The RED perimeter presents the calculated Sound Pressure Level to 70-dB, the minimum recommended SPL in all FEMA guidelines in order to be effective. Anyone within this perimeter should receive 70-dB or greater depending on their location to the nearest siren.
The YELLOW perimeter presents the calculated Sound Pressure Level to 80-dB. Anyone within this perimeter should receive 80-dB or greater depending on their location to the nearest siren. According to FEMA Technical Bulletin Version 2.0 the 80-dB SPL is the minimum required for structure penetration. Building penetration of warning depends on many outside factors and can not be guaranteed at any distance.
Perimeters are based on the 10-dB per distance doubled mathematical model discussed in all FEMA guidelines. It can not take into account the effects of wind, rain, background noises or other factors that will affect sound propagation that may be present at the time of issuing a warning. Systems designed for a minimum of 70- dB typically provide adequate warnings.