Noise Measurement and Surveys

Transcription

1 Chapter 2 Noise Measurement and Surveys Why do we measure sound levels? Sound measurement allows detailed and precise analysis of all sounds. In the case of pleasant sounds this may mean that we can improve the amount of pleasure associated with them. An example of this would be to improve the performance of the components of a Hi-Fi system, or improve the acoustics of an auditorium. When sound measurement is applied to annoying or undesired sounds however, it is used to try and reduce the annoyance. As the characterisation of noise is a highly subject area, not all parties will agree on the level of annoyance of a noise source. Indeed the psychological and physiological differences between human beings almost ensure that scientific measurement will not be able to completely asses what is at least partly a subjective judgement. Measurement does however, give us an object means for comparison of sounds under differing conditions. They also give clear information on when a sound is likely to damage hearing, and therefore suggest when corrective measures such as protective equipment are required. Additionally hearing loss can be assessed, to estimate the amount of damage done to a persons ear. This is done through audiometric testing, which measures individuals hearing performance across a number of frequency ranges. Audiometric testing is an essential component of any successful hearing conservation program. Noise measurement is also essential for noise reduction programs. High noise areas such as factories, airports, busy roads, mines and entertainment centres should be regularly evaluated to assess their impact on the environment. What is the Difference between Sound Power and Sound Pressure? These terms are commonly used in assessment of noise, but they are often used incorrectly. Sound power refers to the amount of sound energy transmitted by the noise source. Sound pressure is the difference between the actual pressure produced by a sound wave and the average or barometric pressure at a given point in space. Another unit, sound level is also commonly used. It refers to the weighted sound pressure level obtained by the use of a sound level meter and frequency weighting network, such as A, B, or C. These will be discussed in more detail later in this chapter. A good analogy for these terms could be obtained by considering a foot tapping into a pond of water. The sound power is equal to the amount of energy the foot uses to set up

2 the ripples in the pond, whereas the sound pressure is the how much a boat floating on the pond is moved up and down by the ripples. Measurement of sound pressure levels The measurement of sound and noise is carried out using sound level meters of which there are several types available. They are designed to respond to sound in approximately the same manner as the human ear, and provide objective and reproducible evaluation of sound pressure levels. Most are based on the simple principle of using a microphone as the detection device that feeds electrical impulses into an amplifier which then passes the signal to a digital meter (normally calibrated in db). Measurements are normally taken over a standard time period ( seconds). Frequency Analysis and Sound Weightings (A, B, C, D, L in ) More expensive sound level meters may be set up to analyse the frequency of the sound waves, or the intensity of each of the different frequencies which make up the noise. Additionally sound signals may be passed through a weighting network, which is an electronic circuit whose sensitivity to different sound frequencies varies in the same way as the human ear. This simulates equal loudness contours. Weighting Networks In the previous chapter it was shown that the apparent loudness of a sound is related to the frequency of the sound as well as the sound pressure level. This is because the human ear s response to sound varies according to the frequency of the sound waves. Sound measuring equipment is now designed to make allowances for the behavior of the ear by using electronic weighting networks, which filter out certain sound frequencies, and favour others. There are several different internationally standardised characteristics used to weight sound. These are termed the A, B, C, D, and L in weightings. The A weighting circuit is designed to approximate the response of the average human ear at low sound pressure levels. Similarly the B and C weighting circuits are intended to approximate the response of the human ear at 55 85dB and 85 db, respectively. The characteristics of these networks are shown in Figure 2.1. Another weighting, the D weighting is sometimes used for aircraft noise. Most sound level meters also have a linear network. This does not weight the signal at all, but rather allows the signal to pass through unmodified. Sound levels obtained in this way are designated L in. Most commonly these days the A weighting network is used and the trend seems to be toward using it almost exclusively when a weighting network is desirable. This is because the B and C weighting networks do not correlate well with subjective listening tests. This is probably because the equal loudness contours that were used to set up the weighting scales used pure tones and most natural sounds are made up of complex signals made from many different tones. 21

3 B,C Figure 2.1 A, B, and C weighting curves for sound level meters (from Bies & Hanson) Table 2.1 shows the corrections in db made to certain frequencies to obtain an A weighting Table 2.1 Corrections (db) made to frequencies to obtain db(a) 1 Frequency (Hz) A weighting Correction Frequency (Hz) A weighting Correction Frequency (Hz) A weighting Correction

4 db(a) Weightings It is possible to use the values from table 2.1 to convert an unweighted (L in ) sound level in db into an A-weighted sound level as shown in the example given below in Table 2.2. Table 2.2 An example of an application of an A-weighting to a linear sound level 1. Octave band centre frequency (Hz) Linear sound level A weighted correction factor A weighted sound level The A-weighted sound levels are found by adding the A-weighted corrections to the linear sound values for each frequency. This produces the values shown in line 3. The sound level values in line 3 are then added to give the A-weighted sound level in units called db(a). When added together the final result is 84.9dB(A), not the 650 odd expected. This is explained later in the section on combining decibel readings. The (A) here simply means that the sound level has been obtained either using an A network weighting on a sound meter or has been converted to A-weighting as shown above. Combining Sound Levels in Decibels Often it is necessary to combine the measurements from several noise sources stated in decibels to obtain a total noise level. For example we may be required to predict the noise level in the work place if a machine of known sound emission levels is added. This can be done by adding the sound level from the machine to the background level already in the workplace. It cannot be done by simply adding the values in db. This is because the db scale is a contrived logarithmic scale. In order to combine sound levels it is necessary to convert them back to energy levels, add them, and then convert them back to db. When this is done you will find that combining an 80dB sound source with another 80dB sound will produce a total sound pressure level of 83dB not the expected 160dB. In other words for every 3dB increase the sound pressure level doubles! 23

5 The mathematics of combining db values is a little complicated. Without trying to explain how it is done the equation for the calculation is as follows. Sound pressure level (db) = 10log 10 [ 10 1st db value/ nd db value/10...]...] Example Calculation For example if you wish to add together the sounds from three noise sources with sound pressure levels of 89dB, 90dB and 91dB you would proceed as follows. Sound pressure level = 10log 10 [10 89/ / /10 ] = 10log 10 [7.94 x x x 10 8 ]* = 10log 10 [ x 10 8 ] = 10 x = 94.85dB * Note: These values are obtained by dividing the indices (for example 89/10 to get 8.9), then taking the anti-log (push inverse then log on calculator) of the result. In this case it is 7.94 x If you wish to subtract sound levels use the same equation except that the values in square brackets are subtracted. An example of this may be if you have a noisy machine in a factory and wish to find out the noise from the machine only. With the machine on the noise level in the factory is 95dB, but when it is turned off it is reduced to 89dB. The noise from the machine is determined as follows: Example Calculation Sound pressure level = 10log 10 [10 95/ /10 ] = 10log 10 [31.62 x x 10 8 ] = 10log 10 [ x 10 8 ] = 10 x = 93.74dB 24

6 If you find all of this mathematics too hard there are charts which can be used to deduce the combined sound level from two sources. This is demonstrated in figure 2.2. Figure 2.2 Chart for combining sound levels To use this to estimate the effect of combining two noise sources one of 88dB and another of 91dB, proceed as follows. 1. Find the difference between the two sound levels 2. Find this value on the X-axis of the Chart 3. Proceed directly up from the X-axis until intersecting the curve. 4. Move straight across to the Y-axis and note the value 5. Add the value obtained from the Y-axis to the noisier source 6. To obtain noise levels for several combined sources repeat the first five steps for the lowest level noise sources, then calculate their combination with the next noisiest source. 25

7 For example if you wish to add together the sounds from three noise sources with sound pressure levels of 87dB, 90dB and 94dB you would proceed as follows 1. The two lowest level noise sources are 87 and 90, therefore the difference is 3dB 2. The corrected value from the chart is 1.7dB (see dashed lines on Figure 2.2) 3. Total noise from these two sources is therefore = 91.7dB 4. Noise level difference between loudest machine and the combined noise of the other two is = 2.3dB 5. The corrected value from the chart is 2.0dB 6. Total noise from these two sources is therefore = 96dB Frequency Analysis, Frequency Bands and Centre Frequency In order to be able to correctly weight sounds to the perception levels of the human ear, or to get more detailed information about complex sounds it is necessary to divided the frequency range of audible sound (20 20,000Hz) up into bands. This is done by using electronic filters that reject all sound with frequencies outside the selected band. These bands normally have widths of 1/3 of an octave or 1 octave. For those not familiar with music an octave is a doubling of frequency (i.e. going from 260 to 520Hz is one octave). On a piano this means moving up eight white keys (hence the term octave). On a sound frequency graph it means a frequency band where the higher frequency is twice the lower frequency. In sound measurement, the values stated for frequency bands are normally centre frequencies. This means that a range of sounds is allowed through the sound filter with the frequency stated being the centre. An example of this is the 1000Hz centre frequency. In this band a filter allows all sound of frequencies between Hz through, but rejects all others. This process of dividing complex sound up into bands is called frequency analysis, and the results of a frequency analysis are presented on a chart called a spectrogram or a frequency histogram. Figure 2.3 shows the spectrogram of a complex waveform. 26

8 Figure 2.3 The Spectrogram of a complex waveform (from Brüel and Kjær Measuring Sound). After a signal has been divided into frequency bands, then weighted, it is generally amplified inside the sound level meter and its Root Mean Square or (RMS) value determined using an RMS detector. This determines the amount of energy in the sound being measured. Measuring Noise Exposure Equivalent Continuous Sound Levels (L eq ) As sound waves transfer energy, the amount of potential hearing damage associated with a noisy environment is directly linked to both the sound level and the amount of time to which a person is exposed. That is a one-hour exposure to a loud sound will do more harm to our hearing than will a one minute exposure to the same sound pressure level. Noise exposure x time = hearing damage 27

9 In order to assess the hearing damage potential of a noisy environment, both the sound pressure level and the time of exposure must be measured. This allows us to determine the amount of energy received by the ears. When sound levels are fairly constant this is a simple process, but if they fluctuate then sound levels must be sampled repeatedly over a set sampling period in order to obtain valid results. It is then possible to derive a single value referred to as equivalent continuous sound level (L eq ) which relates the amount of energy received over time from a fluctuating sound to an equivalent continuous sound level exposure. Hence it relates the relative potential for hearing damage to an equivalent continuous noise dose. A good example of time varying sound is the estimation of traffic noise in urban areas. When the sound level measurement is also A-weighted the dosage is referred to as L Aeq. In addition to determining the hearing damage potential of sound, these types of measurement are also commonly used in community noise annoyance assessments. In order to be able to calculate the L Aeq of a noise source it is essential to use an integrating sound level meter (see section on sound level meters) which accumulate and average the noise dose over a set time period usually 60 seconds. L Aeq values can also be used to estimate the noise dose to a person who moves around in a work environment where noise levels vary greatly. For example if a worker spends one hour in a very noisy workshop (at 100dB(A)), then the other seven hours behind a desk in a quiet office (at 70dB(A)), their equivalent noise exposure level would be 91dB(A). Figure 2.4 Estimation of L Aeq values (from Brüel and Kjær Measuring Sound 2 ) 28

10 Equivalent sound can also be useful in evaluating community noise over a 24hr period. Figure 2.5 shows how this may be used. The dotted line indicates the L eq for the same period, giving an indication of the constant sound level that would have delivered equivalent sound energy to the ears of residents. Figure 2.5 Use of equivalent sound level for evaluating community noise Sound Level Meters These are the most common instruments used to measure sound pressure levels. The main components of a typical sound level meter are shown in Figure 2.6. Although there are many different brands and types of sound meters available, all have the basic layout shown. The microphone is a transducer that converts a physical measurement of sound waves into an electrical signal. The voltage that leaves the microphone is proportional to the sound pressure level. The most suitable type of microphone for sound level meters is the condenser microphone, which is very stable and reliable, but only produces a small voltage. This means that a preamplifier is needed to boost the signal before it can be processed. There are several different types of processing that may be performed on the signal. These will vary according to the type of assessment being conducted. The simplest type of processing involves passing the signal straight through to the detector unmodified. This produces the L in or linear sound rating. It is sometimes referred to as an all pass network. More often though the signal is passed through a weighting network. The function of these has already been discussed, in previous sections. Basically they are just selective electronic filters which provide A, B, or C weightings to the sound frequencies. 29

11 After the sound levels are weighted they are passed on to the amplifier, then the RMS detector, which accurately assess the amount of energy transferred by the sound wave. Figure 2.6 The components of a typical sound level meter (above) Figure 2.7 A typical hand held sound meter (from Brüel and Kjær Instruments) Detector modes for sound meters Sound waves are rarely steady in level. In order to cope with the often large fluctuations of sound levels found during analysis, most sound level meters are provided with two or more response modes. Some of these include slow response mode, fast response mode, impulse mode and peak mode. Fast response mode This means that the meter responds much like the human ear in approximately msec. It allows accurate analysis of sound that does not fluctuate very rapidly. 30

12 Slow response mode This mode uses a time frame of approximately one-second to average the sound level. This does not mimic the response of the human ear, but rather is used to evaluate sound levels with large fluctuations in intensity during measurement. Impulse mode This is a very rapid response mode (typically 35msec). It is used to analyse very short sharp sounds such as dropping objects and impacts which could not be accurately followed by typical sound meter settings. This is referred to as transient noise. In this mode the meter responds like the human ear to transient noise peaks. Peak mode Although the perceived loudness of very short duration sound is lower than that of steady continuous sound, this does not mean that the potential damage to hearing is reduced. For this reason most sound level meters include a circuit for measuring the peak value of the sound. This is the highest sound pressure level reached during measurement regardless of the duration for which it occurred. This is an important parameter in most occupational noise exposure standards. In Australia peak values are referred to as short-term exposure levels or STEL. Hold circuits Most meters also include a special circuit called a Hold circuit which stores the Peak value or the maximum RMS value. Integrating Sound Level Meters Noise levels encountered in practice are rarely constant in level. Often the fluctuations in sound pressure levels are quite large. In order to measure these levels most high quality sound meters are equipped with an integrating facility, which allows mean sound levels over a set time period to be established. More detail on this is given in the section on equivalent sound levels. Calibration of Sound Level Meters As with all scientific instrumentation sound level meters must be calibrated to enable accurate and reliable sound level measurement. A complete procedure involves both electrical calibration and acoustic calibration. Electrical calibration This involves the use of an internal oscillator of known frequency which checks the amplifier, the weighting networks and the output meter. If any of these are incorrect they may be adjusted by controls on the meter. This form of calibration does not however check the performance of the microphone, which must be checked by regular acoustic calibration. 31

13 Acoustic calibration This involves placing a small acoustic calibrator (sometimes called a pistonphone) on the microphone, and comparing the result with the known reference value of the calibrator. These devices provide precisely defined sound pressure levels to which the sound pressure level meter should be adjusted (for example one provides a 94dB output at 1,000Hz). Generally accuracy of up to 0.2dB can be obtained by using these calibrators and a good quality meter. This calibration is normally limited to a few discrete frequencies, so it is by no means totally foolproof. It is good technique to calibrate a meter both before and after a sound measurement session to ensure that valid results have been obtained. Any large errors indicate damage to either the sound meter or the calibrator in which case both should be serviced. Common Errors in Performing Sound or Noise Measurements As with most scientific instruments, unless sound pressure level meters are used correctly, then the data produced with them is meaningless. Some of the more important points with regard to their use are listed below. In general the most important sources of error in obtaining sound levels include mishandling of microphones and meters, wind, temperature, dust, humidity, changes in ambient pressure, vibrations, magnetic fields, and background noise. Mishandling of microphones and equipment If microphones are placed on surfaces that are vibrating, then the microphone will produce signals that register on the meter as sound pressure. Exposure to extremely high noise levels and dropping microphones will also cause great changes in calibration of the instrument. Wind Even a light breeze blowing across the microphone will produce spurious noise. This sounds somewhat like someone blowing in your ear and is principally made up of frequencies below 200Hz. Fitting a porous foam windscreen over the microphone reduces these effects. Additionally this protects the microphone from dust and humidity. Most commercially available windscreens do not work effectively in winds above 20km/h and they do attenuate (reduce) the levels of high frequency sound. If sound levels must be obtained in windy conditions then special acoustic enclosures may be used. Humidity Relative humidity levels of up to 90% have little or no effect on noise measurements. Moisture does however affect the long-term performance of the microphone. For this reason they should always be protected from rain and stored in a dry (or desiccated) environment. 32

14 Temperature Most sound level meters are designed to work in temperature ranges from C. So temperature is not normally a problem. If the instrument is moved from an airconditioned environment into a hot environment however, condensation forming on the microphone may cause problems. Very low temperatures may cause battery failure. Ambient pressure Variations in atmospheric pressure of up to 10% cause little error (0.2dB) on microphone sensitivity, but at high altitudes the sensitivity of the instrument to high frequencies drops off. To get around this special adjustments must be made when calibrating with the pistonphone. Reflected and absorbed sounds Objects near sound sources may greatly affect the sound output. For the purpose of measurement to assess things such as operator levels, these must be left in place to ensure an accurate reflection of the true environment is obtained. If the true sound power output of a particular machine is required however, it may be necessary to remove such items. Background noise One factor that is often overlooked in sound measurement of noise sources is the level of background noise compared to the level of sound being measured. The background noise must be such that it does not overwhelm the sound being generated by the noise source. What this means in practice is that the level of the sound being measured must be at least 3dB higher than background for any sort of measurement to be made. This means that it must be at least twice as loud. Even when there is 3dB or more difference between background and the sound source being measured, it is still generally necessary to apply a correction factor to obtain a valid result. This is done by following the procedure below. 1. Measure the total noise level (L S+N ) 2. Measure the background level only (L N ) by turning off the noise source if possible. 3. Find the difference between the two levels (L S+N - L N ). If the difference is less than 3dB then the background level is too high for reliable measurement. If it is between 3 10 db then a correction factor must be applied. If it is greater than 10dB then correction factors are unnecessary. 4. An example of the use of correction factors is given in Figure Use the correction chart to estimate the correction factor. The value for (L S+N - L N ) is plotted in the x-axis, and its intersecting value from the Y axis obtained. 33

15 6. The value obtained from the y-axis is subtracted from the total noise level (L S+N ). Number of Decibels to be subtracted from total noise level Difference between total and background level (db) Figure 2.8 Applying a correction for background noise. Noise Level Measurement It is important to realise that the type of noise measurement made depends on the purpose of the measurement. For example narrow frequency band determinations may be required to identify the noise from a particular machine in a factory, or maybe only the db(a) level is required to find out whether the factory noise level exceeds the allowable levels according the legislation. In the former case the machine may put out noise in a particular frequency band, and narrow band frequency analysis would allow estimation of its contribution to total noise only. In the latter case only the total noise in the area would be determined, and no specific noise sources examined. The type of noise source will also determine the type of noise measurement made. For example steady noise sources require different types of measurement to impulsive noise sources. The appropriate types of measurement for different types of noise sources are summarised in table

16 Table 2.3 Noise types and their measurement Type of noise Constant continuous (steady) noise Examples of noise source Electric motors and pumps Appropriate type of measurement Direct reading db(a) value Suitable Instrument for Measurement Normal sound level meter Constant intermittent noise Automatic processing machinery db value and exposure time or L Aeq, noise dose Integrating level meter sound Regular impulses Automated drilling or hammering equipment Isolated impulses Manual hammer blows L Aeq, or noise dose and impulse noise level to check peak noise L Aeq and peak value Integrating level meter sound Impulse sound level meter and meter with peak hold facility. Periodically fluctuating Surface grinding equipment db value and L Aeq, or noise dose Integrating level meter sound Non-periodic fluctuating All types of manual machining and grinding work, welding etc. L Aeq or noise dose, and frequency analysis Noise dose meter and integrating sound level meter with frequency analysis. Statistical Analysers (L 1, L 10, L 50 and L 90 ) These are instruments that measure the distribution of fluctuating noise with time for the purpose of assessing community noise and its potential to cause hearing damage. In addition to providing energy averaged noise levels (such as L eq and L Aeq ) they also provide information on how often certain sound levels are exceeded. For example they provide values such as L 1, L 10, L 50 and L 90, which are the sound pressure levels exceeded 1%,10%, 50% and 90% of the time respectively. When these are used with A weightings these values become L A10, L A50 and L A90 values which are commonly used by the NSW EPA for investigating sound levels. Equivalent (average) Noise Levels (L T, L R ) Statistical analysers also allow measurement of cumulative noise doses over time. Where noise levels fluctuate in an unpredictable fashion over time, they are best represented by the equivalent noise level which has the same acoustic energy (or noise dose) as the original fluctuating levels for the same period of time T. All measurements of this type are A-weighted, and so are sometimes represented by the symbol L Aeq,T. Here the A represents an A-weighting, the eq tells us that it is an equivalent noise or sound level, 35

17 and the T is the time is it averaged over. A single L Aeq,T value can for example be used to represent the fluctuating noise levels from an industrial workshop, which can then be compared with the 85dB(A) standard. This indicates whether there is a danger of exceeding the allowable Australian 100% noise dose for an 8 hour day of 85dB(A) L Aeq,8hr if the noise fluctuations continue. Noise doses over 10 hours L 10hr, and 18 hours L 18hr are also commonly determined. In addition to L Aeq, corrections for impulsive noise and pure tone content of noise are sometimes used. When the L Aeq value is corrected for these it is referred to as the L R. Sound Exposure Level (SEL) or (L S ) or (L EA,T ) The levels of many sounds change from moment to moment. This variation must also be accounted for when measuring noise levels. High quality sound integrating level meters have a setting that allows for this a measure referred to as the sound exposure level or SEL. It is also given other symbols such as (L S ) or (L EA,T ). The sound exposure level is defined as that level which lasting for 1 second has the same acoustic energy as a given noise equivalent lasting for time T hence the term (L EA,T ). As it is a measure of acoustic energy it can be used to compare unrelated noise. This is possible because the time element in the definition is always normalised to 1 second. Some documents also refer to this as the single event noise exposure level (SENEL). Background A-weighted Sound Pressure Level (L Abg,T ) This is the A-weighted sound pressure level obtained by using the time weighted F mode of the sound meter and arithmetically averaging the lowest levels of the ambient sound pressure levels without the noise under investigation. The time interval for study (T) must be noted. It is used by the NSW DECC, but is not a common unit overseas. For all intents and purposes it is approximately equal to L A90,T. Day-Night Sound Level (L DN ) The Day-Night Sound Level is the A-weighted equivalent sound level for a 24-hour period with an additional 10dB weighting imposed on the equivalent sound levels occurring during night time hours (10pm to 7am). Hence, an environment that has a measured daytime equivalent sound level of 60 db and a measured night time equivalent sound level of 50 db, can be said to have a weighted nighttime sound level of 60 db ( ) and an L DN of 60 db. Table 2.4 summarizes the use of the four most common sound descriptors used in noise analysis. Summary of Sound Descriptors Table 2.4 A Summary of the most commonly used sound descriptors 36

18 Typical use Name of descriptor Nature of descriptor To describe steady air-conditioning sound in a room or measure maximum sound level during a vehicle pass by with a simple sound level meter. To describe noise from a moving source such as an airplane, train, or truck. To measure average environmental noise levels to which people are exposed. To characterize average sound levels in residential areas throughout the day and night. A-weighted Sound Level A-weighted Sound Exposure Level Equivalent Sound Level Day-Night Sound Level Note: The unit for all sound descriptors is the decibel. The momentary magnitude of sound weighted to approximate the ear's frequency sensitivity. A summation of the energy of the momentary magnitudes of sound associated with a single event to measure the total sound energy of the event. The A-weighted sound level that is "equivalent" to an actual time varying sound level, in the sense that it has the same total energy for the duration of the sound. The A-weighted equivalent sound level for a 24-hour period with 10 decibels added to nighttime sounds (10pm 7am). Noise Dose Meters (noise dosimeters) These are meters used to measure the cumulative noise exposure of individuals who move between different environments with varying noise levels during a normal work period. The person normally wears the dosimeter, with the microphone being placed somewhere near the hearing zone. The meter may be attached to a belt or slipped into a pocket (they are normally small enough). Newer models come in the form of noise badges that are so small they can be attached just like a badge and worn on the lapel. They are normally used with A-weighting filters and assess the noise dose received over the working day to assess the potential of the work environment for hearing damage. Figure 2.9 A typical noise dose meter (from Brüel and Kjær Instruments) Noise dose meters are available in different formats according to where they are being used. They are designed to display the percentage of daily allowable noise dose received (as well as other assessments). This means that they 37

19 are country specific. Meters that comply with Australian regulations have the following characteristics: Sounds below 80dB(A) are not integrated (as they do not make a significant contribution to noise dose An eight hour exposure level of 85dB(A) constitutes a 100% noise dose (displayed in L eq ) Note: in some states this is still 90dB(A). But will soon be changed! For each 3dB increase above 85dB(A) the noise dose is doubled for the same exposure time. From these rules we can see that an increase in noise levels of 3dB halves the permitted exposure period. Measuring Steady Noise Steady noise refers to sound pressure levels that are fairly constant over time generally remaining within a 10dB range. Typically this applies to items such as machines in factories which run at a constant rate. An example would be what we call a constant hum. Steady noise measurement is normally performed using sound level meters set to measure an A-weighting and in slow response mode. Noise should be measured at the position of the exposed person s head. Noise doses from steady noise are relatively easy to calculate and compare to accepted standards. In Australia this is 85dB(A) for an average 8-hour day and 40 hour week. Measuring Discretely Varying Noise Levels This refers to situations whereby persons are exposed to a certain number of varying noise levels. This can occur when a person moves around a workshop, or production line as part of the work process in a systematic fashion. In these cases it is possible to sum the series of partial doses received during the exposure period. For example is a person spends 4 hours exposed to 85dB and four hours exposed to 88dB, they will have received 150% of their daily allowable noise dose. This is calculated as follows: The 100% noise dose for 8 hours is 85dB(A) Each increase of 3dB represents a doubling of the noise dose, therefore 4 hours at 88dB(A) is equal to 100% of the allowable daily noise dose Four hours at 85dB(A) is equal to 50% of the allowable daily noise dose. Total noise dose is therefore = 150% of allowable daily noise dose. These values may be logged on a work card, which might look like that shown in Figure

20 Noise Exposure Record Name G. Fullick Date 9/9/09 Work Location Time in Calculation location (hours) db(a) 8Hr Dose % Partial Dose %* Drill press Office Packing bay Prod. Line *Partial Dose = (%Time/100) x 8 hour dose Total Dose Figure 2.10 A typical workplace exposure record Note: A noise level 15 db(a) below the accepted dose limit makes no effective contribution to noise dose (due to the logarithmic nature of the noise scale). Each increase of 3dB(A) above the limit doubles the exposure level. Measuring Impulsive Noise This is somewhat more difficult than assessing steady or fluctuating noise. The International Standards Organisation (ISO) suggests that it an approximation of the partial noise dose for impulsive sounds (such as hammering) may be obtained by adding 10dB to the measured db(a) value obtained from the slow response mode of the sound level meter. This does not approximate single impulse sound very well however. This has resulted in the issuing of supplementary criteria by noise authorities and standards vary greatly between different countries in this area. New national standards impose impulsive noise limits in terms of db(a) recorded on the Impulse (I) setting of the sound level meter. Impulsive noise must also fit in the peak noise criteria. In most Australian States the maximum allowable short-term (peak) exposure level is 115dB(A). Typical readings for impulsive noise sources are given in Table

21 Table Typical values obtained from the assessment of impulsive noise sources using sound level meters on different settings Meter Setting/Time weighting Noise Source Hammer Drill Press Nail Gun F (fast mode) 105 db(a) 93 db(a) 112 db(a) I (impulse mode) 112 db(a) 97 db(a) 113 db(a) P (peak mode) 131 db(a) 121 db(a) 128 db(a) Frequency Analysis Many sound protection devices are more effective at reducing some frequencies of sound spectrum than others. This means that not only should the sound pressure levels of an environment be measured before suggesting appropriate hearing protection, but the sound pressure levels of individual frequencies as well. This means performing a measurement using a meter capable of frequency analysis. Normally octave band filters are used. These are filters that divided the sound spectrum up into octaves (or 1/3 octaves depending on the cost of the meter), and display the individual intensity of each set of frequency bands. Figure 2.11 shows a typical frequency analysis from a noise source. Figure 2.11 A typical frequency analysis of a noise source 40

22 Noise Mapping This refers to the process of taking noise measurements at several points around an area which is being evaluated for noise, plotting these values on a map, then joining the points of equal sound levels with lines. These lines of equal sound intensity are referred to as noise contours. They are used to indicate the sound distribution pattern. Obviously with this type of procedure the more measurements which are obtained the more accurate the noise map. Noise maps allow an easy visual appraisal of the areas in which sound levels are too high, or are likely to cause harm to hearing. They may even be used to estimate noise dosage if the noise levels do not vary greatly. This is done by marking areas where the sound levels are known and monitoring the amount of time that an individual spends in these areas. This can then be logged on the person s noise dosage card. Noise maps also identify where hearing protection or noise reduction strategies should be used. If any aspect of the area changes new noise maps must be constructed. Even minor changes in layout of an area can greatly modify the noise map as most materials reflect or absorb sound to greatly different degrees. This is discussed in more detail in later chapters. Figure 2.12 shows a typical noise map prepared for two machines set up side by side. Each machine radiates noise in a hemispherical pattern around itself. This can be seen from the noise map. Figure 2.12 A typical noise map from a two-point noise source (from Brüel and Kjær Measuring Sound 2 ) Noise Rating Curves These are extremely complicated in nature so only a brief description of what they are and how they are used will be given here. 41

23 Noise rating curves are used in noise abatement studies to estimate the level of noise annoyance, which is then compared to those accepted in various standards issued by the ISO (International Standards Organisation) or AS (Standards Australia). A noise rating curve is simply a plot of how well the human ear responds to different frequencies at different sound pressure levels. A sound level meter capable of frequency band analysis is required to provide sound levels in each frequency band. These are then plotted on the noise rating curve to give a band noise spectrogram. The spectrogram obtained is then compared to different standard spectrograms and the noise is assigned the number of that one which has the closest match. Figure 2.13 shows a typical noise rating curve. Analysis of this shows that mid range frequencies are far more important in noise annoyance than are lower and higher frequencies. Figure 2.13 A typical noise rating curve and spectrogram (from Brüel and Kjær Measuring Sound 2 ). Sound Attenuation with Distance (Distance Attenuation) You are probably well aware of the fact that the further away you move from a sound source, the softer the sound from that source becomes. This is because the energy transferred in a sound wave is slowly dissipated over distances the further the distance the greater the energy dissipation. The process is known as distance attenuation. Studies have shown that for each doubling of distance from a sound source, the sound pressure level drops by 6dB. Without going into the mathematics of it the relationship between sound power and intensity (loudness) is an inverse square with distance and is called the Inverse Square Law. 42

24 It is not important that you understand the mathematics of the Inverse Square Law only how to use it. Hence if you know that a sound level 50meters from a point source is 90dB, then at 100meters from the same source it will be 84dB. Note that this does not take into account that some materials in the sound path may absorb some of the sound energy (see Chapter 5). If you wish to estimate the sound intensity at a certain distance from a sound source (whose sound power is known) then this can be calculated using the equation Sound pressure level = sound power level - 20log r Where r = distance from the sound source in meters This equation only works for sound emanating from a point source, not a line source or multiple point sources. Another important point to be aware of is that different frequencies are attenuated at different rates by distance. Higher frequencies are attenuated at a greater rate then lower frequencies. Example Calculation Question A point source has a sound power level at the source of 80dB(A). Determine the sound pressure level at a distance of 15 meters from the source. Answer Use the equation Sound pressure level = sound power level 20log r = log = X = = 58.6 db(a) It is also possible to calculate the sound pressure level at a known distance from a point source if you are given sound pressure level at a different distance from the source, but do not know the sound power level at the source. This is done using the equation given below: Sound pressure level (2) = sound pressure level (1) - 10log (r 2 2 ) (r 2 1 ) Where r 1 = closest distance to the sound source in meters r 2 = furthest distance to the sound source in meters 43

25 Example Calculation Question The sound pressure level is measured 20 meters from a point source and found to be 60dB(A). Determine the sound pressure level at a distance of 47.5 meters from the source. Answer Use the equation Sound pressure level (2) = sound pressure level (1) - 10log (r 2 2 ) = log ( / 20 2 ) = log (2256/400) = = 52.5 db(a) (r 1 2 ) General Rules for Making Sound Measurements Every sound level meter will be different, so the most appropriate method for learning to use your sound level meter correctly is to read the manual thoroughly as unappealing as the prospect may sound! There are some general rules for making sound measurements 2 however, which can be highlighted to help simplify the process. 1. Ensure that you have read all the appropriate standards and have the appropriate equipment to perform the measurement required, and that you use the appropriate technique. The state EPA s provide details of how to measure each different type of noise and noise source. In NSW you should consult the EPA Stationary Noise Source Policy (1998). 2. Ensure that you have enough batteries (or power) for all calibration sources and meters 3. Check the calibration of the sound level meter before and after use. 4. Make a map of the area to be measured, and note preliminary sound levels to determine the type of sound field you are working with. 5. Identify the main sound source/s being measured. 44

26 6. Choose the appropriate microphone for the job (either directional or omnidirectional) if you have a choice. 7. Point the microphone at the source being measured (not if you are using special microphones such as ANSI). 8. If you have a choice, decide which weighting network to use. Generally the A weighting will be most appropriate, but this will depend on the standard you are using. 9. Select the correct detector response, either F, S or I. Remember that I measurements can only be made with an impulse sound level meter. 10. When taking measurements hold the meter at arms length away from your body to minimise reflections from your body and blocking reflected sound arriving from behind the meter. 11. During the measurement remember to: - keep away from reflecting surfaces - measure at a suitable distance from the noise source - check the background noise level - use a windshield if necessary - do not accept readings when meter is overloaded 12. Keep a well-documented measurement report. For more detail on making sound measurements you should consult the Australian Standards AS 1055 (1997), AS 1217 (1985) and AS IEC (2004) or their replacements when they are available. Standards for Noise Exposure Standards used for controlling noise exposure vary greatly from country to country. In Europe noise exposure standards are broadly defined by ISO standards, in the USA they are defined by Occupational Safety and Health Administration (OSHA). In Australia, noise exposure standards are set by the Occupational Health and Safety acts of the individual states. This legislation is administered by Health and safety bodies in the states such as WorkCover (NSW), the state Environmental Protection Authorities, and local councils. 45

27 All Australian states now have a maximum permissible noise exposure level of 85dB (LA eq8hr ), and also have a maximum short term permitted exposure (STEL) of 115dB(A) and peak exposure values of 140dB(C or Lin). See National Code of Practice for Noise Management and Protection of Hearing at Work [NOHSC: 2009 (2004)] 3rd Edition or ASCC Noise Control for more details. References 1 Bies, D.A., and Hanson, C.H., Engineering Noise Control Theory and Practice, Chapman & Hall, London, Brüel and Kjær, Measuring Sound, Nærum, NSW DECC, Stationary Noise Source Policy, or later. 4 Standards Australia, AS 1055 Acoustics Description and Measurement of Environmental Noise, Standards Australia, AS 1217 Acoustics Determination of Sound Power Levels of Noise Sources, Standards Australia, AS IEC Electroacoustics Sound Level Meters, oise/nationalstandardforoccupationalnoise.htm