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Basic Information on Noise

Basic Information on Noise
"""What do noise and sound mean?

The thing we hear is sound. Noise is unwelcome audio. The listener and the situation determine the distinction between sound and noise. One person may find enjoyment in rock music while another may find it bothersome. In any situation, if the sound is loud and the person is exposed for a long enough period of time and frequently enough, it can be harmful to their hearing. Vibrating things produce sound, which travels through the air or other media and is heard by the listener. The air pressure somewhat changes as an object vibrates. Sound is created when air pressure changes cause waves to travel through the atmosphere. Imagine using a stick to strike the surface of a drum as an example. The surface of the drum oscillates back and forth. It pushes the air in touch with the surface as it advances. By compressing the air, this raises the pressure to a positive value. By decompressing the air, the surface travels in the opposite direction to produce a negative (lower) pressure. As a result, as the drum surface vibrates, alternating areas of higher and lower air pressure are produced. As sound waves, these pressure variations move across the atmosphere (Figure 1). Table 1 The approximate velocities of sound in air and various mediums are listed in Table 1. Remember the ""Mickey Mouse"" sound individuals make when talking after inhaling helium gas? This is because the higher the sound velocity, the higher the pitch will be in gases. Table 1 provides an approximation of the sound speed in common materials. Medium Sound Velocity in Feet Per Second m/s Dry air (760 mm Hg, 0 °C) 1,100 330 Wood (soft - along the fibre) (soft - along the fibre) 11,100 3,400 (15°C) water 4,700 1,400 Concrete 10,200 3,100 Steel 16,000 5,000 Glass 18,500, 5,500, Lead 3,700, 1,200, and Hydrogen at 0°C and 760 mm Hg. 4,100 1,260 The ear's hearing system detects sound waves and transforms them into information that it communicates to the brain. The information is perceived as sound by the brain. When compared to the surrounding air pressure, even very loud noises cause extremely modest pressure fluctuations (1 in 10,000). (i.e., atmospheric pressure). Even tiny pressure waves can be detected by the ear's hearing mechanism. Due to its fragility, hearing can be harmed by loud noise.

Why is noise a serious occupational hazard?

One of the most prevalent occupational health risks is noise. In settings like fields, cafeterias, or pubs, as well as environments like heavy industry and manufacturing. The biggest health concern is permanent hearing loss. The major issues in noisy workplaces, classrooms, and computer rooms are annoyance, tension, and interference with verbal communication. Noise levels should be brought down to tolerable levels in order to avoid the negative effects of noise exposure. Engineering changes to the noise source or the working environment are the most effective way to reduce noise. Personal hearing protection (such ear muffs or plugs) can be utilized when technology is unable to sufficiently handle the issue. Personal safety, however, should be taken into consideration as a stopgap measure while other strategies for decreasing workplace noise are researched and put into practice. Workplaces need to identify areas or activities where excessive noise exposure occurs as a first step in dealing with noise.

How can I know whether the noise level at work is too high?

If you indicated yes to any of the following, there might be a noise issue at work. Do people need to speak louder? Do those who work in noisy settings experience ringing in their ears after a shift? Do they discover that they need to crank up their car radio more than they did when they left for work when they go home from work? Does a person who has spent years working in a noisy environment have trouble comprehending conversations during gatherings with many of voices and ""competing"" noises, such as parties or restaurants? If there is a noise problem in a workplace, then a noise assessment or survey should be undertaken to determine the sources of noise, the amount of noise, who is exposed and for how long.

What are some measurements-able characteristics of noise?

The properties of noise which are important in the workplace are: frequency sound pressure sound power time distribution

What are pitch and frequency?

Frequency is the rate at which the source produces sound waves, i.e. complete cycles of high and low pressure regions. In other words, frequency is the number of times per second that a vibrating body completes one cycle of motion. The unit for frequency is the hertz (Hz = 1 cycle per second: Figure 1 shows one cycle from an air compression to a minimum pressure and back to a maximum pressure). The Harvard Dictionary of Music defines the pitch as a """"stretch of sound whose frequency is clear and stable enough to be heard as not noise"""". The Oxford Dictionary defines pitch as """"the quality of a sound governed by the rate of vibrations producing it; the degree of highness or lowness of a tone."""" Since the pitch is determined mostly by frequency, it is usually identified with it. The pitch depends also, in a lower degree, on the sound level and on the physiology of the auditory system. Low pitched or bass sounds have low frequencies. High-pitched or treble sounds have high frequencies. A healthy, young person can hear sounds with frequencies from roughly 20 to 20,000 Hz. The sound of human speech is mainly in the range 300 to 3,000 Hz.

What is sound pressure?

Sound pressure is the amount of air pressure fluctuation a noise source creates. We """"hear"""" or perceive sound pressure as loudness. If the drum in our example (Figure 1) is hit very lightly, the surface moves only a very short distance and produces weak pressure fluctuations and a faint sound. If the drum is hit harder, its surface moves farther from its rest position. As a result, the pressure increase is greater. To the listener, the sound is louder. Sound pressure also depends on the environment in which the source is located and the listener's distance from the source. The sound produced by the drum is louder two metres from the drum if it is in a small bathroom, than if it is struck in the middle of a football field. Generally, the farther one moves from the drum, the quieter it sounds. Also if there are hard surfaces that can reflect the sound (e.g., walls in a room), the sound will feel louder than if you heard the same sound, from the same distance, in a wide-open field. Sound pressure is usually expressed in units called pascals (Pa). A healthy, young person can hear sound pressures as low as 0.00002 Pa. A normal conversation produces a sound pressure of 0.02 Pa. A gasoline-powered lawn mower produces about 1 Pa. The sound is painfully loud at levels around 20 Pa. Thus the common sounds we hear have sound pressure over a wide range (0.00002 Pa - 20 Pa).

What is a sound pressure level?

It is difficult to work with the broad range of common sounds pressures (0.00002 Pa - 20 Pa). To overcome this difficulty we use decibel (dB, or tenth (deci) of a Bel)). The decibel or dB scale is more convenient because it compresses the scale of numbers into a manageable range. The decibel is named after Alexander Graham Bell, the Canadian pioneer of the telephone who took great personal interest in the problems of the Deaf or people with hearing loss. Sound pressure converted to the decibel scale is called sound pressure level (Lp). Appendix A gives a detailed explanation of decibels and sound pressure levels. Figure 2 compares sound pressures in pascals and sound pressure levels in decibels (dB). The zero of the decibel scale (0 dB) is the sound pressure of 0.00002 Pa. This means that 0.00002 Pa is the reference sound pressure to which all other sound pressures are compared on the dB scale. This is the reason the decibels of sound are often indicated as dB re 0.00002 Pa. Figure 2

What is sound power?

The sound power is the sound energy transferred per second from the noise source to the air. A noise source, such as a compressor or drum, has a given, constant sound power that does not change if the source is placed in a different environment. Power is expressed in units called watts (W). An average whisper generates a sound power of 0.0000001 watts (0.1 microwatt (µW)), a truck horn 0.1 W, and a turbo jet engine 100,000 W. Like sound pressure, sound power (in W) is usually expressed as sound power levels in dB. Appendix B provides examples of sound power level calculations. Figure 3 relates sound power in watts to sound power level in decibels. Note that while the sound power goes from one trillionth of a watt to one hundred thousand watts, the equivalent sound power levels range from 0 to 170 dB. Figure 3

What is the relation between sound pressure and sound power?

Because the sound power of a noise source is constant and specific, it can be used to calculate the expected sound pressure. The calculation requires detailed information about the noise source's environment. Usually a noise source with a lower sound power generates less sound pressure. For example, from the sound power of a compressor, one can calculate the expected sound pressure and sound pressure level at a certain location and distance. This information can be helpful in determining possible noise exposures and how they compare to the noise guidelines. The manufacturer can often provide noise emission data such as sound pressure levels under standard conditions and the sound power of equipment. A number of international standards are available for labelling machines and equipment with their noise emission levels. In Canada, the CSA Standard Z107.58-15 Noise Emission Declarations for Machinery provides manufacturers with a means to determine and to create noise emission declarations for the machinery they produce. In addition, Health Canada has issued a notice to stakeholders recommending that this noise data be included in the technical sales literature and instructions for use of machinery intended for a workplace.

What kinds of noise are there?

Noise can be continuous, variable, intermittent or impulsive depending on how it changes over time. Continuous noise is noise which remains constant and stable over a given time period. The noise of boilers in a power house is relatively constant and can therefore be classified as continuous. Most manufacturing noise is variable or intermittent. Different operations or different noise sources cause the sound changes over time. Noise is intermittent if there is a mix of relatively quiet and noisy periods. Impulse or impact noise is a very short burst of loud noise which lasts for less than one second. Gun fire or the noise produced by punch presses are examples of such noise.

What are A-weighted decibels?

The sensitivity of the human ear to sound depends on the frequency or pitch of the sound. People hear some frequencies better than others. If a person hears two sounds of the same sound pressure but different frequencies, one sound may appear louder than the other. This difference occurs because people hear high frequency noise much better than low frequency noise. Noise measurement readings can be adjusted to correspond to this peculiarity of human hearing. An A-weighting filter which is built into the sound measuring instrument de-emphasizes low frequencies or pitches. Decibels measured using a sound meter equipped with this filter are A-weighted and are called dB(A). Legislation on workplace noise normally gives exposure limits in dB(A). Table 2 lists examples of typical noise levels. A-weighting serves two important purposes: gives a single number measure of noise level by integrating sound levels at all frequencies gives a scale for noise level as experienced or perceived by the human ear Table 2 Typical Noise Levels Noise Source dB(A) pneumatic chipper at 1 metre 115 hand-held circular saw at 1 metre 115 chainsaw, leaf blower, snowmobile 106-115 ATV, motorcycle 96-100 subway, shouted conversation 90-95 power lawn mower at 1 metre 92 diesel truck 50 km per hour at 20 metres 85 passenger car 60 km per hour at 20 metres 65 conversation at 1 metre 55 quiet room 40

What are basic rules of working with decibel (dB) units?

The decibel [dB, and also dB(A)] is a logarithmic scale. For mathematical calculations using dB units, we must use logarithmic mathematics (see Appendix A). However, in our day-to-day work we do not need such calculations. The use of dB unit makes it easy to deal with the workplace noise level data provided we use a set of simple rules as summarized in Table 3. Table 3 Decibel (dB) basics Change in dB Change in sound energy 3 dB increase Sound energy doubled 3 dB decrease Sound energy halved 10 dB increase Sound energy increased by factor of 10 10 dB decrease Sound energy decreased by factor of 10 20 dB increase Sound energy increased by factor of 100 20 dB decrease Sound energy decreased by factor of 100

How are noise levels added?

Sound pressure levels in decibels (dB) or A-weighted decibels [dB(A)] are based on a logarithmic scale (see Appendix A). They cannot be added or subtracted in the usual arithmetical way. If one machine emits a sound level of 90 dB, and a second identical machine is placed beside the first, the combined sound level is 93 dB, not 180 dB. Table 4 shows a simple way to add noise levels. Table 4 Addition of Decibels Numerical difference between two noise levels [dB(A)] Amount to be added to the higher of the two noise levels [dB or dB(A)] 0 3.0 0.1 - 0.9 2.5 1.0 - 2.4 2.0 2.4 - 4.0 1.5 4.1 - 6.0 1.0 6.1 - 10 0.5 10 0.0 Step 1: Determine the difference between the two noise levels and find the corresponding row in the left hand column. Step 2: Find the number [dB or dB(A)] corresponding to this difference in the right hand column of the table. Step 3: Add this number to the higher of the two decibel levels. For instance, using the example of two machines each emitting a noise level of 90 dB: Step 1: The numerical difference between the two levels is 0 dB (90-90= 0), using the first row. Step 2: The number corresponding to this difference of 0, taken from the right hand column, is 3. Step 3: Add 3 to the highest level, in this case 90. Therefore, the resulting noise level is 93 dB. When the difference between two noise levels is 10 dB(A) or more, the amount to be added to the higher noise level is zero. In such cases, no adjustment factor is needed because adding in the contribution of the lower in the total noise level makes no perceptible difference in what people can hear or measure. For example if your workplace noise level is 95 dB(A) and you add another machine that produces 80 dB(A) noise, the workplace noise level will still be 95dB(A). Appendix A - Sound Pressure Level Calculations Sound pressure level in decibels is defined in the following way: dB = 20 log (Sound Pressure/Reference Pressure) The """"log"""" or logarithm of a number is a mathematical manipulation of the number, based on multiples of 10. It is the exponent that indicates the power to which the number 10 is raised to produce a given number. For example, the logarithm of 10 is 1 since 10 is multiplied by itself only once to get 10. Similarly, the logarithm of 100 is 2 since 10 times 10 is 100. The logarithm of 1000 is 3 since 10 times 10 times 10 is 1000. Therefore log(1) = 0 Since 10 to the exponent 0 = 1, log(10) = 1 since 10 to the exponent 1 = 10, log(100) = 2 since 10 to the exponent 2 = 100, log(1000) = 3 since 10 to the exponent 3 = 1000 The logarithm scale simply compresses the large span of numbers into a manageable range. In other words, the scale from 10 to 1000 is compressed, by using the logarithms, to a scale of 1 to 3. The decibel scale for sound pressures uses as the reference pressure the lowest noise that the healthy young person can hear (0.00002 Pa). It divides all other sound pressures by this amount when calculating the decibel value. Sound pressures converted to the decibel scale are called sound pressure levels, abbreviated Lp. So, the sound pressure level of the quietest noise the healthy young person can hear is calculated in this way: Lp = 20 log ( 0.00002/ 0.00002) = 20 log (1) = 20 X 0 = 0 dB The sound pressure level or Lp in a very quiet room, where the sound pressure is 0.002 Pa, is calculated: Lp = 20 log (0.002/ 0.00002) = 20 log (100) = 20 X 2 = 40 dB The sound pressure level of a typical gasoline-powered lawn mower, which has a sound pressure of 1 Pa, is calculated Lp = 20 log (1/0.00002) = 20 log (50 000) = 20 X 4.7 = 94 dB Appendix B - Sound Power Level Calculations Sound power levels or Lw are determined by the following formula: Lw = 10 log (Sound Power Level / Reference Power Level ) The reference power is one trillionth of a watt (0.000000000001 W). Therefore Lw = 10 log (Sound Power Level / 0.000000000001) Thus, the sound power level associated with an average whisper, which has a sound power of 0.0000001 W, is calculated Lw = 10 log (0.0000001/ 0.000000000001) = 50 dB"""

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"Basic Information on Noise" was written by Mary under the Health category. It has been read 41 times and generated 0 comments. The article was created on and updated on 23 November 2022.
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