Handbook for Sound Engineers

1640 Chapter 47

where low or high frequencies are important.
Remember the microphone response determines the
response. Sound levels measured with the Z weighted
filter are designated as dBZ (dBSPL Z weighted).

Analog Switch. The outputs of the A-, C-, and
Z-weighted filters connect to the analog switch. The
switch is controlled by the microcontroller. The selec-
tion of the desired filter is done at setup using the utili-
ties found in SLARMWatch™.
Selection of the filter as with the other SLARM™
settings is password protected. Permission must be
assigned to the user by the administrator before selec-
tion is possible. This is essential to minimize the possi-
bility of someone changing measurement profiles that
may result in improper ALarm activation or inaccurate
measurements.

RMS Detection and LOG Conversion. The output of
the analog switch goes to the RMS detection and Loga-
rithmic conversion section of the SLARM™. The RMS
detector is a true RMS detector able to handle crest
factors of 5–10. This is different from an averaging
detector set up provide rms values from sine wave (low
crest factor) inputs. The response of the detector
exceeds the response limits of the SLARM™.

The output of the RMS detector is fed to the Log

(Logarithmic) converter. A logarithmic conversion range

of over 100 dB is obtained. The logarithmic output then

goes to the A/D section of the microcontroller.

Microcontroller. The microcontroller is the digital

heart of the SLARM™. A microcontroller (MCU) does

all the internal calculations and system maintenance.

SPL, Leq. The digital data from the internal A/D is

converted by the MCU to supply dBSPL, and Leq

values for both storage in the on-board flash memory

and inclusion in the data stream supplied to the USB

and serial ports. These are complex mathematical calcu-

lations involving log and anti-log conversation and

averaging.

The SPL values are converted to a rolling average.

The results are sent to the on-board flash memory that

maintains a rolling period of about 2 to 3 weeks.

Leq generation in the SLARM™ involves two inde-

pendent calculations with two programmable periods. A

set of complex calculations generates the two Leq

values.

Thresholds and Alarms. The results of the Averaging

and Leq calculations are compared by the micro-

Figure 47-8. SLARM™ functional block diagram. Courtesy ACO Pacific.

Alarm

Output

Connector

Preamp Z Filter

Analog

Switch

RMS-Log

Detector

CPU

Clock/

Calendar

Data

StorageFlash

Memory

Drivers

RS-232

DriverRS-232

USB

Serial

Control

To

PC

USB

Mic/

Preamp

A Filter

C Filter

Opto isolator

Opto isolator

Front Panel

LEDs:

Power

Alarm1

Alarm2

Serial Connect

Power

12 Vdc Conditioning

Alarm 2

Alarm 1

Range Control