Instruction Manual

Sound Level Meter Types 2245 – Instruction Manual Page 25 of 110
Fig. 2.3 Typical variation in sensitivity at 250 Hz from that at 101.3 kPa
as a function of ambient pressure
At the Calibration Check Frequency (1 kHz), the recommended Brüel & Kjær Sound
Calibrator Type 4231 is rather insensitive to variations in the static pressure – the static
pressure coefficient is below 0.001 dB/kPa.
2.12 Mechanical Vibration
Mechanical vibration can affect indicated levels at low levels. Section 4.12.4 gives an
indication of the level of these errors.
The primary source to the vibration sensitivity is the microphone. It is most sensitive to
vibrations coming from a direction perpendicular to the diaphragm.
To reduce this problem the sound level meter should be mounted isolated from the
vibrations if measurements are taken at places where there are strong vibrations.
2.13 Frequency Weightings
Both broadband and spectrum measurements can be frequency weighted with A-, B-,
C- or Z-weighting.
The A- and C-weightings conform to the requirements in IEC 61672–1 and IEC 60651.
The B-weighting is not defined in IEC 61672–1, but conforms to the requirements in IEC
60651. Although B-weighting is not specified in IEC 61672–1, this realization complies
to the same acceptance limits as A- and C-weighting.
The Z-weighting (Zero frequency weighting) is a linear, unweighted frequency
weighting. It conforms to the Z-weighting defined in IEC 61672–1 and the Lin response
defined in IEC 60651.
Table 2.1 states the design goal frequency responses for the frequency weightings. They
are stated for the complete instrument including the microphone. The corresponding
acceptance limits can be found in the standards.