PASS Audibility Project

The passive sonar equation is the fundamental concept guiding our research efforts in analyzing the PASS signal in fireground environments. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

"Passive sonar" refers to a scenario in which a receiver is listening for a signal. To design such a system, the source signal, the transmission through the medium, the receiver and the noise levels must be defined. This information is used in an energy balance using the the passive sonar equation:

 

SL - TL = NL - DI + DT

 

where SL is the signal level, TL is the transmission loss through the medium, NL is the noise level in the environment at the receiver, DI is the directivity index associated with the receiver, and DT is the detection threshold of the receiver. 

 

For example, in underwater passive sonar (pictured above), the receiver is a vessel, usually a ship or submarine using a sonar system to detect, classify and localize a noise-emitting target of interest. The transmission loss (TL) comes from energy lost to the environment before it is received by the vessel. The background noise level (NL) can come from a variety of factors, such as other ships or submarines creating noise, rain, fish, etc. The detection threshold (DT) and directivity index (DI) is determined by the sensitivity of the system and the self-noise of the listening vessel. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

We are applying the passive sonar equation as used in the underwater environment to describe the fireground environment. Here, the signal level (SL) is the source level of the PASS acoustic output as explored in "Characterization of PASS Device". The transmission loss (TL) is the sound energy lost as the signal propagates through the fireground, suffering attenuation due to both the medium (as described in "Modeling and Validation of Sound Transmission") and interaction with boundaries in the environment (as described in "Analysis of Construction Materials"). The noise level (NL) represents all other sounds on the fireground, including other firefighting equipment, the sound of the fire itself, fire alarm sounds, radio communication, etc. These sounds are outlined in the "Analysis of Fireground Sounds" section. The detection threshold (DT) relates to the auditory abilities of firefighters; we studied this using both physical acoustic and audiological methods as described in "Firefighter Response to PASS Signal".