SP395 SoundPro Audio Integrator
Form7492
Operation Manual
60
Note: You must have at least 30dB of range available to compute the RT60 and
ALCON % for the room.
10.
Run test.
Change the “Max” field to “Run” to begin the test. The Critical Distance is
computed, and the correct %ALCONS formula is selected based on whether the
microphone is within the Critical Distance. The resulting loss of consonants is displayed
as a percent.
Speech Intelligibility Results
Use this table as a guideline for evaluating the results of the %ALCONS and RaSTI tests.
%Alcons
RaSTI
Rating
57.70 0.20 Bad
44.00 0.25 Bad
33.56 0.30 Poor
25.59 0.35 Poor
19.52 0.40 Poor
14.89 0.45 Fair
11.35 0.50 Fair
6.60 0.60 Fair
5.04 0.65 Good
3.84 0.70 Good
2.93 0.75 Good
2.23 0.80
Excellent
1.70 0.85
Excellent
1.30 0.90
Excellent
0.99 0.95
Excellent
0.76 1.00
Excellent
Speech Intelligibility – RASTI
Rapid Speech Transmission Index (RaSTI) is one of several speech intelligibility testing
methods. RaSTI calculates an index value (0.00 to 1.00, with 1.00 being perfect intelligibility) by
evaluating the loss of modulation, known as the Modulation Transfer Function (MTF), of special
waveforms.
The RaSTI waveforms are a subset of the Speech Transmission Index (STI) waveforms, which
are based on octave-band limited pink noise, covering 7 octave bands from 125Hz to 8 kHz, each
modulated 100% in amplitude (loudness) by sine waves, in 14 different frequencies ranging from
0.63Hz to 12.5Hz, for a total of 98 waveforms.
STI is calculated by measuring the amount of modulation that is lost (MTF) in each of the 98
waveforms, and converting to an “apparent” signal to noise ratio in dB, limiting these to +/–
15dB, and averaging and normalizing these values.
