Control the noise of work by moving loud jobs to windy, rainy or midday periods, padding every impact, enclosing or screening engines, and measuring how far each job carries by walking away from it. In open air every 6 dB of reduction halves the distance at which a sound is just audible, so a 20 dB cut shrinks that distance to a tenth, but only if exhaust and heat are handled safely.
Part of Camouflage and Concealment
Activity noise control is the reduction of sound from chopping, hammering, sawing, building and engines, using timing, padding and screens, so that a worksite is not audible far from the site.
A Dull Thud Carries Less Than A Sharp Crack, And Distance Does The Rest
The parent article gives chopping as audible at 500 m or more and a generator at 1 km or more, and advises scheduling noisy tasks during wind, rain or midday, muffling impacts with a cloth between hammer and work, and putting a generator in a pit or behind sandbags and earth walls. It also says that a generator audible to you is audible to others and asks whether the power is worth the exposure. That last question is the most important one; the best noise reduction is not running the machine.
The physics sets the limit. Noise Makers states the rule for open air: a sound falls by about 6 dB for every doubling of distance and 20 dB for every tenfold increase. The same rule works backwards. If a barrier or a muffler lowers a sound at the source by some number of dB, the distance at which it falls to the same level at the listener shrinks by the factor 10 raised to (dB ÷ 20).
What You Need
- Padding — folded cloth, leather, sacking, turf or a rubber pad
- Earth, sandbags or a thick bank — to screen a machine on the side facing listeners
- A ventilated, fire-safe enclosure plan if an engine must run
- A calibration helper — someone to walk away until the job can no longer be heard
Method: Cut, Screen, Measure
Step 1: Remove the job if possible. Hand tools, a different time, or stored work done earlier replace a machine that carries a kilometre.
Step 2: Schedule by background. Do the job when wind, rain, running water or midday thermal currents add their own noise.
Step 3: Pad the impact. For hammering and chopping, put a cloth, turf or leather between the tool and the work and, where possible, a mat under the work. This turns a sharp crack into a dull thud, as the parent article says.
Step 4: Screen an engine on the listener’s side, not all round. A bank of earth or a wall of sandbags between a machine and the likely listeners cuts the line-of-sight sound. Leave the machine open to air from above and from the back, so that heat and exhaust leave. A machine in a tight pit overheats and fills the pit with exhaust.
Step 5: Measure, do not guess. Walk away from the job in several directions until you can no longer hear it, and record each distance. Repeat after each change.
Worked Example: What A Given Cut Buys
Free-field sound loses 6 dB per doubling of distance, so the distance factor for a cut of N dB is 10^(N ÷ 20).
- A 6 dB cut: 10^(6 ÷ 20) = 10^0.3 = 2.0. The audible distance halves, so a job heard at 500 m is heard at about 250 m.
- A 10 dB cut: 10^(10 ÷ 20) = 10^0.5 = 3.16. A job heard at 500 m falls to about 158 m.
- A 20 dB cut: 10^(20 ÷ 20) = 10^1 = 10. The audible distance falls to a tenth, so a job heard at 500 m falls to about 50 m.
Area scales with the square of distance, so the 20 dB cut reduces the area in which the job is audible to one hundredth. These figures assume open air, no wind effects and no reflection from buildings, and real conditions bend them: wind carries sound downwind, a hill or a bank blocks it, and a reflecting wall can add to it. The parent article gives a generator-pit reduction of 50-70%, but does not say whether that is a percentage of sound pressure, of loudness or of distance, so it cannot be converted into decibels honestly; calibrate by walking instead.
Common Mistakes
- Closing the engine in. An engine in a sealed box overheats and poisons the box with exhaust.
- Screening the wrong side. A bank on the quiet side does nothing for the listeners.
- Padding only the tool. The bench or log it strikes also rings; pad that too.
- Working in the evening quiet. There is no background noise to hide it.
Safety
Engines in pits kill: exhaust, heat and fuel
An engine running in a pit or a covered hole fills it with carbon monoxide, which is colourless, odourless and deadly; see Carbon Monoxide Risks. Never run an engine in a covered or closed space; duct the exhaust well away from where anyone sleeps and above the level of any pit. Keep fuel away from the machine and from hot exhaust, never refill a hot or running engine, and keep sandbags and earth clear of the exhaust pipe and cooling air. Do not climb into a pit where a machine has been running until it has been aired out.
What’s Next
- Learn next: Voice and Communication — silent ways to coordinate the work
- Combine with: Noise Makers — the same sound-distance rule used in the opposite direction, to make an alarm heard