IET Labs 1986 Omnical, User And Service Manual

Page: 10 / 105

2
1986
Condensed Operation
c. Note reference reading on instrument under
test.
d. Indicator Linearity. Decrease 1986 LEVEL in
10 dB steps, and note if reading on instrument
under test responds in corresponding 10 dB steps.
Tolerances are listed in Tables 2-4, 2-5 of manual.
e. LEVEL RANGE Control (Attenuator)
Tracking Accuracy. Repeat steps a through c;
decrease LEVEL RANGE control on instru-
ment under test in 10 dB (or 20 dB) steps,
and make corresponding changes with 1986
LEVEL control. Readings should agree, within
tolerances listed in Table 2-6 of manual.
Dynamic-Characteristics Test.
a. Set instrument under test : LEVEL RANGE
as desired, FAST response, A WEIGHTING.
b. On 1986: set VARIABLE SPL to set fast/slow
(reference), any FREQUENCY (1 kHz is auto-
matically generated for this test), adjust LEVEL
and SPL ADJUST to set reference level at 4 dB
below upper limit of indicator.
c. On 1986, set VARIABLE SPL to fast to pro-
duce 200 ms tone bursts to test FAST indicator
response. Note maximum reading. (HINT: To
check maximum meter response, it may be
helpful to slide a card parallel with needle
over meter scale to point at which MAXIMUM
excursion of needle just becomes visible.)
d . Set instrument under test to SLOW re-
sponse. On 1986, set VARIABLE SPL to slow
to produce 500 ms tone bursts to test SLOW
response. Note maximum reading.
e. Compare both FAST and SLOW maximum
readings with tolerances in Table 2-7 of manual .
RMS-Accuracy (Crest-Factor-Capacity) Test.
a. Set instrument under test: LEVEL RANGE as
desired ( 100 dB maximum upper limit) , FAST or
SLOW response, C or FLAT WEIGHTING.
b. On 1986: set VARIABLE SPL to set crest fctr
(reference), any FREQUENCY (2 kHz auto-
matically generated), adjust LEVEL and SPL
ADJUST controls to set reference level at 2 dB
below upper limit of indicator.
c. On 1986, set VARIABLE SPL to crest factor
to produce a tone-burst signal with crest factor
of 3; this tests rms accuracy of instrument under
test. Note change in reading.
d . Compare change in reading with tolerances in
Table 2-8 of manual.
MICROPHONE CORRECTIONS.
The following table (Table A) lists corrections,
for various microphones, that are to be applied to
the reading of an instrument being calibrated by
the 1986; these correc tions are only applicable at
the high frequencies (1 kHz, 2 kHz and 4 kHz) of
the 1986. The corrections listed for microphones
designed for free-
fi eld response (either perpendicu-
lar or random incidence) incorporate pressure-
to-free-
fi eld, high -frequency and microphone-
volume corrections; corrections listed for micro-
phones designed for pres sure response incorporate
high-frequency and microphone-volume correc-
tions. These corrections are discussed in para 2.11
and 2.12, and must be applied when either a
Sensitivity Test or a Frequency-Weighting Test is
performed. The applicable correction
(in dB) is
to be added to the reading of the instrument under
test before comparing it with the calibrated output
of the 1986 (shown on its panel by the posi-
tion of the LEVEL switch)