Tyco 579-769 User Manual

Fire Alarm Audio Applications Guide

Guideline for Designing Emergency Voice/Alarm Communications Systems for Speech Intelligibility

579-769 Rev. C

© 2005 Tyco Safety Products - Westminster. All rights reserved. Printed in the U.S.A. All specifications and other information shown were current as of publication, and are subject to change without notice.

Copyrights and Trademarks

Copyrights

Trademarks

Notice

All specifications and other information shown were current as of document revision date, and are subject to change without notice.

Printed in the United States of America.

Tyco, Simplex, and the Simplex logo are trademarks of Tyco International Services (AG) or its affiliates in the U.S. and/or other countries.

All other products are trademarks of their respective manufacturers. All registered and unregistered trademarks are the sole property of their respective companies.

This guide is intended as an informational resource and is not intended to provide definitive legal, engineering, design or architectural advice. Legal, engineering, design, or architectural requirements and interpretations may vary from jurisdiction to jurisdiction or project to project. Therefore, no warranty or representation is made about the sufficiency of any of the contents of this guide. Tyco Safety Products – Westminster, disclaims any and all liability for damages of any sort claimed to result from the use of this guide. This guide is distributed with no warranties whatsoever, including but not limited to, warranties of merchantability or fitness for a particular purpose. Readers with specific questions should consult the appropriate advisor.

Chapter 1 Speech Intelligibility Overview 1-1

Chapter 2 Background Information 2-1

Introduction .............................................................................................................. 1-1

Chapters of this Publication ..................................................................................... 1-1

In this Chapter ......................................................................................................... 1-1

Importance of Audible and Intelligible Emergency Communications .......................... 1-2

Speech Intelligibility Importance .............................................................................. 1-2

Designing for Intelligibility ........................................................................................ 1-2

Introduction .............................................................................................................. 2-1

In this Chapter ......................................................................................................... 2-1

Basic Audio Math......................................................................................................... 2-2

Ohm’s Law and the Decibel..................................................................................... 2-2

Adding Decibels....................................................................................................... 2-3

Sound and Hearing...................................................................................................... 2-4

The Relationship Between Sound and Hearing ...................................................... 2-4

The Nature of Speech ................................................................................................. 2-5

Introduction .............................................................................................................. 2-5

Consonants and Vowels.......................................................................................... 2-5

Room Acoustics........................................................................................................... 2-6

Introduction .............................................................................................................. 2-6

Reverberation .......................................................................................................... 2-6

Estimating Reverberation Times ............................................................................. 2-7

Countering the Effects of Reverberation ................................................................. 2-7

Speaker Basics............................................................................................................ 2-9

Inverse Square Law................................................................................................. 2-9

Sensitivity............................................................................................................... 2-10

Speaker Dispersion Angle and “Q”........................................................................ 2-10

Speaker Coverage................................................................................................. 2-12

Determining Critical Polar Angle............................................................................ 2-13

Determining Critical Polar Angle............................................................................ 2-14

Power Rating ......................................................................................................... 2-15

Speaker Layouts.................................................................................................... 2-15

Distributed Wall Mounted Systems............................................................................ 2-16

Introduction ............................................................................................................ 2-16

Advantages............................................................................................................ 2-16

Disadvantages ....................................................................................................... 2-16

Design of a Distributed Wall Mount System .......................................................... 2-17

Chapter 3 Speech Intelligibility 3-1

Introduction .............................................................................................................. 3-1

In this Chapter ......................................................................................................... 3-1

iii

Influences on Intelligibility............................................................................................ 3-2

Introduction .............................................................................................................. 3-2

Background Noise ................................................................................................... 3-3

Reverberation .......................................................................................................... 3-4

Distortion.................................................................................................................. 3-5

Microphone Technique ............................................................................................ 3-5

Measures of Intelligibility ............................................................................................. 3-6

Introduction .............................................................................................................. 3-6

The Common Intelligibility Scale (CIS) .................................................................... 3-6

The STI Method ....................................................................................................... 3-7

STIpa ....................................................................................................................... 3-7

STITEL..................................................................................................................... 3-7

RASTI ...................................................................................................................... 3-7

Percent (%) ALcons................................................................................................. 3-7

Phonetically Balanced Word Scores ....................................................................... 3-7

Practical Measurement of Intelligibility ........................................................................ 3-8

Introduction .............................................................................................................. 3-8

Tools for Predicting Intelligibility .................................................................................. 3-9

Introduction .............................................................................................................. 3-9

Acoustical Modeling Software ................................................................................. 3-9

Chapter 4 Emergency Voice/Alarm Communications Systems 4-1

Introduction .............................................................................................................. 4-1

In this Chapter ......................................................................................................... 4-1

A Typical Emergency Voice/Alarm Communications System ..................................... 4-2

Parts of an Emergency Voice/Alarm Communications System .................................. 4-3

Command Center .................................................................................................... 4-3

Audio Riser .............................................................................................................. 4-3

Transponder ............................................................................................................ 4-3

Speaker Circuits ...................................................................................................... 4-4

Chapter 5 Regulatory Issues 5-1

Introduction .............................................................................................................. 5-1

In this Chapter ......................................................................................................... 5-1

Audibility ......................................................................................................................5-2

Tones and SPL ........................................................................................................ 5-2

High Background Noise........................................................................................... 5-3

Large Areas ............................................................................................................. 5-3

Intelligibility .................................................................................................................. 5-4

Intelligibility .............................................................................................................. 5-4

Intelligibility Certification .......................................................................................... 5-5

Chapter 6 Speaker System Design Method 6-1

Introduction .............................................................................................................. 6-1

In this Chapter ......................................................................................................... 6-1

Speaker Design Method .............................................................................................. 6-2

Introduction .............................................................................................................. 6-2

Step 1: Room Characteristics................................................................................ 6-2

Step 2: Calculate the Number of Speakers ........................................................... 6-2

Step 3: Audio Power and Individual Speaker Wattage Tap .................................. 6-2

Step 4: Model Design to Predict Intelligibility......................................................... 6-2

Step 5: Verify Final Installation .............................................................................. 6-2

Recommendations for Maximizing System Intelligibility.............................................. 6-3

Maximizing Intelligibility ........................................................................................... 6-3

Applying the Methods .................................................................................................. 6-4

Design Examples..................................................................................................... 6-4

Example 1: Office Space ......................................................................................... 6-4

Example 2: Corridor................................................................................................. 6-6

Example 3: Gymnasium........................................................................................... 6-8

Example 4: Lobby .................................................................................................. 6-10

Conclusion ................................................................................................................. 6-13

In Closing............................................................................................................... 6-13

Chapter 7 Glossary of Terms 7-1

Introduction .............................................................................................................. 7-1

In this Chapter ......................................................................................................... 7-1

Glossary....................................................................................................................... 7-2

Index ...........................................................................................................................IN-1

Related Publications

Refer to the publications and web sites listed below for more information regarding sound, speech, and audio intelligibility:

• Acoustics – The Construction and Calibration of Speech Intelligibility Tests

ISO/TR 4870:1991(E).

• American National Standard Methods for Calculation of the Speech Intelligibility Index

(ANSI S3.5-1997).

• Handbook for Sound Engineers, Third Edition, Glen M. Ballou, Editor, published by

Butterworth-Heinemann, Woburn, MA.

• The Limits of Wide Dispersion (White Paper), Atlas Sound (www.atlassound.com

• National Fire Alarm Code (NFPA 72) 2002 Edition, published by National Fire Protection

Association, (http://www.nfpacatalog.org/

• Objective Rating of Speech Intelligibility by Speech Transmission Index,

International Electrotechnical Commission (IEC), 60268-16, Second Edition, 1998-03.

• Speech Intelligibility – A JBL Professional Technical Note, JBL Professional,

Northbridge, CA.

• Sound System Design Reference Manual, JBL Professional Northbridge, CA.

• Sound Systems for Emergency Purposes, International Electrotechnical Commission (IEC),

60849, 1998-02.

• Sound System Engineering, by Don & Carolyn Davis, published by Howard Sams & Co.

Tyco Safety Products publications:

• STI-CIS System Users Guide (579-377).

• iTool Installation and User’s Guide (579-772).

Chapter 1

Speech Intelligibility Overview

Introduction

Chapters of this Publication

INTELLIGIBILITY – The capability of being understood or comprehended.

In simple terms, intelligibility is an evaluation of changes that occur to speech that impact comprehension. More specifically, intelligibility is concerned with evaluating reductions of the modulations of speech that cause undesired reductions in speech comprehension. These modulation reductions can also be thought of as a degradation of signal (speech) to noise ratio.

Over the last few years, the drive towards intelligible Emergency Voice/Alarm Communications Systems has been gaining momentum throughout the fire alarm industry. NFPA 72

Fire Alarm Code

intelligible and discusses methods for verifying intelligibility.

In the past, the fire alarm industry primarily focused concern on audibility requirements, assuming that if the sound was loud enough it would be sufficiently intelligible. Furthermore, many designs did not take into account ongoing changes in the construction of the building, the construction materials used in a building, or its furnishings. It is possible that many emergency voice/alarm communications systems designed under those conditions do not provide sufficiently intelligible communications. While those systems may provide highly audible alert and evacuation tones, speech information may not be properly delivered.

This guide provides general information on the concepts of intelligibility and on the design of emergency voice/alarm communications systems. It provides you with a better understanding of the factors affecting the intelligibility of these systems in public spaces and is intended to help design a system that meets the requirements for speech audibility and intelligibility in a cost-effective manner.

This guide is separated into the following chapters:

• Chapter 1. Speech Intelligibility Overview: Provides an overview of audio intelligibility and

an introduction to the topics covered in this publication.

now requires that emergency voice/alarm communications systems be

, the National

In this Chapter

• Chapter 2. Background Information: Provides several sections of background material that

are essential to designing an intelligible system. Topics such as room acoustics, speaker design layouts, and audio math are discussed.

• Chapter 3. Speech Intelligibility: Details the influences and measurements of intelligibility.

• Chapter 4. Emergency Voice/Alarm Communication Systems: Details emergency

voice/alarm communications systems and describes the advantages of an emergency system compared to a typical non-emergency paging system.

• Chapter 5. Regulatory Issues: Discusses National Fire Protection Agency (NFPA) Codes.

Several excerpts of the 2002 Code are included.

• Chapter 6. Fire Alarm Audio Speaker System Design Method: Provides examples of

speaker designs created by using the Tyco Safety Products iTool.

• Chapter 7. Glossary of Terms: This chapter includes definitions of the important terms

used in this publication.

Refer to the page number listed in this table for information on a specific topic.

Topic See Page #

Importance of Audible and Intelligible Emergency Communications 1-2

1-1

Importance of Audible and Intelligible Emergency Communications

Speech Intelligibility Importance

Emergency voice/alarm communications systems are used in applications where it is necessary to communicate more detailed information to occupants of a building than the simple evacuation signal provided by horns or bells. For example, in a high-rise building, evacuation of all of the occupants at one time could create an unsafe situation in which the routes to evacuation could be blocked by the sheer number of people trying to exit at once.

An emergency voice/alarm communications system can provide a means to ensure a more orderly and safe evacuation. However, if the emergency voice/alarm communications system is not audible (loud enough), or if it is not intelligible (understandable), then emergency information is not properly communicated. Therefore, a safe response to a fire cannot be reliably achieved. In some ways an inaudible or unintelligible system is worse than not having a system. This is due to a possible false sense of security. Also personnel responding to an incident may operate under the premise that building occupants are getting proper instructions, when in reality they are not.

Historically, the emphasis in emergency voice/alarm communications system design has been on audibility. These systems have been required to have a sound level that is at least 15 dB above the average ambient sound level, or 5 dB above the maximum sound level having a duration of at least 60 seconds, whichever is greater. Starting with the 1999 version of the National Fire Alarm Code (NFPA 72) the fire alarm industry recognized the importance of requiring both audibility and intelligibility.

Although a specific measure of intelligibility is not currently required by NFPA 72, the Code’s Annex recommends the use of International Electrotechnical Commission (IEC) 60849 and a Common Intelligibility Scale (CIS) measurement of 0.70 as a guideline. It is expected that future versions of NFPA 72 will quantify the measurements required to demonstrate intelligibility.

Designing for Intelligibility

Properly designing emergency voice/alarm communications systems for intelligibility requires knowledge of the acoustical factors that influence intelligibility; awareness of the tools available to predict acoustical performance; and the ability to measure the intelligibility of the completed installation. It is also necessary to identify complicated areas where experienced sound professionals using sophisticated audio design tools may be required to achieve the desired intelligibility.

This document is presented as an introductory guide to understanding intelligibility and its importance in achieving successful emergency voice/alarm communications systems. Please refer to the cited references for more information concerning audio systems design.

1-2

Chapter 2

Background Information

Introduction

In this Chapter

There are a few fundamental concepts that are necessary to understand when working with emergency voice/alarm communications systems. This chapter introduces basic concepts of sound, but is not intended to be an exhaustive treatment of the subject.

Note: Refer to the “Related Documentation” section at the beginning of this manual for

publications containing in-depth discussions of sound and speech.

Refer to the page number listed in this table for information on a specific topic:

Topic See Page #

Basic Audio Math 2-2

Sound and Hearing 2-4

The Nature of Speech 2-5

Room Acoustics 2-6

Speaker Basics 2-9

Distributed Wall Mounted Systems 2-15

2-1

Basic Audio Math

Ohm’s Law and the Decibel

Audio engineers use “Decibels” (dB) to express ratios between levels, such as power, Volts, Amps, and Sound Pressure Levels (SPL). The decibel is not an absolute measure like Volts and Amps, rather it is used to make comparisons between two numbers. The decibel is defined as the logarithm of two power levels, shown below in the equation as P

log10 Decibel

Equation 2-1. The Decibel

is the reference power and P1 is the power level used for comparison. The logarithm is used in

the decibel in order to make comparisons of power over a very wide range. This is very useful in audio applications as the ear responds logarithmically to changes in SPL.

You can also use the decibel for voltage comparisons. From Ohm’s Law we know that:

RIV

Equation 2-2. Ohm’s Law

The electrical power equation:

PIVP

Equation 2-3. Power Relationships

Use the following equation to determine the decibel difference between two voltage measurements powering the same load resistance:

⎤

⎥ ⎥

⎥

⎦

Equation 2-4. dB and Voltages

) and the voltage at the last speaker on the circuit is

⎡

()

⎢ ⎢

log10dB

()

⎢ ⎢

⎣

The decibel is often used to make comparisons between two different numbers, neither of which is at an absolute reference level. For instance, if we take two voltage measurements along the length of a speaker circuit, the power lost to the wiring can be calculated directly. If the voltage at the amplifier driving a speaker circuit is 25 V (V 15 V (V

) the power loss due to the wiring is 4.4 dB.

and P0:

⎞

⎛

⎟

⎜ ⎝

⎠

Where:

• V = Volts

• I = Amps

• R = Resistance

RIP

×==×=

⎞

⎛

⎟

⎜

log20dB:tosimplifiedbecanwhich

Continued on next page

⎟

⎜

⎠

⎝

2-2

Basic Audio Math, Continued

Ohm’s Law and the Decibel,

(continued)

Adding Decibels

-6

When the decibel is used to express SPL, the reference sound pressure is 20 x 10 which is approximately the threshold for hearing for a normal listener. When using a dB meter to measure sound, the meter is performing the calculation between the received SPL and the reference SPL:

spl

Equation 2-5. dB and Sound Pressure Levels

When multiple sound sources are combined, there is an increase in SPL. However, you cannot add decibels directly:

90 dB + 90 dB is not 180 dB but 93 dB. Doubling the power results in a +3 dB SPL increase.

To add SPL decibels:

1. Convert the decibels back to the original power value (which for SPL, is referenced to 1pW

2. Add the numbers together.

3. Convert the numbers back to decibels.

To add 90 dB + 90 dB:

or 10

-12

W).

⎜

1020

⎝

SPL

⎛

log20dB

−6

⎞ ⎟

⎠

Newtons/m²

⎞

⎛

⎟

⎛ ⎜

⎝

log10dB

log1090

log10dB

⎜

;

⎟

⎜

⎠

⎝

⎞ ⎟

⎠

⎞ ⎟

−

⎠

;

⎛ ⎜

⎝

−

101010

002.0

⎛ ⎜

⎝

⎞ ⎟

−

⎠

pW1

⎛

⎜ ⎝

3129

−−

===

⎞ ⎟

−

⎠

W002.0W001.0x2P2

log9

()( )( )

Equation 2-6. Adding Decibels

−

W001.0

2-3

Sound and Hearing

The Relationship Between Sound and Hearing

Sound is created by mechanical vibrations that displace air molecules to create repetitive changes in air pressure. The ear detects these changes in air pressure, with the magnitude of the pressure perceived as loudness and the frequency of the changes perceived as pitch.

Due to the physiology of the ear, sound pressure does not correlate directly to the perceived loudness over all SPL and frequencies. The ear is most sensitive to frequencies between 3 to 5 kHz and much less sensitive to low frequencies. For a low frequency tone to be perceived as loud as a high frequency sound, it must have a much higher SPL. In addition, the ear’s sensitivity to the low frequencies also depends on the SPL. At high sound volumes, the loudness difference between the most sensitive frequencies and low frequencies is reduced.

The non-linear nature of the ear’s response to frequencies and loudness is well documented in the Fletcher-Munson equal loudness curves, updated in the Robinson and Dadson equal loudness curves that were adopted in the ISO (International Standards Organization) Recommendation R-226.

Note: The MAF Curve in Figure 2-1

represents the “Minimum Audible Field” Curve.

Figure 2-1. Robinson and Dadson Equal Loudness Curves

The equal loudness curves are used when sound levels are measured with a sound level meter. If the meter has a “flat” response, then the displayed result shows a larger than perceived level when sounds with significant low frequencies are measured. For this reason, sound level meters have a correction or “weighting” filter built-in. This filter can more closely match the displayed reading with the ear’s response. The most widely used weighting curve (and the one required by NFPA 72) is the “A” weighted curve, which is approximately the inverse of the 40 phon equal loudness curve. Meters configured with the “A” weighted filter read out in units of dBA, short for “A” weighted decibels.

Other common weighting curves are the “B” and “C” curves, which approximate the ear’s response at higher decibel levels. From a practical standpoint these curves are useful for estimating the frequency content of the background noise during a room survey, but cannot be used to validate the audibility of an emergency voice/alarm communications signal.

Note: The ear is capable of perceiving a difference in the sound level only when the sound level

has roughly doubled or halved. The dBA scale is a logarithmic scale, so a doubling of sound power represents a 3 dBA increase in the SPL of the sound. Therefore, most listeners can not perceive changes in SPL of less than 3 dBA. For a sound to be perceived as twice as loud, the power must be increased 10 fold.

2-4

The Nature of Speech

Introduction

The frequency of speech ranges over seven octaves from 125 Hz to 8,000 Hz, with the majority of frequencies contributing to intelligibility falling between 500 Hz and 4,000 Hz. The creation of “phonemes,” or the sounds that make up words is created by amplitude modulation of those frequencies. Amplitude modulations of speech patterns are seen as the peaks and valleys of the waveform. These modulations range from 0.63 Hz to 12.5 Hz. A typical fragment of speech: “an emergency has been reported” is shown in the figure below.

Consonants and Vowels

Figure 2-2. Speech Pattern that Illustrates Modulations

Consonants generally have the lowest power contribution to speech, but are extremely important to intelligibility. Consonants like the “T” and “S” sounds are relatively high in frequency, but of a short duration. Vowels (A, E, I, O, U sounds) carry most of the power of the speech signal.

2-5

Room Acoustics

Introduction

Reverberation

This section is provided as a summary of room acoustics. See the references in the “Related Documentation” section earlier in this manual for a list of publications containing more thorough discussions of this subject.

Reverberation is one of the most important contributors to reduced intelligibility, and is the result of sound being reflected off floors, walls, ceilings and other surfaces. When a message is broadcast over a speaker system, the listener hears a combination of the direct sound from the speakers plus the reflected or delayed sound from the reverberation. Reverberation should not be confused with echoes. An echo is a delayed but distinct reproduction of an original sound, where reverberation contains the original sound jumbled into something not distinctly identifiable as part of the original signal.

Note: In the distributed speaker system typical of fire alarm applications, echoes are generally

not a problem, but reverberation can have a major impact on intelligibility.

Reverberation Time (also known as T60 times) is the amount of time it takes for a sound to diminish to 60 dB below the original level. For example, to estimate a room’s reverberation time, pop a balloon in a room and time how long it takes for the sound to diminish.

The reverberation in a room is dependent on its dimension, construction, materials, and objects within the room, including the room’s occupants. People and furnishings are good sound absorbers. Reverberation levels in occupied and/or furnished rooms can be significantly lower than levels in unoccupied/unfurnished rooms.

Each surface in a room absorbs or reflects a certain percentage of sound, characterized by the “Absorption Coefficient” of the material. The absorption coefficient is the ratio of absorbed to reflected sound, and has a range of 0 to 1. A hard surface, such as glass or marble has a low absorption coefficient. This indicates that most of the energy is reflected back into the room. Soft surfaces, such as thick carpeting and acoustic ceiling tiles, have high absorption coefficients.

Frequency content of reverberation depends on the surfaces as well. Very hard surfaces such as tile reflect most of the frequencies, while soft surfaces like drapes absorb most frequencies. Most surfaces fall in between, where higher frequencies are absorbed readily and lower frequencies are either passed through or reflected.

Reverberation is also affected by the room dimensions. In general, the larger the room, the higher the reverberation times. More precisely, reverberation is dependent on the distance between opposing surfaces. Two rooms with the same volume (L x W x H) and the same surface materials can have dramatically different reverberation times.

Continued on next page

2-6

Room Acoustics, Continued

Estimating Reverberation Times

Countering the Effects of Reverberation

Several equations are available for estimating the amount of reverberation that can be expected in a room. The equations take into account the room dimensions and surface materials to provide a reasonably accurate estimation of a rectangular room’s reverberation time. The formulas below are commonly used Sabine and Eyring equations:

The Sabine Equation, used when α < 0.2:

V049.0

English Units (ft)

α=S

The Eyring Equation, used when α > 0.2:

= English Units (ft)

Where: V = Room Volume (L x W X H) S = Total Surface Area (2LH +2LW + 2WH) α = Average absorption coefficient, equal to the area of each surface multiplied by the

absorption coefficient for that surface, divided by the total surface area of the room

Equation 2-7. Sabine and Eyring Formulas for Calculating Reverberation Times

• Acoustical treatment:

Adding drapes, wall hangings, carpeting, or specially designed diffusers can absorb sound and reduce reverberation. If possible, this is perhaps the best method in combating reverberation.

V049.0

)]1([ln)S(

α−−

V16.0

Metric Units (m)

α=S

= Metric Units (m)

V16.0

)]1([ln)S(

α−−

Note: Final room acoustics are often unknown at the time of the system design.

• Speaker Placement:

Because reverberation is caused by reflections, it is important to select speaker locations that minimize stray energy. Sound system designers are often heard saying “put the sound where people are and do not put sound where people are not.” This usually implies locating speakers toward the center of the room, away from walls and other hard surfaces. When possible, aim speakers towards soft surfaces such as rugs or upholstered furnishings. These soft surfaces absorb direct sound coming from the speaker, preventing the sound from scattering throughout the room.

Continued on next page

2-7

Room Acoustics, Continued

Countering the Effects of Reverberation,

(continued)

• Increasing the Signal-to-Noise Ratio:

Intelligibility degradation from reverberation is essentially a signal-to-noise issue, however when the noise is specifically caused by reverberation it is referred to as the “Direct-toReverberant” ratio. Increasing the direct sound field at the listener improves the direct to reverberant ratio and therefore the signal-to-noise ratio. You can increase the direct sound in several ways:

1. Move the speaker closer to the listener and reduce the wattage of the speaker:

This places the sound where it is needed and minimizes excitation of the room’s reverberation, at the expense of additional speakers.

2. Increase the speaker density and reduce the wattage to each speaker:

This increases the direct sound heard by the listener by creating overlapping regions of coverage.

3. In areas with high ceilings, specify a more directional speaker:

A speaker that is more focused (has a higher “Q”) concentrates most of the sound energy in a tighter beam than low “Q” devices. This is important in areas with high ceilings to reduce the effect of multiple late arriving sounds.

Note: See the section later in this chapter entitled “Speaker Dispersion Angle and ‘Q’” for more information.

2-8

Speaker Basics

Inverse Square Law

Speakers are essentially “point sources” of sound. Sound radiates outward in all directions, creating a spherical sound pattern. The sound pressure is spread over an increasingly larger surface area as the sound moves away from the source. This causes a drop in loudness per unit area. The drop in SPL is referred to as the “Inverse Square Law,” and originates from the fact that as the diameter of the sound-sphere doubles, the surface area increases by a factor of four. This behavior of outwardly radiating sound causes a drop in SPL of –6 dB per doubling of distance. You can calculate the change in SPL at any distance from a speaker as follows:

⎞

⎛

Δ dB

⎟

= 20 log

spl

Equation 2-8. The Inverse Square Law

⎜

⎟

⎜

⎠

⎝

D1 D2

The figure below illustrates how SPL decreases with distance as you move away from a speaker:

Figure 2-3. dB and Distance Chart

Continued on next page

2-9

Speaker Basics, Continued

Sensitivity

The amount of sound that a speaker can be expected to produce is found in the speaker’s sensitivity rating provided in the manufacturer’s literature. “Sensitivity” is the amount of sound (SPL) produced by the speaker with a known signal frequency, power level and distance from the speaker. For fire alarm listed speakers approved under UL Standard 1480, the sensitivity is rated at 1 W of power and 10 feet (3 meters) from the speaker. By knowing the speaker’s sensitivity, you can determine the on-axis SPL (SPL measurements taken directly in-line with the speaker) at any distance from the speaker using the following equation:

Where:

• SPL = Sound Pressure Level

• D = Distance from the speaker

• Dr = The reference distance

• Sensitivity = The SPL at the

reference distance.

⎤

⎡

SPL = Sensitivity + 20 log

⎥

⎢

⎦

⎣

Equation 2-9. On-Axis SPL Calculation

Simplex speakers have two sensitivity ratings listed on their respective data sheets, a reverberant chamber test as required by UL Standard 1480 and an anechoic rating as defined by ULC-S541. The reverberant chamber specification is derived from a test where the speaker’s sound is emitted in a chamber specifically designed to reflect all of the sound so that a total sound power measurement can be made. Correlating the speaker’s reverberant chamber sensitivity rating with real-world acoustics has proven to be difficult. Typically, the anechoic rating at 1 kHz is more representative of real world performance.

The speaker sensitivity rating, while useful for comparing speaker models, tends to oversimplify the true response of a speaker. Speakers “beam” sounds analogous to the way a flashlight produces light: the beam of sound is loudest directly in-line with the device and becomes quieter the farther the listener moves away from the center. This beaming effect is also dependent on the frequency of the signal.

Speaker Dispersion Angle and “Q”

The beaming effect is referred to as the directivity or “polar response” of the speaker, and is occasionally provided by manufacturers in the form of “polar plots.” For typical fire alarm speakers the beam is very wide for low frequencies (low directivity) and becomes more focused for higher frequencies. When determining coverage area, it is common practice to use the directivity information at 2 kHz: a critical band for intelligibility. Fire alarm speakers produce the highest output in the 800Hz to 4 kHz frequency range.

Continued on next page

2-10

Speaker Basics, Continued

Speaker Dispersion Angle and “Q”

(continued)

Note: See Figure 2-5 on

the following page for a more detailed view of a Speaker Polar Plot.

The figure below includes a typical polar plot graph and the interpretation of the dispersion angle.

Simplex 4902-9721

Ceiling Mount Speaker

Polar Plot - 2kHz 0

6dB/division

87dB

Dispersion Angle

Sensitivity = 93dB @10 Feet, 1 W

Figure 2-4. Speaker Polar Plot Interpretation

The “Coverage Angle” is defined as the angle where the speaker SPL drops 6 dB from the on-axis SPL. For the speaker above, the coverage angle is 150 degrees.

Another common representation of speaker directivity is “Directivity Factor” or “Q.” For

speakers having a conical coverage pattern (typical of single driver speakers used in fire alarm applications), “Q” is determined by:

Equation 2-10. Directivity Factor “Q” for a Conical Source

For the speaker above, the coverage angle (θ) is 150 degrees at 2 kHz, resulting in a “Q” of 2.7.

Q =

−

⎛

cos1

⎜ ⎝

Ceiling

off

87dB

⎞ ⎟

⎠

Continued on next page

2-11

+ 49 hidden pages

Tyco 579-769 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

Copyrights and Trademarks

Table of Contents

Related Publications

Speech Intelligibility Overview

Background Information

Basic Audio Math

Sound and Hearing

The Nature of Speech

Room Acoustics

Speaker Basics

Inverse Square Law

Sensitivity