Measurement Errors - BRUEL & KJAER 4297 User Manual

32 Sound Intensity Calibrator Type 4297 – User Manual
The low vent sensitivity leads to more accurate intensity measurements in the field. It has also
made it possible to develop Sound Intensity Calibrator Types 3541 and 4297 for measurement of
pressure-residual intensity index over a wide frequency range. Types 4297 and 3541 apply sound
pressure to the microphone diaphragms only, and therefore, can only be used with microphone
pairs such as Types 4181 and 4197.
A.1

Measurement Errors

Pressure-Residual Intensity at Low Frequencies and High Indices
Even though Type 4181 and 4197 microphones have very low vent sensitivities, an error may
occur if the pressure-residual intensity index is measured for a system having a high index at low
frequencies. This error can be evaluated when Type 4181 or 4197 is used because their vent
sensitivities are stated on their calibration charts. In practice, high field indices at low frequencies
cannot be measured accurately in the field as pressure gradients may cause errors.
Use of Nomogram
Fig.A.2 is a nomogram for determining the error in a pressure-residual intensity index
measurement due to the vent sensitivity of Microphone Types 4181 and 4197.
A range of frequencies is shown on the x-axis. A range of pressure-residual intensity index values
is shown on the y-axis. The diagonal lines correspond to values of vent sensitivity. The vent
sensitivity values are in dB with respect to diaphragm sensitivity at 20 Hz. To determine the
measurement error using the nomogram:
1) Read the vent sensitivity values of the two microphones used from the calibration chart.
Select the higher value
2) Place a line on the nomogram so that it corresponds to the selected value of vent sensitivity
and microphone separation. It must be parallel to the preprinted lines
3) Locate the point representing the measured pressure-residual intensity index at the
frequency at which the measurement was taken
If the point of the intersection is to the right of the line for the vent-sensitivity value, then the
measurement error is less than 1 dB.
If the point of intersection is on the line for the vent-sensitivity value, then the measurement error
is 1 dB.
If the point of intersection is to the left of the line for the vent-sensitivity value, then the
measurement error is greater than 1 dB.