SOUND ENERGY: CHARACTERISTICS, TYPES, USES, ADVANTAGES, EXAMPLES - PHYSICAL - 2025

The sound energy or acoustic that is carrying the sound waves as they propagate in a medium, which may be a gas such as air, a liquid or a solid. Humans and many animals use acoustic energy to interact with the environment.

For this they have specialized organs, for example the vocal cords, capable of producing vibrations. These vibrations are transported in the air to reach other specialized organs in charge of their interpretation.

Acoustic energy is translated into music through the sound of the clarinet. Source: Pixabay

The vibrations cause successive compressions and expansions in the air or the medium that surrounds the source, which propagate with a certain speed. It is not the particles that travel, but they simply oscillate with respect to their equilibrium position. The disturbance is what is transmitted.

Now, as is known, objects that move have energy. Thus, the waves as they travel in the medium also carry with them the energy associated with the movement of the particles (kinetic energy), and also the energy that said medium intrinsically possesses, known as potential energy.

characteristics

As is known, objects that move have energy. Likewise, the waves as they travel in the medium, carry with them the energy associated with the movement of the particles (kinetic energy) and also the energy of deformation of the medium or potential energy.

Assuming a very small portion of the medium, which can be air, each particle with velocity u, has kinetic energy K given by:

In addition, the particle has potential energy U that depends on the change in volume it experiences, where Vo is the initial volume, V is the final volume and p is the pressure, which depends on position and time:

The negative sign indicates an increase in potential energy, since the propagating wave does work on the volume element dV when compressing it, thanks to a positive acoustic pressure.

The mass of the fluid element in terms of the initial density ρ _o and the initial volume V _o is:

And how the mass is conserved (principle of conservation of mass):

Therefore the total energy is like this:

Calculating potential energy

The integral can be solved using the principle of conservation of mass

The derivative of a constant is 0, so (ρ V) '= 0. Therefore:

Isaac Newton determined that:

(dp / dρ) = c ²

Where c represents the speed of sound in the fluid in question. By substituting the above in the integral, the potential energy of the medium is obtained:

If A _p and A _v are the amplitudes of the pressure wave and velocity respectively, the average energy ε of the sound wave is:

Sound can be characterized by a quantity called intensity.

The intensity of sound is defined as the energy that passes in one second through the unit area that is perpendicular to the direction of sound propagation.

Since the energy per unit time is the power P, the intensity of sound I can be expressed as:

Each type of sound wave has a characteristic frequency and carries a certain energy. All this determines its acoustic behavior. As sound is so important to human life, the types of sounds are classified into three large groups, according to the range of frequencies audible to humans:

- Infrasound, whose frequency is less than 20 Hz.

- Audible spectrum, with frequencies ranging from 20 Hz to 20,000 Hz.

- Ultrasound, with frequencies greater than 20,000 Hz.

The pitch of a sound, that is, whether it is high, low or medium, depends on the frequency. The lower frequencies are interpreted as bass sounds, approximately between 20 and 400 Hz.

Frequencies between 400 and 1600 Hz are considered midtones, while highs range from 1600 to 20,000 Hz. High-pitched sounds are light and piercing, while bass is perceived as deeper and booming.

The sounds you hear every day are complex overlays of sounds with various frequencies in close proximity.

Sound has qualities other than frequency, which can serve as criteria for its classification. Examples of them are timbre, duration and intensity.

The equalizer consists of filters that remove noise and boost certain frequencies to improve sound quality. Source: Pixabay.

Noise

It is also important to make the distinction between wanted sounds and unwanted sounds or noise. Since noise is always sought to be eliminated, it is classified according to intensity and period in:

- Continuous noise.

- Fluctuating noise.

- Impulsive noise.

Or by colors, linked to their frequency:

- Pink noise (similar to a "shhhhhh").

- White noise (similar to a "psssssss").

- Brown noise (by Robert Brown, the discoverer of Brownian motion, is a noise that greatly favors low frequencies).

Applications

The use that is given to acoustic energy depends on the type of sound wave used. In the range of audible waves, the universal use of sound is to allow close communication, not only between people, since animals also communicate by emitting sounds.

The sounds are versatile. Each differs according to the source that emits it. In this way the variety of sounds in nature is infinite: each human voice is different, as well as the characteristic sounds that animal species use to communicate with each other.

Many animals use the energy of sound to locate themselves in space and also to capture their prey. They emit acoustic signals and have receptor organs that analyze the reflected signals. In this way they obtain information about distances.

Human beings lack the organs necessary to use sonic energy in this way. However, they have created orientation devices such as sonar, based on these same principles, to facilitate navigation.

On the other hand, ultrasounds are sound waves whose applications are well known. In medicine they are used to obtain images of the interior of the human body. They are also part of the treatment of some conditions such as lumbago and tendonitis.

Some applications of acoustic energy

- With high-energy ultrasound, stones or calculi that form in the kidneys and gallbladder can be destroyed due to the precipitation of mineral salts in these organs.

- In geophysics, ultrasound is used as prospecting methods. Its principles are similar to those of seismic methods. They can be used in applications ranging from determining the shape of the ocean to relief to calculating elastic moduli.

- In food technology they are used to eliminate microorganisms resistant to high temperatures, as well as to improve some textures and qualities of food.

Advantage

Acoustic energy has advantages that are largely due to its short range. For example, it is inexpensive to produce and does not generate chemical or other waste, as it dissipates in the medium quickly.

As for the sources of acoustic energy, they are numerous. Any object capable of vibrating can become a source of sound.

When used in medical applications, for example ultrasound imaging, it has the advantage of not using ionizing radiation, such as x-rays or tomography. It is a fact that ionizing radiation can cause cell damage.

Its use does not require the protection measures that are needed when ionizing radiation is applied. The kits are also cheaper.

Likewise, ultrasonic energy is a non-invasive method to eliminate the aforementioned kidney and gallstones, thus avoiding surgical procedures.

In principle it does not generate pollution neither in the air nor in the waters. But it is known that there is noise pollution in the seas, caused by human activities such as intensive fishing, geophysical prospecting and transportation.

Disadvantages

It is difficult to think about the disadvantages that a phenomenon as natural as sound can have.

One of the few is that loud sounds can damage the structure of the eardrum, and over time make continuously exposed people lose their sensation.

Very noisy environments end up causing stress and discomfort in people. Another disadvantage is perhaps the fact that acoustic energy is not used to move objects, making it very difficult to take advantage of vibrations to affect solid objects.

This is because sound always requires the existence of a medium to be able to propagate, and therefore it is easily attenuated. In other words, sound energy is absorbed in the medium more quickly than that of other types of waves, for example electromagnetic ones.

For this reason the energy of sound waves is relatively short-range in air. Sound is absorbed by structures and objects as it propagates, and its energy gradually dissipates into heat.

Of course, this is related to the conservation of energy: energy is not destroyed but changes form. The vibrations of the molecules in the air are not only transformed into pressure changes that give rise to sound. Vibrations also give rise to heat.

Sound absorption in materials

When sound waves hit a material like a brick wall, for example, a part of the energy is reflected. Another part is dissipated in heat, thanks to the molecular vibration of both the air and the material; and finally the remaining fraction passes through the material.

Thus, sound waves can be reflected in the same way that light does. The reflection of sound is known as "echo". The more rigid and uniform the surface, the greater the ability to reflect.

In fact, there are surfaces that are capable of producing multiple reflections called reverberations. Usually this occurs in small spaces and is avoided by placing insulating material, so that in this way, the emitted and reflected waves do not overlap, making hearing difficult.

During all its propagation, the acoustic wave will experience all these successive losses until finally the energy is completely absorbed in the medium. Which means that it has been transformed into heat energy.

There is a magnitude to quantify the ability of a material to absorb sound. It's called the absorption coefficient. It is denoted as α, and it is the ratio between the absorbed energy E _abs and the incident energy E _inc, all referred to the material in question. It is expressed mathematically like this:

α = E _abs / E _inc

The maximum value of α is 1 (completely absorbs sound) and the minimum is 0 (lets all sound through).

Sound can be a disadvantage on many occasions when silence is preferred. For example, cars are fitted with silencers to dampen engine noises. To other devices like water pumps and power plants as well.

Sound insulation is important in a recording studio. Source: Pixabay.

Examples of sound energy

Sound energy is everywhere. Here is a simple example that illustrates the properties of sound and its energy from a quantitative point of view.

Exercise resolved

A pin of mass 0.1 g falls from a height of 1m. Assuming that 0.05% of its energy is converted into a sound pulse of 0.1 s duration, estimate the maximum distance at which the pin drop can be heard. Take as the minimum audible sound intensity 10 ^-8 W / m ².

Solution

The equation given above will be used for the intensity of the sound:

A good question is where the sound energy comes from in this case, that whose intensity the human ear detects.

The answer is in gravitational potential energy. Precisely because the pin falls from a certain height, at which it had potential energy, as it falls it transforms this energy into kinetic energy.

And once it hits the ground, the energy is transferred to the air molecules surrounding the crash site, giving rise to the sound.

The gravitational potential energy U is:

Where m is the mass of the pin, g is the acceleration of gravity, and h is the height from which it fell. Substituting these numerical values, but not before making the corresponding conversions in the International System of units, we have:

U = 0.1 x 10 ^-3 x 9.8 x 1 J = 0.00098 J

The statement says that of this energy, only 0.05% is transformed to give rise to the sound pulse, that is, to the tinkling of the pin when it hits the floor. Therefore the sound energy is:

E _sound = 4.9 x 10 ^-7 J

From the intensity equation, the radius R is cleared and the values ​​of the sound energy E _sound and the time that the pulse lasted are substituted: 0.1 s according to the statement.

Therefore the maximum distance at which the pin drop will be audible is 6.24 m in all directions.

References

1. Giancoli, D. 2006. Physics: Principles with Applications. Sixth Edition. Prentice Hall. 332-359.
2. Kinsler, L. (2000). Fundamentals of Acoustic. 4th Ed. Wiley & Sons. 124-125.