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

9.8 Proper Procedures for Using the Sound Level Meter 183Type 1 meters generally should measure with 1 dB accuracy and are employedto obtain accurate data for noise control purposes. The corresponding error forType 2 meters, which are used for quick surveys, may not exceed 2 dB.Figure 9.7(a) shows a variety of digital-readout SLMs and Figure 9.7(b) illustratesan SLM with a bar-graph spectrum display. Note that the contour of a metercase slopes away from the microphone in order to minimize reflections from itssurfaces. Both Types 1 and 2 generally incorporate A-, B-, and C- weighting networks.The A and C networks are far more commonly used than B-weighting,particularly if low-frequency acoustic energy is present. Also a “fast” or “slow”response setting is generally available. The former setting, corresponding to a timeconstant RC ≈ 0.1 s, responds more quickly to changes in noise levels, but thereadings become more difficult to ascertain with very rapid fluctuations. The slowsetting (RC ≈ 1) reduces the response speed, as attested by the slower movementof the needle in the older-type analog-readout SLMs, and a better grasp of typicalsound levels can be obtained for rapidly fluctuating sounds. More elaborate versionsof sound level meters include 1/3-octave and octave filters to enable soundpressure measurements in various frequency bands.In the block diagram of Figure 9.8, a typical SLM is shown to contain the followingcomponents: a 4-cm or 2-cm microphone feeding a preamplifier (whichfunctions as a cathode follower), which in turn, relays to the one of the weightingnetworks that is selected by a switch. The weighted (or unweighted, if none ofthe weighting curves has been selected) signal then becomes amplified and thenpasses through a root-mean-square amplifier, becomes converted to logarithmic(i.e., decibel) form, and fed to either a digital or analog readout device. Some SLMmodels contain output jacks so that AC and DC signals from the meter can serve asinputs to other instruments, such as Fast Fourier Transform (FFT) analyzers, printers,or graphic plotters. By taking advantage of the PCMCIA modular technology,a meter can be designed to provide a variety of other functions such measuringL eq , L n , reverberation times, and so on.9.8 Proper Procedures for Using the Sound Level MeterTo ensure that a sound level meter is in proper working order, an acoustic calibratorshould be employed just prior to beginning a series of measurements and aftercompleting the series. The calibrator is a single-tone, battery-driven device thatfits over the microphone and produces a precise reference sound pressure level forcalibrating the meter. The calibrator, illustrated in Figure 9.9, employs a zenerstabilizedoscillator to provide impetus to a piezoelectric driver element that causesa diaphragm to vibrate at 1 kHz ±1.5%. The diaphragm produces a sound pressurelevel of 1 Pa (corresponding to rms SPL of 94 ± 0.3 dB) in the coupler volume.The cavity at rear of the diaphragm behaves as a Helmholtz resonator with anatural frequency of 1000 Hz. Calibrators operate at 1 kHz, the internationalreference frequency for weighting networks. Therefore, no correction is requiredfor calibrating instruments for weighted and unweighted measurements.