Noise pollution, unwanted or excessive sound levels can have harmful effects on human he alth and environmental quality. It commonly occurs in many industrial facilities and some other workplaces. As well as noise pollution associated with road, rail and air traffic and outdoor activities.
Measurement and perception of loudness
Sound waves are vibrations of air molecules carried from a noise source to the ear. It is usually described in terms of loudness (amplitude) and pitch (frequency) of the wave. Sound pressure level, or SPL, is measured in logarithmic units called decibels (dB). The normal human ear can detect a tone ranging from 0 dB (threshold of hearing) to 140 dB. At the same time, sounds from 120 dB to 140 dB cause pain.
What is the sound level, for example, in the library? It is about 35 dB, while inside a moving bus or subway train it is about 85. Construction workbuildings can generate up to 105 dB SPL 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, a 10-point increase represents a 10-fold increase in sound intensity. At 20, it transmits 100 times more. And 30dB represents a 1000x increase in intensity.
On the other hand, when the tension doubles, the volume level of the sound only increases by 3 points. For example, if a construction drill produces 90 dB of noise, then two identical tools working side by side will create 93 dB. And when two sounds that differ by more than 15 points in SPL are combined, the weak tones are masked (or drowned out) by the loud sound. For example, if a drill is running at 80 dB on a construction site next to a bulldozer at 95, 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 a sound wave is expressed in cycles per second, but hertz is more commonly used (1 cps=1 Hz). The human tympanic membrane is a highly sensitive organ with a large dynamic range, capable of detecting sounds at frequencies ranging from 20 Hz (low pitch) to approximately 20,000 Hz (high pitch). The tonality of the human voice in normal conversation occurs at frequencies from 250 Hz to 2000 Hz.
Accurate sound level measurement and scientific description is different from most subjective human notions and opinions about it. Individual human responses to noise depend on both pitch and loudness. People with normal hearing usually perceive high frequency sounds louder than low frequency sounds of the same amplitude. For this reason, electronic noise meters take into account changes in perceived loudness with pitch.
Frequency filters in the meters serve to match the readings with the sensitivity of the human ear and the relative loudness 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 a 6-10 dBA increase in SPL as doubling "loudness". 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 averages to express overall sound levels. There are several ways to do this. For example, a series of repeated sound level measurements could be reported as L 90=75 dBA, meaning that the values were equal to or greater than 75 dBA for 90 percent of the time.
Another unit called sound equivalent degrees (L eq) can be used to express the average SPL over any period of interest, such as an eight-hour workday.(L eq is a logarithmic value, not an arithmetic value, so loud events dominate the total result.)
A unit of sound level called Day-Night Noise Value (DNL or Ldn) takes into account the fact that people are more sensitive to tone at night. So, 10-dBA is added to SPL values measured between 10 am and 7 am. For example, DNL measurements are very useful in describing overall exposure to aircraft noise.
Working with effects
Noise is more than just a nuisance. At certain levels and durations of exposure, it can cause physical damage to the eardrum and sensitive hair cells in the inner ear, resulting in temporary or permanent hearing loss.
It usually doesn't occur below 80 dBA SPL (8-hour influence levels are best kept below 85). But most people repeatedly exposed to more than 105 dBA will have some degree of permanent hearing loss. In addition, excessive exposure to noise can also increase blood pressure and heart rate, cause irritability, anxiety, and mental fatigue, and interfere with sleep, relaxation, and intimacy.
Noise pollution control
Therefore, it is important to maintain the utmost silence in the workplace and in society. Noise regulations and laws adopted at the local, regional and national levels can be effective in mitigating the negative effects of noise pollution.
Environmental andindustrial hum is regulated under the occupational safety and he alth law and the law against it. Under these regulations, the Occupational Safety and He alth Administration established criteria for industrial noise to impose limits on the intensity of sound exposure and the duration for which this intensity can be allowed.
If a person is exposed to different levels of noise at different times during the day, the total exposure or dose (D) of the noise is obtained from the ratio,
where C is the actual time and T is the allowed time at any level. Using this formula, the most possible daily noise dose would be 1, and any exposure above that would be unacceptable.
Max sound level
Criteria for indoor noise are summarized in three sets of specifications that were obtained by collecting subjective judgments from a large sample of people in various specific situations. These have evolved into Noise Criteria (NC) and Preferred Tone Curves (PNC), which set limits on the level introduced into the environment. NC curves, developed in 1957, aim to provide a comfortable working or living environment by specifying the maximum allowable sound level in octave bands across the entire audio spectrum.
A complete set of 11 curves define noise criteria for a wide range of situations. PNC graphics, developed in 1971, add limits to low frequency hum and high frequency hiss. Therefore, they are preferredolder NC standard. Summarized on the curves, these criteria provide design targets for noise levels for various ideas. Part of the job or habitat specification is the corresponding PNC curve. In the event that the level exceeds the PNC limits, sound-absorbing materials may be introduced into the environment as needed to comply with the standards.
Low noise levels can be overcome with additional absorbent material such as heavy draperies or indoor tiles. Where low levels of identifiable noise may be distracting, or where the privacy of conversations in adjacent offices and reception areas may be important, unwanted sounds may be masked. A small source of white noise, such as static air, placed in a room can mask conversation from nearby offices without being a lethal sound level to the ears of people working nearby.
This type of device is often used in the offices of doctors and other professionals. Another method of noise reduction is the use of hearing protectors, which are worn over the ears in the same way as earmuffs. By using commercially available protectors, tone reduction can be achieved in the range typically from 10 dB at 100 Hz to over 30 dB for frequencies above 1,000 Hz.
Detect sound level
Outdoor noise limits are also important for human comfort. The construction of a building will provide some protection from external sounds if the building meets minimum standards and ifthe noise level is within acceptable limits.
These limits are usually specified for certain periods of the day, such as during daylight hours, in the evening and at night while sleeping. Due to refraction in the atmosphere caused by the temperature inversion at night, relatively loud sounds can be emitted from a rather 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, which prevail on roads and are inherent in large vehicles. To be effective, they must be as close as possible to the source or observer of the noise, thereby maximizing the diffraction required 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
Decibel (dB) is used to measure sound levels, but it is also widely used in electronics, signals, and communications. DB - logarithmic way of describing tangency. The ratio can manifest itself as power, sound pressure, voltage or intensity, or several other things. Later we associate dB with the phone and sound (in relation to loudness). But first, to get an idea of logarithmic expressions, let's look at some numbers.
For example, we can assume that there are two speakers,the first of which plays a 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 the same.
The decibel difference between them is defined as
10 log (P 2 / P 1) dB where log is for base 10.
If the second produces twice as much energy as the first, the difference is in dB
10 log (P 2 / P 1)=10 log 2=3 dB,
as shown in the graph that plots 10 log (P 2 / P 1) vs P 2 / P 1. To continue the example, if the second has 10 times the power of the first, the difference in dB would be:
10 log (P 2 / P 1)=10 log 10=10 dB.
If the second had the same strength a million times, the dB difference would be
10 log (P 2 / P 1)=10 log 1 000 000=60 dB.
This example shows one feature of the decibel scales that is useful when discussing sound. They can describe very large relationships using modestly sized numbers. But you need to pay attention that the decibel represents the ratio. That is, it will not be said how much power any of the speakers emits, only from the difference. And also pay attention to the factor 10 in the definition, which stands for deci in decibels.
Acoustic pressure and dB
Frequency is usually measured with microphones and they respond (approximately) proportional to pressure, s. Now the power of the sound wave at otherunder the same conditions is equal to the square of the head. Similarly, electrical power in a resistor goes like voltage multiplied. The logarithm of the square is only 2 log x, so when converting pressure to decibels, a factor of 2 is introduced. Therefore, the difference in degree of acoustic pressure between two levels of sounds with p 1 and p 2 is:
20 log (p 2 / p 1) dB=10 log (p 22 / p 1 2) dB=10 log (P 2 / P 1) dB.
What happens when the sound power is halved?
The logarithm of 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 you do this operation again, then the acoustics will decrease by another 3 dB.
Decibel size
You can see above that halving the power reduces the pressure on root 2 and the sound level by 3 dB.
The first sample is white noise (a mixture of all audible frequencies). The second sample is the same tone with the voltage reduced by a factor of the square root of 2. Its reciprocal is approximately 0.7, so 3 dB corresponds to a reduction in voltage or pressure of up to 70%. The green line shows the nozzle as a function of time. The red one outlines a continuous exponential decline. Note that the voltage drops by 50% for every second sample.
Sound files and flash animation by John Tann and George Hatsidimitris.
How big is a decibel?
Bin the following series, consecutive samples decrease by only one point.
What if the difference is less than a decibel?
Sound levels are rarely given in decimals. The reason is that those that differ by less than 1 dB are difficult to distinguish.
And you can also see that the last example is quieter than the first, but it's hard to see the difference between consecutive pairs. 10log 10 (1.07)=0.3. Therefore, to increase the sound level by 0.3 dB, you need to increase the power by 7% or the voltage by 3.5%.
