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Bedrock SMxx operating manual
© Bedrock Audio BV, 2019
The frequency weighting and time averaging options are selected in the third screen of the
module, in the same fashion as the standard sound pressure level module. Two sets of time
and frequency settings can be chosen (main and aux), respectively corresponding to the
three fields on the left side and right side of the first screen. The measuring time is set on
the second screen, in the same way as all other modules.
Note that each level measurement is defined by three different settings:
• Frequency weighing (A, C, Z)
• Time averaging (FAST, SLOW)
• Statistic (MAX, MIN, CURR, PEAK, EQ, or any percentile between 1 and 99%)
Not all implementations of percentile level measurements explicitly allow time
averaging settings to be chosen. The time averaging window is then (often im-
plicitly) set by the manufacturer of the device. The length of the time window
may vary widely, from less than a millisecond to several seconds. If the stand-
ards and documentation that you are using to guide your measurements are un-
clear on the topic of time averaging, we recommend choosing FAST. The time
averaging window is then set to 125 ms and opt for a measurement duration of
60 seconds or longer.
The percentile levels are computed according to the following procedure:
• A level measurement takes place across audio frames of 512 samples, at a sampling rate
of 48 kHz. A computation takes place after the acquisition of every new audio frame
(so 93.75 times per second).
• For each channel (Main and Aux), the frequency-weighted and time-averaged level is
calculated and rounded to 0.1 dB.
• A level histogram is compiled with bins (categories) that are 0.1 dB wide. Each meas-
urement is sorted into this histogram. I.e., 93.75 measurements per second are added
to the histogram.
• The percentile levels are calculated from the histogram by determining which level
constitutes the threshold which is exceeded more than the chosen percentile (e.g. for
L95 more than 95% of the samples in the histogram exceed the threshold level).
• Max and Min are respectively the highest and lowest bin in the histogram for which
data is recorded. EQ is the energetic average across all measurements. By selecting
CURR, the current level is shown according to the selected time averaging option
(FAST or SLOW).