Audio signals between posts are whistle blasts, short horn calls and soft taps that work in the dark and without a sight line, at the price of being heard by everyone in earshot, including outsiders who now know where the sentry is. Use a single table of counted patterns, decide in advance when a quiet means such as a pull-line cord replaces a loud one, and test the audible range at night in the wind of a bad night.
Part of Watch and Patrol Scheduling
Audio signals between posts are the sounds, from whistles to cord tugs, by which sentries alert each other and the camp without needing to see them.
The Cost Of A Sound
A whistle blast at the parent’s range of about 500 m reaches the camp. It also tells every listener where the sentry stands and that something has happened. The camouflage article gives ordinary conversation as audible at 100 to 200 m and a gunshot at 3 to 5 km (Camouflage and Concealment); a whistle is louder than a voice. The same decision exists for guard dogs, where the article asks the household to decide in advance whether a loud alert or a quiet one is wanted (Night Alert Behavior). Posts need the same decision.
Worked Example: Hearing A Whistle At Distance
In open air a sound falls by 20 x log10 of the distance ratio (6 dB for each doubling; see Noise Makers). A whistle just audible to the camp at 500 m is about 6 dB too quiet at 1,000 m and 12 dB too quiet at 2,000 m, ignoring wind. So:
- A sentry who whistles toward the camp from 500 m can be heard by a listener on the far side only if the listener is within about 500 m.
- A post 1,000 m away needs a signal 6 dB, about 4 times, stronger, or a relay.
- A downwind listener hears farther and an upwind listener less (recalled from acoustics); a gale can cut the working range to a fraction of the calm-air figure.
The Signal Table
The watch-scheduling article gives different whistle meanings for posts and for patrols, so the same two blasts mean acknowledged at a post and come to me on patrol. Use the single table in Simple Code Tables: one blast for attention or halt, two for acknowledged, three for danger, one long blast for come to me. Counts, not lengths, because a sentry who is out of breath cannot send a clean long blast.
Quiet Means
- Pull-line. A cord strung between two posts; a tug pattern is a message. Silent and hard to see, but it can snag, stretch or be cut, and works over short spans.
- Hand signals at close range, which carry no sound.
- Soft tap on wood or metal, audible at close range only.
- A runner, who is silent but slow (Runner Systems).
Method: Setting Up Audio Signals
Step 1: Decide the loud and quiet rules. At threat Green the loud signals are used freely; at higher levels a contact is first signalled by quiet means.
Step 2: Set the table and print it for every sentry and patrol member.
Step 3: Test the range at night from the receiver’s end, in the wind of a bad night.
Step 4: Keep spares. Whistles lose their peas and crack; carry two.
Step 5: Acknowledge. Every signal is answered by the acknowledgement pattern.
Common Mistakes
- Different codes for posts and patrols. The collision in the parent’s two tables.
- A loud signal for a minor matter. It reveals the post and teaches the camp to ignore it.
- Assuming sound reaches evenly. Wind, hills and trees carve gaps; test them.
- A metal whistle in frost. Bare lips can stick to cold metal in hard frost (recalled); carry a plastic whistle or a cloth.
Safety
Ears and cords
Blowing a whistle close to someone’s ear can damage hearing. A taut pull-line across a path can trip a person; mark it and keep it above or beside paths, never across the track a patrol uses at night.
What’s Next
- Learn next: Runner Systems — messages longer than a signal
- Combine with: Whistles and Bells — the instruments themselves