Data obtained from measurements of siren performance depend not only on the characteristics of the siren tested, but additionally on the test procedures and the characteristics of the measurement instrumentation and test environment. These additional factors must be well defined and controlled to obtain reliable data. Detailed test methods are described here, which include specifications for a laboratory environment, to minimize the measurement uncertainty and obtain accurate and reproducible measurement results. Such results are necessary to qualify the performance of all sirens as equally as practicable. Requirements have been established based on the laboratory-measured performance of sirens that have been effective in emergency service.
Whether a person will hear, recognize, and react quickly enough to the warning sounds produced by a siren during an emergency depends on many factors in addition to the sound pressure level (SPL) it produces in a controlled test environment. Reflection, scattering and attenuation caused by objects such as buildings, trees, road surfaces and vehicles contribute to sound propagation losses. Absorption of sound by the atmosphere itself also results in losses. Windows, soundproofing and other materials that are part of a vehicle further decrease sound levels. Background noises also interfere with the audibility of acoustical signals, an effect called masking. Siren sounds are masked by traffic and community noise, and noise produced by car stereos, air conditioning, wind and rain. There are also variations in how well different people can detect, identify and localize sounds, which are partly due to their ability to hear as a function of frequency. Finally, how effectively someone can react to a detected sound depends on the proximity and speed of the emergency vehicle, the speed of their vehicle, and their reflexes.
Emergency vehicle sirens do not produce sounds that are loud enough to warn effectively in all circumstances. A report prepared for the U.S. Department of Transportation by Bolt, Beranek, and Newman Inc. concluded that the sound levels produced by sirens would have to be increased greatly, to the point where these levels would be intolerable to the community, to be loud enough to warn effectively in all ordinary circumstances. There is no assurance that all other motorists will always hear, recognize, or react quickly enough to the warning sounds produced by a siren to take appropriate action. It is necessary for emergency vehicle operators to watch for the reaction of other motorists to the siren and be prepared to maneuver accordingly. Sirens have been effective in calling for the right-of-way by an emergency vehicle, but must always be used in conjunction with effective visual warning devices and operated only by properly trained personnel who are aware of the limitations noted here.
There is an additional concern for emergency vehicle operators and others exposed to siren noise. Sounds produced by emergency vehicle sirens are loud enough to increase the risk of temporary or permanent hearing loss. Appendix A contains information regarding occupational hearing loss and exposure to siren noise.
Appendix B is a data sheet that includes Tables B1 through B5C and descriptive statements for use when documenting test results. It outlines the data that must be recorded when performing the measurements specified in this document.