BRUEL & KJAER 4226 User Manual page 12

4 Multifunction Acoustic Calibrator Type 4226 – User Manual
1.2.5 Checking Time Weighting Networks
This is a check in accordance with IEC 60651. It is made by comparing the instrument readings for
short pulses with the reading obtained for a continuous signal of the same level. The difference
depends on the time weighting. The preset test frequency is 2 kHz and each pulse is an integer
number of cycles, whatever the frequency. The pulses are automatically repeated: one pulse of
approximately 200 ms every 2 s for the F (Fast) time weighting; and one pulse of approximately
500 ms every 8 s for the S (Slow) time weighting. The delay between pulses ensures that the
detector of the instrument under test has time to decay before the next pulse.
To check the time weighting, the feedback loop is disconnected since its response time is
incompatible with this test. The pulses in this case are obtained by an intermittent 20 dB
attenuation of the constant level set by a potentiometer.
1.2.6 Checking Crest Factor Capabilities
The signal used to check an instrument's crest factor capability has the same rms level as the
constant test signal set on the calibrator. The check is made by comparing the readings with both
signals, which should be identical within specified tolerances. The signal applied in the acoustic
coupler has a crest factor of 3.
To check the crest factor capability of sound level meters (standards require a crest factor
capability of 10 for type 0 and 1 instruments), the signal from the AC output socket (crest factor:
10) can be applied directly to the sound level meter input stage. It is valid to check a sound level
meter in this way as the crest factor capability is normally limited by the detector of the sound
level meter and not by the microphone.
1.2.7 Calibrating a Noise Dose Meter
Noise dose meters are calibrated using one of the three calibrated levels and Type 4226's built-
in timer. The timer automatically switches off the stabilised SPL after 30 s. This gives a well-
defined noise dose to calibrate the meter. The frequency response of the noise dose meter is
checked by applying the inverse A-weighted level and by changing the signal frequency.
1.2.8 Using AC Output
The AC Out socket outputs a signal similar to the signal normally sent to the acoustic transducer.
Consequently, all the electrical tests that can be carried out with this signal are similar to the
acoustic tests described above. The only difference is the signal for crest factor capability checks,
which has a crest factor of 10. The electrical test signals are, for example, convenient for
calibrating or checking recorders, filters and other instruments related to acoustic work.