SPEECH INTELLIGIBILITY and Fire Alarm Voice Communication Systems WILLIAM KUFFNER, M.A. Sc., P.Eng, PMP Senior Fire Protection Engineer Director Fire Protection Engineering October 30, 2013
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. 2
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 clipping) 3
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) Diagnostic Rhyme Test (DRT) 4
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 5
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. 6
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. 7
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 8
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. empirical method - louder is better 9
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 10
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 ) modeling software (Modeler ) 11
Algebraic Equations Absorption Coefficient Add the area weighted average of absorption coefficients of surface materials Material α 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 battens 0.6-0.07 1 n α S n S n 12
Algebraic Equations Reverberation time for a<0.2, (Sabine) T 0.16V Sα T 0.16V - S[ln(1- α)] (for a>0.2 (Eyring) Where T (s) is the reverberation time, V (m 3 ) is the room volume, S (m 2 ) 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 13
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 = 2r 2 14
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 15
Modeler Software Prepare floor plan layout Select and place speakers Select and place listeners Select and place objects Set surface features 16
Modeler Software Use view tools to confirm space and speaker layout Run model software and analyse results 17
Modeler Software Sound pressure levels 18
Modeler Software STI levels 19
Design Results When compared to traditional design methods, designing to achieve intelligibility means at least double the number of speakers will be installed. 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. 20
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 normal operations. 21
Tools Goldline http://www.gold-line.com/dsp2.htm Quest http://www.questtechnologies.com/sound/advanced/spro_ SE_DL/index.htm 22
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% confidence) 23
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 24
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. 25
Field Notes 26
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 27
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 equipment. 28
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 29
Questions? william.kuffner@genivar.com