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OUTDOOR WARNING SYSTEMS GUIDE

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CPG 117 MARCH 1, 1980 OUTDOOR WARNING SYSTEMS GUIDE federal emergency management agency CPG 117..,, {t ':'; :':ii....: ': t!!..i,. '. tt.:::}: .. : OUTDOOR WARNING SYSTEMS GUIDE..... FEDERAL EMERGENCY
CPG 117 MARCH 1, 1980 OUTDOOR WARNING SYSTEMS GUIDE federal emergency management agency CPG 117..,, {t ':'; :':ii....: ': t!!..i,. '. tt.:::}: .. : OUTDOOR WARNING SYSTEMS GUIDE..... FEDERAL EMERGENCY MANAGEMENT AGENCY Washington, D.C March 1980 CPG 117 NOTE TO USERS OF THE FEMA CPG 117 This publication supersedes the following portions of the Federal Civil D'efense Guide ;:.'.. ' Part E, Chapter 1, Appendix 3 Part E, Chapter 1, Appendix 4, nnex 1 Also superseded are any other publications of FEMA and FEMA Regional Offices which are inconsistent with CPG 117 i ..' CPG 117 OUTDOOR WARNING SYSTEMS GUIDE Abstract This practical guide has been developed to aid public officials in determining the requirements for outdoor warning systems. The guide covers,.in a simplified fo, the.principles of sound, outdoor warning systems and devices, pr'opagation and detection of sound out of doors, avoiding hazardous noise exposures, and warning system planning, testing, and use. The guide is adapted from Report No. 4100, Bolt Beranek and. Newman Incorporated, produced under Contract No. DCPA0178C0329, Work Unit No. 2234E. Report No is based upon a survey of the current literature 'on the subject, and upon discussions with Civil Preparedness personnel and vendors. No experimental work has been performed. The guide is a replacement for Federal Guide, Part E.Chapter 1, Appendix 3, Principles of Sound and Their Application to Outdoor Warning Systems, and Part E, Chapter 1, Appendix 4, Annex 1, General Instructions for Determining Warning Coverage, If both published in December 't7. :,'. :.'::;' . :t':' ::i ',' ::'.. :.. :.':.'. : ii CPG 117 CONTENTS I. II. III. PURPOSE INTRODUCTION PRINCIPLES O F SOUND. A. TERMINOLOGY B. ATTENUATION... PAGE C HEARING I IV OUTDOOR WARNING SYSTEMS AND DEVICES A. SIRENS 3 B. ELECTRONIC LOUDSPEAKER (OR VOICE/SOUND) SOURCES.. 3 C. HORNS AND WHISTLES 4 D. RATINGS AND SPECIFICATIONS 4 E WARNING. SIGNALS V. BASIC FACTS ABOUT SOUND OUT OF DOORS A. ATTENUATION WITH DISTANCE 5 1. DIVERGENCE ATTENUATION CAUSED BY GROUND EFFECTS 5 3 BARRIERS j:.: 4. EFFECTS OF VERTICAL TEMPERATURE AND WIND GRADIENTS: ATMOSPHERIC REFRACTION 6 5 FOL IAGE ABSOPTION OF SOUND IN THE ATMOSPHERE 7 7 SRY 7 :...:..... :... B HEARING ' LOCAL BARRIERS 7. iii CPG BACKGROUND NOISE AND DETECTABILITY. 3. DELETERIOUS EFFECTS OF WARNING SOUNDS. 4. HEARING DAMA.GE SRY C. ESTIMATING RANGE OF COVERAGE. VI. PLANNING AN OUTDOOR WARNING SySTEM. A. DETERMINING WARNING COVERAGE. B. SITING TO AVOID HAZARDOUS EXPOSURE. VII. SYSTEM TESTING AND USE A. KNOWLEDGE OF WARNINGS. B TESTING/ ALERT Ii C. PUBLIC INFORMATION CAMPAIGN. PAGE f I iv CPG 117 r;i:i ':,.... ;:c:.'. 'r:q'.:::.... ...,,'i; ; LI ST OF FIGURES FIGURE 1 RATED OUTPUT OF WARNING DEVICE 10 IN db(c) AT 100 FT. (30 m) FIGURE 2 MAP WITH CIRCLES CENTERED ON 12 SINGLE WARNING DEVICES FIGURE 3 MINTIMUM MOUNTING HEIGHT OF A 15 TYPICAL WARNING DEVICE TO AVOID RISK OF HEARING DAMAGE TO PEDESTRIANS (FOR HORIZONTAL BEAM). f FIGURE 4 MINIMUM DISTANCE TO AVOID RISK OF 16 HEARING DAMAGE TO OCCUPANTS OF ADJACENT BUILDINGS LOCATED IN SOUND BEAM OF WARNING DEVICES PAGE ; :: tr..:;...:::... v OUTDOOR WARNING SYSTEMS GUIDE I. PURPOSE. e The purpose of this guide is to set forth the basic principles of sound that are applicable to audible outdoor warning devices and to describe a method for planning and laying out an effective outdoor w'arning system. This gui.de concentrates on the selection, siting, and operation of audible outdoor warning devices. II. INTRODUCTION Audible outdoor warning systems (sirens, air horns, etc.) are an essential component of the Civil Defense Warning System (CDWS) established by the Federal Government to advise government agencies and the public of impending enemy attack or other disaster. Following the detection of an attack or other hazard, information is disseminated over the Federal Emergency Management Agency (FEMA) dedicated communication network The National Warning System (NAWAS) to more than 2,000 locations throughout the United States.. From these locations, the public can be informed of a potential hazard through the Emergency Broadcast System (EBS), TV stations, the news media, and other means. Outdoor warning systems can advise people that a hazard exists and that they should determine the nature of the hazard by listening to the radio, etc. For more information on other aspects of the CDWS, see CPG 114, Civil Preparedness, Principles of Warning, June 30, III. PRINCIPLES OF SOUND A. Terminology Since outdoor warning devices use sound to alert listeners to danger, this section starts with a brief introduction to the vocabulary and principles of sound. Sound is a fo of mechanical energy that moves from a source (a voice, a musical instrument, a siren) to a listener as tiny oscillations of pressure just above and below atmospheric pressure. When people hear sounds, they can distinguish their loudness, their tone or pitch, and variations of loudness and pitch with time. The loudness and pitch variations of some sounds are recognized as having certain meanings, such as with speech sounds. Instruments used to measure sounds give the magnitudes of sounds in decibels (abbreviated here as db(c». This magnitude is closely related to what we hear as loudness. Thus, an audible warning device that produces 110 db(c) at 100 ft. (30 m) away sounds louder than one that produces only 100 db(c) at the same distance. All audible outdoor warning devices are rated in terms of their sound output at 100 ft. in db(c). 1 Instruments can also measure the frequency components of a sound in Hertz (Hz). They are closely related to what we hear as pitch. As discussed below, the frequency components of the sound from an audible outdoor warning device are important in determining how far that sound will carry through the air and how well it will be heard. Most audible outdoor warning devices produce sound within the frequency range from about 300 Hz to about 1,000 Hz. B. Attenuation It is well known that sound decreases in magnitude (in loudness and in db(c)) at greater distances from its source. This decrease is called attenuation with distance, and it is caused by a number of factors described in Section VA. The amount of sound available to warn a listener can be calculated simply with the following equation: [ Amount of Sound) ound Output Of) ttenuation ] Available to Warn, = udible Warning minus with Distance, in db(c) device, in db(c) n db(c) Thus, if it is known that an audible outdoor warning device produces 110 db(c) at 100 ft. (30 m), and that the attenuation with distance is 25 db(c), then the amount of sound left over to warn people is db(c), or 85 db(c). c. Hearing Whether the amount of sound available to warn people will indeed be sufficient to do the job depends upon several J factors. First, the warning sound must be audible above the ambient, or background, noises. These ambient noises change constantly in loudness and pitch, depending upon noiseproducing activities in the vicinity of the listener. Second, the warning sound must get the attention of the listener away from what he is doing. Normally, people close out of their minds distracting sounds that are not pertinent to what they are doing. A warning sound must penetrate this mental barrier. Tests have shown that to attract a listener's attention away from what he is doing, a warning sound must be about 9 db(c) greater than would be sufficient to make it audible to someone who was concentrating on listening for it, and not doing anything else. All of these factors suggest that a warning sound must be loud: loud enough to overcome attenuation with distance, to exceed the b ackground noise, and to attract attention. Yet it cannot be too loud, or there is risk of injuring the hearing of some people who listen to it. This risk, which is discussed in greater detail in Section VB, can occur wb.en people are exposed to audib,le warn ing sounds excee'ding 123 db (C). 2 ' ..._ IV. OUTDOOR WARNING SYSTEMS AND DEVICES* When a civil preparedness official buys an audible outdoor warning system for his community, he will be purchasing: The soundmaking devices. The controls and equipment that operate the devices. In this manual, the controls and equipment are not discussed. These vary with the manufacturer and are completely described in vendors' literature. The civil preparednes official should be aware, however, that the costs of the system will include both kinds of components, as well as installation costs. The soundmaking devices themselves can be of three different types:, 1 II i Sirens Electronic (loudspeaker) devices. Horns and whistles A. Sirens Sirens are by far the most widely used soundmaking devices for outdoor warning systems. Sirens are capable of producing very intense sounds by chopping the flow of compressed gas (usually air). The fundamental frequency (pitch) of a siren sound is determined by the rate at which the flow is chopped, in cycles per second.** Sirens are powered by electric motors, gasoline engines, compressed air, or steam. Electricmotordriven sirens are the most common for civil preparedness purposes. Some sirens are nondirectional that is, they continuously produce. the same sound in all directions horizontally from the source. The most powerful sirens, however, use a horn that radiates a beam of sound in a single direction. The horn is then rotated several times a minute, so' that the beam sweeps through the entire area around the siren. For a stationary listener, the sound from such a siren goes up and down in loudness as the horn sweeps around.. B. Electronic Louds eaker or Voice/Sound Sources Loudspeaker soun sources ave tea vantage t at t'ey can roadcast voices as well as sirenlike sounds. Therefore, they can be used to issue messages as well as warning sounds to the public. However, their soundoutput capability is less than that availabl from siren sources, so that more sources may be required to cover the same area. *While in the past there were Federal matching funds for this purpose the current FEMA budget contains no such funds and future budgets ' may not include such funds. **Some sirens, known as twotone sirens, generate two frequencies simultaneously by using two airflow chopping rates. 3 Furthermore, sound reflections from large surfaces or simultaneous messages rom several loudspeaker sources at different distances may garble the signal so badly that some listeners will not be able to understand voice messages. c. Horns and Whistles Air horns have the advantage that the sounds they produce 'cannot be confused with those of emergency vehicles or fire department sirens. When a suitable air supply is already available, the cost of a horn installation is very low. In addition,. the air horn requires a minimum of maintenance and. because it weighs very little, is easily installed. In the absence of an air supply or commercial storage cylinders, a compressor, storage tanks, and related appurtenances are necessary. These increase costs substantially, for horns require more power than many outdoor warning devices of the same decibel (db(c» rating. In general, the comments on air horns apply to steam whistles as well. However, steam supplies are even more expensive than air supplies. It is generally not practical to install steam whistles unless an adequate steam supply is already available. D. Ratings and Specifications The sound outputs of acoustic outdoor warning devices are given in terms of their maximum decibels (db(c» measured at 100 ft. (30 m) from the device. The siting guide r1' lines in this manual are based upon this figure. ' t ' The fundamental sound frequencies of almost all outdoor warning devices are in the range from 300 to 1,000 Hz. (Some devices warble up and down in pitch within this frequency range. See Subsection E.) 1 Below 300 Hz, reduced human.hearing sens.itivity and h.igher background nois'e levels combine to restrict warning rang.es. Aoove 1,000 Hz I sounds. are mc?'re rapily t.tenuated in the atmosphere, so the warning range 18 agan restricted. The sounds from audible outdoor warni:t;tg device,s are generally focused into the horizontal plane surrounding the device. Sound radiated upward would be wasted, and sound radiated downward close to the aevice is unnecessary and may be hazardous. (See Section VIB.) As indicated above, some sirens may radiate a beam of sound in one direction horizontally, and have a mechanical means for rotating this beam around a vertical axis. E. Warning Signals Different cities and towns use their.,qutdoor warning systems in diferent ways. Most lac.al goveritmeri'ts', however, follow th:e Federal Emergenc'y Management Agency (FEMA) guidance and use a certain signal to warn.pe.ople of an enemy:' attack, and a different signal to notify th.em of a' peacet'ime dis aster. These warning signals are:... rr 4 e Attack Warning This is a 3 to 5minute wavering (warbling in pitch) tone on sirens, or a series of short blasts on horns or other devices. The Attack Warning signal shall mean that an actual attack against the country has been detected and that protective action should be taken immediately. The Attack Warning signal shall be repeated as often as warnings are disseminated over the National Warning System or as deemed necessary by local government authorities to obtain the required response by the population, 'including taking protective action related to the arrival of fallout. The meaning of the signal protective action should be taken immediately is appropriate for the initial attack warning and any subsequent attacks. This signal will also be used for accidental missile launch warnings. Attention or Alert Warning This is a 3 to 5minute steady signal from sirens, horns, or other devices. This signal may be used as authorized by local government officials to alert the public to peacetime emergencies. In addition to any other meaning or requirement for action as determined by local government officials, the Attention or Alert signal shall mean to all persons in the United States, Turn on radio or TV. Listen for essential emergency information. A third distinctive signal may be used for other purposes, such as a local fire signal. v. BASIC FACTS ABOUT SOUND OUT OF DOORS A. Attenuation with Distance As sound moves away from an outdoor warning device toward potential listeners, t can be greatly altered by the atmosphere. For example, everyone knows that the loudness of a sound decreases as the listener gets further from the source. Also, beyond a few undred feet from a steady sound source, the loudness varies withtime, being unnoticeable at some times and quite pronounced at others. Such effects, which are characteristic of the propagation of sound outofdoors, are caused by the factorsdescribed below. 1. Divergence As sound radiates away from a source, its intensity decreases with di_tance because its energy is spread over a larger and larger area. r6m a pintsource, this decrease is called spherical divergence or inverse square loss, because the sound intensity decreases 'inversely with the square of the distance from the source to the receiver (sound level decreases 6 db for each doubling of sourcereceiver distance). 2. Attenuation Caused B Ground Effects The ground produces a number 0 e ects on t e propagaton 0 sound over its surface. Perhaps the simplest of these is the interferometer effect, which occurs when sound is propagated over a hard, flat surface. For any 5 given source and receiver height, there are two soundwave paths between the source and the receiver: one direct, and the other somewhat longer reflected off the ground surface. Under some conditions, the sound waves arriving at a listener along these two paths interfere with each other, and cancel out. The opposite effect can also occur: the two sound waves can add, and a gain (negative attenuation) is observed. When the ground is soft and absorbs some sound, this effect becomes even more complicated. 3. Barriers A barrier is any large solid object that breaks the line of sight between the sound source and the listener. In general, a barrier can introduce p to 20 db of attenuation. The sound available behind the barrier comes from diffraction around the barrier, or from sound energy scattered into the region behind the barrier from other wave paths. / 4. Effects of Vertical Temperature and Wind Gradients: Atmospheric Refraction The speed of sound in air increases with temperature. Furthermore, when the wind is blowing, the speed of sound is the vector sum of the sound speed in still air and the wind speed. The temperature and the wind in the atmosphere near the ground are frequently nonuniform. This atmospheric nonuniformity produces refraction (bending) of sound wave paths. Near the ground, this refraction can have an effect on the attenuation of sound propagated through the atmosphere. During the daytime in fair weather, temperature normally decreases with height (lapse), so that sound waves from a source near the ground are bent upward. In the absence of wind, an acoustic shadow, into which no direct sound waves can penetrate, forms around the source. Large attenuations are observed at receiving points well into the shadow zone just as if a solid barrier had been built around the source. On clear nights, a temperature increase with height is common near the ground (inversion) and the barrier disappears. Wind speed almost always increases with height near the ground. Because the speed of sound is the vector sum of its speed in still air and the wind vector, a shadow zone can form upwind of a sound source, but is suppressed downwind. The combined effects of wind and temperature are usually such as to create acoustic shadows upwind of a source, but not downwind. Only under rare circumstances will a temperature lapse be sufficient to overpower wind effects and create a shadow completely surrounding a source. It is less rare, but still uncommon, for a surface inversion to be sufficiently strong to overcome an upwind shadow entirelyo 5. Foliage Large amounts of dense foliage (100 ft. (30 m) or more) can attenuate sound somewhat, although small amounts of foliage have no effect.,.., 6 . .. 6. Absorption of Sound in the Atmosphere Sound is absorbed in the atmosphere in a way that depends upon the humidity. In general, this loss is most pronounced at high frequencies and is of lesser importance at the sound frequencies produced by outdoor warning devices. 7. Summary The combination of all the factors that cause sound to be attenuated in the atmosphere is both complicated and unpredictable. If one were to observe the sound from a warning device 1,000 ft. (300 m) or farther away, he would find that it varies with t'ime as much as 20 to 30 db, depending upon the conditions of the atmosphere and the ground. This manual provides (Section VC) a simple and conservative method for estimating warning ranges. It is important to realize, though, that this is an estimate whch like the weather cannot be guaranteed. B. Hearing The most important factors determining the ability of a warning sound to alert a potential listener are the barriers to sound in the listener's immediate vicinity, and the background or masking noise at his location. 1. Local Barriers A potential listener indoors or inside a motor vehicle is much less likely to be alerted by a warning sound of a given loudness than someone out of doors. This is, of course J because of the attenuation of the sound as it comes through the walls of the structure surrounding him. In general, an outdoor warning device cannot be counted on to alert people in vehicles or buildings unless they are very close to the device. It is interesting to note that the current activity toward improving the energyconservation properties of buildings will have the concomitant effect of increasing their soundattenuating properties. Thus,
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