Comparing Results from two monitors at the same place

Background

Every now and then Texcel is faced with the question ‘we put monitor X next to your monitor, measured an event, and got different number. Please explain.’

This document provides two statements to answer this question in a consistent manner. One for Air Overpressure (microphones) and one for ground vibration (geophones).

Air Overpressure Comparisons

It is very difficult to get close correlation between vibration levels, and even more so, air blast levels, in co-located monitors in real world situations.

AS2187 Airblast Measurement (Appendix J3.3 in AS2187-2006) indicates a tolerance +/- 1 dBL, and the ISEE Performance Specification (2000) states an accuracy of “+/- 10% or +/- 1dBL whichever is larger”. This leads to discrepancies of up to 2 dBL to be expected between co-located airblast measurements, and achieving this requires careful attention to microphone placement, nearby structures, air blast signal paths and more.

Ground Vibration Comparisons

Ground vibration measurements are very dependent on the coupling of the sensor to the ground.  AS2187-2006 recommends an embedded mount (for example, 200 mm concrete cube or similarly sized cylinder) for consistency of results.

AS2187 Ground Vibration Measurement (Appendix J3.2 in AS2187-2006) indicates a tolerance +/- 10%, and the ISEE Performance Specification (2000) states an accuracy of “+/- 5% or +/- 0.5mm/s whichever is larger”. This leads to discrepancies of up to 1 mm/s to be expected between co-located ground vibration measurements, and achieving this requires careful attention to sensor coupling, nearby structures and more.

Further Assistance

If you need any further assistance, please contact Texcel:
Phone: +61-7-3237 8111
Email: team@texcel.com.au