SOUND: HISTORY, CHARACTERISTICS, HOW IT IS PRODUCED, TYPES - PHYSICAL - 2024

The sound is defined as a perturbation to the propagating in a medium such as air, alternately it produces compressions and expansions in it. These changes in air pressure and density reach the ear and are interpreted by the brain as auditory sensations.

Sounds have accompanied life since its inception, forming part of the tools that animals have to communicate with each other and with their environment. Some people claim that plants also listen, but in any case they could perceive the vibrations of the environment even if they do not have an auditory device like higher animals.

Figure 1. Rupture of the sound barrier

In addition to using sound to communicate through speech, people use it as an artistic expression through music. All cultures, ancient and recent, have musical manifestations of all kinds, through which they tell their stories, customs, religious beliefs and feelings.

History

Due to its importance, humanity became interested in studying its nature and created acoustics, a branch of physics dedicated to the properties and behavior of sound waves.

It is known that the famous mathematician Pythagoras (569-475 BC) spent a long time studying the differences in height (frequency) between sounds. On the other hand, Aristotle, who speculated on all aspects of nature, correctly asserted that sound consisted of expansions and compressions in the air.

Later the famous Roman engineer Vitruvius (80-15 BC) wrote a treatise on acoustics and its applications in the construction of theaters. Isaac Newton himself (1642-1727) studied the propagation of sound in solid media and determined a formula for its speed of propagation.

Over time, the mathematical tools of calculation made it possible to adequately express all the complexity of wave behavior.

Sound characteristics (properties)

In its simplest form, a sound wave can be described as a sinusoidal wave, propagating in time and space, like the one shown in figure 2. There it is observed that the wave is periodic, that is, it has a a way that repeats itself in time.

Being a longitudinal wave, the direction of propagation and the direction in which the particles of the vibrating medium move are the same.

Sound wave parameters

Figure 2. Sound is a longitudinal wave, the disturbance propagates in the same direction in which the molecules experience their displacement. Source: Wikimedia Commons.

The parameters of a sound wave are:

Period T: is the time it takes to repeat a phase of the wave. In the International System it is measured in seconds.

Cycle: is the part of the wave contained within the period and covers from one point to another that has the same height and the same slope. It can be from one valley to the next, from one ridge to the next, or from one point to another that meets the specification described.

Wavelength λ: is the distance between one crest and another of the wave, between one valley and another, or in general between one point and the next with the same height and slope. Being a length it is measured in meters, although other units are more appropriate depending on the type of wave.

Frequency f: is defined as the number of cycles per unit of time. Its unit is the Hertz (Hz).

Amplitude A: corresponds to the maximum height of the wave with respect to the horizontal axis.

How is sound produced and propagated?

Sound is produced when an object that is immersed in a material medium is vibrated, as shown at the bottom of Figure 2. The taut membrane of the loudspeaker on the left vibrates and transmits the disturbance through the air until reaches the listener.

As the disturbance spreads, energy is transmitted to the molecules in the environment, which interact with each other, through expansions and compressions. You always need a material medium for the propagation of sound, be it solid, liquid or gas.

When the disturbance in the air reaches the ear, variations in air pressure cause the eardrum to vibrate. This gives rise to electrical impulses that are transmitted to the brain through the auditory nerve, and once there the impulses are translated into sound.

Speed ​​of sound

The speed of mechanical waves in a given medium follows this relationship:

For example when propagating in a gas like air, the speed of sound can be calculated as:

As the temperature increases, so does the speed of sound, since the molecules in the medium are more willing to vibrate and transmit the vibration through their movements. The pressure on the other hand, does not affect its value.

Relationship between wavelength and frequency

We have already seen that the time it takes for the wave to complete a cycle is the period, while the distance traveled in that period of time is equal to one wavelength. Therefore the speed v of sound is defined as:

On the other hand, the frequency and the period are related, one being the inverse of the other, like this:

Which leads to:

The audible frequency range in humans is between 20 and 20,000 Hz, therefore the wavelength of sound is between 1.7 cm and 17 m when substituting the values ​​in the above equation.

These wavelengths are the size of common objects, which influences the propagation of sound, since being a wave, it experiences reflection, refraction and diffraction when it encounters obstacles.

Experiencing diffraction means that sound is affected when it encounters obstacles and openings that are close to or smaller in size as its wavelength.

Bass sounds can spread better over long distances, which is why elephants use infrasound (very low-frequency sounds, inaudible to the human ear) to communicate across their vast territories.

Also when there is music in a nearby room, the bass is heard better than the treble, because its wavelength is about the size of doors and windows. On the other hand, when leaving the room, the high-pitched sounds are easily lost and therefore stop being heard.

How is sound measured?

Sound consists of a series of compressions and rarefactions of the air, in such a way that as it propagates, the sound causes increases and decreases in pressure. In the International System, pressure is measured in pascals, which is abbreviated Pa.

What happens is that these changes are very small compared to atmospheric pressure, which is worth about 101,000 Pa.

Even the loudest sounds produce fluctuations of as little as 20-30 Pa (pain threshold), a fairly small amount in comparison. But if you can measure those changes, then you have a way of measuring sound.

Sound pressure is the difference between atmospheric pressure with sound and atmospheric pressure without sound. As we have said, the loudest sounds produce sound pressures of 20 Pa, while the weakest ones cause about 0.00002 Pa (sound threshold).

Since the range of sound pressures spans several powers of 10, a logarithmic scale should be used to indicate them.

On the other hand, experimentally it was determined that people perceive changes in low-intensity sounds more noticeably than changes of the same magnitude but in intense sounds.

For example, if the sound pressure increases by 1, 2, 4, 8, 16…, the ear perceives increases of 1, 2, 3, 4… in intensity. For this reason, it is convenient to define a new quantity called sound pressure level (Sound Pressure Level) L _P, defined as:

Where P _o is the reference pressure that is taken as the hearing threshold and P ₁ is the mean effective pressure or RMS pressure. This RMS or average pressure is what the ear perceives as the average energy of the sound signal.

Decibels

The result of the above expression for L _P, when evaluated for various values ​​of P ₁, is given in decibels, a dimensionless quantity. Expressing the sound pressure level like this is very convenient, because logarithms convert large numbers into smaller, more manageable numbers.

However, in many cases it is preferred to use sound intensity to determine decibels, rather than sound pressure.

Sound intensity is the energy that flows for one second (power) through a unit surface oriented perpendicular to the direction in which the wave is propagating. Like sound pressure, it is a scalar quantity and is denoted I. The units of I are W / m ², that is, power per unit area.

It can be shown that the intensity of sound is proportional to the square of the sound pressure:

In this expression, ρ is the density of the medium and c is the speed of sound. Then the sound intensity level L _I is defined as:

Which is also expressed in decibels and is sometimes denoted by the Greek letter β. The reference value I _o is 1 x 10 ^-12 W / m ². Thus, 0 dB represents the lower limit of human hearing, while the pain threshold is 120 dB.

Since it is a logarithmic scale, it must be emphasized that small differences in the number of decibels make a big difference in terms of sound intensity.

The sound level meter

A sound level meter or decibelmeter is a device used to measure sound pressure, indicating the measurement in decibels. It is designed to respond to it in the same way that the human ear would.

Figure 3. The sound level meter or decibelmeter is used to measure the sound pressure level. Source: Wikimedia Commons.

It consists of a microphone to collect the signal, more circuits with amplifiers and filters, which are responsible for adequately transforming this signal into an electric current, and finally a scale or a screen to show the result of the reading.

They are widely used to determine the impact that certain noises have on people and the environment. For example noises in factories, industries, airports, traffic noise and many others.

Sound types (infrasound, ultrasound, mono, stereo, polyphonic, homophonic, bass, treble)

Sound is characterized by its frequency. According to those that the human ear can capture, all sounds are classified into three categories: those that we can hear or the audible spectrum, those that have a frequency below the lower limit of the audible spectrum or infrasound, and those that are above the audible spectrum. upper limit, called ultrasound.

In any case, since sound waves can overlap linearly, everyday sounds, which we sometimes interpret as unique, actually consist of different sounds with different but close frequencies.

Figure 4. Sound spectrum and frequency ranges. Source: Wikimedia Commons.

Audible spectrum

The human ear is designed to pick up a wide range of frequencies: between 20 and 20,000 Hz. But not all frequencies in this range are perceived with the same intensity.

The ear is more sensitive in the frequency band between 500 and 6,000 Hz. However, there are other factors that affect the ability to perceive sound, such as age.

Infrasound

They are sounds whose frequency is less than 20 Hz, but the fact that humans cannot hear them does not mean that other animals cannot. For example, elephants use them to communicate, since infrasound can travel long distances.

Other animals, such as the tiger, use them to stun their prey. Infrasound is also used in the detection of large objects.

Ultrasound

They have frequencies greater than 20,000 Hz and are widely used in many fields. One of the most notable uses of ultrasound is as a tool of medicine, both diagnostic and treatment. The images obtained by ultrasound are non-invasive and do not use ionizing radiation.

Ultrasounds are also used to find faults in structures, determine distances, detect obstacles during navigation, and more. Animals also make use of ultrasound, and in fact that is how its existence was discovered.

Bats emit sound pulses and then interpret the echo they produce to estimate distances and locate prey. For their part, dogs can also hear ultrasounds and that is why they respond to the dog whistle that their owner cannot hear.

Monophonic sound and stereophonic sound

Figure 4. In a recording studio, sound is appropriately modified by electronic devices. Source: Pixabay.

Monophonic sound is a signal recorded with a single microphone or audio channel. When listening with headphones or sound horns, both ears hear exactly the same thing. In contrast, stereophonic sound records signals with two independent microphones.

The microphones are located in different positions so that they can pick up different sound pressures of what you want to record.

Then each ear receives one of these sets of signals, and when the brain gathers and interprets them, the result is much more realistic than when listening to monophonic sounds. It is therefore the preferred method when it comes to music and film, although monophonic or monaural sound is still used in radio, especially for interviews and conversations.

Homophony and polyphony

Musically speaking, homophony consists of the same melody played by two or more voices or instruments. On the other hand, in polyphony there are two or more voices or instruments of equal importance that follow melodies and even different rhythms. The resulting ensemble of these sounds is harmonious, such as the music of Bach.

Bass and treble sounds

The human ear discriminates audible frequencies as high, low, or medium. This is what is known as the pitch of the sound.

The highest frequencies, between 1600 and 20,000 Hz, are considered as acute sounds, the band between 400 and 1600 Hz corresponds to sounds with a medium tone and lastly, the frequencies in the range of 20 to 400 Hz are the bass tones.

Bass sounds differ from treble in that the former are perceived as deep, dark, and booming, while the latter are light, clear, joyous, and piercing. Also, the ear interprets them as more intense, unlike bass sounds, which produce the sensation of less intensity.

References

1. Figueroa, D. 2005. Waves and Quantum Physics. Series: Physics for Science and Engineering. Edited by D. Figueroa.
2. Giancoli, D. 2006. Physics: Principles with Applications. 6th. Ed Prentice Hall.
3. Rocamora, A. Notes on musical acoustics. Recovered from: eumus.edu.uy.
4. Serway, R., Jewett, J. (2008). Physics for Science and Engineering. Volume 1. 7th. Ed. Cengage Learning.
5. Wikipedia. Acoustics. Recovered from: es.wikipedia.org.