Instruction manual

117
Technical Notes
Measurement Functions
L
Aeq
(equivalent sound pressure level)
For a sound pressure level signal that changes over time, the L
Aeq
(equivalent sound pres-
sure level) is a hypothetical constant sound pressure level that has the same energy as the
actually measured signal in the measurement interval. It is determined by the following
equation.
t
1
: Measurement start time
t
2
: Measurement end time
Tm: Measurement time (integrated time) Tm = t2 - t1
p0: Reference sound pressure 20 µPa (2 × 10
-5
N / m
2
)
pA(t): Instantaneous sound pressure measured with sound pres-
sure level meter A weighting
Expressing the above equation for sound pressure level yields the following equation.
LA(t) : Instantaneous sound pressure level
In the sound pressure level meter NA-27, this statement is used as reference, and digital
processing to determine L
Aeq
is carried out according to the following equation.
N: Number of samples
Using the output signal of the rms detection circuit, digital processing is performed to
determine the L
Aeq
value. For this purpose, a suitable rms detection time weighting and
sampling interval for L
Aeq
processing must be chosen. In the NA-27, the sampling interval
for A/D conversion is 10 ms (100 samples per second), and L
Aeq
processing is carried out
every interval. The L
Aeq
reading can therefore be displayed already during measurement.