Introduction

A bird scare gun is a relatively simple device which produces impulsive noise by means of a gas explosion. It is used for scaring birds away from areas where their presence is unwanted, like orchards, airfields, or oilfields. It is meant to operate automatically for long periods of time, with little or no human intervention. The construction is simple and robust, with as few as possible moving parts, so that a lifecycle in the order of 100,000–200,000 explosions is attainable.

The mechanism is simple. Acarefully controlled mixture of air and propane or butane gas (stoichiometric mixture, or a little bit richer than that) is periodically (every 5 or 10 minutes) blown into a semi-open pot, which is the combustion chamber. This pot is connected via a small diaphragm or iris (a small hole in the wall of the combustion pot) to an exhaust pipe. After ignition, the gas burns quickly (but without detonation, i.e. with a subsonically moving flame front) so that pressure and temperature increase quickly. This high pressure drives the gas out of the pot via a hot jet, which issues from the diaphragm into the pipe. Acting like a piston, this jet creates a pressure wave in the cold exhaust pipe. Part of the wave reflects at the exit, and part radiates, nearly spherically, away into the open air.

An interesting detail in the design gives, without any further analysis, insight in the gasdynamic behavior. In order to vary the noise that is produced, the length of the exhaust pipe is made variable. The pipe consists of two shorter pipes, one of which slides inside the other, like a telescope.