Data Acquisition and Analysis
Data Acquisition: Portable Sound Level Meter
The measurement system used to analyze acoustical data in the field consists of portable sound level meter and analyzer with an instrumentation grade condenser microphone. LSG&A has portable analyzers that are capable of one-third and full octave band sound and vibration measurements, long-term monitoring, reverberation time measurements, sound and impact isolation measurements, and real-time FFT analysis. At the beginning and end of each measurement period, the entire system calibration is verified by means of a sound level calibrator. All of LSG&A's sound level meters meet ANSI Type 1 criteria, and are calibrated annually in a laboratory which provides NIST traceable calibration certificates.
The long-term data acquisition systems consist of ANSI Type 1 instruments capable of sampling the sound levels over long periods of time, selected by the engineer, and provides the statistical sound level results. The monitors can operate from external power or from internal or external battery power. Weather resistant windscreens are utilized for extended outdoor monitoring periods. In addition, the equipment can be programmed to simultaneously capture and record events that exceed a desired threshold level. A threshold level is set, and when the threshold is exceeded, the event time, sound level, and audio file is recorded. The analyzer is secured inside a tamper proof container, and is locked to a stationary object, such as a fence.
Data Acquisition: Digital Tape Recording
The measurement system used to record acoustical data for later analysis consists of an instrumentation grade condenser microphone, used in conjunction with a precision sound level meter which serves as a linear amplifier and step attenuator.
The output is fed to a digital audio tape recorder (DAT). The sound level meter and tape recorder are adjusted to optimize the dynamic range of the recording, in order to eliminate signals out of range. Headphones are used to monitor the signals being recorded. This allows the notation of various identifiable noise sources present during the sampling period and to confirm the correct operation of the recording system.
At the beginning and end of each measurement period, the entire system is calibrated by means of a sound level calibrator. The calibration consists of introducing a signal at a precise sound pressure level and a known frequency into the system through the microphone and recording this signal on the tape. As well as providing a reference tone for later adjustment of the analysis equipment, this method ensures that no change in calibration of the system occurred during the measurement. The engineer records all relevant equipment settings and notes overall sound pressure levels to ensure the accuracy of the recordings.
Computer Controlled Real Time Analysis
The data analysis system consists of a digital audio tape recorder (DAT), a Brüel and Kjær PULSE™ real-time analyzer, and a Windows workstation. The tape recorder is used to play back the data recorded in the field, and the output signal is fed to the real-time analyzer for processing. If necessary, adjustments can be made using a precision graphic equalizer and parametric equalizer to compensate for measuring system response, windscreen attenuation, and microphone response, in order to obtain frequency analyses which would be obtained with a flat record/playback system response.
During playback, the recorded data are sampled by the real-time analyzer in all one-third octave bands using an averaging time constant selectable by the operator, depending upon the application. The system is adjusted using the field recorded calibration signals to ensure the accuracy of the analysis. The system, under computer control, uses the sampled data to provide octave, 1/3 octave, and A-weighted sound levels of the recorded data. For time varying signals, the system can be programmed to measure continuously and provide either time histories, level distribution histograms and statistics in selected bandwidths and frequency ranges, or in terms of the A-weighted sound level.
Computer Controlled Narrow Band Analysis
The data analysis system consists of a digital audio tape recorder (DAT) and an FFT based dual channel spectrum analyzer. During playback, the recorded data are sampled by the signal analyzer using an analysis bandwidth, selectable by the user, as narrow as a fraction of a hertz. This allows very accurate identification of a signal's spectral content. The system is adjusted using the field recorded calibration signals to ensure the accuracy of the analysis. The FFT analyzer can also be used to determine system characteristics, such as resonances in structures, gain and phase characteristics in electronic systems, and other transfer functions.
Room Acoustics Analysis
Much of our architectural design work involves traditional techniques for ray-tracing and calculating reverberation times. In addition, we use a room acoustics software program to create three-dimensional models to predict acoustical characteristics such as reverberation time, clarity, lateral reflections, and other parameters which quantify the liveness, loudness, spaciousness, warmth and intimacy of a space.
Environmental Acoustics Analysis
Our environmental design work involves traditional techniques for calculating sound propagation, shielding, and reflection, as well as using advanced computer modeling software. We use a three-dimensional modelling program to calculate expected sound levels for complex outdoor situations, considering the effects of distance, topography, ground cover, structures, and source directivity. We are able to produce contour maps to show the expected sound levels across the site and surrounding areas.