Integrating-averaging Sound Level Meter Type 2240 – Instruction Manual16
4.5 Microphone
Microphone Type 4188
Type: Prepolarized Free-field 1/2″ Condenser Microphone
Open-circuit Sensitivity: –30 dB ±2 dB re 1 V/Pa, 31.6 mV/Pa
Capacitance: 12 pF (at 1000 Hz)
Microphone reference point: Centre-point of protection grid’s exterior
Reference direction of sound incidence: Frontal
4.6 Frequency Response
The frequency responses are given in tabular form in Appendix A and in graphical form in this section.
Limit curves are drawn on some of the frequency response graphs in the following sections. These
curves represent the IEC 61672-1 limits, reduced by the Maximum Expanded Uncertainties of Measure-
ment from Appendix A of IEC 61672-1. The Maximum Expanded Uncertainties of Measurement used
here are the maximum uncertainties that a test organisation may have on its measurements when it
performs conformance tests according to IEC 61672.
The Expanded Uncertainties of Measurement in the tables are the two-sigma limits maintained in the
Brüel & Kjær production for the specific product. This means that the correct value lies in the range of
the measured value plus/minus the Expanded Uncertainties of Measurement with a probability of 95%.
Note: This range is not related to the production spread. The range is for a specific typical unit. The
factory acceptance tests at Brüel & Kjær ensure that the ranges for all units are within the IEC 61672-1
limits.
4.6.1 Free-field Frequency Responses
The deviation from the ideal frequency weighted Free-field Frequency responses for plane progressive
sinusoidal sound waves incident from the reference direction with A or C frequency weighting are
provided in Fig.4.1. Table A.2 in Appendix A provides the Microphone Free-field 0° Response, the
Body Influence, the frequency weighted Electrical Responses, the overall frequency weighted Acoustical
Responses and the Expanded Uncertainties of Measurement.
Fig.4.1 Free-field 0° deviation from ideal A- or C-weighted frequency responses, for Sound Level Meter Type
2240, with Microphone Type 4188
10 100 1000 10000 100000
Hz
-4
-2
0
2
4
dB
070017
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