Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
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software has no knowledge or control over the microphone sensitivity and the preamplifier on
the audio interface, it has no way of knowing which analog voltage levels and corresponding
digital sample values represent actual SPL levels. To solve this problem, an SPL calibrator is
used. An SPL calibrator is a device that generates a 1 kHz sine wave at a known SPL level
(typically 94 dBSPL) at the transducer. The analysis microphone is inserted into the round
opening on the calibrator creating a tight seal. At this point, the tip of the microphone is up
against the transducer in the calibrator, and the microphone is receiving a known SPL level. Now
you can tell the analysis software to interpret the current signal level as a specific SPL level. As
long as you don’t change microphones and you don’t change the level of the preamplifier, the
calibrator can then be removed from the microphone, and the software is able to interpret other
varying sound levels relative to the known calibration level. Figure 4.22 shows an SPL
calibrator and the calibration window in the Smaart analysis software.
SPL Calibrator
SPL calibration settings in Smaart analysis software
Figure 4.22 Analysis software needs to be calibrated for SPL
4.2.2.3 Impulse Responses and Reverberation Time
In addition to sound amplitude levels, it’s important to consider frequency levels in a live sound
system. Frequency measurements are taken to set up the loudspeakers and levels such that the
audience experiences the sound and balance of frequencies in the way intended by the sound
designer.
One way to do frequency analysis is to have an audio device generate a sudden burst or
“impulse” of sound and then use appropriate software to graph the audio signal in the form of a
frequency response. The frequency response graph, with frequency on the x-axis and the
magnitude of the frequency component on the y-axis, shows the amount of each frequency in the
audio signal in one window of time. An impulse response graph is generated in the same way
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