Sound intensity is the amount of energy that a sound wave carries 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: sound wave, its frequency, acoustic pressure, flow of sound energy. To understand what intensity is, we will analyze in detail each term associated with it.
How does the sound appear
Sound can arise from an oscillating body. It must vibrate fast enough to create 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 withstand compression or any other kind of deformation (if we talk about solids). Yes, solids, and liquids, and gases, and air (as a mixture of different gases) possess elasticity, but to different degrees.
The amount of elasticity is determined by the 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 worst of all.
The elasticity of air and denser media is due to various reasons. In liquids and solids there are forces of intermolecular interaction. They hold the particles together in the form of a crystal lattice, and the sound wave is very easy to propagate through its nodes.
Air molecules are not interconnected, they are separated by large distances. Particles do not scatter due to continuous and random motion, as well as gravity. It has long been noticed: the more rarefied air (for example, in the upper layers of the atmosphere), the lower the intensity, volume of sound. There is complete silence on the Moon, but not because there is nothing to sound there, but because of the lack of air.
As a sound wave runs through the air
Of greatest interest to us is the propagation of a sound (acoustic) wave in air. When the body deviates from its initial position, it compresses nearby air on one side of itself. On the contrary, 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? Oscillatory is added to their chaotic movement. Unlike the constant thermal motion of molecules, the vibrational one has one direction. In a layer of air that is perpendicular to the direction of body deflection, particles begin to push each other. They move with the sound source in one direction. Thus, alternating compression-rarefaction of air is transmitted from one layer of air to another. This is an acoustic wave. Sound intensity is a quantity that depends on the basic characteristics of the wave — frequency and length.
Sound frequency
The frequency of the wave depends on how quickly the sound source vibrates . All bodies oscillate with different frequencies, but not every frequency is available to our perception. Those waves that we hear are called sound. The frequency of an acoustic wave is measured in hertz (1 Hz is equal to 1 oscillation per second).
The layers of compressed and rarefied air alternate. The wavelength is equal to the distance between adjacent layers in which the same pressure. Sound does not spread infinitely far, because with increasing distance the wave weakens. How far it goes will depend on the length and frequency of the acoustic wave. These values are directly proportional: high-frequency waves are shorter than low-frequency ones. We speak of high-frequency sounds as high, low-frequency waves generate low sounds.
The sound intensity level 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 lowing of a cow is an intense sound that can be heard from at least 10 meters. Its frequency is 30 Hz.
Acoustic pressure
In each layer of air that the sound wave has reached, the pressure changes either up or down. The value by which it, in comparison with atmospheric, increases, is called acoustic (sound) pressure.
Our ear is amazingly sensitive. It’s hard to believe, but it distinguishes between a change in pressure of 0.01 millionths of a gram per unit area. The rustle creates a very small pressure, it is 3 * 10 -5 N / m 2 . This value is 3 * 10 10 times less than atmospheric pressure. It turns out that human hearing is more accurate than chemical weights. Physiologists have studied the elasticity of the eardrum and the pressure that the quietest sound exerts. Comparing the data, they came to the conclusion that the eardrum bends to a distance that is less than the size of an atom.
Sound intensity and sound pressure are directly related. When the body oscillates at a low frequency, it significantly increases pressure - the sound comes out strong. The intensity (strength) of the sound is proportional to the square of the acoustic pressure.
Flow of sound energy
Sounds of various frequencies and intensities 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 diverges, 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 a wave carries through a surface area per second. This refers to the surface of the medium, for example, an air layer located at right angles to the direction of the elastic wave. Energy flow is measured in watts (W).
Sound power
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 m 2 ).
The sound intensity is indicated by the letter I. The minimum value (I 0) is 10-12 W / m 2 . The higher the intensity, the louder the sound. The dependence of sound power and volume was established experimentally. It was noticed that when the intensity rises 10 times, the volume increases by 10 decibels (dB), when 100 times - by 20 dB.
Audible and Inaudible Sounds
Physiology allows a person to hear sounds only within certain limits. If the body oscillates with a frequency of more 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 interconnected. High frequency sound waves transmit very little energy. It is not enough to change the acoustic pressure so that our eardrum vibrates. They say about such sounds: they are beyond the threshold of audibility.
A wave with a frequency of less than 16 thousand Hz is called ultrasound. The most famous creatures that “speak” with ultrasound are dolphins and bats. Infrasound, although we don’t hear it, at a certain intensity (190-200 dB) can lead to death, because it increases the pressure in the pulmonary alveoli too much.
Interestingly, at different frequencies, the dependence of the volume and sound intensity is different. At medium frequencies (about 1000 Hz), a person feels changes in intensity by only 0.6 dB. Boundary frequency levels are another matter. On them, we barely distinguish between a change in sound intensity by 3 units.
Sound classification
Sound intensity is measured in W / m 2 , 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 different intensities.
Permissible Norms
A constant noise or one that persists for a long time is called background. For an apartment, 20-30 dB is a normal level of background noise. It is perceived by man as silence. Sounds of 40 dB are also acceptable, but a volume of 60 dB is acceptable for offices, institutions. Long-term exposure to sounds with a volume of 70 dB leads to disturbances in 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 its complete loss.
Sound intensity is a value whose permissible values are prescribed in the sanitary rules and norms (SanPiN). The period of time in which it is allowed to include noisy household appliances, talk loudly, make repairs, etc., is determined by the Law "On Ensuring Peace and Silence." It is taken separately for each area. The time in each region may differ: somewhere, daytime hours begin at 7:00 in the morning, and somewhere at 9:00. For example, in the Moscow region, quiet night time is considered to be the interval from 21:00 to 8:00 on weekdays and from 22:00 to 10:00 on weekends. In addition, there is a quiet hour from 13:00 to 15:00.