SP395 SoundPro Audio Integrator
Form7492
Operation Manual
65
STI-PA Function Operation
To use STI-PA, you need to play the STI-PA test signal through a speaker, and then use the
SP395 microphone to measure the STI-PA signal in the room. If you are testing a specific
installed system, such as a fire evacuation sound system, send the signal through the system
amplifier and speaker.
•
Set up the connections
. Connect the STI-PA test signal to the speaker system. Play the
test signal CD and adjust for appropriate level. In some cases, a test level SPL will be
specified. Otherwise, typically a nominal level such as 70-80 dB SPL is used.
•
Start the test.
Click on the Run field to begin the test. Wait for 15 seconds for the test to
complete. While the test is running, the test microphone should remain in the same
position, and the noise level in the room should remain stable.
•
Read the results.
After the test is complete, you will see a set of signal to noise (s/n) and
m values. Also, the final STI value is shown. If a condition was detected that prevents
getting an accurate value, you may see the message “low input” or “overload” on the
screen, and the data values are not shown.
Noise Curves
All rooms have noise: sound that is present without any energy applied by the audio system.
Noise affects the dynamic range of the reproduced audio. Noise levels that are too high interfere
with our ability to hear sounds, especially softer sounds, and may require extra volume from the
amplifier.
How our ears respond to noise depends on its frequency and loudness. Noise measurements
follow ANSI standard S12.2-1995 Noise Criteria (NC) weighting curves (see Appendix C) that
are weighted for the frequencies and sound pressure levels where our ears are most sensitive. NC
curves specify equal-loudness levels of noise over the audio spectrum. They are numbered for
easy reference, with lower numbers corresponding to less noise. The NC rating of a room is the
number of the lowest curve that is not exceeded by any of the background noise.
There are actually several standards used to measure noise curves. The SP395 supports three of
these standards. SP395 Noise Curve tests are based on standard NC or NCB (balanced noise
curves), RC (Room Criteria), and PNC (preferred noise curve) ANSI tests.
The NC (also known as NCB, or balanced noise curves) value is based on ANSI standard S12.2-
1995 (ASA 115-1995) Criteria for Evaluating Room Noise, developed first by Beranek in 1957.
It uses ANSI Class 1 octave-band filters to divide the sound spectrum into 9 octave bands,
measuring the SPL level of each band. Then, the results are compared to a table of values
defined in the specification. The table lists noise criteria numbers by row, with each row having
values defined for each octave band. The NC band is defined as the lowest band number for
which none of the octave-band SPL values for the row are exceeded.
A similar computation is RC (Room Criteria), based on the same ANSI standard. For this
measurement, a different table of values is used, based on a study by Blazier in 1981, primarily
to rate noise in offices. This system was not intended for use in very quiet spaces below RC 25.
Typically, RC is used in evaluating HVAC systems, while NC is used for more general purposes,
including motion picture theater spaces and offices.
An additional method is to compute PNC (preferred noise criteria). This computation was
derived by Beranek, Blazier, and Figwer in 1971, but is not based on an ANSI standard, and has
not been widely accepted.
