Casella CEL CEL-414 - Manual (Page 44)

In some situations, impulsive noise is con-
sidered to be more of a problem than a
similar amount of energy delivered at a
steady rate. The noise rating formula
must be amended so that the time aver-
aged integral will be based on the im-
pulse weighted level, the result being
calculated from:
L Alm = 10log 1
T
∫
0
T
10 (0.1L AI ) dt
Where:
T = measured duration,
L AI = A-weighted impulse
sound level.
With these sound level meters, exactly
the same method used to determine L
eq
will yield the alternative L AIm value when
the impulse time weighting has been se-
lected.
For steady state noise, then both L eq and
L AIm measurement methods give the
same answer, but the results will show a
growing divergance as the impulsive con-
tent of the signal increases.
Individual L eq (or L AIm ) values may be
added together into a single cumulative
value using a relationship of the form:
10log 1
T
[
t 1 10 ( 0.1L eq1 )
+t 2 10 ( 0.1L eq2 ) + ..... t n 10 ( 0.1L eqn ) ]
Where:
L eq1 = L eq value accumu-
lated in period t 1 ,
t 1 = duration 1........etc,
T = sum of durations.
When results are added together in this
way, the general instrument reset will be
found more useful, as it synchronizes the
L
eq and duration measurements automat-
ically.
9.4Sound Exposure Level
(SEL or L AE )
The sound exposure level is a very con-
venient unit for the assessment of single
events. It allows a direct comparison of
energy levels to be made without the
need to take the different durations of
the events into consideration. It is de-
fined as follows:
SEL = 10log 1
t ref
∫
(
t 1
t 2
P
P 0
) 2 dt
Where:
t ref = 1 second,
t 2 - t 1 = the duration of the
event in seconds
It will be seen that this formula bears a
close resemblance to that for L
eq , and
can be re-written as:
L eq = SEL − 10log T
So that the sound exposure levels can be
produced by adding 10log (t
2 - t 1 ) to the
L eq values calculated.
SEL = L
eq
+ 10log (t
2
−t
1
)
Individual SEL values may be added to-
gether using the relationship:
SEL Tot = 10log
[
10 ( 0.1SEL 1 )
+ 10 ( 0.1SEL 2 ) + ..... 10 ( 0.1SEL n ) ]
Where:
SEL 1 = SEL value
accumulated in
period 1,....etc,
SEL Tot = total value.
The energy in the event may be related
to longer time spans to give an average
energy level L
eq (T e ) over a selected pe-
riod by using the relationship:
Page 40 - General CEL Sound Level Meters Handbook