Acoustic Calibrators - A new "old" design

Abstract

When IEC standard 60942: 20031for Acoustic Calibrators was published, it offered a significant improvement in accuracy over previous versions. Not only were the tolerances tightened, but measurement uncertainties were included. As the accuracy of the acoustic calibrator controls the final measuring accuracy of a sound level meter, this was an important step forward. The general opinion at the time was that the new standard would make obsolete traditional calibrators that simply used a stabilised oscillator to power a transducer, and would most likely be met by a feedback design where a precision microphone is used to control the acoustic pressure in the cavity. In the event, the new technology of the feedback design gave new problems that were far less easy to solve than at first thought; but by paying attention to fine details and by correcting for static pressure and temperature, the more traditional design could easily meet the new requirements. In 2008, five years after the standard was published only one commercial design appeared to have been formally Pattern Approved as meeting IEC 60942: 2003 at both Class 1 and Class 2 accuracy and this is a simple, reliable design using a small microcontroller to make the various corrections needed. A special transducer was designed for the calibrator as existing ones were inadequate for reliable, long-term accuracy. The paper describes the steps taken to produce the device and gives results showing the stability after the first full year of operation.