BRUEL & KJAER 2221 User Manual

18
The sample of text was typed twice; the first time slowly (taking 3 min), and the second
time quickly (taking 1 min), i.e. the same key strokes were used in both cases resulting in
the same noise but over different time periods. Readings were taken during both trials
after 1 and 3 min. In the second trial (fast typing) the 1 min of typing was followed by 2 min
of silence. Background noise levels were minimized and assumed negligible. The results
are shown in Table 3.1.
leq
The Leq values obtained from the trials were notably different after one minute, reaching
almost the same value after 3 minutes.
The noise doses referred to the measurement periods of 1 min. are different. In the first
trial the noise dose corresponds to part of the total text, whilst in the second trial it
corresponds to the complete text. Both noise doses referred to a measurement period of
3 min. correspond to the complete text.
SEL
The SEL values measured at 3 min. for the first trial and at 1 and 3 min. for the second tri-
al, are almost identical
Assuming no background noise, the noise doses for the three SEL calculations (after
3 min. in the slow trial and after 1 and 3 min. in the fast trial) are identical. The time period
to which they are referred is by definition 1 s, therefore the three SEL values should be
identical.
Considering the two trials as two independant noise events of different durations and
measuring the SEL from the begining of the event until its end, i.e. during 3 and 1 min., the
values obtained in both cases are identical. This is because both noise events contain the
same noise dose. This shows how the SEL can be used to directly estimate the noise dose
produced by events of any duration.
Max Hold "Slow"
The two Max. Hold "Slow" values obtained are different.
The Max. "Slow" value is the maximum RMS value measured with a 1 s time constant. In
the second trial the strokes from the typewriter are closer than in the first one and
therefore the Max. -value is higher.
Max. Hold "Peak"
The two Max. "Peak" values are identical for both trials
The "Peak" value is the maximum level of the signal (rise time <301Ls), and for a
typewriter it is determined by a single stroke. The strokes are identical in both cases
(electricaly determined), only the repetition rate changes.

BRUEL & KJAER 2221 Specifications

General

Brand	BRUEL & KJAER
Model	2221
Category	Measuring Instruments
Language	English