Checking The Differential Level Linearity Of Brüel & Kjær Sound Level Meters - BRUEL & KJAER 4226 User Manual

18 Multifunction Acoustic Calibrator Type 4226 – User Manual
the sound pressure on the microphone's diaphragm is influenced by the geometry of the
microphone, which can vary from type to type, and significant deviations can occur at higher
frequencies.
In many cases a particular microphone can be calibrated by obtaining its pressure response using
Type 4226 and comparing this with the pressure response, also obtained Type 4226, of another
microphone of the same type known to be functioning properly. This too is a form of test by
comparison.
3.6 Checking the Differential Level Linearity of Brüel & Kjær Sound Level
Meters
Type 4226 may be used to check that the differential level linearity of a Brüel & Kjær sound level
meter is within the tolerances specified by IEC and ANSI standards. Table 3.2 shows these
tolerances for the two specified frequency ranges for types 0, 1 and 2 sound level meters. The
tolerances shown are for an applied signal which changes by 10 dB steps, a full test also involves
step changes of 1 dB for closer tolerances.
Table 3.2
Tolerances (in
Level inside primary
decibels) for
differential level
linearity for 10 dB step
changes inside and
outside of the primary
indicator range and
two frequency ranges
The checking procedure requires determining what, if any, differences exist when the applied
sound level is stepped by 10 dB for a fixed level range control (used to select full-scale deflection
or range) on Brüel & Kjær sound level meters. The procedure calls for the application of the
stepped sound level at various frequencies to cover the ranges shown in Table 3.2. An external
generator will be necessary for frequencies below 31.5 Hz.
✐
Since only differences are to be determined, it is immaterial what the settings of the Sound
Please note:
Field and Microphone switches are on Type 4226 once they have been fixed.
Ensure that both Type 4226 and the sound level are switched on.
1) Set up the sound level meter as follows:
a) Set to measure SPL (sound pressure level).
b) Set the dynamic range to a range that will prevent overload at 114 dB SPL.
c) Set the frequency weighting to Lin or aW.
d) Set the time weighting to Fast or Slow.
Level
indicator range
Level outside
primary
indicator range
Frequency range 20 to 12500 Hz
Type 0 Type 1
±0.4 dB ±0.4 dB
±0.6 dB ±1.0 dB
Type 2
±0.6 dB
±1.5 dB
31.5 to 8500 Hz