WILLIAM KUFFNER, M.A. Sc., P.Eng, PMP
Senior Fire Protection Engineer
Director – Fire Protection Engineering
SPEECH INTELLIGIBILITY and
Fire Alarm Voice Communication
Systems
Code Reference
OBC Clause 3.2.4.23.(2)
– (2) The voice communication system … shall be
capable of broadcasting … messages with voice
intelligibility meeting or exceeding the equivalent of a common intelligibility scale score of 0.70.
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Intelligibility
Assuming that speech, hearing etc. are normal, intelligibility is affected by
– signal to noise level (sound pressure levels)
– reverberation (physical property of room)
– echo (reflection arrives much later than original sound [50+ ms])
– distortion (sound system property like amplifier
Measuring Intelligibility
Several methods, some subjective, others objective
– Speech Transmission Index (STI)
– Speech Intelligibility Index (SII)
– Articulation Loss of Consonants (%Alcons)
– Phonetically Balanced Word Scores (PBWS)
– Modified Rhyme Test (MRT)
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Speech Transmission Index (1/3)
The speech transmission index (STI) is an objective method.
measurement of the amount of modulation preserved in the transmission of an artificial speech signal through a system.
IEC standard 60268-16 Objective rating of speech intelligibility by speech transmission index
Speech Transmission Index (2/3)
Physical quantity representing the transmission quality of speech with respect to intelligibility.
Accounts for signal to noise ratio as well as reverberation and distortion
98 frequencies accounting for 7 audible octaves (125 Hz to 8 kHz) and 15 modulation
frequencies (0.63 Hz to12.5 Hz)
Weighted average of the sum of modulation frequencies signal to noise ratios of audible spectrum.
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Speech Transmission Index (3/3)
Score of a space is determined by producing a characteristic signal which represents the
combinations of the audible and modulation spectra to be tested for
Signal transmitted through the system under test and then measured in the space under consideration
The difference represents a reduction in the intelligibility score of the space.
Common Intelligibility Scale
• Common Intelligibility Scale (CIS) relates a
speech intelligibility score from one method to
another.
• A suggested pass/fail
value for CIS is 0.70 for a given space (relates to 50% score on the STI scale)
• Building Code Requirement
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Design of Voice Communication Systems
Speech intelligibility from a voice communication system perspective - past
– designs based on signal to noise ratios to meet sound pressure level requirements of Building Codes.
Design of Voice Communication Systems
Speech intelligibility from a voice communication system perspective – present
– current designs based on signal to noise ratios to meet sound pressure level requirements of Building Codes.
– empirical method - lower power, denser placement of speakers
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Design of Voice Communication Systems
Speech intelligibility from a voice communication system perspective – future
– Design for signal to noise ratio or intelligibility measured
from the speaker to the listener’s ear.
• algebraic equations
• design software (iTool®)
Algebraic Equations
Absorption Coefficient
– Add the area weighted average of absorption coefficients of surface materials
S
S
α
α
1 n n n
Material Sound Absorption Coefficient - α - Plaster walls 0.01 - 0.03 Unpainted brickwork 0.02 - 0.05 Painted brickwork 0.01 - 0.02 3 mm plywood panel 0.01 - 0.02 6 mm cork sheet 0.1 - 0.2 6 mm porous rubber sheet 0.1 - 0.2 12 mm fiberboard on battens 0.3 - 0.4 25 mm wood wool cement on13
Algebraic Equations
Reverberation time
for a<0.2, (Sabine)
(for a>0.2 (Eyring)
• Where T (s) is the reverberation time,
• V (m3) is the room volume,
• S (m2) is the total surface area of all room boundaries, and
• a is the average absorption coefficient
– Less than 1.5 seconds in reverberation time indicates that
sound pressure level will be the dominant factor in intelligibility
Sα
0.16V
T
α)]
-S[ln(1
-0.16V
T
Algebraic Equations
Sound pressure level
– Determine coverage of speaker based on a spl loss of 6 dB as a maximum.
• Speaker polar plot (performance chart) provides spl losses at various angles (θ).
• Add inverse square law to loss data 20 log (3/d).
• Interpolate data to determine angle where spl loss is 6 dB
• Determine coverage radius r = d tan(θ/2)
• Use coverage radius to project a square that fits inside the circle and determine the coverage area of the speaker as a square A = 2r2
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Itool® Software
• Each simple room (6 surfaces) is input into the system by providing dimensions (length, width, and height),
• Surface characteristics of each of the room surfaces are selected from an available list (user defined surfaces and unique surface combinations can be created),
• A speaker type is selected form an available list (user defined speakers can be created),
• A speaker layout pattern which represents speaker density is selected,
• Ambient and required minimum
differential sound pressure levels for the room are entered, and
• Listener height is selected
• Software calculates reverberation time and creates a speaker layout
Modeler® Software
• Prepare floor plan layout
• Select and place speakers
• Select and place listeners
• Select and place objects
• Set surface features
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Modeler® Software
• Use view tools to confirm space and speaker layout
• Run model software and analyse results
Modeler® Software
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Modeler® Software
Design Results
Simplex predicts coverage areas using their 4902 speaker
For a typical 3 m ceiling and a listener height of 1.5 m, a
speaker will be placed every 3 m on the ceiling to achieve a 6 db loss coverage pattern.
When compared to traditional design
methods, designing to achieve intelligibility means at least double the number of
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Measure Speech Intelligibility
• Use handheld tools to measure speech
intelligibility in finished spaces accounting for the background noise that can be expected to be present during
Tools
• Goldline – http://www.gold-line.com/dsp2.htm • Quest – http:// www.quest-technologies.com/Sound/Advanced/Spro_ SE_DL/index.htm23
Measuring Method
• Obtain STI or CIS measurements to
determine compliance. – Generally requires two
or more
measurements for
each speaker to get an overall score for a
space
• Send the signal through the sound system and measure the response in the space under
consideration.
• Take a statistical average of all the floor area
readings minus one
standard deviation as the overall score (84%
Equipment Setup – STI-CIS Analyzer
Check the batteries
Check the calibration date
Check the plan for measurements (locations, quantities, etc.)
Verify proper equipment operation
Background noise measurement
Sound pressure calibration
– Measure spl using analyzer with a voice message playing
– Initiate the test tone and adjust volume until spl measurement is the same as that taken during voice message test
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Taking readings
Take spl and cis measurements at each planned test location
– Test locations should be equal distance between speakers
– Take two measurements facing in different directions each time
– If cis readings differ between measurements by more than 0.3 take a third reading
– Record spl and cis readings.
– Note any reading errors that occur, background noise or incidental noises heard during testing.
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Determining Results
Each acoustically unique space should be evaluated separately
– Rooms separated by walls
– Areas where significant changes in finishes occur
– Areas where ceiling height changes by more than 20%
Determine the average of all readings taken in the space
Determine the standard deviation of the readings
The overall intelligibility score is the average less one standard deviation.
– An individual reading failure should not be taken to mean the area fails
Determining Results
The overall sound pressure level is the average of all readings
– The values are not intended to be compared to spl
requirements in the building code. Those values are for fire alarm signalling equipment not voice communication
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Reporting
No. Floor Area Input SPL
Speaker
Power Location Reading
(dBA) (W) CIS/SPL Location Room
a b c Average Average Std. Dev CIS Score
1 Office A 70 0.5 1 0.09 0.22 0.155 41 39 40 2 0.16 0.19 0.175 0.165 0.055678 0.11 39 40 39.5 2 Office B 70 0.5 1 0 0.22 0.11 41 41 41 2 0.05 0.24 0.145 0.1275 0.120381 0.01 39 39 39 3 Office C 70 1 1 0.16 0.12 0.14 40 39 39.5 2 0.09 0.19 0.14 0.14 0.04397 0.1 39 39 39 4 Office D 70 1 1 0.27 0.27 0.27 41 39 40 2 0.36 0.09 0.225 0.2475 0.113248 0.13 40 39 39.5
william.kuffner@genivar.com