CPG-1-17 contains specifics on how to determine the calculated range of a device. It contains specifics on how to insure the device is both effective and safe for the public. It is very beneficial to read and understand the entire publication and we recommend it highly.
We will touch on two aspects of this publication in this document; how to calculate range, and the requirement of height.
The publication spells out a 10dB per distance doubled model of calculating range. Simply put this means you begin with a measured point of sound. This point of measurement is specifically determined by the American National Standards Institute (ANSI) specification S12.14-1992, titled, "Methods for the Field Measurement of the Sound Output of Audible Public Warning Devices Installed at Fixed Locations Outdoors". This specification is the STANDARD and specifies the exact procedures to follow when determining the measured output of the device at 100 feet. The manufactures literature should indicate this specification is followed. (CAUTION: Not all manufactures test to this standard and may mislead you in the literature.)
According to CPG-1-17, with a known starting point, that being 100 feet, every time you double the distance you subtract 10dB from the previous level beginning at 100 feet.. For example let us assume the device is tested following the standard and has a published measured output of 120dB at 100 feet. At 200 feet the sound is reduced to 110dB. It is 100dB at 400 feet, 90 dB at 800 feet, 80 dB at 1600 feet and 70 dB at 3200 feet. The siren is said to have a 3200 foot range to the 70 dB perimeter.
We believe the FEMA publication is conservative in the real world. If you follow this guideline in designing your system, you should be covered. Remember, the range of the unit is purely calculated and NOT guaranteed. If anyone guarantees the 70 dB perimeter to be absolute, they do not understand the guideline, they are mistaken, they do not know what they are talking about or they are deliberately misleading you.
The second area discussed here is the height requirements of CPG-1-17. Paragraph 4. of section V of CPG-1-17 states, "Hearing Damage - For test purposes, audible warning devices should be so located and operated that no person is likely to be subject to a sound level great enough to cause hearing damage. A suitable limit for this purpose, based upon recommendations of the Committee on Hearing, Bioacoustics and Biomechanics (CHABA) of the National Academy of Sciences, is 123 dB(C)."
Sound producing devices which generate more than 123dB of sound is considered to be dangerous to human hearing. Therefore, FEMA guidelines recommend that sound producing devices which generate more than 123dB of sound be mounted at such a height that not more than 123dB of sound will be at ground level. There is a chart contained in CPG-1-17 which shows the recommended mounting height of devices which produce 123dB, or greater, levels of sound. Equipment is manufactured by some which claim to produce levels of 134+dB of sound when measured in accordance with ANSI specifications. IF this is true and if you select one of these units for your city, you would have to mount the siren at a height of 140 feet above ground level IF you are going to follow FEMA guidelines. We have seen some of these units mounted on poles at a heights of 50 feet and less. If damage occurs to a persons hearing which can be attributed to the siren mounting, there could be some liability exposures for the city. This should be a consideration in designing your siren system.
FEMA publication can be obtained by contacting FEMA in Washington, D.C., or IF you are a person charged with Emergency Management within your City E-mail us with your NAME, CITY, YOUR POSITION and your TELEPHONE NUMBER. American Communications will supply you with a copy.