Sound and Vibration Instrument Hire and Calibration
Bruel & Kjaer 4297 Sound Intensity Calibrator
| B&K 4297 : Sound Intensity Calibrator | £ 11 a day * |
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Sound Intensity Calibrator type 4297 is used for on-site sound pressure calibration and Pressure Residual Intensity Index verification, with the Bruel & Kjaer 2260E Sound Intensity Meter and Bruel & Kjaer 3595 Sound Intensity Probe Kit.
UsesIntroduction
Sound Intensity Calibrator Type 4297 enables instruments which measure sound intensity to be accurately calibrated.
Type 4297 is intended for use with Bruel & Kjaer Sound Intensity Probes Type 3595, 3583 and 3584 (or earlier Types 3545 or 3548) with Sound Intensity Microphone Pair Type 4197 (or earlier Type 4181). The microphones must be used with 1/4" preamplifiers.
The Sound Intensity Calibrator can be used for calibration of sound-pressure sensitivity. To do this, the microphones are both positioned in the calibration chamber. There is no need to dismantle the probe, and both microphones are exposed to exactly the same sound pressure amplitude and phase.
The broad-band sound source is provided for measurement of the Pressure Residual Intensity Index spectrum. This is used to assess the accuracy of sound-intensity measurements.
A calibration chart is supplied which states the levels that should be detected during calibration.
A barometer is not needed because an accurate feedback system holds the sound-pressure level at a constant value.
Calibration of an intensity measuring instrument includes:Sound Pressure Calibration
With the probe in the calibrator as shown in Fig. 1, the sound source produces the same sound-pressure level at each microphone. The microphone channels are calibrated against this known sound-pressure level.
Pressure Residual-intensity Index Measurement
Small differences in the phase responses of the microphones and input channels result in the detection of 'residual intensity'.
Residual intensity is a parameter that should be taken into account when interpreting measured intensity data. The
Pressure Residual Intensity spectrum is not fixed; it is 'tied' to, and rises and falls with measured sound pressure level
Residual Intensity Level
If a pressure residual intensity index spectrum is to be used to assess the accuracy of sound intensity measurements, then the mean sound-pressure spectrum in the field must also be measured. The
Residual Intensity level is then quickly established by subtracting the pressure residual-intensity index spectrum from the measured, mean sound pressure spectrum.
The residual intensity level is then compared to the measured sound-intensity level. It can be shown that, for a certain frequency, the residual-intensity level must be at least 7 dB lower to ensure a measurement error of less than 1 dB.
Microphones and Vent Sensitivity
Type 4297 has been designed to work with Microphone Pair Types 4197 and 4181, which have an extremely low sensitivity to sound pressure at the equalization vents due to their patented acoustical filters. When microphones are inserted into the coupler, their diaphragms are exposed to the sound pressure in the coupler but their pressure equalization vents are not. Type 4297 cannot be used to measure the
Pressure Residual Intensity Index with conventional microphone pairs as they have vent sensitivities several orders of magnitude higher than that of type 4197
