average value of the pressure and the maximum value of the pressure. We define the resulting strength in the better-known term decibels. This term is not insignificant either: from 120 decibels, hearing damage will occur with short-term exposure. However, the pain threshold is 134 decibels: which explains why hearing damage can also occur unnoticed.

Finally, we cited earlier that sound effects behave much like water waves. For example, they can curve around an object, bounce off, change direction or come into contact with other sound waves, then amplify or cancel each other out. The latter is also how specialized headphones work: they emit a sound to cancel out other sound effects.

Speed of sound is not a constant

Before this, we cited that the speed of sound is about 300 m/s. In fact, this is not quite correct. It is more or less correct for displacements in air, but sound can also travel through solids and liquids. For example, the speed of sound in aluminum is 6,260 m/s at 293 degrees Kelvin (+/- room temperature) and 1120 m/s in methanol at a similar temperature. In carbon dioxide, on the other hand, sound is much slower, travelling at 259 m/s at 273 degrees Kelvin (+/- freezing point).

Even in air, the statement that the speed of sound would be about 300 m/s is not entirely true. Indeed, here one uses a complicated formula to determine the speed of sound.

Formula speed sound:√[γ(RT/M)]

We will omit a detailed discussion, but above all, remember that temperature and humidity play an important role here. At a temperature of 233 degrees Kelvin (about - 40 °C), sound will indeed be about 307 m/s, but at a warmer 313 degrees Kelvin (about + 40 °C), it will already rise to a much higher 354 m/s. Thus, in air, sound will move faster if the temperature is warmer. In practice, however, sound travels so fast that after an avalanche accident that certainly need not be your biggest concern.

Finally, if we talk about the speed of sound in water, the formula becomes even more complicated. After all, here we have to take into account not only temperature, but also salinity and water depth.

Sound for humans and animals

To human hearing, sound vibrations are noticeable within a range of about 20 to 20,000 Hertz (20 kHz). However, this varies from person to person. The elderly in particular will find it more difficult to notice sound vibrations at high frequencies.

Wondering to what 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 limit with infrasonic sound. We cannot hear such infrasonic sound, but sometimes we can feel the vibrations. Infrasound is used by some animals, though. After all, it helps them communicate over a long distance. Elephants, rhinos and giraffes, among others, use infrasound for this reason. Yet it can also serve another purpose. Indeed, some species of whales use infrasound to paralyze their prey, mainly squid.

When the upper hearing threshold (20 kHz) is exceeded, we speak of ultrasonic sound. Some animal species use ultrasound to orient themselves (e.g., bats) or to detect prey (e.g., dolphins). Yet humans have also come up with a number of applications. One important ultrasound application is ultrasound (visualizing differences between soft and hard tissue via ultrasound). Another example can be found in dentistry (cleaning dental instruments and removing tartar). For another ultrasound application, however, we have to go back to the past: in the 1970s, for example, we used it to operate television sets.

Finally, there is a final category of sound: hypersonic sound. This involves sound with frequencies from 800 MHz and up. In fact, the term came v because for a long time humans were unable to generate such sound frequencies. Today, thanks to the piezoelectric effect, this is no longer an obstacle. Among other things, it is used to study solids.

The importance of noise level to humans and animals

Not only the sound frequency plays a role, but also the sound level (dB). In principle, a person can also hear a sound level between 0 and 130 dB, but this is not always the case. In that case, we speak of noise-related deafness. With a hearing loss of thirty to sixty dB, one will usually use a hearing aid. By comparison, the rustling of leaves has a sound level of approximately 10 dB, while a television at living room level (+/- 1 meter away) has a sound level of approximately 60 dB.

In addition to this, we must also consider the unpleasant nature of loud noises. Indeed, starting at 90 dB, hearing damage already occurs with prolonged exposure. This is approximately the sound you experience when standing next to a freeway. A disco, on the other hand, has noise levels that reach 100 dB, similar to the sound of a jackhammer one meter away. Here we are exceeding the maximum decibel for long-term exposure: so be sure to pay attention with festival weekends where you are exposed to high noise levels non-stop.

From a maximum decibel of 120 dB, hearing damage then occurs again with short-term exposure. This is approximately the sound of a jet engine at +/- 100 meters away. Finally, the pain threshold is 134 dB, lower than the sound we experience from a gunshot at one meter distance. In such cases, without protective devices, hearing damage will almost always occur.

Various applications of sound

Sound is used by humans not only to experience music. It is also very important for mutual communication, for example. Yet the number of (potential) applications are much more extensive than that. Among others, the ultrasound mentioned earlier demonstrates that. After all, if we know the speed of sound or other characteristics, we can determine a lot. Earlier we cited, for example, that we can calculate the speed of sound in water. Indeed, by sending a sound pulse under water and measuring the time until the pulse reflects, we can calculate the distance to an object or the sea floor. In other words, sound is more than a musical gift, a means of communication or a warning signal: it is an integral part of our contemporary technical knowledge.

The audible nature of sound

Finally, all this brings us back again to those two magical boundaries: 20 Hz and 20 kHz. After all, the sound between these two boundaries is the most "tangible" form of sound for us. It is the car horn honking, but it is also the Ninth Symphony in d minor. It is sound in its most narrow form: sound that is simply perceptible by humans. Ultimately, the question remains as to how we can now perceive that sound.

In fact, it is the auricle that collects the sound vibrations and carries them through the ear canal to the eardrum. In the eardrum, the sound is transmitted to the ossicles (hammer, anvil and stirrup), which act as amplifiers. Next, the fluid in the cochlea will move, causing the hair cells to follow suit as well. Finally, those movements excite the nerve endings with which there is a connection to the brainstem. The brain will then "translate" the incoming stimuli and send them to the thalamus. This part of the brain also processes the information from what we see or feel and will interpret the "translated information." Finally, this interpretation is transmitted to the cerebral cortex.

In the cerebral cortex, audible sound arrives at the end of its journey. This is where it acquires meaning. It links it to our memories. It helps us recognize sound: a mother's voice or the sound of a train approaching. Those memories can also trigger feelings. For example, hearing the voice of a deceased person can trigger dejection or happiness, even if we do not recognize the voice or dwell on it. Just as sound is miraculous, so is our brain. It is also precisely why we love sound so much and famous musicians can live rich lives.

Making more meaning out of sound is part of what makes people human.

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