THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

9.11 Noise Measurement in Selected Frequency Bands, Band Pass Filters 189Administration (MSHA, 1982). The exchange rate may also be 3 or 4 dB dependingon the application. ANSI S1.25 also specifies limits for the effects of changes inair pressure, temperature, vibration, and magnetic field on the noise dosimeter.The British standard BS 6402 calls for a personal sound level meter to measuresound exposure, i.e., the timed-integrated A-weighted sound pressure level witha 3-dB exchange rate and without exponential time weighting (British StandardsInstitution, 1983). A latching overload indicator is also mandated by the Britishstandard to provide warning that the sound level received at the microphone hasexceeded the measuring range of the instrument (up to 132 dB peak).An IEC international standard for personal sound exposure meters defines awider operating range for a Type-2 integrating averaging SLM. Under this specification,measurements are made of the exposures produced by impulsive, fluctuating,and intermittent sounds over a range of A-weighted sound levels from 80 dBto 130 dB. Excessive input sound levels should trigger the mandatory overloadindicator.9.11 Noise Measurement in Selected Frequency Bands,Band Pass FiltersA valuable means of analyzing noise is the evaluation of sound in each frequencyband. If a machine emits a noise that indicates it is malfunctioning, the analysisof the sound output according to frequency can provide vital clues as to whichcomponent of the machine is defective. This situation calls for the use of aspectrum analyzer, which is a device that analyzes a noise signal in the frequencydomain by electronically separating the signal into various frequency bands. Thisseparation is executed through the use of a set of filters. A filter is a two-portelectrical network with a pair of terminals at each port. A filter can be constructedwith as few as two passive electrical components (e.g., a resistor and a capacitor),or it can be more complex involving a large number of passive components, or acombination of passive elements operating in conjunction with active components(e.g., op amps). Analog filters embody electronic circuitry tuned to pass certainfrequencies, whereas digital filters make use of active electronic elements.Figure 9.12 illustrates the effects of ideal and real filters. An ideal bandpassfilter is a circuit that transmits only that part of the input signal within its bandpass( f 1 ≤ f ≤ f 2 ) and completely attenuates all of the components at all frequenciesoutside of the bandpass ( f ≤ f 1 and f ≥ f 2 ). The ideal low-pass filter passes allsignals up to frequency f lp and rejects all signals having frequencies above f lp .The ideal high-pass filter passes all signals above frequency f hp and rejects all ofthe contents of the input signal below f hp . In the real world, filters alter the shapesof the input signals to some degree. The amplitude and phase characteristics of thefilter can be ascertained by computing the transfer function (or filter response),which is the ratio of the filter output to the filter input for all possible values offrequency. Both ideal and actual frequency responses are shown in Figure 9.12.The actual filters have characteristics of the form shown on the right-hand side