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

208 9. Sound-Measuring Instrumentation1. The spatial average sound power level ¯L p generated by the subject machine ismeasured in each octave band.2. The reference source is placed near the machine and both are operated simultaneously.The reference source is adjusted until it produces an additional 3 dBin the sound pressure level in each octave band than when the machine wasoperating by itself. The reference source is then producing a sound power levelin that band equal to that generated by the machine under test. The sound powerlevel indicated on the meter of the reference power source is recorded. Theprocedure is repeated for each of all the other octave bands.3. The total sound power level can now be calculated from the use of Equation(9.26).9.24 Data Acquisition SystemsFor a number of years, magnetic recorders provide a permanent record of noisedata taken in situ for subsequent analyses on instruments located elsewhere and forarchival purposes. Currently, digital recorders are being used for acoustical dataacquisition. Very few, if any, analog recorders (either AM or FM) are currentlysold. Computer technology has advanced to the stage where measured data andits analytical results can be acquired and digitally stored in computer memoryor on recordable CD-ROMs, DVD-R, and removable cartridges or disks. A CD-ROM can hold approximately 700 MB of data, but the DVD (which can be usedto contain digital data and not just video programs) holds nearly ten times asmuch. Newer storage disks are soaring past the gigabyte range to encroach inthe terabyte territory. We are witnessing the rapid demise of the magnetic taperecorder in favor of computerized acquisition devices and digital storage units. Therecent introduction of flash disks provides the potential for even greater portabilityof data. Transient sounds, analyzed on a cascade-type FFT analyzer, are moreconveniently archived in a nonvolatile memory medium such as a random-accessremovable cartridge than they would be on a magnetic tape (which obviously doesnot provide random access), for the purpose of later retrieval.9.25 Integration of Measurement Functions in ComputersAdvances in computer technology and software development tools make it possibleto integrate measurement functions into a personal desk computer or a laptopequipped with the appropriate acquisition printed circuit boards, high quality soundcards, and the applicable sensors. A specially equipped personal computer can executeDSP-based (diagnostic signal processing) signal generation, filtering, andspectrum analysis. Hence, a single computer can replace a whole rack of dedicatedanalog units linked together by BNC cables. In testing the performance ofa loudspeaker through traditional analog means, a sine wave generator provides asignal to the loudspeaker and a calibrated microphone picks up the loudspeaker’s