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SPEECH INTELLIGIBILITY and Fire Alarm Voice Communication Systems

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(1)

WILLIAM KUFFNER, M.A. Sc., P.Eng, PMP

Senior Fire Protection Engineer

Director – Fire Protection Engineering

SPEECH INTELLIGIBILITY and

Fire Alarm Voice Communication

Systems

(2)

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.

(3)

3

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

(4)

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)

(5)

5

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

(6)

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.

(7)

7

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.

(8)

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

(9)

9

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.

(10)

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

(11)

11

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®)

(12)

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 on

(13)

13

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

0.16V

T

α)]

-S[ln(1

-0.16V

T

(14)

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

(15)

15

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

(16)

Modeler® Software

• Prepare floor plan layout

• Select and place speakers

• Select and place listeners

• Select and place objects

• Set surface features

(17)

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Modeler® Software

• Use view tools to confirm space and speaker layout

• Run model software and analyse results

(18)

Modeler® Software

(19)

19

Modeler® Software

(20)

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

(21)

21

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

(22)

Tools

• Goldline – http://www.gold-line.com/dsp2.htm • Quest – http:// www.quest-technologies.com/Sound/Advanced/Spro_ SE_DL/index.htm

(23)

23

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%

(24)

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

(25)

25

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.

(26)
(27)

27

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

(28)

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

(29)

29

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

(30)

william.kuffner@genivar.com

References

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