Mine Acoustics. Ventilation Lab. #4. March 4, Introduction

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1 Ventilation Lab. #4 March 4, 2002 Introduction Rock concerts and mining operations have one big thing in common: both activities produce sounds at levels which can permanently damage your hearing. To protect miners, federal government has set standards to limit the maximum noise level a miner can be exposed to. The purpose of this lab is to have a basic understanding of how is sound created, how is it measured, how it behaves, and the federal noise regulations. Noise Unwanted Sound Sound is any source that produces a pressure wave in the audible range. To a scientist, sound is vibrating power or, vibrational energy, created when something moves back and forth rapidly and creates waves. Sound waves can travel through any substance rock, metal, water, air. They can't be seen, but they can be felt. When you hit something, you are generating vibrating waves. The reason you can hear the noise when someone hits something is because the vibrating waves transmitted through air reach your ear drum and creating some type of pressure. The amount of pressure, or power of sound depends on the amount of energy reaching your ear: loud noises are sounds with a lot of pressures; soft noises are sounds with little pressure measure pressure changes and you measure sound power. Fundamentals The movement of pressure waves away from a source means that there is a net flow of momentum or energy. The amount of energy flowing through unit area in unit time is known as the intensity (I) and is usually expressed in W/m 2. The magnitude of the pressure variation of a sound wave is known as the sound pressure or acoustic pressure. The intensity is directly proportional to the sound pressure squared and can be obtained from the following equation: I = p2 wc where I = intensity (W/m 2 ) p = sound pressure (Pa) w = density (kg/m 3 ) c = velocity of sound (m/sec) The ear is very sensitive and responds to intensity i.e., sound pressure squared rather than pressure in a non-linear manner. This is fortunate because although the actual sound pressures are very small the range is very large and a linear scale would be cumbersome. Page 1 of 12

2 Sound Power Levels Sound power is not measured directly, but by comparison with a reference point. Further, as the range of sound "power" is so wide (1 billion to one), a logarithm value is used. Power in dba = 10 log 10 W/Wr Wr = reference level = Watts Sound power is measured scientifically in decibels (db), but because the human ear is so complex, what you hear not only depends on the power but also the frequency. The dba scale takes into account the different sensitivity to different frequencies. Being a log 10 scale, dba values cannot be added/subtracted in the normal way. A simple rule is: 3 dba difference doubles or halves the power level 6 dba difference gives 4 or 1/4 of its original power level 9 dba difference gives 8 or 1/8 of its original power level 10 dba difference gives 10 or 1/10 of its original power level. Frequency This term refers to number of complete cycles of sound rise and fall, normally measured in Hertz or cycles per second. The audible frequency range is 20 to 20,000 Hertz. Increase the number of cycles per second and high frequency sounds are produced screeches, for example, or the scream that a jet engine makes. Decrease or reduce the number of cycles per second and low frequency sounds are produced. The rumble of thunder, for example, is an example of a low frequency sound. Table - 1 Relationship between Sound Level and Sound Energy Sound Level Ration of Sound Energy 140 dba - 100,000,000,000, dba - * 10,000,000,000, dba - 1,000,000,000, dba - 100,000,000, dba - 10,000,000, dba - ** 1,000,000, dba - 100,000, dba - 10,000, dba - 1,000, dba - 100, dba - 10, dba - 1, dba dba - 10 * Threshold of pain ** Hearing damage can occur with continued exposure at or above this level 2 of 12

3 The response to changes of intensity can be represented by a logarithmic scale and related to a datum value to avoid negative values. If the datum is taken as the threshold of hearing i.e. 2 x 10-5 Pa the unit of sound becomes: log (pressure)2 (2 x 10-5 ) 2 This unit is known as bel. To avoid the excessive use of decimals it is normally multiplied by 10 and the resultant unit of sound is the decibel or db. With respect to acoustic pressure and acoustic power level of watts corresponds to the lowest intensity the ear can normally detect i.e W/m 2. Sound Pressure Level SPL = 10 log Pressure2 (2 x 10-5 ) 2 db Sound Power Level SWL = 10 log Power db For example, a pneumatic hammer generates 130 dba's of sound power. Assuming that you had excellent hearing (which you wouldn't have for long if you worked around a chipper for several hours), the lowest limit of sound power that you could detect would be 1 dba. Since there are 13 units of 10's between 1 dba and 130 dba, the difference in sound power between the faintest sound you could hear and a pneumatic chipper would be ten trillion! Also, because human ears respond differently to differently frequency levels, the db scale must be adjusted to take this into account. 1 This adjusting to take into account the different sensitivity to different frequencies is called weighting. Most of the noise survey instruments presently available are equipped with three weighting: A, B, and C, with the A weighting discriminates quite heavily against frequencies less than 1,000 Hz resulting in a frequency response approximately that of the human ear. The noise level limits established by the U.S. Congress for all industrial working environments are expressed in dba's, and all sound measuring devices that have been approved for use in or around the mines give readings in dba's. How is noise measured? The features of sound that are important for noise control are frequency and sound power. The sound measuring devices must adjust mid-frequency readings the frequencies to which the ear is most sensitive so that the readings of the instrument approximate the effect that sound would actually have on a human. This process of automatically adjusting frequency readings is called "weighting". All the sound readings taken in or around the mine are the weighted readings. For our instrument, the "A" weighting response emulates the response of the human ear and is used for 1 The loudness of a sound depends not only on the db level but also on the frequency of the sound. For example, a 50 Hz sound and a 5,000 Hz sound appear equally loud to a normal human ear, only if the 50 Hz sound is at a 95 db level and the 5,000 Hz sound is a 76 db level. 3 of 12

4 most industrial and community noise measurements. "C" weighting is used for measuring noise reduction in hearing protectors and for other scientific purposes. The purpose of this lab is to familiarize the students with the principles of mine acoustics and to perform a noise level survey in the Experimental Mine where mining equipment are in operation. Instructions and Equipment The noise level survey to be performed in this lab will be accomplished with the use of a sound level meter. This device will be read at several different locations in the drift while the drill and mucking machine are operating, first individually, then simultaneously. The separation distances of these points, in addition to their respective distances from the drift walls and the machinery, need also to be recorded. Procedure 1. Lecture and sound level meter demonstration. 2. Establish points at places which sound level readings will be taken. 3. Accurately measure distance between noise source(s) and point of measurement and mark clearly on the mine map. 4. Making noise measurements with various combinations of the following three pieces of equipment located at various points ("+": in operation; " ": idle): Mine fan Drill Dozer a) b) + + c) + + d) + Report There are two parts to this lab, the understanding of the federal noise regulations and actual noise measurements underground. Part A: Part B: Briefly outline the noise regulations as specified by CFR (a maximum of 1 page for metal and nonmetal and 2 pages for coal). Noise measurements, including but not limited to, 1) Sound pressure readings (dba) with various combinations where the equipment are run separately as well as together. 2) Describe what things besides distance can affect noise levels. 3) Discuss the topic of noise in the mine environment and provide at least three specific recommendations on how various noise levels in the mine might be reduced use specific equipment/facility to make your case. 4 of 12

5 SOME SOUND INFORMATION For most people, sounds above 130 dba's cause pain. The following table gives some ideas as to the sound levels with which we are familiar with: Table - 2 Different Sound Levels 130 dba - pneumatic chipper 126 dba - large jet motor 120 dba - an active rock band 118 dba - drill on a pneumatic roof bolter 108 dba - loading machine (loading) 107 dba - continuous miner (cutting and loading) 104 dba - wood planer 98 dba - shuttle car (loading) 65 dba - normal conversation 60 dba - large department store 43 dba - a room in an average home 25 dba - whisper 10 dba - sound motion picture studios The way the sound level is calculated is rather unique. For every 10 dba increase in sound level there is really a tenfold increase in sound power. Noise Exposure Levels The Walsh-Healy Public Contracts Act of 1969 sets the standard for noise exposure. The selection of the appropriate standards was complicated due to variations in the response of humans to different noise levels. For instance, the sensitivity of an individual's ear to noise differs from one person to another. Some people are capable of withstanding high levels of noise, whereas others are not capable of tolerating even low levels. In addition, the sensitivity of the ear is different with ranges of frequency. High frequencies can be damaging to the ear. Ear protection can reduce the effect of high frequencies by as much as 20 to 40 dba. The time of exposure to noise was an important factor in determining these standards. The sensitivity of the ear will be recovered slowly after high levels of noise exposures. The selection of appropriate standards as set by the Walsh-Healy Public Contracts Act are complicated by variations in human response. 1. Individual sensitivity to noise varies. Some people can stand very high levels without damage, others not even low levels. 2. Sensitivity of ear varies according to different frequencies. High frequency levels are more dangerous than low ones. Importance of ear protectors is demonstrated for the high frequency sound. Protection normally reduces the effect of high frequencies by 20 to 40 dba, but at low frequencies by only 10 dba or less. 5 of 12

6 3. Time of exposure to noise is important. Further, the sensitivity of ear will slowly recover after high level exposure. 4. Hearing naturally deteriorates with age. 5. As a general rule, sound waves move with equal intensity in all directions from the source of the noise. As the sound waves move outward from their source they cover a larger area and strength diminishes. The sound intensity is at full strength for a distance that is three times the size of the noise source itself. For example, if noise source is four feet in diameter, then the sound is at full strength for approximately 12 feet from the source. With each doubling of the distance from the sound source, the sound strength is reduced by one half. Noise levels over 90 dba are considered to cause hearing damage and miners are not allowed to work in areas above 90 dba for a prolonged periods of time. What this means is, that if a miner works in an area of average noise level of 85 dba through the day (8 hours), he is considered OK even though sometimes he is exposed to noise reading of 100 dba or more. This average can be found by the following equation. C 1 T 1 + C 2 T 2 + C 3 T < 1 C = Time of exposure at a measured level T = Time permitted for exposure at that level Example: For example, the power of the sound made when loading a shuttle car (98 dba) has more than 50 dba than an average home. The sound energy produced for a 50 dba difference is to multiply 10 x 10 x 10 x 10 x 10. In other words, loading a shuttle car produces 100,000 times the sound power that is produced in an average home. With just a little effort most noises can be reduced at least a few decibels. By reducing the noise only three dba's will reduce the sound power to one half of its previous level. A few dba's can make a tremendous difference 6 of 12

7 Exposure limits for noise Federal Regulations on Noise Exposure Limit Metal and Non-metal Mines a) No employee shall be permitted an exposure to noise in excess of that specified in the table below. Noise level measurements shall be made using a sound level meter meeting specifications for type 2 meters contained in American National Standards Institute (ANSI) Standard S , "General Purpose Sound Level Meters," approved April 27, 1971, which is hereby incorporated by reference and made a part hereof, or by a dosimeter with similar accuracy. This publication may be obtained from the American National Standards Institute, Inc., 1430 Broadway, New York, New York 10018, or may be examined in any Metal and Nonmetal Mine Health and Safety District or Subdistrict Office of the Mine Safety and Health Administration. Permissible Noise Exposures Duration per day, hours of exposure Sound level dba, slow response Duration/day, hrs dba / / /4 or less 115 No exposure shall exceed 115 dba. Impact or impulsive noises shall not exceed 140 db, peak sound pressure level. Note: When the daily exposure is composed of two or more periods of noise exposure at different levels, their combined effect shall be considered rather than the individual effect of each. If the sum (C 1 /T 1 )+(C 2 /T 2 )+...(C n /T n ) exceeds unity, then the mixed exposure shall be considered to exceed the permissible exposure. C n indicates the total time of exposure at a specified noise level, and T n indicates the total time of exposure permitted at that level. Interpolation between tabulated values may be determined by the following formula: log T = SL Where T is the time in hours and SL is the sound level in dba. b) When employees' exposure exceeds that listed in the above table, feasible administrative or engineering controls shall be utilized. If such controls fail to reduce exposure to within per- 7 of 12

8 missible levels, personal protection equipment shall be provided and used to reduce sound levels to within the levels of the table. For Coal Mines Definitions. As used in this subpart F, the term-- (a) dba means noise level in decibels, relative to a reference level of 20 micro pascals, as measured by the use of an A-weighting and slow metering characteristic as specified in American National Standards Institute (ANSI), "Specification for Sound Level Meters," S (Type S2A). (b) Noise exposure means a period of time during which the noise level is 90 or more dba; (c) Multiple noise exposure means the daily noise exposure is composed of two or more different noise levels; (d) Noise level is the average dba during a noise exposure; and, (e) Qualified person means, as the context requires, an individual deemed qualified by the Secretary and designated by the operator to make tests and examinations required by this Act. (f) Personal noise dosimeter means equipment worn by an individual, which performs noise level measurements along with exposure time measurements. The circuitry of the instrument is such that it automatically performs the computation of the multiple noise exposure specified in Requirements. Every operator of an underground coal mine shall maintain the noise levels during each shift to which each miner in the active workings of the mine is exposed at or below the permissible noise levels set forth in Table I of this subpart. Example: If a noise is recorded to be 110 dba then exposure shall not exceed 30 minutes during an 8-hour shift Computation of multiple noise exposure. The standard will be considered to have been violated in the case of multiple noise exposure where such exposure totals exceed one as computed by adding the total time of exposure at each specified level (C1, C2, C3, etc.) divided by the total time of exposure permitted at that level (T1, T2, T3). Thus, [C1/T1]+[C2/T2]+[C3/T3] must not exceed 1. Example I: Exposure of 2 hours at 92 dba and 1 hour at 100 dba during an 8-hour shift. Total minutes of noise exposure at dba level/total minutes of permissible noise exposure at dba level [120 min./360 min min./120 min.] = 2/6 + 1/2 = 2/6 + 3/6 = 5/6 The sum of the fractions does not exceed one; hence the exposure for the shift would not violate the standard. Example II: Exposure of 3 hours at 95 dba and 1 hour at 100 dba during an 8 hour shift. 3/4 + 1/2 = 3/4 + 2/4 = 5/4 The sum of the fractions exceeds one; hence the exposure for the shift would violate the standard Noise exposure measurements; general. Every coal mine operator shall take accurate readings of the noise levels to which each miner in the active workings of the mine is exposed during the performance of the duties to which he is normally assigned. 8 of 12

9 Noise exposure measurements; by whom done. The noise exposure measurements required by this subpart F shall be taken by, or as directed by, a person who has met the minimum requirements set forth in , and has been certified by the Assistant Secretary of Labor for Mine Safety and Health, Mine Safety and Health Administration as qualified to take noise exposure measurements as prescribed in this subpart F Persons qualified to measure noise exposures; minimum requirements. The following persons shall be considered qualified to take noise exposure measurements as prescribed in this subpart F; (a) Any person who has been certified by the Mine Safety and Health Administration as an instructor in noise measurement training programs; (b) Any person who has satisfactorily completed a noise training course conducted by the Mine Safety and Health Administration and has been certified by the Administration as a qualified person; and, (c) Any person who has satisfactorily completed a noise training course approved by the Mine Safety and Health Administration and has been certified by the Administration as a qualified person Certification of qualified persons by the Mine Safety and Health Administration. Upon a satisfactory showing that a person has met the minimum requirements for taking noise exposure measurements set forth in , the Mine Safety and Health Administration shall certify that such person has the ability and capacity to conduct tests of the noise exposure in a coal mine and to report and certify the results of such tests to the Secretary and the Secretary of Health and Human Services Noise exposure measurement equipment. Noise exposure measurements shall be taken only with equipment which is approved by the Mine Safety and Health Administration as permissible electric face equipment under the provisions of part 18 of this chapter and which in the case of sound level meters, meets American National Standards Institute (ANSI), "Specification for Sound Level Meters," S (Type S2A), or in the case of personal noise dosimeters, has been found to be acceptable by the Mine Safety and Health Administration Noise exposure measurement procedures; instrument setting; calibration. (a) Noise exposure measurements made with sound level meters shall conform to the following: (1) Noise exposure measurements shall be made at locations where the noise is typical of that entering the ears of the miner whose exposure is under consideration. (2) Five measurements shall be made for each type of noise exposure producing operation to which the miner under consideration is exposed. (3) Each measurement shall be made by observing the A-scale readings for 30 seconds and recording the noise level. (4) The average of the five noise level measurements shall be considered as the noise level measurement which is representative of the operation. (5) Where different and distinct noise levels occur at various phases of an operation, noise exposure measurements shall be made in accordance with this section for each distinct phase. (6) The noise levels and the estimated length of time the miner is exposed to each level during a normal work shift shall be reported for the operation. (b) Noise exposure measurements made with personal noise dosimeters shall conform to the following: 9 of 12

10 (1) For the miner whose noise exposure is under consideration, noise exposure measurements shall be made with the personal noise dosimeter microphone located at the top of the shoulder, midway between the neck and the end of the shoulder with the microphone pointing in a vertical upward direction in accordance with the diagram shown below: [See illustration P70.PCX] (2) To the extent practical, the personal noise dosimeter instrument case and microphone cable shall be positioned underneath exterior clothing so as to minimize potential safety problems and damage to the instrument. The microphone shall not be covered by clothing. (3) The personal noise dosimeter shall be worn by the miner whose noise exposure is under consideration for an entire normal work shift and the accumulated per centum of the noise exposure shall be reported. (c) Noise exposure measurement instruments specified in shall be set to operate with the A-weighted network and slow response. (d)(1) Sound level meters and personal noise dosimeters used by an operator in fulfilling the requirements of this subpart shall be acoustically calibrated in accordance with the manufacturer's instructions before and after each shift on which the meter is used. (2) Sound level meters and personal noise dosimeters used by an authorized representative of the Secretary shall be acoustically calibrated in accordance with the manufacturer's instructions or by another equivalent procedure before and after each shift on which the meter is used. (3) Personal noise dosimeters shall be recalibrated annually, including, as a minimum, the following: (i) Visual inspection of the microphone for any foreign matter or damage, (ii) Comparison of the dosimeter, at 1000 Hz, with a laboratory type condensor microphone of known sensitivity, and (iii) Frequency response testing in a free or diffused field where the sound field is established using a laboratory type condensor microphone of known sensitivity. (4) A document containing the date of the annual recalibration of each personal noise dosimeter and the names of the individual and organization performing the calibration shall be kept on file at each mine office. (e)(1) Acoustical calibrators which are used to calibrate sound level meters and personal noise dosimeters shall be recalibrated once a year using a laboratory type condensor microphone of known sensitivity as determined by a National Bureau of Standards calibration. (2) A document containing the date of the annual calibration of each acoustical calibrator and the names of the individual and organization performing the calibration shall be on file at each mine office Initial noise exposure survey. On or before June 30, 1971, each operator shall-- (a) Conduct, in accordance with this subpart, a survey of the noise levels to which each miner in the active workings of the mine is exposed during his normal work shift; and, (b) Report and certify to the Mine Safety and Health Administration, and the Department of Health and Human Services, the results of such survey using the Coal Mine Noise Data Report, Figure 1. Reports shall be sent to: Division of Automatic Data Processing, Mine Safety and Health Administration, Building 53, Denver Federal Center, Colo Periodic noise exposure survey. (a) At intervals of at least every 6 months after June 30, 1971, but in no case shall the interval be less than 3 months, each operator shall conduct, in accordance with this subpart, periodic surveys of the noise levels to which each miner in the active workings of the mine is exposed and shall report and certify the results of such surveys to the Mine Safety and Health Administration, and the Department of Health and Human Services, using the Coal Mine Noise Data Report Form. Reports shall be sent to: 10 of 12

11 Division of Automatic Data Processing, Mine Safety and Health Administration, Building 53, Denver Federal Center, Colo (b) Where no A-scale reading recorded for any miner during an initial or periodic noise exposure survey exceeds 90 dba, the operator shall not be required to survey such miner during any subsequent periodic noise level survey required by this section: Provided, however, That the name and job position of each such miner shall be reported in every periodic survey and the operator shall certify that such miner's job duties and noise exposure levels have not changed substantially during the preceding 6-month period. (Approved by the Office of Management and Budget under control numbers and ) Supplemental noise exposure survey; reports and certification. (a) Where the certified results of an initial noise exposure survey conducted in accordance with , or a periodic noise exposure survey conducted in accordance with , show that any miner in the active workings of the mine is exposed to a noise level in excess of the permissible noise level prescribed in Table I, the operator shall conduct a supplemental noise exposure survey with respect to each miner whose noise exposure exceeds this standard. This survey shall be conducted within 15 days following notification to the operator by the Mine Safety and Health Administration to conduct such survey. (b) Supplemental noise exposure surveys shall be conducted by taking noise exposure measurements in accordance with , however, noise exposure measurements shall be taken during the entire period of each individual operation to which the miner under consideration is actually exposed during his normal work shift. (c) Each operator shall report and certify the results of each supplemental noise level survey conducted in accordance with this section to the Mine Safety and Health Administration and the Department of Health and Human Services using the Coal Mine Noise Data Report Form to record noise level readings taken with respect to all operations during which such measurements were taken. (d) Supplemental noise exposure surveys shall, upon completion, be mailed to: Violation of noise standard; notice of violation; action required by operator. (a) Where the results of a supplemental noise exposure survey conducted in accordance with show that any miner in the active workings of the mine is exposed to noise levels which exceed the permissible noise levels prescribed in Table I, the Secretary shall issue a notice to the operator that he is in violation of this subpart. (b) Upon receipt of a Notice of Violation issued pursuant to paragraph (a) of this section, the operator shall: (1) Institute promptly administrative and/or engineering controls necessary to assure compliance with the standard. Such controls may include protective devices other than those devices or systems which the Secretary or his authorized representative finds to be hazardous in such mine. (2) Within 60 days following the issuance of any Notice of Violation of this subpart, submit for approval to a joint Mine Safety and Health Administration-Health and Human Services committee, a plan for the administration of a continuing, effective hearing conservation program to assure compliance with this subpart, including provision for: (i) Reducing environmental noise levels; (ii) Personal ear protective devices to be made available to the miners; (iii) Pre-employment and periodic audiograms. (3) Plans required under paragraph (b) (2) of this section shall be submitted to: Administrator, Coal Mine Health and Safety, Mine Safety and Health Administration, Department of Labor, 4015 Wilson Boulevard, Arlington, Va of 12

12 Table I--Permissible Noise Exposures Duration/day, hrs dba / / /4 or less Incorporation by reference. In accordance with 5 U.S.C. 552(a)(1), the technical publication, "Specification for Sound Level Meters", S (Type S2A), issued by the American National Standards Institute (ANSI), April 27, 1971, referenced in this subpart F is hereby incorporated by reference and made a part hereof. The incorporated technical publication is available for examination at MSHA, 4015 Wilson Blvd., Arlington, Va ; the National Institute for Occupational Safety and Health, 5600 Fishers Lane, Rockville, Md ; each Coal Mine Health and Safety District and Subdistrict Office and the Federal Register library. In addition, copies of the document can be purchased from the American National Standards Institute (ANSI), 1430 Broadway, New York, N.Y of 12