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docdroid NOISE DOSE METER.docx Report Share o o o o o http://docdro.id Twitter Facebook Embed Download o DOCX o o o o o PDF DOC ODT TXT ACCELERATED MOTION, LAMINAR AND TUBULENT FLOW BY GILA ARHIWO MANGA (UAM/APP/P/1491) AND OGAH PETER EJEGWOYA (UAM/APP/P/3564) A PAPER PRESENTATION ON PHY 687: ADVANCED AEROSOL PHYSICS. FEDERAL UNIVERSITY OF AGRICULTURE MAKURDI APRIL, 2013 1 ABSTRACT The need to ascertain the noise exposure of workers during their n ormal working day, has led to the development of the noise dosimeter. This is a small, light and compact instrument to be worn by the worker. It measures the total Aweighted sound energy received and expresses it as a proportion of the maximum A- weighted energy that can be received per day. This study was carried out on Noise Dose Meter and Audiometer. The working principle of a NDM and the Audiometer, the difference between NDM and SLM where all vividly explained. 2 1.1 NOISE DOSE METER A noise dosimeter (American) or noise dose meter (British) is a specialized sound level meter intended specifically to measure the noise exposure of a person integrated over a period of time; usually to comply with Health and Safety regulations such as the Occupational Safety and Health (OSHA) 29 CFR 1910.95 Occupational Noise Exposure Standard or EU Directive 2003/10/EC. Basically, there are two different instruments to measure noise exposures: the sound level meter and the dosimeter. A sound level meter is a device that measures the intensity of sound at a given moment. Since sound level meters provide a measure of sound intensity at only one point in time, it is generally necessary to take a number of measurements at different times during the day to estimate noise exposure over a workday. If noise levels fluctuate, the amount of time noise remains at each of the various measured levels must be determined. To estimate employee noise exposures with a sound level meter it i s also generally necessary to take several measurements at different locations within the workplace. After appropriate sound level meter readings are obtai ned, people sometimes draw "maps" of the sound levels within different areas of the workplace. By using a sound level "map" and information on emplo yee locations throughout the day, estimates of individual exposure levels can be developed. This measurement method is generally referred to as "area" noise monitoring. A dosimeter is like a sound level meter except that it stores sound level measurements and integrates these measurements over time, providing an average noise exposure reading for a given period of time, such as an 8-hour workday. With a dosimeter, a microphone is attached to the employee's clothing and the exposure measurement is simply read at the end of the desired time period. A reader may be used to read-out the dosimeter's measurements. Since the dosimeter is worn by the employee, it measures noise levels in those locations in which the employee travels. A sound level meter can als o be positioned within the immediate vicinity of the exposed worker to obtain an individual exposure estimate. Such procedures are generally referred t o as "personal" noise monitoring. Area monitoring can be used to estimate noise exposure when the noise levels are relatively constant and employees are not mobile. In workplaces where employees move about in different areas or where the noise intensity tends to fluctuate over time, noise exposure is generally more accurately estimated by the personal monitoring approach. 3 In situations where personal monitoring is appropriate, proper positioning of the microphone is necessary to obtain accurate measurements. With a dosimeter, the microphone is generally located on the shoulder and remains in that position for the entire workday. With a sound level meter, the microphone is stationed near the employee's head, and the instrument is usually held by an individual who follows the employee as he or she moves about. Manufacturer's instructions, contained in dosimeter and sound level meter operating manuals, should be followed for calibration and maintenance. To ensure accurate results, it is considered good professional practice to calibrate instruments before and after each use. 1.2 TYPES OF NOISE DOSE METER 1.21 Personal Noise Dose Meter Personal sound level meters are in fact integrating sound level meters designed as dosimeters in order to be worn by the worker during his regular work. These instruments make it possible to record on almost any increment of time the equivalent level, the peak level or any statistical parameter. Typically it will record the LAeq,T (in dB(A)) and Lpeak (in dB) every second. This is extremely interesting as it makes it possible to analyse the evolution of the noise exposure during the day and to correlate it to the type of work or the location of the worker. This type of instrument makes use of the equal energy principle and offers generally a much broader dynamic range than dosimeters. They are definitely expected to replace dosimeters in the near future and in fact are already referred to as dosimeters by some manufacturers and users. Personal sound level meters or personal sound exposure meters conform to the IEC 61252 standard. 1.3 WORKING PRINCIPLE OF A NOISE DOSE METER The need to ascertain the noise exposure of workers during their normal working day, has led to the development of the noise dosimeter. This is a small, light and compact instrument to be worn by the worker. It measures the total A-weighted sound energy received and expresses it as a proportion of the maximum A-weighted energy that can be received per day. This instrument is particularly useful whenever the exposure varies appreciably during the working day. The maximum A-weighted energy that is permitted to be received per day is defined in standards or regulations: it is absolutely necessary that the dosimeter be calibrated on the basis of the adopted standard (e.g. 85 dB(A) or 90 dB(A) for an 8-hour exposure), including the accepted trading rule, which is 3 dB(A) in accordance with the ISO 1999 - 1990 standard (and for most European countries) and 5 dB(A) for the OSHA Standard (USA). The 3 dB(A) trading rule is consistent with the equal energy principle: 96 dB(A) during 2 4 hours providing the same energy as 93 dB(A) during 4 hours or 90 dB(A) during 8 hours. The 5 dB halving rate assumes that 90 dB(A) during 8 hours is equivalent to 95 dB(A) for 4 hours or 100 dB(A) for 2 hours. Dosimeters are actually sound level meters having a DC output signal converted into a series of impulses which are counted to provide the dose. The technical characteristics of dosimeters must then be the same as for type II sound level meters. 1.4 USES OF NOISE DOSE METER If mobile work patterns exist in your workplace and a noise dosimeter fits the bill, it is important to realize precisely how these instruments must be used and understand their limitations. Given the logarithmic nature of the decibel scale, a variance of only 1 or 2dB can often mean serious misinterpretation of noise levels. This margin of error should be accounted for and the worst-case scenario measurement taken as the reading, particularly when close to an action level. A noise dosimeter consists of a microphone on a cable, which can be clipped to a collar. The microphone cable is then passed under the clothing to the unit itself, which is small enough to be located in a pocket or clipped to a belt. The dosimeter can then be started at the beginning of the shift. If it runs until the end of the working day, the noise dose can be directly read from the instrument or downloaded without the need for calculations. Another useful feature of noise dosimeters is that they will log the noise data so that when downloaded to a PC, the time history of the noise can be viewed. This gives the ability to analyze when and where high noise exposures occur. This can be even more useful when the dosimeter is placed on an employee who is prepared to make a list of the times and jobs he or she was performing throughout the day. This will give the employer the ability to see which operations most need noise control in order to reduce exposure. A traditional noise dosimeter is fixed to the worker's belt, and then a microphone on a cable is attached to the collar near to the ear. You should make allowances for human nature. Employees fitted with dosimeters and their colleagues will often shout into the microphones, distorting the readings, so it's best to ignore the first few days' results until the novelty wears off. One advantage of dosimeters is that if employees wear them for complete working shifts, the noise dose is measured in full. However, if you need to make several measurements of different employees in the same day, a dosimeter can be moved to different employees, as long as the measurements taken for each employee are representative of their working day. Most modern dosimeters also 5 will project the noise dose forward to the standard eight hours, so no calculations are needed. With innovations in digital technology, noise dosimeters are becoming smaller and smaller. The latest "badge" dosimeters have certain advantages over traditional dosimeters. Because the dosimeter is small and light enough to be worn on the shoulder, it means there are no cumbersome microphone cables. If there are no cables to get in the way, not only is it safer to wear, but also employees are less resistant to wearing it and much more likely to forget it is there. This means the quality of the noise data collected will be improved. Because of the small size of badge-type products, it is also possible to mount them in more innovative ways, such as on a hard hat, close to the ear, without interfering with an employee's working process in any way. This allows the dosimeter to not be mounted on clothing at all, therefore completely removing it from the employee's mind. The dose provided by the instrument is of course dependent on the duration during which the instrument is used. Therefore, it should first be corrected for an 8 hour period and then converted to the daily noise exposure (LEX,8) level according to the relevant formula (ISO or OSHA). It is important to know that some old dosimeters do not take into account levels below 89 dB(A) or 80 dB(A), as they assume that lower levels do not lead to hearing impairment. The LEX,8 is then physically not correct. These dosimeters are obsolete and should be discarded. On certain instruments, a warning marker is activated if the peak level ever exceeds 140 dB. It is worth noting that the characteristics of the dosimeters have never been standardized. 6 2.1 AUDIOMETER An audiometer is a machine used for evaluating hearing loss . Audiometers are standard equipment at ENT (ear, nose, throat) clinics and in audiology centers. They usually consist of an embedded hardware unit connected to a pair of headphones and a test subject feedback button, sometimes controlled by a standard PC. Such systems can also be used with bone vibrators, to test conductive hearing mechanisms. Audiometer requirements and the test procedure are specified in IEC 60645, ISO 8253, and ANSI S3.6 standards. An alternative to hardware audiometers are software audiometers, which are available in many different configurations. Screening PC-based audiometers use a standard computer and can be run by anybody in their home to test their hearing, although their accuracy is not as high due to lack of a standard for calibration. Some of these audiometers are even available on a handheld Windows driven device. Clinical PC-based audiometers are generally more expensive than software audiometers, but are much more accurate and efficient. They are most commonly used in hospitals, audiology centres and research communities. These audiometers are also used to conduct Industrial Audiometric Testing. Because these audiometers can typically be calibrated to an accuracy of fractions of a decibel , calibration is more accurate than hardware audiometers. Some audiometers even provide a software developer's kit that provides researchers with the capability to create their own diagnostic tests. 2.1 WORKING PRINCIPLE OF AN AUDIOMETER Any audiometer that is used to test hearing should be calibrated regularly to ensure that the level shown on its display is equal to the actual stimulus the subject is exposed to. Accurate and reliable measurements are the first stage in characterising and quantifying hearing loss; furthermore, proper calibration ensures that the measurements are consistent, regardless of the clinic or indeed the district or nation where the measurements are carried out. Audiometers are calibrated using an Ear Simulator System (or ‘Audiometric Calibration System’). For testing air-conduction hearing mechanisms, the systems include an ear simulator or acoustic coupler, conforming to either IEC 60318-1 (a.k.a. artificial ear) or IEC 60318-3 (reference coupler) respectively. These devices essentially consist of a calibrated microphone with an associated coupling volume, which is open on one side to allow application of headphones when testing. For testing bone-conduction hearing mechanisms, the ‘system’ will include a mechanical coupler (a.k.a. artificial mastoid) conforming to IEC 60318-6. This device uses a series of rubber layers to couple the bone-vibrator (which in testing delivers the stimulus to the patient) to a force transducer. This enables the device to mimic the way in which sound is transferred through the mastoid part of the temporal bone , to which the bone-vibrators are applied when testing. 7 2.2 ARTIFICIAL EAR / REFERENCE COUPLER (AIR-CONDUCTION EAR SIMULATOR SYSTEMS) Both the ‘artificial ear’ and ‘acoustic reference coupler’ are designed for the purpose of calibrating (air conduction) audiometer outputs. When calibrating, the audiometer headphones are applied to the open side of the coupler, and the internal (calibrated) microphone is used to detect the resulting sound pressure level in the cavity. The electrical output from the microphone is usually measured with a sound level meter , and gives an indication of the sound pressure level that a real listener’s eardrum would be exposed to by the same signal and headphones when used in hearing tests. It is the complete combination of artificial ear, sound level meter and associated leads etc. that is referred to as the ‘Audiometric Calibration System’. The microphone and associated coupling volume comes in two distinct types: the IEC Acoustic Reference Coupler (specified in IEC 60318-3) is simply a cylindrical coupling volume (volume depending on application) with a 1-inch microphone at the far end. The second, the IEC Artificial Ear (specified in IEC 60318-1) is designed to be used with a ½-inch microphone, and has various extra cavities and portholes adjoining the main coupling volume between the headphone and microphone. These serve to alter the acoustical transfer impedance (with varying frequency ) in the main cavity in a way that mimics that of a human outer ear . Though both devices are still relevant, and comply with ISO 389-1 (for the calibration of supra-aural headphones), the more sophisticated Artificial Ear provides a better indication of the actual sound pressure level at the average patient’s eardrum . The calibration of audiometers using an ear simulator is a form of secondary calibration ; the ‘audiometric calibration system’ (ear simulator system) is used to validate / calibrate the output from an Audiometer. Bear in mind, however, that the sensitivity of the testing system itself is liable to drift with time (due to factors such as instability in the sound level meter , microphone drift) and hence also requires regular calibration. The calibration of ear simulator systems is often carried out by specialist test and calibration laboratories, or a National Measurement Institute, such as NPL in the UK. Calibration is by comparison; in the case of air-conduction systems this means comparison of the ‘system’ output against known levels as measured by a calibrated reference microphone . Note that this includes specifying a method for setup of the sound level meter , as well as testing the sensitivity of the microphone and associated connectors and pre-amplifier . Once the ear simulator system has been calibrated, it can be used to calibrate audiometers ‘in the field’. 8 A similar calibration procedure to the above takes place for bone conduction audiometry, using a device called an artificial mastoid. As per air-conduction ear simulator systems, the system can be used to test the force hearing level delivered by an audiometer at a particular frequency, but the artificial mastoids must themselves be regularly calibrated. 2.3 ARTIFICIAL MASTOID / MECHANICAL COUPLER (BONE-CONDUCTION EAR SIMULATOR SYSTEMS) An artificial mastoid (or IEC Mechanical Coupler, as specified in IEC 60318-6) is an acoustic device that is designed to present a frequency dependent mechanical impedance approximating that of the mastoid part of the temporal bone . They are primarily used to calibrate the bone-conduction channel of an audiometer, which themselves are used to test patient’s hearing in response to bone-conduction mechanisms by presenting known force levels at specified frequencies. The device consists of a ‘dome’ that represents a mechanical simulation of the human mastoid, in terms of its frequency dependent mechanical impedance, and has a force transducer to measure the force level delivered when bone-vibrators are applied. The electrical output from the force transducer is usually measured with a sound level meter , and gives an indication of the force level, (or force hearing level when referred to the RETFL, ‘Reference Equivalent Threshold Force Level’) that a real listener would be exposed to by the same signal and bone vibrator when used in hearing tests. It is the complete combination of artificial mastoid, sound level meter and associated leads etc. that is referred to as the ‘Audiometric Calibration System’. Whilst the artificial mastoid, as part of an audiometric calibration system, is used to calibrate the bone vibrator force output of audiometers, the sensitivity of the testing system and the mechanical impedance of the ‘dome’ is prone to drift and hence also requires regular calibration . The calibration of artificial mastoid systems is usually carried out by specialist test and calibration laboratories, or a National Measurement Institute, such as NPL in the UK. A bone-conduction audiometric calibration system must be calibrated in two specific ways: firstly to ensure that the mechanical impedance of the ‘dome’ is within the tolerance specified in IEC 60318-6, and secondly to ensure that the reading on the sound level meter is representative of the force hearing level which would be delivered to the patient. Calibrating the system includes specifying a method for setup of the sound level meter , as well as testing the sensitivity of the force transducer and associated connectors and pre-amplifier. Once the ear simulator system has been calibrated, it can be used to calibrate audiometers ‘in the field’. It is also worth noting that the artificial mastoid is 9 very sensitive to temperature ; special precautions such as temperature isolated enclosures are usually necessary when transporting the device in hot or cold conditions. A similar calibration procedure to the above is used for air conduction audiometry, using either an Artificial Ear or Acoustic Reference Coupler. As per bone-conduction ear simulator systems, the system can be used to test the sound pressure level delivered to the patient by an audiometer at a particular frequency, but the system itself requires regular calibration. 10