Checking The Crest Factor Handling - BRUEL & KJAER 4226 User Manual

7) Set the display on the sound level meter to show and hold the maximum rms level. Reset to
clear the display, then note what is displayed when the LED on Type 4226 flashes. Subtract
106.0 from this and compare the result with Table 3.3.
8) If required, repeat steps 4, 5, 6 and 7 for other frequencies of interest.
9) Repeat steps 5, 6, 7 and 8 for the other time weighting.
3.8

Checking the Crest Factor Handling

Type 4226 may be used to check the crest factor handling of sound measuring instruments,
measuring amplifiers, level recorders, etc. Type 4226 does this by providing two test signals: an
acoustic one in the coupler with a crest factor of 3 and an electrical one via the
a crest factor of 10. In each case, this test signal consists of a series of well-defined tonebursts and
intervals repeated 40 times per second (see the Specifications).
Crest factor handling limitations arise mainly in the electronic detector circuit of a sound
measuring instrument and not in its microphone or input stage. Therefore, a test signal can be
applied either acoustically via the coupler or electrically from the
for the acoustic and electrical test signals on Type 4226 are 3 and 10, respectively.
The crest factor of a signal is defined as its peak:rms ratio. For example, a pure tone has a crest
factor of 1.414. In terms of decibels this works out to 3 dB, that is, 20 log
Since the crest factor of the test signal generated by Type 4226 is known (either 3 or 10), the
procedure merely requires applying this test signal with a known rms level to the measuring
instrument under test and observing any error in the measured rms level. Table 3.4 shows the
maximum errors allowed by IEC and ANSI for the detector-indicator systems of type 0, 1 and 2
sound level meters.
Table 3.4
Maximum allowable
errors (in decibels) for
detector-indicator
systems
On Type 4226, the rms level of the applied test signal is always the same as that set up using the
Test Level facility. The magnitude of an error is equal to any deviation observed in the measured
rms level (on the instrument under test) when moving the
Level to Crest Factor.
Acoustic Coupler
Type
0 I
0
1 I
1
2 I
2
socket. The crest factors
AC Out
10
AC Output (C. F. 10)
(C. F. 3)
±0.5
±0.5
±1
switch on Type 4226 from Test
Function
CHAPTER 3
21
Operation
socket with
AC Out
1.414.
±1
±1.5
–
–
–