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Sunday, December 13, 2009

Determining Direction of Sound During A Search

I recently received an email from a young woman who, along with her hiking partner, was rescued by SAR volunteers after getting lost in the mountains of northern California's Angelo Coast Range Reserve. According to the woman's account, searchers were fairly close all night, yet, despite having voice contact with the subjects, they weren't able to locate them until morning.

As the two women huddled together for warmth in the intermittent rain, they periodically saw the SAR team's lights and heard their shouts, though they couldn't make out their words (or vice versa, apparently). Without a light source of their own other than the small screen of a cell phone, which they didn't try to use for signaling, the ladies had no way to indicate their location other than to shout back at searchers.

With what little battery power she had left on the phone, one of the lost hikers made calls to 9-1-1 in an attempt to have the dispatcher relay to the searchers where the subjects were in relation to the team's lights and shouts. Eventually, the cell phone went dead, and the shouting continued. Searchers were having a lot of difficulty determining the direction of the subjects' voices in that mountainous terrain.

Having had similar experiences during SAR missions, when it was difficult to pinpoint a subject's location despite being within hearing range, I thought I'd look into the fickleness of sound and how we as searchers can better hone in on the source of that sound without the aid of a visible light source. I didn't find a whole lot.

First, I read an article called The Direction of Sound, which explains the interplay between time lag, wavelength, and tone. This all makes perfect sense, but it's not always so easy in practice.

Then, in another article I came across about auditory perception, the writer states that a listener can acquire more information about the direction of a sound by moving his or her head, adjusting the orientation until the source of the sound seems to be located out front, where a human's localization sensitivity is greatest.  So if searchers stop moving and take time to call back and forth with the subject, turning our heads as we listen to each call, this would be better than continuing to move while exchanging shouts or whistle blows with the person/s we're trying to locate.

I also found an electronic device--a "bionic" ear with a booster dish--which is said to increase sound by up to 40 decibels and apparently is highly directional, allowing the user to locate the source of a sound. Search and rescue is listed as one of its intended uses; however, I've been unable as of yet to find any comments or reviews by a SAR team that's used this technology.

During one search I was on in the company of a SAR K-9, the handler closely watched his dog's ears each time we heard a shout somewhere in the distance. Just as the dog heard the sound, her ears would twitch in a particular direction. Her handler told me this was a very good indication of the direction the sound was coming from.

If anyone has any additional suggestions or feedback on what little I've discovered so far, please leave a comment. 


7 comments:

Ian Turner said...

It might be completely unrelated, but I have some problems when working with vibraphones that might be worth reflecting on. The vibraphone is basically a device that converts vibrations into sound, and by using two sensors, the source of the sound can be gradually pin pointed. This sort of equipment is used in collapsed building serches to sense the vibrations caused by a casualty banging on something in the rubble pile when they are trapped.

The way we work with the vibraphone is to gradually move the sensors to a position where the vibrations have the same strengtth on each sensor. Once we have done this we then know that the casualty is positioned somewhere on a line that runs halfway between each sensors.

The problem comes, usually in training, when we work on large intact slabs. Intact slabs tend to resonate under the vibrations caused by a casualty. What this means is that there are a number of louder spots on the slab, not just over the casualty, but sometimes a signficant distance away from the casualty. Using normal practise with the vibraphone it is easy to mistake one of these louder points for a casualty and then try and rescue the casulaty at this location.

I suspect there are similar analogues for auditory sounds as well, where the environment can set up some some situations where the sound seems to be coming from more than one direction, whereas other environments result in a much clearer direction ofr the sound.

Jane Mackay said...
This comment has been removed by a blog administrator.
Jane Mackay said...

Thanks for doing this research, Deb. I was particularly interested to read that sound could be most easily located by staying in one spot and turning one's head until the sound seems to be right in front. It makes sense if you think about it. I'll store that away for future use.

Ian, your description of using vibraphones to locate people trapped under rubble is really interesting.

eluctor.com said...

Great article. I was just starting to work on an article about ways to signal rescuers. This story just drives home the need to carry devices to help rescuers find you (mirrors, lights, whistles, etc.)

Deb Lauman said...

Thank you for your comments Ian, Jane, and eluctor. And, yes, the more the subjects can assist rescuers with light sources and other signaling devices the better. It's also REALLY helpful if they stay put and not try to move in the direction towards where they THINK rescuers are located. Moving targets are a real challenge.

Dee's Daddy said...

Deb,
I studied this in some detail many years ago about the same time the "Sound Sweep" came forward and we introduced a search tactic something we call - whistle stop.

What we discovered were:
1] we noticed that when our searchers heard a whistle while on assignment this was immediately followed by radio traffic trying to isolate if a team blew this and where they were. Very problematic and confused issues.

2] we teach children (and in the peripheral their parents)via Hug-A-Tree to carry a whistle to blow - therefore because of this and #1 we now teach our searchers not to blow their whistles - a whistle heard means either it is your subject or it is another searcher with the subject and of course an emergency - exception of course our whistle stop

3] the whistle higher freq does not travel as far as lower freq in vegetation and/or fog - it takes a higher decibel to penetrate the forest

4] our ear, by evolutionary means, is tuned to the frequency of the human voice we hear a yell more routinely than a whistle. However it takes so much energy to generate the decibels vocally to generate the sound coming through a whistle and direction of sound was bounced around by the environment

5] terrain features; we discovered that if your subject was in a drainage blowing a whistle, teams that were in the drainages (either end)could readily determine which direction the subject was in. Teams up on the ridges of this same drainage could not determine finitely where the whistle blow had come from.

6] sound direction needs to be assisted by a) listening more b)directionally cupping one's ears or c)using terrain features to assist us ... all of this led us to our whistle stop

I will look for my paper ...

Dee's Daddy said...

Oh I just found my paper from 2003, I will copy and paste here:
WHISTLE STOP
by Del Morris , Sonoma County Sheriff’s Department SAR/Helo Unit

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.

More efficient.