A noise maker is whatever makes the sound in a warning system, from tin cans with stones to bells and gravel paths, and the sound must reach a person who can act on it. In open air a sound falls by about 6 dB for every doubling of distance and 20 dB for every tenfold increase, so a noise line 100 m from the house is about 40 dB quieter than it is 1 m from the can; use metal and gravel, not glass.
Part of Early Warning Systems
Noise makers are the sound-producing parts of an alarm, either objects set off by a trigger, such as cans, bells and metal lids, or surfaces such as gravel, dry leaves and brush that are loud under any footstep.
A Sound Nobody Hears Is Not An Alarm
The parent article gives two kinds of noise maker. Objects set off by a trigger: tin cans with pebbles, glass bottles that crash, bells, stacked rocks. And surfaces that are loud under any step: a layer of crushed gravel about 10 cm deep around the perimeter, dry leaves raked into the approach path (the parent article says a person walking through them is audible at 50 m or more), dry branches across paths, crushed shell.
The sound has to reach somebody. A noise line out at the edge of the outer zone makes its noise a long way from the sleepers, and the sound dies with distance.
What You Need
- Metal objects: tin cans, lids, pans, bells.
- Stones or gravel to fill and strike with.
- A surface that rings: a flat stone, a sheet of metal or a board.
- A way to hear it: a test from the place people sleep, with doors shut.
- A cord and a bell indoors if the noise line is far from the house.
Method: Make The Sound, Then Test How Far It Goes
Step 1: Use metal, gravel and shell. Cans struck on a flat stone give a sharp, carrying clang. A gravel path gives a continuous crunch under a footstep.
Step 2: Keep glass out of any surface people may step on. The parent article suggests glass bottles that shatter, and crushed glass mixed into the gravel. I advise against both: broken glass cuts bare feet, dogs’ paws and hooves, and stays in the ground for years. Use clean crushed shell or coarse gravel, which are as loud.
Step 3: Test from the bed. Have someone trigger the noise maker at night and listen from the bed with the door shut. If you cannot hear it clearly, move it closer or add a relay.
Step 4: Relay a distant sound. A cord run from the noise line to a bell by the bed carries the pull instead of the sound. Check it after wind and animals.
Step 5: Make the sounds different. A can line at the gate and a bell on the orchard path should sound different, so you know which route was touched.
Worked Example: How Far The Sound Carries
In open air with no wind and no obstacles, sound spreads over a sphere, and its level in decibels (dB) falls by 20 × log10(r2 ÷ r1) from distance r1 to r2. That is standard acoustics and does not depend on the device.
- Doubling: 20 × log10(2) = 20 × 0.301 = 6 dB quieter.
- Tenfold, from 1 m to 10 m: 20 × log10(10) = 20 dB quieter.
- From 1 m to 100 m: 20 × log10(100) = 20 × 2 = 40 dB quieter.
A noise line 100 m from the house sounds 40 dB lower than at the can, before the walls, the wind and the sleeper’s own deep sleep take their share. Each 20 dB is a factor of 100 in sound power. So the noise maker should be loud at its own source and close to the listener, or relayed.
Gravel quantity: a path 1 m wide and 0.10 m deep over 100 m takes 1 × 0.10 × 100 = 10 m³ of gravel.
Common Mistakes
- Putting the can line at the edge of hearing. It then fails unheard.
- Glass in the path. Cuts the household before it warns about anyone.
- Gravel that has been rained flat. Rake and refresh it.
- Same noise for every route. You lose the direction.
Safety
Glass, falling objects and hearing
Broken glass on ground that people, dogs or livestock walk on causes deep cuts that fester. Do not hang anything heavy above head height. If an alarm sounds near a sleeping child at close range, loud metal on metal can damage hearing; keep noise makers away from bedrooms.
What’s Next
- Learn next: Balance Triggers — a way of setting off a noise maker with a touch
- Combine with: Tripwire Systems — the trigger that pulls the release