Casella CEL CEL-160 User Manual

Page 22

When the L

option is selected the CEL-160

prints 'LEQ ON' and waits for the beginning of

the next whole minute interval before taking

any samples. The program then antilogs and

accumulates over the L

period and then

averages the result prior to displaying as either

LOG or LIN bar graph. Annotation is similar to

normal SLM mode. Time markers are output

every L

with a time print every four L

periods. During the first period no result is

produced as the calculation is still in progress.

The result for the first period is

displayed throughout the second period; and

so on throughout the measurement.

Since an L

value is calculated after a

period has ended then bar graph period L

values would be out of synchronisaton with

the time axis of the recorder.

This problem is overcome by

reprogramming of the time clock when

selecting L

mode and results in complete

synchronisation of the bar chart L

and time

annotation. When the L

option is deselected

the CEL-160 will print 'LEQ OFF' and return

the clock to its original state.

10.3.2 Operation

To commence profile L

operation select

RMS level recording (see section 5.3) and

select the required L

profile by paper speed

selection as outlined in the above table. During

the minute preceding the required start time

select the OPTION position with the option

switch.

The Leq output can be annotated in

logarithmic or linear axes and determined by

the LOG/LIN switch. Once the L

option

program is running any switch or key change

will terminate the measurement.

If the required operating changes are

made and the recorder is left in RMS and

OPTION modes the program will restart a new

measurement at the beginning of the next

whole minute.

10.4 CEL 160/2B Reverberation Time

(RT60) Measurement Option

10.4.1 Introduction

This option package extends the range of

functions provided by the instrument to

include a direct printout of the rever-

beration time (RT60) in both analogue and

digital formats. It also includes all the

necessary command functions to

automatically control the CEL-213 Random

Noise Generator and CEL-196 Third Octave

Filters in both the source and receiving signal

paths. It will therefore provide a very compact

and convenient measurement package that is

ideally suited to field measurements.

The instrument may be operated in

either an impulse or continuous excitation

mode. In the former a signal line on the

recorder auxillary socket is raised when the

system is ready for the excitation and will then

wait until it detects the rapidly rising level and

when its maximum value is passed the decay

calculation procedure is initiated. In the

continuous mode the same line is used to

switch on and off the noise source. When the

measurement procedure commences in this

mode the source is activated and a period of

approximately 2.2 seconds is allowed for the

levels to normalise; then the average value of

the generated level is calculated and this value

subsequently used in the RT60 calculation. The

'source off' signal is then activated and the

calculation procedure commenced.

In both modes of operation prior to the

sound source being activated a short period of

background level is measured and averaged

and this information is subsequently used to

validate results obtained. When the CEL-213

Random Noise Generator is used in a

measurement configuration the source will be

automatically switched by the programme and

in addition a 'ready' light is provided for the

impulse excitation mode.

The reverberation time may be

calculated from measurements made over

either a 20 or 30 dB decay range and this

parameter should be preset at the beginning of

a measurement sequence. In the case of

impulse exicitation 8 mS is allowed after the

detection of the maximum level to ensure that

the shock wave has passed and then decay is

timed from a point -5 dB on that level. For the

Paper

Speed
mm/S

period

Column
Width mm

Number of
Samples

1B - 4B

10 S

2 479

0.3

1 min

15 215

0.1

5 min

76 559

0.03

10 min

153 455

0.01

1 hr

921 455

Figure 13: Leq period determination

Page 22 - CEL-160 Graphic Recorder