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

9.12 Real Time Analysis 193analyzers become more apparent in the detection of prominent components ofextremely narrow-band noise.9.12 Real Time AnalysisReal time analysis entails the evaluation of a signal over a specified numberof frequency bands simultaneously. This type of parallel process is schematicallyillustrated in Figure 9.15. Most real time analyzers rely on digital filteringof a sampled time series. A fluctuating noise signal is converted by a microphoneassembly into an electric signal that is entered simultaneously into aparallel set of filters, detectors, and storage components. The scanned output isdisplayed as a bar graph of sound pressure levels versus frequency on a cathoderay tube (CRT) monitor or another type of display such as liquid crystal display(LCD) or plasma screen. The display must be refreshed several times persecond as the level and frequency content of the input signal change. The scanninginterval may be adjusted to meet different conditions, becoming longer tocover the cycling of a specific machine operation, or shortened to evaluate impactsounds.A considerable variety of features are available on current real-time analyzers.Some features may include alphanumeric displays, choice of weighting and linear(unweighted) sound levels, linear and exponential averaging, time constants, spectrumstorage for recall and comparison with other data, and integrated or externalsoftware packages for further analysis of analyzer output. A number of real timeanalyzers operate on batteries, and models are available that provide a choice ofbattery or AC operation.Figure 9.15. Schematic of the processing of a signal through a parallel bank of filters ina real-time analyzer.