Chapter 2. Safety 2 – 33
2.5.1.15. Acoustic Measurement Precision and Uncertainty
The Acoustic Measurement Precision and Acoustic Measurement Uncertainty are described
below.
Quantity Precision Total Uncertainty
Ipi,
α
(attenuated pulse intensity integral)
3.2% +21% to -24%
P (acoustic power) 6.2% ±19%
P
r,α
(attenuated rarefaction pressure) 5.4% ±15%
f
awf
(acoustic working frequency) < 1% ±4.5%
■ Systematic Uncertainties
For the pulse intensity integral, derated rarefaction pressure Pr.3, center frequency, and
pulse duration, the analysis includes considerations of the effects on accuracy of:
Hydrophone calibration drift or errors.
Hydrophone/Amp frequency response.
Spatial averaging.
Alignment errors.
Voltage measurement accuracy, including.
– Oscilloscope vertical accuracy.
– Oscilloscope offset accuracy.
– Oscilloscope clock accuracy.
– Oscilloscope Digitization rates.
– Noise.
The acoustic power is measured using a Radiation Force for systematic uncertainties
through the use of calibrated NIST acoustic power sources.
We also refer to a September 1993 analysis conducted by the working group of the IEC
technical committee 87 and prepared by K. Beissner, as a first supplement to IEC publication
1161.
The document includes analysis and discussion of the sources of error/measurement effects
due to:
– Balance system calibration.
– Absorbing (or reflecting) target suspension mechanisms.
– Linearity of the balance system.
– Extrapolation to the moment of switching the ultrasonic transducer (compensation for
ringing and thermal drift).
– Target imperfections.
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