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Fire Alarm Audio Applications Guide Guideline for Designing Emergency Voice/Alarm Communications Systems for Speech Intelligibility 579-769 Rev. C © 2005 Tyco Sa fety Products - Westminster.
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Copyright © 2 005 Tyco Safety Products – West minster. All rights reserved. All specificatio ns and ot her inform ation sho wn were curre nt as of doc ument revi sion date , and are subject to ch ange wi thout notice. Printed in the United States of America.
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iii Chapter 1 Speech Intelligibility Overvie w 1-1 Introductio n .............................................................................................................. 1-1 Chapters of this Publication ..........................................
iv Influences on Inte lligibility ............................................................................................ 3-2 Introductio n ..........................................................................................................
v Step 1: Room Ch aracteristics ................................................................................ 6-2 Step 2: Calculate the Number of Speakers ........................................................... 6-2 Step 3: Audio Power and Indivi dual Speaker Watta ge Tap .
vi Refer to the publications and web sites listed belo w for more inform ation regarding sound, speech, and audio intelligibility: • Acoustics – The Construction and Calibration of Speech In telligibility Tests ISO/TR 4870: 1991(E). • American National Stan dard Methods for Calculation of the Speech Intelligibility Index (ANSI S3.
1-1 INTELLIGIBILITY – The capability of b eing understood or comprehended. In simple terms, intelligibility is an evaluation of chang es that occur to speech that impact comprehension.
1-2 Emergency voice/alarm comm unications systems are us ed in applications where it is necessary to communicat e more detail ed informati on to occupa nts of a bui lding than t he simple evac uation signal provi ded by ho rns or bel ls.
2-1 There are a fe w fundamental con cepts that are necessary to un derstand when working with emergency voice/alarm communications system s. This chapter introduce s basic concepts of sound, but is not intended to be an exhaustive treatment of the subject.
2-2 Audio enginee rs use “Decibels” (dB ) to express ratios between levels, such as power, Volts, Amps, and So und Pressure Le vels (SPL). T he d ecibel is not an absolute measure like Volts and Amps, rather i t is used t o make com parisons between t wo num bers.
2-3 When the deci bel is used to express SPL, the reference sound pressure is 20 x 10 -6 Newtons/m² which is approximately the thresho ld for hearing for a normal listener.
2-4 Sound is created by mechanical vibrations that displace air m ol ecules to create repetitive changes in air pressure. The ear detect s these changes in air pressure, with the magnitude of the pressure perceived as lo udness and t he frequency of the change s perceived a s pitch.
2-5 The frequency of speech ra nges over seven o ctaves from 125 Hz to 8,0 00 Hz, with t he majority of frequencies contributing to intelligibility falling betw een 500 Hz and 4,000 Hz. The creation of “phonemes,” or the sounds that make up wor ds is created by amplitude modu lation of those frequencies.
2-6 This section is provided as a su mmary of room acoustics. See the references in t he “Related Documentation” section earlier in this manual for a list of publications co ntaining more thorough discussions of this subject.
2-7 Several equations are availa ble for estim ating the am ount of re verberatio n that can be e xpected in a room. The equations take into acc ount the room dim ensions and surface m aterials to provi de a reasonably accurate estimation of a rectangular r oom’s reve rberation t ime.
2-8 • Increasing the Signal-to-Noise Rati o: Intelligibility deg radation from reverberation is essentially a signal-to-noise issue, however when the noise is specifically caused by re verb eration it is referred to as the “Direct-to- Reverberant” ratio.
2-9 Speakers are e ssentially “point sources” of sound. S ound radiates outward in al l directions, creating a sphe rical sound pat tern. The s ound pr essure is s pread over an increasingly larger surface area as the sound m oves away from the so urce.
2-10 The amount of sound t hat a speaker can be e xpected to p roduce is f ound in the speaker’s sensitivity rating provided in the m anufacturer’s lite rature. “Sensitivity” is the amount of sound (SPL) produce d by the spea ker wit h a known si gnal freque ncy, powe r level and di stance from the speaker.
2-11 The figure below includes a t ypical polar plot gra p h and the interpretation of the dispersion angle. 87dB 87dB Sensit i vit y = 93dB @1 0 Feet, 1 W Si m pl ex 4902- 9721 C e ilin g M ou n t S p e a k e r Pol ar Pl ot - 2k Hz 0 º 6dB/divi si on 7 5 º o ff a x i s Disper sion Angl e Figure 2-4.
2-12 Using the pol ar inform ation of the speaker, i n combi nation with the distance bet ween the spea ker and the listener, you can determin e the area that a speaker can cove r.
2-13 Real world spe akers have some polar l oss at angles less than the rated dis persion angl e. In orde r to determine the actual coverage area for a partic ular speaker, the “Critical Polar Angle” for the speaker must be found.
2-14 Once the criti cal polar angle has been deter mined , calculate the cove rage area for a given s peaker- to-listener distance: Coverage Circle Diameter = 2 D 2 tan ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ 2 θ Co.
2-15 Speakers used for emergency voi ce/alarm communication sy stem ar e wired as “Constant Voltage” systems, whe re the m aximum power out put of t he amplifier is obtained at a certain speaker voltage, such as 25 V or 70 .
2-16 The preceding sections apply prim arily to ceili ng mounted speakers, generally referre d to as “Distributed O verhead System s.” Another usef ul mounting strategy is the “Distributed Wall Mount System.” Under this configuration, the speakers are placed on walls or columns, and are aimed into t h e room .
2-17 The design o f a distribute d wall mount system is similar to an over head system , with some important differences. In a wall mount syst em the speaker -to-listener distance depends on the listener location in the room. Therefore the audibility calculatio ns must be done with the listener at the farthest distance from the speaker.
2-18 The coverage patterns for a distributed wall mount system are similar to ceiling mount designs, except only a single row is used in the pattern. Because of the typically larger potential speaker- to-listener di stance, only edge-to-edge a nd tighter spacing pat terns should be used to provide adequate intelligibility.
3-1 Intelligibility is a measure of the capability of a message to be comprehended. In simplest terms, it is the reduction of the modulations of speech that reduce speech intelligibility. The m odulation reductions can also be thou ght of as a reduction in the signal (the speech) to noise r atio.
3-2 The figure bel ow lists the relative contri butions of eac h frequency band: Octave Band Contribution t o Intelligibility 0% 5% 10% 15% 20% 25% 30% 35% 125 250 500 1000 2000 4000 8000 Frequency component of speech Relative Contribution to intelligibility Figure 3-1.
3-3 Background no ise causes a reducti on in signal -to-noise ratio over al l frequencies and m odulations. Consider the comparison of t h e speech si gnal below with and without added noise: No Noise With Added Noise Figure 3-2.
3-4 Some types of background noise have a greater impact on intellig ibility than others depending on the frequency content of the noise. N oise generated by several c onversations occurrin g simultan.
3-5 Distortion of the speech waveform can come from many sources, however it is usually exhibited by an overdriv en signal, causing the pea ks of the wavef o rm to be clipped. “Clipping” is c aused by some part of the electrical signal pat h within th e fire alarm syste m exceed i ng the capacity of the component s.
3-6 International Electrotechn ical Commission (IEC) 60849 defines intellig ibility as: “a measure of the proportion of the content of a speech message that can be c o rrectly understood.” Because “understanding” involves evaluation by a human, intelligib ility is by definition difficult to quantify abso lutely.
3-7 As described i n Chapter 2, speech consists of the frequency of the sound being uttered and the amplitude modulation of that sound into the p honemes that create words. The STI (Speech Transmissi on Index) met hod measures the modul ation transfer fu nction for 14 modulation frequency bands spaced at 1/ 3-octave intervals from 0.
3-8 Measurement of intelligibility can be complicated, and it sometimes includes subj ective analysis. To effectively implement intelligible systems in r eal buildings requires that a simple, accurate, and repeatable method of measuring intelligib ility must be available.
3-9 Several tools varying in levels of co mplexity can assist the sound system designer in producing a system of acceptable intelligibility. These range fr om simple layout guides for speaker placem ent to complex c omputer modeling to ols which can accurately simulate and predict s ound system performance in complex spaces.
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4-1 An emergency voice/alarm communica tio ns system is designed to provide a highly reliable voice reinforcement and distribution net work. Th ese systems must del iver messages to building occupants fo r evacuation i n an organized and sa fe manner.
4-2 The figure below illustrates a typical emergency voice/alarm co mmunications system: Figure 4-1. Typical Emergency Vo ice/Alarm Communications System While an emergency voice/alarm co mmunications.
4-3 A command center shoul d be located at the buildin g entranc e and act as a communications center for emergency personnel. T he command center is use d to display the sys tem status an d control the annunciation system.
4-4 Speaker circuits convert electrical power from am p lif iers into sound. These circuits are wired in a daisy-chain fashion, with a single path of electrical continuity from the NAC to the last speaker in the circuit. The speaker circ uits can be wired in Class A or Class B configurations.
5-1 The governing specifications for the US Fire Alar m Market are found in the installation standa rd, NFPA 72 ® “National Fire Alarm Code.” The fire alarm audio system is defined within the class of “Notification Appliances.” NFPA 72 defines, am ong other things, requirements for audibility and intelligibility.
5-2 For emergency messages to b e heard, NFPA 72 su ggests that the sound level of t h e emergency evacuation tone to be measured at 5-feet. This is the average “ear l evel” of someone standi ng. The messages must be 15 dBA above normal ambient sound or 5 dBA above sounds lasting longer than 60 seconds.
5-3 To meet the 15 dBA requirement, there are cases where high levels of ba ckground noise requ ire extremely hi gh levels of emergency an nunciation t o overcome the noise. Whe n backgr ound noise exceeds 105 dBA or when the SPL calculations requ ire greater than 110 dBA, the use of visual notification applian ces is warranted.
5-4 Intelligibility has historically been a difficu lt parameter to measure. Unlike SPL that can easily be measured with a relatively common dBA meter, intelligibility measurements have previously required trained aco ustical engineers or sophisticated/high end evaluations.
5-5 There is significant explanatory in formation in Annex A, recently revised for the 2002 edition : From NFPA 72, 20 02 Edition: A.7.4.1.4. The designer of an intelligib le voice/alarm system should possess skills sufficient to pr op erly design a voice/ alarm system f or the occupancy to be prot ected.
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6-1 This chapte r covers a de sign methodology t hat can be used to design a speaker system for an emergency voice/alarm communications system . The ability to design an emergency voice/alarm communication s system which is highly intelligible at a reason able cost, represents a significant advan tage to the customer.
6-2 The steps below summarize the s p eaker system de sign method. Use these steps in conjunctio n with the Tyco Safety Products iTool (described later in this chap ter). Determine if the room requires advanced desi gn methods. Some characteristics of a difficult location/space include: • High Backgroun d Noise Levels (Greater than 80 dB) .
6-3 Use the following recommendatio ns to maximize system intelligibility: • Ensure at least an 8 dBA signal-to-noise ratio with regard to the speech signal. Note: This can resu lt in a higher th an 15 dB signal-to -noise ratio for notificatio n tones.
6-4 The following examples illustrate the design methodology outlined earlier in this chap ter. For these exam ples, computer based m odeling was employed usi ng Tyco Safet y Products “i Tool” to demonstrate intellig ibility. Note: See the iTool Installation and User’s Guide (579-772) for iTool installation an d operation instructions.
6-5 Click the “Speaker Location” tab on th e iTool for more detailed information. The following screen shows a speaker locatio n guide for the office space: Figure 6-2.
6-6 In this example, consi der a standard offi ce co rridor with the following specifications: • Dimensions = 100’ L x 12’ W x10’ H • Flooring = T i l e • Ceiling = Acoustic Tile • Walls = Gypsum over 2” x 4”, (16” on center) • Ambient Noise = 60 dB Figure 6-5.
6-7 Click the “Speaker Location” butto n on the iTool for more detailed information. The fo llowing screen shows a speaker locat ion guide for the corridor: Figure 6-6.
6-8 Gymnasiums are notoriousl y bad acousti c environm ents. Extremely high reverberation times can be expected because of the la rge room volume pl us the hard walls, wood floors, and plast er or metal ceilings.
6-9 Click the “Speaker Location” butto n on the iTool for more detailed information. The fo llowing screen shows a speaker locatio n guide for the gymnasium: Figure 6-10.
6-10 During an intelligibility surv ey in an office building, an employee lobb y area measured 0.60 CIS intelligibility, failing the NFPA suggested 0.70. This room is characterized by tile floor, hard walls, and one wall made up mostly of glass, see t he figure below.
6-11 The existing design had two wall mounted s peakers, to the left and right of the entrance doo rs. Figure 6-15. Lobby Layout The following screens below show the lobby speaker location guid e and t he SPL distribution for the lobby: Continue d on next page Applying the Methods, Continued Example 4: Lobby, ( continued ) Figure 6-17.
6-12 The followin g screen shows t he reverberati on time and s peaker coverage inform ation: Figure 6-18. Lobby Reverberation Time and Speaker Cov erage Information Applying the Methods, Continued Ex.
6-13 Designing Emergency Voice/Alarm Communicati ons Systems for Speech Intelligibility requires awareness of the area dimensions, a nticipated background noise level; wall, ceiling , and floor materials; anticipated occupancy, and any other characteristics that may influence the desired acoustical properties.
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7-1 This chapter contains a glos sary of technica l terms that are used throughout th is manual. Refer to the page num ber listed i n this tabl e for information o n a specific topi c.
7-2 This list provides brief descriptions of va rious terms relating to this publication: ABSORPTION COEFFICIENT – The rat io of absorbed-to-re fl ected sound.
7-3 DISTOR TION – The undesired change i n the waveform of a signal that can lead to dimini shed clarity in rece ption or reproduct ion. ECHO – The repetition of sound by reflectio n of sound waves from a surface. FIBER OPTIC RISER – An a nalog or digital risers that uses fi ber optic distribution media.
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IN-1 % %ALcons, 3-7 A acoustical treatment, 2-7 acoustics, 2-6 AHJ, 7-2 Amplifiers, 4-3 audibility, 5-2, 7-2 audio riser interface m odules, 4-3 B background noise, 3-3, 5-3 C Ceiling hei ght, 2-14 CI.
IN-2 speaker dispers ion angle, 2- 10 Speaker layout patterns, 2-15 speaker placem ent, 2-7 speaker, sensitivity, 2-10 speech pattern, 3-4 speech pattern , modulations , 2-5 STI method, 3- 7, 7-3 STI-.
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7-1 579-769 Rev. C Printed in the U.S.A . Specifications and other information shown were current as of pub lication, and are subject to change wit hout notice.
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