CRL 222 Integrating Sound Level Meter. CRL 222 Integrating Sound Level Meter

Transcription

1 CRL 222 Integrating Sound Level Meter This manual, the software to which it relates, the program code and drawings are all: Copyright Cirrus Research plc

2 The content of this manual, any illustrations, technical information and descriptions within this document were correct at the time of going to print. Cirrus Research plc reserves the right to make any changes neccessary, without notice, in line with the policy of continuing product developement and improvement. No part of this publication may be duplicated, reprinted, stored in a data processing system or transmitted by electronic, mechanical, photographic or other means, or recorded, translated, edited, abridged or expanded without the prior written consent of Cirrus Research plc. No liability is accepted for any inaccuracies or omissions in this manual, although due care has been taken to ensure that is it complete and accurate as possible. Accessories supplied by Cirrus Research plc have been designed for use with the instrumentation manufactured by Cirrus Research plc. No responsibility is accepted for damage caused by the use of any other parts or accessories. Produced by Cirrus Research plc, Acoustic House, Bridlington Road, Hunmanby, North Yorkshire, YO14 0PH. Copyright Cirrus Research plc 2003 Reference Number 01/222/A/98 Document Printing Date 5 December 2003 Page 2

3 1. The CRL 222 Sound Level Meter Page Uses Page Features Page L Measurements Page SEL Measurement Page Sound Pressure Levels (SPL) Page CRL 222A Measurement Kit Page 6 2. Controls Page Weighting Switch Page Mode Selection Switch Page Range Attenuator Page Calibration Page Reset Page Indicators Page Analogue Display Page Batteries Page Microphone Page 8 3. Operation Page Preliminary Page Operation Of The CRL 222A Page Measuring Sound Levels Page Sound Measurements and Survey Page The Choice and Use of Sound Level Meters Page Microphone Choice Page Microphone Height Page Microphone Position Page Observer Page Instrument Precautions Page High Sound Levels - Microphonics Page Interpretation of the Meter Pointer Page Tones and Beats Page Varying Speed Sources Page Random Noise Page Sound Surveys Page Record of Measurements Page Specifications Page Guarantee Page Cirrus Research Offices Page 25 Page 3

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5 1. The CRL 222 Sound Level Meter 1.1. Uses Determination of L for assessment of hearing loss risk or noise annoyance. Measurement of machine noise. Determination of Sound Exposure Level (SEL). Measurement of Environmental Noise Levels. Sound Power Level measurements according to survey method Features Noise is an important consideration in many areas of the environment. The CRL 222A has all of the features necessary for a wide variety of noise measurements and complies with type 2 of both IEC 651 Sound Level Meters and IEC 804 Integrating Sound Level Meters standards L Measurements By convention - and in all national and international standards - L is the A-weighted sound level which would have the same total A-weighted acoustic energy as the real fluctuating sound over the same period of time. The measurement of L is based on the ual energy principle: The A weighted L may be calculated directly by integrating the squared signal according to the formula: 1 t L A '10log 2 P 2 A (t) 10 dt dba t 2 &t 1 mt 2 1 P 0 Where P o is the reference sound pressure (20µPa), P A(t) is the A- 2 where pa is the weighted time varying sound pressure and t2-t 1, is the time interval over which it is measured. Where (t2-t 1) is an actual measurement time interval without interruption in the measuring process; and where P A(t) is the instantaneous sound pressure of the sound signal. This method gives true integration of the squared sound pressure of a sound whose level varies with time SEL Measurement Sound Exposure Level (SEL) is the constant level which if maintained for a period of 1 second would have the same A-weighted acoustic energy as the measured noise event, and is defined as: Page 5

6 %4 1 SEL ' 10Log t ref m t ref ' 1sec &4 p(t) 2 p o dt The SEL is used primarily to describe and compare transient sounds or single events (e.g. vehicle pass-by) Sound Pressure Levels (SPL) The CRL 222A features a true analogue display to allow the reading of rapidly varying signals. The 30dB scale, 50dB dynamic span and 110dB acquisition range ensure that the CRL 222A can be used without the errors associated with older generation, low dynamic span instruments. The CRL 222A is set for SLOW response - on SPL only. The meter is, therefore, an ideal instrument for persons involved with community and industrial noise measurements. To be formally correct, the CRL 222A actually measures Sound Level, as it is weighted, but the term SPL was preferred by users. True Sound Pressure Level is the unweighted level in decibels CRL 222A Measurement Kit The CRL 222A is normally supplied as a measurement kit, incorporating a CRL 222A Sound Level Meter, CRL 511E Calibrator, windshield, screwdriver, spare calibrator batteries and a strong ABS case. Page 6

7 2. Controls 2.1. Weighting Switch This is a 3 position slide switch. The left hand position disconnects all power from the CRL 222A, the centre position, marked dba, gives the 'A' weighting filter, while the right position, marked dbc, gives the 'C' weighting filter. The CRL 222A has the 'A' and 'C' weighting networks built-in. The 'A' weighting best expresses the human ear's response to loud sounds. Use of this weighting is generally preferred for sound measurement. The 'C' weighting is the response which would normally be selected when significant low fruencies are involved and need to be assessed. The 'A' and 'C' weighting networks are shown in Fig Mode Selection Switch This 3 position slide switch selects the mode or function. In the left hand position, the meter is switched to measure L, in the centre SPL (Sound Pressure Level), and in the right position SEL (Sound Exposure Level). The 3 functions are computed continuously and the switch merely selects what is fed to the meter. The Sound Pressure Level (SPL) gives the weighted r.m.s. level. This varies in proportion to the actual level at the instant of measurement. The other 2 functions are L and SEL both of which integrate (average) the level from the beginning of a measurement period. L is, in effect, the average sound level over a period and thus the longer the period of measurement, the more stable the readings. SEL (Sound Exposure Level) is a measure of the amount of energy received at the microphone each second and thus it can only increase during the measurement period Range Attenuator This is a 6 position switch controlling the instrument gain after any fruency weighting is selected. For example, if the meter reads + 17 and the range switch is at 70, the SPL or L is 87 dba or dbc - depending upon the weighting switch position Calibration This is a potentiometer which allows fine adjustment of the amplifier gain of the CRL 222A. This is normally used for adjusting the acoustic calibration with the appropriate calibrator (CRL 511E). Note that the CRL 222A has a ½" microphone. The CAL control is normally covered by a plug which must be removed to adjust calibration using a small screwdriver Reset This button, when pressed, returns the L and SEL circuitry to zero. In other words, it makes a new reading commence and destroys the data currently in the in- Page 7

8 strument. It should be used with caution because several hours work can be destroyed by pressing 'RESET' accidently. The button does NOT cause the meter to go to zero. While RESET is pressed the needle will be in an undefined random position. RESET has no effect on SPL. Do not hit the button hard or the CRL 222A will integrate the resulting acoustic noise Indicators Stop (Green). This indicates that either of the dose or time registers inside the CRL 222A is full and that no further SEL or L readings can take place. However, the readings when this indicator light is on are correct and the accuracy is not affected; it is simply that data acquisition stops. The CRL 222A does however still function on SPL Overload (Yellow). This indicates that an overload of the amplifier has been applied to the unit. This can be an overload before weighting or an overload of the rms signal. In either case, the light will latch on and will only go out on pressing RESET. This starts a new reading. Do not operate for long periods with the O/L or STOP indicators lit as this shortens the battery life Battery Low (Red). When this indicator comes on the batteries are discharged and the unit should not be used because erroneous results will occur. The batteries must be replaced. However, if the batteries are totally discharged, the red light will have no power and cannot come on, if the meter does not read at all and the red light is out the batteries are totally discharged. Remove discharged batteries immediately from the CRL 222A to prevent possible leakage and damage Analogue Display The display is a 30dB linear scale with SLOW response as standard Batteries The CRL 222A is powered from two 9V batteries which are located in the battery compartment at the lower end of the instrument. Alkaline batteries are recommended and, from new, will provide approximately 16 hours continuous operation. Before fitting or replacing batteries, ensure that the instrument is switched off. Access to the battery compartment is gained by removing the battery cover plate which is not symmetrical, and held in place by two screws. The unit operates on 13 to 20 volts and will operate on 12 volts (an automobile battery) but the red "BATT LOW" LED may stay on. A current of 33mA (Maximum) is ruired. Neither side of the battery is grounded Microphone Page 8

9 The CRL 222A has a removable ½" electret condenser microphone, MK202A on all models. Before connecting, or disconnecting the microphone, ensure that the initrument is switched off. Light finger torque only should be used when screwing the microphone onto the fixed input stage adaptor. In dry weather, it is recommended that static electricity is discharged from the body before fitting or removing the microphone. NOTE: The microphone protection grid is not removable. A fine wire gauze filter inside the protection grid prevents dust and particle penetration to the diaphragm. DO NOT attempt to remove the protection grid. Page 9

10 3. Operation 3.1. Preliminary This instruction manual describes the specific procedures necessary to use the CRL 222A Integrating Sound Level Meter correctly. However, other more general aspects of sound measurements must be considered in order to be sure of accurate reproducible measurements. Many of these are discussed in "Noise" by Rupert Taylor (Penguin Books) Operation Of The CRL 222A 1. Make sure the microphone is attached. 2. Push WEIGHTING slide switch from Off to dba. 3. Three indicators should glow: STOP (green), O.L (yellow) and BATT LOW (red), If they do not glow then the batteries have discharged and MUST be replaced. The BATT LOW indicator should glow momentarily and then go out. If this indicator stays on then the batteries are nearly discharged and must be replaced. 4. Press RESET. All the indicators should now be out. 5. Sound level measurements can now be carried out Measuring Sound Levels General Check 1. Has the battery sufficient charge? Replace if low. 2. Is the correct weighting network selected? Select weighting : A or C. 3. Has the correct mode been selected? Select mode: L, SPL or SEL. 4. Has the CRL 222A been calibrated? Calibration is essential before any measurement is made After turning on the CRL 222A push RESET. Select SPL and point the meter towards the sound source. If the needle is reading off scale to the right, turn the RANGE switch clockwise until a reading is obtained as close as possible to midscale. If either of the stop or O/L indicators have come on, push RESET again. If the reading is off scale to the left, turn the RANGE switch anti-clockwise. Sound Level is the sum of the RANGE and meter reading. If L is ruired, push the MODE switch to the left and the measured L will be displayed. To start another measurement period simply press RESET again. It is vital to remember that RESET starts a new integrating period AND DESTROYS Page 10

11 PREVIOUS DATA. The range control should be set so the initial L reading is in the mid-scale of the meter; this allows the optimum measuring choice between off-scale (low) and over-range (high). Make a note of the EXACT time the L or SEL measurement is started Calibration The CRL 222A is calibrated acoustically using an external reference, e.g., the CRL 511E Calibrator, which is placed over the microphone. The calibrator generates a stabilised Sound Pressure Level of 94dB (± 0.3dB) at a fruency of 1kHz. A 104dB output is also fitted to the CRL 511E. The 104dB allows calibration in high ambient levels and also gives a 2 point scale check. The calibration procedure is as follows: PROCEDURE: Ensure that the calibrator adaptor Q3 is fitted to the CRL 511E. Turn on the calibrator and check that the battery condition is aduate. A 1kHz signal should be audible and the green LED in the end should light. Select 94dB on the CRL 511E. Place the CRL 511E over the microphone and select the 80 range on the CRL 222A. Now adjust the front panel CAL potentiometer so that the reading is +14 on the scale. If this cannot be achieved the microphone or the instrument could be at fault and a check should be made by Cirrus Research plc. VERIFICATION: To calibrate, the SPL mode should be selected. When calibration has been achieved on SPL it is sensible to check that L gives the same readings. Similarly SEL can be checked which should read 24 on scale after 10 seconds at 94dB SPL SPL and L Measurements The various national and international standards and codes of practice state the correct measurement mode to meet their recommendations and methods of analysis. Sound Pressure Level (SPL) readings on the CRL 222A may be taken as instantaneous or as maximum or minimum levels, depending upon the standard or code being used. Similarly, levels may be analysed statistically as L levels, such as n L and L. This versatility is due to the analogue meter system that is able to detect and display widely fluctuating sound levels. The uivalent continuous sound level is used in many different measurement situations because the widely fluctuating display of the SPL readings makes it difficult to determine the correct sound level. L levels are often stated, therefore, for long-term exposure of hours or days. The CRL 222A has a 10 hour (nominal) measurement time. The advantages and disadvantages of the various indices are compared in Table 1. TABLE 1: COMPARISON OF NOISE MEASUREMENT INDICES Page 11

12 Index Advantage Disadvantages SPL Measures instantaneously. Often difficult to measure (In "Slow" response in the long term levels if the case of the CRL 222A) sound fluctuates widely. L90 Widely used internationally A statistical parameter that and is supported by extensive is time consuming to derive research. Used as an index in from sound levels unless a many noise assessment standards. noise analyser is used. Does not include the effect of short duration peak levels or tonal characteristics. Previously quoted in ISO 1966 and now replaced by L A. L10 Widely used for road traffic Time-consuming to derive measurements internationally from sound levels unless a and used as an index in many noise assessment standards. noise analyser is used. Does not include the effect of short duration impulse noise or tonal characteristics. CNL Defined in various national Time-consuming to derive Corrected standards for assessment of from sound level unless a Noise Level nuisance noise. Takes into noise analyser is used. (SPL) account total characteristics Difficult to assess rapidly and duration. Well tested in fluctuating noise sources legal proceedings for "nuisance" assessment. by this method or assessing effect of tonal characteristics. LA Widely used internationally Takes into account short and quoted in environmental duration peaks but not the standards. Correlates well with effect of tonal community response. Equates characteristics. Can be well for many rapidly misleading without time fluctuating noise levels. Used history data. in occupational noise assessment Sound Exposure Level (SEL) For the meter to measure a single impulse such as a rifle shot or a door slamming, it must have the ability to respond extremely fast to the rapidly changing level. Normal Sound Level Meter speeds of Slow (S), Fast (F) or even Impulse (I) are simply not aduate as the sound is gone before the instrument has time to integrate the level. Some instruments can measure the actual peak of the impulse (called Peak meters), Page 12

13 but this alone is of limited use. On SEL, the CRL 222A accepts all of the incoming energy in 1 second and stores it as a "Dose" in a register. Thus, if a single event occurs, the energy due to that event can be readily measured. (Sound Exposure Level is defined as that constant Sound Level which has the same amount of energy in one second as the original noise event). To measure an SEL it is necessary that a reasonable estimate of the likely level of the event be known, in order to set the correct range and avoid the needle being off scale after the event. To make the measurement, select a range that you think will probably be correct and then select SEL on the lower slide switch. Hold RESET in until just before the event occurs. On release of RESET the CRL 222A will integrate all of the energy it receives. Hopefully, it will be on scale immediately after the event. The CRL 222A should be read as soon as possible after the event as any further noise will also be integrated. Some people, particularly in the United States, call Single Event Sound Exposure Level L AX. This is strictly incorrect as L AX should have a threshold below which no integration takes place while SEL has no such threshold. Otherwise L AX and SEL are Identical. On SEL for a constant sound level commencing at scale zero the following readings will be obtained: SCAL SEC MIN HRS Reading Time E SEL Sec K 1000 = 16,6 _ * ,000 = 166 = ,000 = 1,660 = Table 2 Scale times SEL: TABLE OF SEL AGAINST TIME FOR A 94dB SIGNAL * Maximum reading time with the analogue display The relationship between SEL and L is illustrated in Fig 7, (Section 6). These figures are derived from the basic data that:- Page 13

14 1. SEL is identical to L after 1 second. 2. For a given constant signal level SEL increases by 3dB for a doubling of the time or increases by 10dB for 10 fold increases in time. Both L and SEL measurements must be related to "start" and "finish" times in order to determine the exact measurement time Wind Shielding Wind noise can cause measurement problems: the air-stream creates a vortex in the microphone grill and the noise of this is picked up by the microphone. The MK 202 capsule has a special grill to minimise this effect but it is still significant. A small windshield Ref. UA240 is provided in the CRL 222A/CRL 511E kit which not only gives some protection against wind noise but also keeps out dust and protects against minor knocks. A large 90mm windshield, UA237, is significantly better and can be fitted onto the CRL 222A. It is recommended that measurements are not taken when the wind speed exceeds 5 ms Changing Functions (MODE): Most measurements are made in 'A' weighting to give the Sound Level, SEL or L in dba, although naturally dbc can be used in any mode. It is vital that when the unit is integrating, i.e on L or SEL, neither the WEIGHTING switch nor the RANGE switch should be touched, OTHERWISE THE DATA IS MEANINGLESS. The MODE switch CAN be moved during a measurement period to read all 3 parameters without causing measurement error. Page 14

15 4. Sound Measurements and Survey 4.1. The Choice and Use of Sound Level Meters Although the use of a precision instrument does not guarantee that a measurement will be valid or accurate, a precision instrument will help to yield more consistent results than is possible with a less precise instrument and improved results can be obtained more readily. An obvious but important rule in any measurement task is to review the results to see if they are reasonable. If they are not, try to track down possible sources of trouble, particularly simple things like poor connections, plugs in the wrong places, no power, low batteries, controls set incorrectly, damaged uipment and so on. If nothing can be found that can be corrected to bring the data into line, perhaps the data seems unreasonable only because of limited understanding of the phenomena or of the measurement process. The results of a noise measurement may be a key factor in resolving a noise problem. Careful records of noise measurements can be valuable for future reference on subsuent problems and this possibility should be kept in mind when tackling noise problems. The integrating Sound Level Meter is a normal Sound Level Meter but with the capability of storing a large number of samples and calculating a linear average for any length of time, the "uivalent continuous" (L ) level. By holding the instrument in a fixed position and direction, an average value over a time period can be obtained which is a very important feature, especially when measuring noise of a fluctuating character. A recognition of the accuracy limitation of acoustic and vibration measurements is important, in order to be reasonably confident in the approach to a measurement problem. Consistency to 0.1dB or better is attainable in only a few laboratory calibration procedures in acoustics and not in general acoustical measurements. Field calibrations of Sound Level Meters at one fruency with a calibrator may be consistent to 0.5dB or slightly better. A consistency of 1dB is difficult to achieve in general measurements even under carefully controlled conditions, but is a more reasonable goal than 0.1dB. Evidence of this is given in the latest revision of BS4142 (the U.K.'s version of ISO used to assess noise nuisance) which ruires that "the specific noise level is quoted to the nearest whole number of decibels". It is useful to think of the measured result as an "estimate". It is an estimate not only because of the uncertainty in the measurement, but also because the phenomenon being studied is not absolutely stable. For example, wind effects can be significant in determining the noise level that occurs at an observation point. The sound level can change appreciably from time to time even with constant source level. One should correspondingly avoid unusual conditions when making measurements. Sometimes, however, it is helpful to Page 15

16 measure under certain low velocity wind conditions, and the wind blowing in a particular direction could very well be the one that shows the most serious effects of the noise. A particular wind direction may possibly illustrate the most serious effect of an individual noise source. This is most useful in environmental noise investigation Microphone Choice The CRL 222A is a Type 2 Integrating Sound Level Meter and is fitted with a freefield microphone. This microphone meets the response specifications for use in both "free-field" and "diffuse field". Background: The IEC Publication 651 (1979) and the American National Standard ANSI 1.4 (1983) for Sound Level Meters are identical, except for one important point: Microphones. While the IEC standard ruires sensitivity calibration in a reference direction ("perpendicular" or "frontal" response) specified by the manufacturer, the ANSI standard prescribes 75 per cent random incidence ("diffuse field") sensitivity. Unfortunately, discrepancies can occur between results obtained by the two Sound Level Meters. However, there are well documented reasons why IEC and ANSI have chosen their respective philosophies. There are merits and disadvantages behind both philosophies. It has been shown that the use of a microphone made to IEC 651 gives more accurate results for free-field measurements when the direction of sound is known as is the case for most environmental noise. A microphone made according to ANSI S1.4 is the better choice for measurements in random fields where the sound arrives from several directions, such as in a reverberant room. An integrating Sound Level Meter WITHOUT a random incidence corrected microphone (i.e the free-field microphone fitted as standard to the CRL 222A) should be pointed TOWARDS the noise source over the entire measurement period. NOTE: Both types of microphones have similar response up to 2kHz, however, the free-field microphone will be approximately 4dB low at 10kHz. This is within the acceptable tolerances for a Type 2 Integrating Sound Level Meter and so there is NO PRACTICAL NEED to fit random incidence correction to the CRL 222A Microphone Height For outdoor noise measurements the microphone should be placed 1.2 to 1.5m above the ground and at least 3.5m away from walls, buildings or other sound reflecting surfaces Microphone Position When studying community noise it is necessary to decide what is to be measured. If traffic noise is to be measured, then a location for the microphone about 15m away from the travel lane and away from buildings and other reflecting surfaces is a logical location. But if traffic noise is only one part of the community noise, the microphone should not be located in a position too close to a highway or it may give undue emphasis to traffic noise. Page 16

17 In a city, traffic noise can hardly be avoided but its contribution to the noise impact will vary significantly from one location to another. Thus, if a general survey is desired measurements at a number of places are necessary. If the enforcement of a noise ordinance regarding noise at the boundary line of a factory is ruired, the proper measurement points are as close to the boundary line as feasible. The position of the survey points on that line would also be determined by the points near which disturbances are likely to occur. Thus, if residents are nearby, the points nearest to the residences are logical selections. Further checks at the boundary points nearest the major noise sources of the factory are also necessary Observer If noise is coming with reasonable uniformity from many directions to the point of measurement, the location of the observer is not critical, although the observer should never stand in front of the microphone. In general, the observer should not stand close to the microphone in order to avoid interfering with the sound field near the microphone. Experiments have shown that at fruencies of approximately 4000Hz reflections from the body of an operator can cause errors of up to 6dB when measuring less than one metre from the body. If the meter is hand-held it should be held firmly at arm's length. The use of a tripod mount with a connecting cable between the microphone and the Sound Level Meter allows the observer to be several meters away from the microphone. It also allows easy manipulation of the controls and reading of the level without disturbing the microphone Instrument Precautions Low noise levels - Effects of Circuit Noise When low noise levels are to be measured, the inherent circuit noise may contribute to the measured level. If the microphone is directly on the Sound Level Meter, the level at which this effect may be important is below 30dB, if the A weighting is used. A microphone cable of up 5m may be used before circuit noise may affect readings Hum Pickup When noise is measured near electrical uipment, a check should be made that there is no appreciable pickup of electro-magnetic field in the sound measuring system. The procedure depends on the directional character of the field. The orientation of the instruments should be changed to see if there is a significant change in level High Sound Levels - Microphonics Some electronic devices are affected by mechanical vibration. Trouble from this effect, which is called microphonics, is not usually experienced until the sound levels are well above 100dB, unless the instruments are placed on supports that carry vibrations directly to the instruments. Mechanical vibration also affects the microphone itself, in that the output of the microphone is dependent on the airborne and solid-borne vibrations that are impressed upon it. Often merely suspending the microphone by means of its con- Page 17

18 necting cable is aduate Interpretation of the Meter Pointer Two ballistic characteristics are available for the meter on the Sound level Meter: when the optional "Fast" response is provided it will be noticed that most sounds do not give a constant level reading. The reading fluctuates often over a range of a few decibels and sometimes over a range of many decibels particularly in analysis of low fruencies. The maximum and minimum readings should usually be noted. These levels can be entered on the data sheet as, e.g 85-91dB or 88 ±3dB Tones and Beats The indicated sound level of a constant-amplitude pure tone is steady, and so is that of a mixture of tones, unless at least two components are close together in fruency. Examples of sounds that have a constant indicated sound level are transformer hum and noise from some rotating electrical machinery. When the combined noise of several machines is measured, the indicated level is also constant, unless the speed of machines is such that some of the major noise components are only a few cycles apart in fruency. In this situation an audible beat, a periodic rise and fall in amplitude, occurs and the indicated level also rises and falls Varying Speed Sources Machinery that operates at a varying speed usually produces a noise that fluctuates in level. If speed varies periodically the level will also vary periodically Random Noise The indicated sound level of a random noise, such as that produced by jets, blowers, combustion chambers, ventilating systems etc., is not steady. In fact, all sounds contain some random noise energy and most have enough so that the indicated level fluctuates noticeably. The extent of the fluctuation is a clue to the nature of the sound. The fluctuations in level are ordinarily not a result of erratic behavior of the measuring uipment but rather reflect the irregularities in the process of noise production. This process can often be considered as a combination of many sources that produce sound at random time intervals. The measurement of such noises can be treated on a simplified statistical basis that is satisfactory for almost all sounds. Page 18

19 5. Sound Surveys The objective when measuring sound is to make valid measurements. In order to achieve this objective, it is helpful to recognize that the results of a measurement are determined by a number of factors among which are:- 1 The phenomenon being measured. 2 The effect of the measurement process on the phenomenon being measured. 3 The environmental conditions. 4 The characteristics of the uipment being used. 5 The way the uipment is used. 6 The observer. Although many useful measurements are made by those with little background in acoustics, the chances of making valid measurements are increased as the understanding of these factors becomes more thorough. Thus, a good knowledge of vibrations and acoustics, of instrumentation and of measurement techniques, is helpful in making noise measurement. Even when one does not need to measure noise according to a standard procedure, it is often wise to try to do so if an appropriate standard can be found. The standards have been prepared to help obtain valid data and they help to make comparisons of measured results more meaningful Record of Measurements The following points should be taken into account before venturing out into the fields: Initial Decisions 1. Why are the measurements being made? Is it just a simple sound level that is really ruired? Is an octave band analysis necessary to reach conclusions about possible noise control methods? Will a sophisticated analysis have to be carried out later in the laboratory in any case? 2. Do the measurements and the instruments have to meet certain standards such as ISO, ANSI, DIN etc., with regard to instrumentation quality, measuring techniques, layout of measuring site etc? 3. What type of noise is to be measured? Is it impulsive? Is it statistically varying? Does it contain significant pure tone(s)? 4. With the above three points in mind, choose the most suitable instrumentation to make the noise measurements ruired and to be able to produce the necessary results. This should take into account the fruency range of the measurements, the possible statistical analysis of those measurements and whether special instrumentation is ruired e.g for sonic booms, shocks or impulsive noise. Page 19

20 Occupational and Machine - Noise Survey These levels will usually be taken as "L ". The following list of items will be helpful in preparing survey data sheets: 1. Describe the space in which measurements were made: nature and dimensions of floor, walls and ceiling. Describe and locate any nearby objects and personnel. 2. Describe the device under test (primary noise source). Dimensions, name-plate data and other pertinent facts including speed, power rating and kinds of operations. 3. Note the type and serial numbers on all microphones, Sound Level Meters and analysers used. Length and type of microphone cable used. 4. Positions of observer during the survey. 5. Positions of microphone. Direction of arrival of sound with respect to microphone orientation. Tests of standing-wave patterns and decay of sound level with distance. 6. Temperature. 7. Calibration tests. 8. Weighting network and meter response used. 9. Measured overall and band levels at each microphone position. Extent of meter fluctuation (Device under test operating). 10. Background overall and band levels at each microphone position. (Device under test not operating). 11. Date and time of survey. 12. Make a note of any significant variation from the norm during measurement e.g. machines being repaired or factory radio switched off. When the measurement is being made to determine the extent of noise exposure of personnel, the following items are also of interest: 1. Personnel exposed - directly and indirectly. 2. Time pattern of the exposure. 3. Attempts at noise control and personnel protection. 4. Audiometric examinations and records of the personnel exposed. Example of L Measurement A worker is engaged in an activity which exposes him to a noise level of 97dBA for a 2 hour period each day. Is the worker at risk for noise induced hearing loss? Page 20

21 The allowed exposure for a 2 hour period is 91dBA (ISO 8 hour day maximum of 85dBA, with a halving of exposure time for every 3dBA increase in noise above 85dBA). Therefore, the worker is at risk because the maximum permissible daily noise exposure is exceeded and hearing impairment is likely to result Environmental Survey These levels will usually be taken as Sound Pressure Levels (SPL) although some latest standards ruire "L ". 1. Having chosen the instrumentation, check and calibrate the entire arrangement. 2. Make a list of the instruments used and a note of the reference numbers of all the instruments 3. Sketch the measurement situation, the position of the source, the microphone and any reflecting or significant surfaces. 4. Note the weather conditions, including wind direction and strength, rain, temperature, humidity. 5. Measure the noise, noting the relevant uipment settings, such as dba, response (Slow for a CRL 222A), range selector settings etc. 6. Check the background noise level, either overall or in the same bands which will be ruired in the subsuent analysis (if an octave band analyser is available). 7. Keep a log, including any changes made to uipment settings and any unusual occurrences. Page 21

22 6. Specifications The specifications of the CRL 222A in accordance with IEC 651 and IEC 804 for a Type 2 Integrating Sound Level Meter are as follows: 1. The type of microphone and method of mounting in order to attain the tolerances ruired for Type 2: * Perma-charged electret condenser mounted on the meter. 2. The reference direction of incidence: * 0E of incidence (frontal or perpendicular response) 3. The range of weighted sound pressure levels which the instrument is designed to measure within the tolerances of this standard: * 30 to 120dBA 40 to 120dBC 4. The reference value of sound pressure level: * 94dB 5. The nominal fruency weighting characteristics: * A weighting and C weighting 6. The detector-indicator characteristics are SLOW as standard. 7. The effect of vibration on the operation of the sound level meter: -2 * 1ms will give an output of less than 60dBC 8. The effect of magnetic fields: * 80 A/m at 50Hz gives a reading of less than 30dB 9. The effects of temperature: * Error is less than 0.5dB for the range -10 to +50EC. 10. The effect of the presence of the operator on a free field measurement: * No effect when the meter is tripod mounted 11. The effects of humidity: * The unit is to specification for a relative humidity of 0 to 90% 12. The limits of temperature and humidity beyond which permanent damage to the sound level meter may result: * 70EC and saturation 13. Any correction to calibration ruired when a microphone extension cable is used: * 0.5dB for a cable length of 5m or less 14. The effect on the performance of the instrument caused by the use of recommended microphone accessories such as windscreens etc., * No effect 15. The calibration procedure necessary to maintain instrument accuracy: Page 22

23 16. The position of the instrument case and observer relative to the microphone in order to minimize their influence on the measured sound field: 17. A procedure to ensure optimum operating conditions when the Sound Level Meter is used with external filters or analysers if applicable: * Not applicable 18. The limitations on the electrical impedance that may be connected to the output connector: 19. The reference fruency: Used for calibration: 1000Hz 20. The reference range: For calibration purposes: The warm-up time before valid readings can be made: (A maximum change in reading of 0.5dB during one hour of operation): 5 minutes. 22. Correction information between the sensitivity in a diffuse field and that in the reference direction as a function of fruency: Per microphone data sheet, available on ruest 23. The directional response of the sound level meter at various fruencies: Per microphone data sheet, available on ruest 24. The electrical impedance which shall be substituted for the microphone for testing purposes: 27pf in series with 10ohm 25. The primary indicator range as ruired by Sub-clause 7.6: 26. The lower indicator range as ruired by Sub-clause 7.6: +10 to -20 on scale 27. The lowest fruency for which the error resulting from non-linear distortion is less than 1dB: An error of 0.4dB at 31.5Hz at 110dB 28. The range of uivalent continuous A-weighted sound pressure level and the upper peak level which the instrument is designed to measure: 120dBA weighted L with an upper peak level of 143dB(the same for A and C weightings). 29. The linearity range and pulse for each indicator range: -5 or -50 rms (53 peak) on each range 30. The fixed integration periods provided: None 31. The response setting time before valid readings are obtained after starting the Page 23

24 integrating meter: Less than 1 second to 0.5dB for CRL 222A. Page 24

25 7. Guarantee Cirrus Research plc offers a 12 month guarantee on all of their units. This covers all parts and labour excepting only damage caused by the user. Because of the unique fragility of microphones, only internal short or open circuits are accepted as faults and not accident damage. The guarantee ruires the user to return the unit to their nearest authorised Cirrus Research plc Agent. This guarantee is in addition to any statutory rights in your country. 8. Cirrus Research Offices The addresses given below are the Cirrus Research plc offices. Cirrus Research plc also have approved distributors and agents is many countries worldwide. For details of your local representative, please contact Cirrus Research plc at the address below. Contact details for Cirrus Research authorised distributors and agents are also available from the Internet Web site at the address shown below. Main Office Cirrus Research plc Acoustic House Bridlington Road Hunmanby North Yorkshire United Kingdom YO14 0PH Telephone: Fax: sales@cirrusresearch.co.uk Technical Support support@cirrusresearch.co.uk Web Site: Germany Cirrus Research Buro Dresden Schlueterstrasse Dresden Germany Telephone: (+49) Fax: (+49) Cirrus.Akustik@t-online.de USA Cirrus Research PO Box Renton WA USA Telephone: (+1) Fax: (+1) Page 25