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7^:^v <br />L-' ^ <br />18 <br />*£*-* -T' 'f fc :■ i ■ f-t. ;--*^**•^' ■.^..* ’JC'■‘•Vi- r^'.^' ?v - • -■’■> -^* ^..^■‘il .'"-^'?. -■•'-; '“ ^■*‘ ■'•'^-•;4^-v>»** -1 •'-jTr.TTr <br />SjL^ ifa« <br />You will note that any references to sound <br />ievel measurements in this brochure involve <br />several factors: <br />Sound pressure level: <br />Measured in decibels on the C weighted scale. <br />Free Field: <br />Obstruction free, non-reve ’ berant anectioic <br />chamber. <br />100': Distance from siren to sound receiver <br />point. <br />On Axis: <br />Sound receiver is at same height as siren <br />speaker in a direct path with the speaker <br />projector. <br />Frequency: <br />The number of sound waves produced per <br />second (cycles) by a sounding body. <br />Decibel: <br />Unit of sound measurement. A change in <br />decibel reading of + 3db results in a 100% <br />increase in sound pressure level. <br />Inverse Square Loss <br />(Spherical Divergence): Each time the dis­ <br />tance from a sound source is doubled in a free <br />field, a 6 db loss in sound pressure level will <br />result. <br />VVhelen High Powered Electronic Sirens <br />.lave been tested and certified in an anechoic <br />chamber. 'I'liis measurement has been e.\- <br />trapolated to a 100' a.xial measurement, and <br />is confirmed by field testing. <br />Our free field measurements are verifia ­ <br />ble under similar conditions. No reference to <br />increased sound pressure level (GAIN) as a <br />result of a questionable “ground reflection <br />factor ” «s included. A siren installed at a <br />standard mounting height from 35' to 60' is <br />unlikely to ger erate reflections adding to the <br />free field sound level. Such a dependence <br />upon reflection to inflate or adjust the sound <br />level is an unreliable method for use in <br />system planning. <br />Frequency has an effect on the range of a <br />siren; higher frequencies will attenuate or <br />lessen more over distance than sounds pro­ <br />duced at lower frequencies and should be a <br />consideration when planning a system. <br />For a guide to planning your outdoor warn ­ <br />ing system you may refer to a document pre-' <br />pared for the Federal Emergency Management <br />Agency (FEMA) entitled “Outdoor Warning <br />Systems Guide” #CPG 1-17. This docipent <br />reviews the many procedures involved in <br />site selections, sound coverage and system <br />planning.