Noise pollution, unwanted or excessive sound levels can have harmful effects on human health and the quality of the environment. It usually occurs in many industrial facilities and some other workplaces. Also, noise pollution is associated with road, rail and air traffic and with outdoor activities.
Volume measurement and perception
Sound waves are vibrations of air molecules carried from a noise source to the ear. This is usually described in terms of loudness (amplitude) and wave heights (frequencies). Sound pressure level, or SPL, is measured in logarithmic units called decibels (dB). A normal human ear can detect tones in the range of 0 dB (hearing threshold) to 140 dB. In this case, sounds from 120 dB to 140 dB cause pain.
What sound level, for example, in the library? It is about 35 dB, and inside a moving bus or metro train it is about 85. The building construction work can generate SPLs of up to 105 dB at the source. SPL decreases with distance from the subject.
The speed at which sound energy is transmitted is called intensity, proportional to the square of the SPL. Due to the logarithmic nature of the decibel scale, an increase of 10 points represents a 10-fold increase in sound intensity. 20 transmits more than 100 times. And 30 dB represents a 1000-fold increase in intensity.
And on the other hand, when the tension doubles, the sound volume level is amplified by only 3 points. For example, if a construction drill causes 90 dB of noise, then two identical tools working side by side will create 93 dB. And when two sounds that differ in SPL by more than 15 points are combined, weak tones are masked (or muffled) by a loud sound. For example, if an 80 dB drill is running at a construction site next to a 95 bulldozer, the combined pressure level of these two sources will be measured as 95. A less intense tone from the compressor will not be noticeable.
The frequency of the sound wave is expressed in cycles per second, but hertz is often used (1 cps = 1 Hz). The eardrum of a person is a very sensitive organ with a large dynamic range, capable of detecting sounds at frequencies from 20 Hz (low pitch) to about 20,000 Hz (high sound level). The tone of the human voice in ordinary conversation occurs at frequencies from 250 Hz to 2000 Hz.
Accurate measurement of sound level and scientific description differ from most subjective human perceptions and opinions about it. Individual human reactions to noise depend on both height and volume. People with normal hearing usually perceive high-frequency sounds louder than low-frequency sounds of the same amplitude. For this reason, electronic sound level meters take into account changes in perceived volume as a function of pitch.
Frequency filters in the meters are used to harmonize the readings with the sensitivity of the human ear and the relative volume of various sounds. The so-called A-weighted filter, for example, is commonly used to diagnose the surrounding community. SPL measurements made with this filter are expressed in A-weighted decibels or dBA.
Most people perceive and describe an increase in SPL of 6–10 dBA as a doubling of “volume”. Another system, the C-weighted (dbs) scale, is sometimes used for impact noise levels, such as shooting, and tends to be more accurate than dBA for the perceived loudness of sounds with low-frequency components.
Noise levels tend to change over time, so measurement data are presented as averaged values to express overall sound levels. There are several ways to do this. For example, the results of a series of repeated measurements of sound level can be presented as L 90 = 75 dBA, which means that the values were equal to or higher than 75 dBA for 90 percent of the time.
Another unit called equivalent degrees of sound (L eq) can be used to express the average SPL for any period of interest, for example, an eight-hour work day. (L eq is a logarithmic rather than an arithmetic value, so loud events prevail in the overall result.)
The unit of sound level, called the day-night noise value (DNL or L dp), takes into account the fact that people are more sensitive to tone at night. So, 10-dBA is added to the SPL values, measured from 10 hours to 7 in the morning. For example, DNL measurements are very useful for describing the overall noise exposure of aircraft.
Working with effects
Noise is more than just a nuisance. At certain levels and duration of exposure, this can cause physical damage to the eardrum and sensitive hair cells of the inner ear, and lead to temporary or permanent hearing loss.
It usually does not occur with SPLs below 80 dBA (eight-hour exposure levels are best maintained no more than 85). But most people who are repeatedly exposed to more than 105 dBA will, to some extent, have permanent hearing loss. In addition to it, the excessive influence of noise can also increase blood pressure and heart rate, cause irritability, anxiety and mental fatigue, as well as disturb sleep, rest and personal communication.
Noise pollution control
Therefore, it is important to maintain utter silence in the workplace and in society. Noise regulations and laws adopted at local, regional and national levels can be effective in mitigating the negative effects of noise pollution.
Ecological and industrial buzz is regulated in accordance with the law on occupational safety and health and the law on combating it. In accordance with these acts, the Occupational Safety and Health Administration has established criteria for industrial noise in order to introduce restrictions on the intensity of sound exposure and the duration during which this tension can be resolved.
If a person is exposed to different noise levels at different time intervals throughout the day, the total effect or dose (D) of noise is obtained from the ratio
where C is the actual time and T is valid at any level. When using this formula, the most possible daily dose of noise will be 1, and any exposure from above is not permissible.
Maximum sound level
The criteria for indoor noise are summarized in three sets of specifications, which were obtained by collecting subjective judgments from a large sample of people in various specific situations. They turned into noise criteria (NC) and preferred tone curves (PNC), which set limits on the level introduced into the environment. NC curves, developed in 1957, are aimed at providing a comfortable working or residential area by determining the maximum allowable sound level in octave bands throughout the audio spectrum.
A complete set of 11 curves defines noise criteria for a wide range of situations. Developed in 1971, PNC charts add limits to low-frequency hum and high-frequency hiss. Therefore, they are preferable to the older standard NC. Summarized on curves, these criteria provide design goals for noise levels for various ideas. Part of the job or habitat specification is the corresponding PNC curve. In case the level exceeds the limits of PNC, sound-absorbing materials can be introduced into the environment as necessary to meet the standards.
Low noise levels can be overcome with additional absorbent material, such as heavy draperies or indoor tiles. Where a low level of identifiable noise can be distracting or where confidentiality of conversations in adjacent offices and receptions can be important, unwanted sounds can be masked. A small source of white noise, such as static air in a room, can mask conversations from neighboring rooms without being a deadly sound level for the ears of people working nearby.
This type of device is often used in the offices of doctors and other specialists. Another method to reduce noise is to use hearing protection that is worn over the ears just like headphones. By using commercially available protective devices, it is possible to reduce the tone level in the range usually from 10 dB at 100 Hz to more than 30 dB for frequencies above 1 thousand Hz.
Determine sound level
Outdoor noise restrictions are also important for human comfort. The construction of the building will provide some protection against external sounds if the house meets the minimum standards and if the noise level is within acceptable limits.
These restrictions are usually indicated for certain periods of the day, for example, during daylight hours, in the evening, and at night during sleep. Due to atmospheric refraction caused by temperature inversion at night, relatively loud noises may be emitted from a fairly distant highway, airport or railway.
One of the interesting methods of noise control is the construction of noise barriers along the highway, separating it from adjacent residential areas. The effectiveness of such structures is limited by the diffraction of sound more at low frequencies that prevail on roads and are inherent in large vehicles. To be effective, they should be as close as possible to the source or observer of the noise, thereby maximizing the diffraction necessary for the sound to reach the observer. Another requirement for this type of barrier is that it must also limit the number of sound levels in order to achieve significant noise reduction.
Definition and examples
A decibel (dB) is used to measure sound level, but it is also widely used in electronics, signals and communications. DB is the logarithmic way of describing tangency. A relationship can manifest itself as power, sound pressure, voltage or intensity, or several other things. Later we associate dB with the phone and sound (due to volume). But first, to get an idea of the logarithmic expressions, let's look at some numbers.
For example, we can assume that there are two speakers, the first of which reproduces sound with a power of P 1, and the other a louder version of the same tone with a power of P 2, but everything else (how far, frequency) remains unchanged.
The difference in decibels between them is defined as
10 log (P 2 / P 1 ) dB, where log for base 10.
If the second produces twice as much energy as the first, the difference in dB
10 log (P 2 / P 1 ) = 10 log 2 = 3 dB,
as shown in the graph that displays 10 log (P 2 / P 1 ) versus P 2 / P 1 . To continue the example, if the second one has 10 times the power of the first, the difference in dB will be:
10 log (P 2 / P 1 ) = 10 log 10 = 10 dB.
If the second had the same power a million times, the difference in dB would be
10 log (P 2 / P 1 ) = 10 log 1,000,000 = 60 dB.
This example shows one feature of decibel scales that is useful in discussing sound. They can describe very large relationships using modest sized numbers. But you need to pay attention that the decibel represents the ratio. That is, it will not be said what kind of power is emitted by any of the speakers, only from the difference. And also pay attention to the coefficient 10 in the definition, which stands for decibels in decibels.
Acoustic Pressure and dB
Frequency is usually measured using microphones, and they respond (approximately) in proportion to pressure, s. Now, the power of a sound wave, ceteris paribus, is equal to the square of the pressure. Similarly, the electrical power in a resistor goes like a multiplied voltage. The logarithm of the square is only 2 log x, therefore, when converting the pressure into decibels, a factor of 2 is entered. Therefore, the difference in the degree of acoustic pressure between the two sound levels with p 1 and p 2 is:
20 log (p 2 / p 1 ) dB = 10 log (p 2 2 / p 1 2 ) dB = 10 log (P 2 / P 1 ) dB.
What happens when the sound power is halved?
Logarithm 2 is 0.3, so 1/2 is 0.3. Thus, if the power is reduced by 2 times, then the sound level will be reduced by 3 dB. And if such an operation is done again, then the acoustics will decrease by another 3 dB.
Decibel size
It can be noted above that a decrease in power by half reduces the pressure on root 2 and the sound volume level by 3 dB.
The first sample is white noise (a mixture of all audible frequencies). The second sample is the same tone with a voltage reduced by the square root of 2 coefficient. Its inverse value is approximately 0.7, so 3 dB corresponds to a decrease in voltage or pressure to 70%. The green line shows the nozzle as a function of time. Red outlines a continuous exponential decline. It is worth noting that the voltage drops by 50% for every second sample.
Sound files and flash animations of John Tann and George Hatsidimitrisa.
How big is the decibel?
In subsequent series, consecutive samples are reduced by only one point.
What if the difference is less than a decibel?
Sound levels are rarely given with decimal places. The reason is that those that differ by less than 1 dB are difficult to distinguish.
And you can also notice that the last example is quieter than the first, but it is difficult to see the difference between consecutive pairs. 10 * log 10 (1.07) = 0.3. Therefore, to increase the sound level by 0.3 dB, it is necessary to increase the power by 7% or the voltage by 3.5%.