While it looks like it’s been on the drawing board since at least 2004, today I ran across a news report on what appears to be an increasingly well-defined plan to outfit surface ships with a new Continuous Active Sonar (CAS) system.  In the face of what is perceived as an increasing threat from quiet subs, along with the increasing costs of compliance with environmental regulations that now govern current sonar systems (primarily the mid-frequency active sonar deployed on hundreds of ships, along with gradually increasing use of low-frequency active sonar on two US and a few UK ships), research and development efforts are turning toward the possibility of using a continuous, lower power sonar system to protect Naval vessels.

The sinking of a South Korean vessel, along with the embarrassing appearance of a Chinese sub that surfaced, undetected, within firing range of a US aircraft carrier group in 2006, has highlighted the gaps in detection that are inevitable using a sonar surveillance system that is only activated at times.  While none of the news or technical pages that I found specified the projected source level or frequency range of the new CAS system, one of its benefits was repeatedly claimed to be that it would broadcast at a lower power level, thus reducing impacts on marine species.  It sounds like they would trade the drawbacks of high sound levels for the different but not insignificant effects of adding more continuous background noise to the ocean environment.  One of the rarely-noted factors that may well contribute to the behavioral responses seen in reaction to the the current generation of mid-frequency active sonar is the chaotic, and quite disturbing, nature of the sound source; this is sometimes referred to as the kurtosis of the sound (how many sharp or abrupt elements there are in the sound itself, including sudden rise times rather than more rounded sine-like changes in intensity that are heard in most naturally-produced sounds).  If there is to be a new generation of continuously-broadcasting sonars, let us hope that the sounds themselves are more closely modeled on naturally-occuring sound patterns.

For more on the Continuous Active Sonar system, see:
The recent news report, about research at Alion Science and Technology in Mystic, CT
These two pages from Signal Systems Corporation, also researching the system
A recent Congressional earmark for research
This 2006 news report on a Signal Systems contract that mentions CAS
A September 2009 Navy powerpoint presentation on advanced development plans for undersea systems, which includes a slide on CAS

Ocean Conservation Research has been studying the implications of kurtosis and rise times to ocean noise sources, especially sonar.  For more, see this research page, and especially these two papers and this audio-video demonstration.