Specifications

7
B. INTRODUCTION
1. Description
The Model 140 Sound Analyzer is a real time analyzer for signals in the acoustic range of
35Hz to 18kHz. It contains a high-gain preamplifier matched to a dynamic microphone of
150 to 200ohms source-impedance, as well as a higher level input characteristic of a
600ohm line. It contains twenty-seven 1/3 octave filters on ISO centers from 40Hz to
16kHz, operating in parallel for continuous display on all channels, plus a broad-band
channel calibrated in dB-SPL. Each channel has an AC/DC converter with a smoothing
time long enough to give readily interpretable indications when white or pink noise is
used as the input. The smoothing times vary from 5 seconds on the lower channels down
to 2 seconds on the high channels.
The 1/3 octave filters are active stagger-tuned pairs to give a two-pole pair response. A
response curve is enclosed. The selectivity of the two-pole pair response is essential in
resolving spectra that vary considerably in amplitude from one channel to the next.
An unusual feature of the Model 140 is the display consisting of an array of light emitting
diodes (LED's) which is 11 diodes high by 28 diodes wide. Although the diodes are
lighted in sequence by internal scanning circuitry, they appear to be continuously lighted
and the display is a row of dots across the raster that gives a graph of the spectral
amplitudes.
The
LED
display raster has two ranges. The first is 10dB in one dB steps, and the second
is 20dB in 2 dB steps. The "Display Range" switch on the front panel selects the range
sensitivity. A wider range of spectral amplitudes can be read by using the 10dB steps in
the
"dB-SPL"
gain control, to move the graph up or down across the diode array, for a
useful display range of over 40dB.
The instrument contains an internal pink noise generator, independent of the other
functions, which may be used as an energizing source for a system under test. This
generator can also be used as an analyzer self-check, as described later.
2. Applications Summary
The Model 140 may be used for conventional analysis of time-varying signals in the
audio range which are suitable for 1/3 octave resolution. In addition to the
LED
display
as a graphic readout, the scanned outputs are available on the back of the instrument for
use in auxiliary displays such as
cathode ray oscilloscopes*.
The controls and gain
ranges are particularly adapted to problems met in the audio and acoustic fields.
In the equalization of a sound reinforcement system for a room or auditorium, the internal
pink noise generator of the Model 140 may be used to energize the amplifier-speaker
system. A single dynamic microphone may be connected to the microphone input to
analyze the response of the room and its reinforcement system. Alternately, the
microphone multiplexor accessory may be used to time-share the inputs of three
microphones to give a space average across the room from three identical microphones.
These microphones may be "standard" microphones or microphones from the equipment
supplied for the final installation. With the display of the room response from the pink