12-08-1986 Council Packet - Laserfiche WebLink

Home Browse Search

You will note that any references to sound level measurements in this brochure involve several factors: Sound pressure level: Measured in decibels on the C weighted scale Free Field: Obstruction free, non -reverberant anecno►c chamber. 100': Distance from siren to sound receiver point. On Axis: Sound receiver is at same height as siren speaker in a direct path with the speaker projector. Frequency: The number of sound waves produced per second (cycles) by a sounding body. Decibel: Unit of sound measurement. A change in decibel reading of + 3db results in a 100% increase in sound pressure level. Inverse Square Loss (Spherical Divergence): Each time the dis- tance from a sound source is doubled in a free field, a 6 d loss in sound pressure level will result. Whelen High Powered Electronic Sirens have been tested and certified in an anechoic chamber. This measurement has been ex- trapolated to a 100' axial measurement, and i,� confirmed by field testing. Our free field measurements are verifia- ble under similar conditions. No reference to increased sound pressure level (GAIN) as a result of a questionable "ground reflection factor" is included. A siren installed at a standard mounting height from 35' to 60' is unlikely to generate reflections adding to the free field sound level. Such a dependence upon reflection to inflate or adjust the sound level is an unreliable method for use in system planning. Frequency has an effect on the range of a siren: higher frequencies null attenuate or lessen more over distance than sounds pro- duced at lower frequencies and should be a consideration when planning a system. For a guide to planning your outdoor warn- ing system you may refer to a document pre- ' pared for the Federal Emergency Management Agency (FEMA) entitled "Outdoor Warning Systems Guide" #CPG 1-17. This document reviews the many procedures involved in site selections, sound coverage and system planning. 1K