Modern Physics

# Sound energy: characteristics, types, uses, advantages, examples

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

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

Vibrations cause successive compressions and expansions in the air or environment around the source, which are spreading with some velocity. It is not the particles that travel, but they are limited to oscillation relative to their equilibrium position. Disturbance is what is transmitted.

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

## Characteristics

As is well known, moving objects have energy. So also waves, while traveling in the medium, carry with them the energy associated with the movement of particles (kinetic energy) and also the deformation energy of the average or potential energy.

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

K = ½ mu 2

In addition, the particle has potential energy U that depends on the volume variation it experiences, with Vo being the initial volume, V the final volume and p the pressure, which depends on the position and time:

The negative sign indicates an increase in potential energy, as the propagation wave works on the dV volume element when compressed, thanks to a positive acoustic pressure.

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

o = ρ or V or

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

ρV = ρ or V o = constant

So the total energy looks like this:

### Calculation of potential energy

The integral can be solved with the help of the mass conservation principle.

o = m f

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

dV = (-V / ρ) dρ

Isaac Newton determined that:

(dp / dρ) = c 2

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

If A p and A v are the pressure and velocity wave amplitudes, respectively, the mean energy ε of the sound wave is:

Sound can be characterized by a magnitude called intensity .

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

As the energy per unit of 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 amount of energy. All of this determines its acoustic behavior. As sound is so important to human life, the types of sounds are classified into three broad groups, according to the range of frequencies audible to human beings:

– 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. 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 sounds are light and penetrating, while bass sounds are perceived as deeper and louder.

The sounds heard daily are complex overlays of sound with several frequencies close together.

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

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

### the noise

It is also important to distinguish between desired and unwanted sounds or noises. As noise is always sought to be eliminated, it is classified according to intensity and period in:

– continuous noise.

– Floating 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 noise that greatly favors low frequencies).

## Uses

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

Sounds are versatile. Each differs according to the source that issues 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.

Many animals use sound energy to locate themselves in space and also to capture their prey. They emit acoustic signals and have receiving organs that analyze the reflected signals. That way they get information about distances.

Humans do not have the necessary organs to use sonic energy in this way. However, they created guidance devices like sonar, based on these same principles, to facilitate navigation.

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

### Some Acoustic Energy Applications

– With high energy ultrasound stones or stones 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 oceanic relief to witnesses 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.

## Benefits

Acoustic energy has advantages that are due in large part to its limited scope. For example, it is not expensive to produce and does not generate chemical or other residues, as it dissipates quickly in the environment.

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, eg imaging using ultrasound, it has the advantage of not using ionizing radiation such as x-rays or tomography. It is a fact that ionizing radiation can damage cells.

Its use does not require the necessary protective measures when ionizing radiation is applied. The equipment is also cheaper.

Furthermore, ultrasonic energy is a non-invasive method to eliminate the mentioned kidneys and gallstones, avoiding surgical procedures.

In principle, it does not generate air or water pollution. But it is known that there is noise pollution in the seas, due to human activities such as intensive fishing, geophysical prospecting and transport.

It’s hard to think about the disadvantages that a phenomenon as natural as sound can have.

One of the few is that high-intensity sounds can damage the structure of the eardrum and eventually cause people who are continually exposed to lose their sensitivity.

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

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

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

Obviously, this is related to energy conservation: energy is not destroyed, but it changes form. The vibrations of molecules in the air don’t just turn into pressure changes that give rise to sound. Vibrations also give rise to heat.

### Sound absorption in materials

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

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

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

During its propagation, the acoustic wave will experience all these successive losses until finally the energy is completely absorbed in the medium. Which means it was turned into caloric 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 is the ratio between the absorbed energy E abs and the incident energy E inc , all referring 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 (allows all sound to enter).

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

## 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 solved

A 0.1 g dough pin drops from a height of 1 m. Assuming that 0.05% of its energy becomes a sonic pulse of 0.1 s in duration, estimate the maximum distance at which the pin drop can be heard. Take 10 -8 W / m 2 as the minimum audible sound intensity .

#### Solution

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

A good question is where does sound energy come from, in this case, the one whose intensity is detected by the human ear.

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

And once it hits the ground, energy is transferred to the air molecules around the crash site, which gives rise to sound.

The gravitational potential energy U is:

U = mgh

Where m is the mass of the pin, g is the acceleration due to gravity, and h is the height from which it fell. Substituting these numerical values, but not before doing the corresponding conversions in the International System of Units, you 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, the touch of the pin when it hits the ground. Therefore, the sound energy is:

Sound E = 4.9 x 10 -7 J

From the intensity equation, the radius R is erased and the sound energy values ​​E sound and the pulse duration are replaced: 0.1 s according to the statement.

Therefore, the maximum distance at which pin drop will be audible is 6.24 m round.