CHAPTER 2
Hardware Setup
7
2.3 Measured Quantities
The measured quantities are also described in the User Guide in practical terms. For completeness, the
formal definitions of the relevant quantities are given here.
2.3.1 Overload
For Running Measurements, the Overload indication is displayed for as long as the overload
condition exists or for a period of 1 s, whichever is the greater. Overload is indicated at full-scale
+0.3 dB. An overload deteected during a timed measurement is latched and displayed with the relevant
parameters until these are cleared by initiating a new measurement.
2.3.2 Under-Range
For Running Measurements, the Under-range indication is displayed for as long as the under-
range condition exists or 1 s, whichever is the greater. Under-range is indicated at scale lower limit
–0.5 dB.
2.3.3 F Time-weighted Sound Level
The time-weighted sound level, Lxy(t), is defined as twenty times the logarithm to base ten of the ratio
of a given root-mean-square sound pressure to the reference sound pressure, root-mean-square sound
pressure being obtained with a frequency weighting, x, and standard time weighting, y, where:
• x is A for A-weighted, C for C-weighted
• y is always F for Fast-weighted using a 0.125 s time constant
The time-weighted sound level is a continuous function of time and is expressed in decibels (dB).
Lxy(t) is not displayed, but is the base for LxyInst(T
n
) and LxyMax(T).
In symbols, frequency-weighted and time-weighted sound level, Lxy(t), at any instant of time, t, is
represented by:
where:
• τ is the exponential time constant in seconds for time weighting F
•
ξ is a dummy variable of time integration from some time in the past, as indicated by –∞ for the lower
limit of the integral, to the time of observation, t
• p
x
(ξ) is the x frequency-weighted instantaneous sound pressure
• p
0
is the reference sound pressure, equal to 20 μPa
2.3.4 Instantaneous Time-weighted Sound Level
The instantaneous time-weighted sound level, LxyInst(T
n
), is defined as the time-weighted sound level,
Lxy(t), sampled at t=T
n
where:
• x is A for A-weighted, C for C-weighted (available for test purposes only)
• y is F for Fast-weighted
• T
n
= t
0
+n·Δt
• t
0
is some starting time;
• n is an incrementing integer
•
Δt is the display update interval
The instantaneous time-weighted sound level is, in other words, updated every
Δt seconds, and
expressed in decibels (dB).
Lxy t() 20lg 1 τ⁄()p
x
2
–∞
t
∫
ξ()e
– t ξ–()τ⁄
dξ p
0
⁄ [dB]=
To Next Page
Loading...
