Sound intensity is the amount of energy that a sound wave transfers in 1 second through a unit area of the medium. The intensity depends on the frequency of the wave, on the acoustic pressure. As you can see, many other concepts are associated with intensity: a sound wave, its frequency, acoustic pressure, the flow of sound energy. In order to understand what intensity is, we will break down each term associated with it in detail.
How sound appears
Sound can come from a vibrating body. It must vibrate fast enough to create a disturbance in the medium and generate an acoustic wave. However, for its occurrence, one more condition is necessary: the medium must be elastic. Elasticity is the ability to resist compression or any other kind of deformation (if we talk about solids). Yes, solids, liquids, gases, and air (as a mixture of different gases) have elasticity, but to varying degrees.
Elasticity valuedetermined by density. It is known that solid media (wood, metals, earth's crust) conduct sound much better than liquid ones. And if we compare water and air, then in the second medium, the sound wave diverges the worst.
The elasticity of air and denser media is due to different reasons. In liquids and solids there are forces of intermolecular interaction. They hold the particles together in a crystal lattice, and it is very easy for a sound wave to propagate through its nodes.
Air molecules are not connected to each other, they are separated by large distances. Particles do not dissipate due to continuous and erratic motion, as well as gravity. It has long been noticed: the more rarefied the air (for example, in the upper layers of the atmosphere), the less the intensity, the loudness of the sound. There is complete silence on the moon, not because there is nothing to sound, but because of the lack of air.
How a sound wave travels through the air
The greatest interest for us is the propagation of a sound (acoustic) wave in the air. When the body deviates from its initial position, it compresses nearby air on one side of itself. On the other hand, the medium is rarefied. Returning to its original position, the sound source deviates to the other side and compresses the air there. This continues until the body stops moving.
How do particles behave? An oscillatory one is added to their chaotic movement. In contrast to the constant thermal motion of molecules, the vibrational motion has one direction. In a layer of airwhich is perpendicular to the direction of deflection of the body, the particles begin to push each other. They move with the sound source in the same direction. Thus, alternating compression-rarefaction of air is transmitted from one layer of air to another. This is the acoustic wave. Sound intensity is a value that depends on the main characteristics of the wave - frequency and length.
The frequency of the wave depends on how fast the sound source vibrates. All bodies vibrate with different frequencies, but not every frequency is available to our perception. The waves we hear are called sound. The frequency of an acoustic wave is measured in hertz (1 Hz equals 1 oscillation per second).
Layers of compressed and rarefied air alternate. The wavelength is equal to the distance between adjacent layers in which the pressure is the same. Sound does not propagate infinitely far, because as distance increases, the wave weakens. How far it travels depends on the length and frequency of the acoustic wave. These quantities are directly proportional: high-frequency waves are shorter than low-frequency ones. We talk about high frequency sounds as high, low frequency waves generate low sounds.
The level of sound intensity is directly dependent on the frequency of acoustic vibrations and wavelength. So, a mosquito squeak sounds with a frequency of 10 thousand Hz and has a wavelength of only 3.3 cm. The mooing of a cow is an intense sound that can be heard from at least 10 meters. Its frequency is 30 Hz.
In every layerthe air that the sound wave has reached, the pressure changes either up or down. The amount by which it increases compared to atmospheric pressure is called acoustic (sound) pressure.
Our ear is amazingly sensitive. It's hard to believe, but it distinguishes between a pressure change of 0.01 millionth of a gram per unit area. The rustle creates very little pressure, it is equal to 310-5 N/m2. This value is 31010 times less than atmospheric pressure. It turns out that human hearing is more accurate than chemical scales. Physiologists have studied the elasticity of the tympanic membrane and the pressure exerted by the quietest sound. After comparing the data, they came to the conclusion that the tympanic membrane bulges to a distance that is less than the size of an atom.
Sound intensity and sound pressure are directly related. When the body vibrates at a low frequency, it increases the pressure significantly - the sound comes out strong. The intensity (strength) of sound is proportional to the square of acoustic pressure.
Sonic energy flow
Sounds of varying frequency and intensity are determined by the flow of sound energy. The sound wave propagates in all directions in the form of a ball. The further the wave travels, the weaker it becomes. The energy that it carries is distributed over an increasing area - the sound subsides. The square of sound energy is inversely proportional to the square of the distance to the vibrating body.
The flow of sound energy is the amount of kinetic energy that carrieswave across a surface area per second. This refers to the surface of the medium, for example, a layer of air located at right angles to the direction of the elastic wave. Energy flow is measured in watts (W).
Power of sound
The strength (intensity) of sound is a quantity, to find which you need to know what the flow of energy is. Its value should be divided by the surface area perpendicular to the wave propagation (in m2).
Sound intensity is indicated by the letter I. The minimum value of (I0) is 10-12 W/m2. The higher the intensity, the louder the sound appears. The dependence of the strength of sound and loudness was established empirically. It has been observed that when the intensity is increased by 10 times, the volume increases by 10 decibels (db), when by 100 times - by 20 dB.
Audible and inaudible sounds
Physiology allows a person to hear sounds only within certain limits. If the body vibrates at a frequency greater than 16-20 kilohertz (kHz) and less than 16-20 Hz, our ear will not be able to perceive it.
The frequency and intensity of sound are related. High frequency sound waves transmit very little energy. It is not enough to change the acoustic pressure enough to make our eardrum vibrate. Such sounds are said to be beyond the threshold of hearing.
A wave with a frequency of less than 16 thousand Hz is called ultrasound. The most famous creatures“talk” with ultrasound, these are dolphins and bats. Infrasound, although we do not hear it, at a certain intensity (190-200 dB) can lead to death, because it increases pressure in the pulmonary alveoli too much.
Interestingly, at different frequencies, the dependence of loudness and sound intensity is different. At medium frequencies (about 1000 Hz), a person feels changes in intensity by only 0.6 dB. Limiting frequency levels are a completely different matter. On them, we can barely distinguish a change in sound intensity by 3 units.
Classification of sounds
Sound intensity is measured in W/m2, however, decibels are used to compare sounds with each other and with a minimum level of intensity.
Sounds are divided into:
- very weak (0-20 dB);
- weak (21-40 dB);
- moderate (41-60 dB);
- loud (61-80 dB);
- very loud (81-100 dB);
- deafening (more than 100 dB).
The figure shows examples of the most common sounds of varying intensity.
Constant noise or one that persists for a long time is called background noise. For an apartment, 20-30 dB is a normal level of background noise. It is perceived by a person as silence. Sounds of 40 dB are also acceptable, but a volume of 60 dB is acceptable for offices and institutions. Prolonged exposure to sounds with a volume of 70 dB leads todisorders of the central nervous system. It is with such a loudness that the street "sounds", and on busy avenues the noise reaches 85-90 dB. Sounds of 100 dB reduce hearing and can lead to complete hearing loss.
Sound intensity is a value whose permissible values are prescribed in sanitary rules and regulations (SanPiN). The period of time during which it is allowed to turn on noisy household appliances, talk loudly, make repairs, etc. is determined by the Law on Ensuring Peace and Quiet. It is taken separately for each area. The time in each region may differ: somewhere the daytime hours start at 7:00 am, and somewhere at 9:00. For example, in the Moscow region, the interval from 21:00 to 8:00 on weekdays and from 22:00 to 10:00 on weekends is considered to be quiet at night. In addition, there is a quiet hour from 13:00 to 15:00.