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Tuesday, February 2, 2010

A Follow-Up To Determining Direction Of Sound

My previous blog post, Determining Direction of Sound During a Search received some very interesting comments, including a couple from Del Morris of the Sonoma County Sheriff's Search & Rescue / Helo Unit. Del wrote an article (2003) about the use of sound during searches:


The controlled use of sound during searches for a responsive subject is a stratagem that should be a practice of every SAR Team.

At a "Sound Sweep" exercise, we were debriefing our "victims" and our searchers, graphing/mapping the time and location of each "whistle blast" and the victim's perception of direction from where the sound was coming from. We discovered that the distance of sound travel and perception of sound direction was so variable that it deserved further study.

After some additional field testing and research in acoustics, I offer the following suggestions:

1] Shift to lower frequency whistles 2000-3000 Hz or fog horns (marine supply) Plastic horns(sports events) Compressed air horns (marine supply). The higher frequencies are “absorbed” by humidity and vegetation.
The lower frequencies will more readily bend around vegetation and rocks.

2] “Whistle Blasts” a.k.a. “Whistle Stops” can be done during the normal course of a Team Assignment whenever you are "inline" with a drainage *** Contact SAR Base and request a "WHISTLE STOP". SAR Comm. will then orchestrate a "Whistle Stop" by contacting all Teams, giving all teams time to get "inline" with other drainages, and doing a count down ( 3...2...1) to a great blast" just like during a "sound sweep" ... followed by that all important listening period.

3] Teams that are on ridges will be at a disadvantage. Their whistles will be difficult for the subject to discern direction of the team. The subjects return whistle blows or yells for help will be equally difficult for the team to accurately determine the location of the subject.

4] Teams that are at the bottom (below the subject) of a drainage will a)have the best chance of hearing the subject in the drainage and b) have the most accurate direction of where the subject is. They will hear the subject better than the subject will hear them.

5] Teams that are at the top (above the subject) of a drainage will a)have the best chance of the subject in the drainage hearing them and b) have the second most accurate direction of where the subject is. The subject will hear them better than they will hear the subject.

6] Frequency is more important than additional decibels. 3 -110db whistles does not equal 330db. The combined decibel level is only about 117db. Sound level does not build exponentially. Some frequencies “blown” side by side can actually cancel each other do some degree.

DISTANCE was greatly increased with the lower frequencies of sound (yelling was heard a lot further than a whistle). The lower feqs are not gobbled up as readily by vegetation and the moisture in the air. I reference the use of low freqs in Fog Horns. Our “victims” all heard "yelling" before they heard a discernible "whistle" at a time when the "yelling" teams were further away. Whistles seemed to be blended with the crickets and other background noises.

DIRECTION or the perception of direction was more accurate when teams were "blowing" or yelling while inline with the drainage where the victim was. Blows that were blown by teams that were on the sides of hills or tops of ridges ... the victims found the sound very difficult discerning from "forest" noise and on the whole non-directional.

Sound seems to carry better up hill than down hill, better with the wind than against.

WHISTLE STOP can be used during any "phase" of a search for a responsive subject. Hasty teams running trails, dog teams doing area searches, etc. ... WHISTLE STOP is a more effective use of person power.
A comparison could be made between "closed grid" search lines (Sound Sweep) and "open grid" purposeful wandering (WHISTLE STOP) searching methods.

Del Morris
Sonoma County (CA) SAR Unit

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