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Sound generating bomb

The audio bomb

Many kinds of powerful devices are used in war or for civil purposes. It would be called a "bomb" when the release of energy is very sudden and linked to the detonation of some explosive. Like for example:
  • Simple blast devices, like grenades, that produce a shock wave.
  • Flash bombs that blind the enemy for a short while.
  • Smoke bombs, that produce a sudden massive amount of smoke. Most commonly they are phosphor bombs (not to be confused with phosphor bombs used to mutilate and inflict cruel and lasting wounds).
  • Schrapnel devices, that project pieces of metal all around. A modern version being the claymore, just metal spheres spread over a layer of high explosive. A gun would be a pricision one-schrapnel device.
  • EMP bombs (electromagnetic pulse). They contain an electric magnet, through which a high current is driven, creating a strong magnetic field. The magnet is exploded by high explosives and the magnetic field spreads around as an electromagnetic shock wave, destroying electronic devices.
  • Rock penetration bombs. They arrive on the target at such a high speed that the pressure of their nose melts any rock or concrete. The bomb penetrates the rock like if it was hard butter. It ultimately explodes deep underground.
  • Cutting bombs. A fuse of high explosive is paste in the shape of the hole that must be cut away in a door or wall.
  • Metal plastifying bombs. Either their pressure molds a metal plate to the shape of a shell and propels it at high speed, or the metal is pressed to a rod that extends forward at hypersonic speed due to the pinch effect.

Can a bomb play an audio signal? The idea would be a long explosive fuse whose diameter varies along the length. The variations of the diameter are comparable to the oscillations of the groove in a recorded audio disc. The explosion front speeding along the fuse, would produce a variating pressure on the surrounding air, like the needle of the disc player reads the record.

For example the fuse would unroll from a parachute and then would be lit from the bottom.

What can such an explosive device be used for?
  • The first idea was to broadcast "surrender!" loud out above an enemy army. Whatever the message broadcast, the force of it should have a strong psychological effect. It should be hearable deep inside bunkers.
  • A less sympathetic usage would be the generation of simple tones, whose frequencies would be aimed at structures, devices or people. That would for example allow to burst open precise objects, whose resonance frequency is known. It would allow to knock out people with less overall damage.

The difficulty lays in the audio frequency that can be attained.

A few kHz are needed to reproduce a human voice. Frequencies in the order of 10 to 100 Hz are typical to make structures resonate or cause physiological or psychological effects.

I read that a block of TNT of 500 kg would produce a positive pressure pulse af about 20 ms. Allowing another 20 ms for an optimal negative pulse, this yields a period of 40 ms hence a frequency of 25 Hz. That would be for packs of 500 kg of TNT placed along a fuse and the target being aside of the fuse and far from it. A typical quantity of explosive for this application would rather be less than a gram. Asuming that the volume of the sphere of hot explosion gas is proportional to the mass of explosive, the duration of the blast is proportional to the radius of the sphere and the radius is proportional to the cubic root of the volume, then a reduction of mass of 500 kg down to 0.1 gram yields an increase in frequency of 170 times. Then 25 Hz x 170 yields 4250 Hz. This makes the system seem possible (don't trust but verify).

Technical remarks:
  • The fuse need not necessarily change diameter. It can change composition, which alters the volume of gas produced and/or the speed of the explosion front.
  • Detonating fuses burn faster than the speed of sound in the air. This has several disadvantages: concentrated explosion fronts in some directions may be created, the pitch of the sound will strongly vary according to the direction and the record will even be played reversed as heard from some directions. So a slow explosive fuse should probably be used. Besides, a record of 1 second on a detonating fuse that has a speed of 1 km/s would need... 1 km of fuse.
  • The fuse need not necessarily be deployed. It can stay wrapped around a container.
  • Electronic regulation of the "soundtrack" is possible. The fuse can be molded while hanging from the parachute, like manufacturing a spaghetti with a variating diameter. Or a device can shoot explosions at a variating interval.

Shock wave canons were used during WWII. They proved to be not very effective. The ones I read about used compressed fuel and air. Using detonating explosives and higher frequencies and durations may make a difference but I don't know.

Eric Brasseur  -  November 30 2009