Noise measurements usually fall into one of two categories. The first category consists of sound level measurements usually made of the dBA level. Common measuring instruments are sound level meters and dosimeters. The second category consists of analyses made in support of environmental noise reduction programmes and product design programmes. In addition to measurement of dBA level, these analyses require frequency analysis or amplitude distribution analysis.
1. Microphones:
Microphones are an essential part of all acoustical measurements and microphone characteristics control the sections of sound level meter specifications and ANSI standards that deal with such parameters as frequency range, directivity, temperature stability and effects of humidity. In fact, the microphone is the main distinguishing factor among different types of sound level meters.
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Ideally, the microphone should produce an electrical signal that is an exact replica of the acoustical disturbance. It must operate over a wide dynamic range and a wide frequency range and it must be stable under severe changes in environmental conditions. Of the many different principles of microphone construction, only condenser and piezoelectric microphones are used for instrumentation purposes.
The condenser microphone can be of the stretched metal diaphragm or the electret type. The stretched metal diaphragm condenser microphone is the best choice for accurate and repeatable measurements, while the piezoelectric and electret condenser are second-best choices if price is important.
The construction of a stretched metal diaphragm microphone, commonly called a condenser microphone. The microphone cartridge consists of a thin metallic diaphragm in close proximity to a rigid backplate. These two elements are electrically insulated from each other and form the plates of a capacitor. A DC polarising voltage is applied across plates of the plates. Variations in pressure due to sound waves will move the diaphragm, thus varying the width of the air gap.
Consequently, an alternating charge is generated on the capacitor. By careful design it is possible to keep the electrical output proportional to sound pressure over a wide range of frequencies and dBA levels. In order that changes in atmospheric pressure will not change the static position of the diaphragm and cause a change in microphone sensitivity, the air inside the microphone is vented to atmosphere.
Therefore, the static pressure remains the same on both sides of the diaphragm. A common temperature coefficient for the metals in the diaphragm and housing is selected so that temperature stability of the microphone can be guaranteed for long periods.
There is another type of condenser microphone, which is commonly called the ‘Electret’ microphone. Its schematic construction is similar to a stretched metal diaphragm condenser microphone in that it has a diaphragm separated from the backplate by a thin air gap. The diaphragm of an electret microphone, however, is made up of a polymer film whose outside surface is metal plated.
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The plating and the backplate form the electrodes of a condenser with each raised section of the metal plate electrode representing a miniature microphone. The outputs are all summed to produce the effective output of the microphone. Since the polymer has electrical charges embedded into it, thereby containing its own charge, no polarisation voltage is required.
The most common form of a piezoelectric microphone uses a piezoelectric ceramic element to which a bending movement is applied when the diaphragm is exposed to sound pressure. The ceramic bender element is supported at both ends. The conical diaphragm applies a force to the centre of the bender, thus producing an output voltage proportional to the amplitude of the diaphragm motion.
Piezoelectric microphones are often used in general purpose sound level meters. They are relatively inexpensive and have the advantage that they do not need a power supply. For precision sound level meters, however, piezoelectric microphones are generally not suitable. Size-for-size, piezoelectric microphones have poorer frequency response and lower sensitivity than condenser microphones.
2. Sound Level Meters:
A sound level meter is an acoustical measuring instrument consisting of a microphone, amplifier, weighting filters and read-out meter. It will meet one or more governing standards: ANSI S1.4-1971 Specification for Sound Level Meters dominates in the United States, but IEC-179, an international specification for a precision sound level meter, is often specified for more exacting work.
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S1.4-1971 specifies three types of sound level meters:
1. Type 1 Precision sound level meter
2. Type 2 General purpose sound level meter
3. Type 3 Survey sound level meter.
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The standards outline straightforward electrical specifications such as meter damping, response of frequency weighting networks and certain environmental characteristics. But the distinguishing factor among the three types of sound level meters is the tolerance allowed on frequency response and directional response. Therefore, the microphone determines whether a sound level meter meets Type 1, 2 or 3 requirements.
Note that precision sound level meters can have no response above 15 kHz and general purpose and survey sound level meters are guaranteed to operate only up to 10 kHz. Most users demand more than the guaranteed minimum, so the prospective user must check the specifications for individual sound meters.
The ANSI standards permit use of grazing incidence microphones, which overestimate sound waves with perpendicular incidence to the sensing surface. The Type 3 sound level meter is permitted to have positive errors ranging up to 14 dB at 8 kHz. To protect the user, OSHA has disapproved use of Type 3 meters for operator exposure surveys. The tolerances for Type 2 meters are much tighter.
Tolerances for Type 1 meters are similar to Type 2, but are extended to 15 kHz. Many organisations are not satisfied with Type 1 directional tolerances so the IEC-179 standard is often specified. The IEC-179 standard essentially requires that the most sensitive axis coincide with the main axis response. The intent is to specify a perpendicular-incidence microphone that can never overestimate sound level.
One other distinction in sound meters should be made. If the meter is to be used for noise reduction programmes, a meter should be selected that has optional octave filters available. These meters are generally more expensive than those designed solely for noise surveys.
Sound level meters measure the rms level of the acoustical signal. Two meter damping positions are required-‘fast’ and ‘slow’-corresponding to averaging times of approximately 200 and 800 ms. Even in the ‘slow’ response the meter will often fluctuate. In this case the operator must note the range of levels indicated on the meter.
For transient noise he notes the maximum reading during the event. But OSHA imposes a measurement requirement for impact noise that is not covered in the ANSI and IEC standards. For impact noise such as forging, stamping and press operations, an OSHA noise limit is imposed based upon the peak sound pressure level.
A peak-responding detector with a rise time not exceeding 50 microseconds is needed to measure this and measurements must be made in the C or linear response mode. Furthermore, the meter must have a hold-circuit that captures the peak level and decays at a rate less than 0.05 dB/second. This capability may be built into the sound meter or the manufacturer may supply a separate accessory.
3. Calibration:
Common practice and most industry standards require that sound level meters be calibrated with an acoustical signal before and after each day’s use. Most calibrators use an electrical signal to derive a diaphragm that serves as a loudspeaker in the calibration cavity. Because the calibration level is a function of the applied voltage, a regulating circuit is used to maintain the supply voltage at a constant level.
Most calibrators of this type operate at 1 kHz. At this frequency the weighting filters have no gain, offering the advantage that a 1 kHz calibrator can be used with the sound meter in the A-weighting mode without using any correction factors. A typical electrically controlled acoustical calibrator is shown in Fig. 32.8. Here a piezoelectric ceramic element is used to drive the metallic diaphragm, which acts as a loudspeaker.
4. Acoustical Measurement:
Today’s acoustical measurements are made either to evaluate an environment or to measure the noise output of a machine. Generally, there are several noise sources in the area and the operator’s machine may not be the major noise source. It is important that the microphone for measuring operator environment be omnidirectional-that it accurately measures all sound sources, regardless of their direction from the operator.
In contrast to operator position measurements, machine measurements call for measuring only the noise emanating from a single source. Machine measurements must usually be made on the factory floor, particularly frequency analyses in support of enclosure design.
In these cases, measurements must be made close to the machine to ensure that the machine sound dominates the environment. This measurement is best made when all other machines are shut down, such as during nonworking periods, so that the most accurate characterisation of the machine can be made.
With the exception of dose measurements for which mobile employees wear a microphone on their lapel, the person making the measurement should be behind the microphone so that his own body does not act as a reflecting object to disrupt the measurement.
5. Dosimeters:
The OSHA law requires characterisation of varying noise environments by single number criteria. The environment may be that experienced by a mobile employee whose work takes him into many different factory areas or it may be a fixed work area where intermittent machine operations produce unpredictable variations in noise level. OSHA specifies a maximum daily noise dose (D) of unity –
Where C is the total exposure at a given steady dBA level and T is the maximum allowable exposure time at that level during a 8-hour work day. The relationship between dBA level and allowable duration are shown in Fig. 32.9.
Noise dose can be predicted for mobile workers by studying their movements and estimating the total time they are exposed to each different dBA level they experience. Alternatively, dosimeters can be worn by mobile employees and by the stationary employees to obtain a direct measure of noise dose. Dosimeters are integrating sound meters that operate over fixed time periods, usually 8 hours.
They convert dBA level to frequency in accordance with the OSHA dBA/exposure time relationship and produce a composite exposure number on a digital read out. Their advantage is that they are simple instruments that convert time-varying sound level to an equivalent steady level. Simplified block diagram of a personal noise dosemeter is shown in Fig. 32.10.
6. Frequency Analysers:
Basic sound level meters are commonly used to determine if there is an OSHA violation of steady noise exposure or if the overall noise level of a machine meets its guarantee. As-such, they are go-no go monitors. The readings give little assistance to the person responsible for reducing noise levels.
He must usually know the frequency content of the sound because the design of efficient barriers and enclosures is heavily dependent upon the frequency spectrum and absorbing materials on walls or ceilings only work if high frequencies dominate.
Because sound level meters are portable instruments, portable filters are also required. Filters take the form of an octave or a ⅓”-octave (Fig. 32.11) sequence of bandwidths that permit the wide audio range to be reduced to a practical number of segments. The centre frequencies and band pass characteristics of octave and ⅓” -octave filters are controlled by ANSI S1.11-1966. Although analysers used in the laboratory contain a large number of individual filters operating in parallel, portable analysers consist of a single filter that the operator tunes or steps from one position to the next.
Sound level meters have been designed so that octave analyses can be made only when the meter is in its flat frequency-response mode, thus preventing analysis of A-weighted signals. But today almost all noise reduction tasks are directed toward reducing dBA level.
Therefore, it is necessary that today’s instruments permit series-connection of the A-weighting filter and the octave filters. In this way, the dominant A-weighted octave band is directly identified. A typical octave analysis is shown in Fig. 32.12. The dBA level is plotted in octave bands to show the beneficial effects of lining the enclosure with sound-absorbing materials.
7. Amplitude Distribution Analysers:
As octave analysers supplement sound level meters in programmes to reduce dBA level, the dosimeter also needs a support instrument to identify dominant machines. Management needs an economical solution to noise problems; therefore, dominant noise components must be identified before any noise reduction programme is initiated.
The machine that is the noisiest one in the shop when it operates is not necessarily the dominant noise source in a mixed noise environment. Its contribution can only be determined by integrating the total time that the machine operates during the day. And, it is inappropriate to build an enclosure for a noisy machine if an equal noise reduction can be achieved by reducing background noise, such as by conveyors or metal tote pans.
One application of the Environmental Noise Classifier is in the measurement of the OSHA mixed exposure level where it serves as an area dosimeter and as a guide to the most economical solution to noise reduction problems. Consider the data shown in Table 32.1. The coefficient values are the ratios of the time spent in each band divided by the maximum allowable time in the band during an 8 hour day.
If the sum of all coefficients (mixed exposure level) exceeds unity, the noise is in violation of the OSHA code. In each example, the times spent in the individual bands are read from the Environmental Noise Classifier in minutes and recorded in the table in hours. In each example, the mixed exposure level is about 1.25.
Without going into great detail, it can be seen that quieting the obvious machine that operates for ½ hour at the 97-100 dBA level in Case 1 is not an economical solution to the problem. If reduced to below 90 dBA, the OSHA coefficient would still be 1.00. Further, quieting a large machine by 7 to 10 dBA is a major undertaking and may require construction of costly enclosures. The enclosures create problems of their own, such as access, increased labour and temperature build-up. The data suggest the reduction of ambient noise and lower level machine noise to accomplish the goal.
In Case 2, there is no choice but to work on the noisiest machine. But the histogram suggests how many dB the level must be reduced. In order to reduce the mixed exposure level to less than unit, the offending machine must be reduced to the 90-92 dBA bands or a reduction of 3 to 5 dBA. The same reasoning can be applied to reducing noise of machine tools where the sound level changes with each cycle.