Fictional Sonar System, how much power.

[Graham1973]

In the 'Quackers' thread I mentioned originally I'd been trying to confirm if a fictional high-powered active sonar system I'd seen in a novel had any basis in fact.

This is the fictional system.

...an ultra-low frequency sonar net called the Avacha Sea Monster had been developed to sanitize the sea of invading Americans.

Huge sonar transmitters the size of rail cars were seeded across the ocean bottom off Petropavlosk, extending south to Cape Lopatka, north to Cape Kamchatka. When the Sea Monster roared, it emitted a rich deep bellow powerful enough to boil seawater."

Typhoon by Robin White, pg 344

Later on it is described as producing a 100 hertz pulse with a sound level of 243 decebels at a range of 200 miles (321.9km) from the emitters.

The author does show that he's considered the practical limitations of this system by having a character point out that the power bill must be immense and there's a limit to how many pulses you can send out before the 'side effects' blind the sensors.

The interesting question are, how many transmitters would actually be needed and how much power?

 

[aam641]

Have a look here for calculating the power required.
http://www.animations.physics.unsw.edu.au/jw/sound-impedance-intensity.htm

 

[Hobbes]

Sound levels are limited by the air or water pressure at the transmitter. A sound wave is a series of high and low pressure areas that travel. In air at 1 bar, the low pressure areas cannot achieve less than 0 bar. A conventional sound producing mechanism (loudspeaker, sonar transducer etc) will produce pressure peaks and troughs at the same delta relative to the ambient pressure, so it'll never produce peaks of more than 2 bar. That's still quite a lot (something like 196 dB according to a quick search).

It's possible to create 180 dB in a small volume (a car interior) using 5-10 kW of conventional amplifiers and speakers.

It is possible to produce louder sounds than 196 dB if you only produce a peak (via an explosion). I found one quote for 210 dB from 1 ton of TNT measured at 250 ft.

Due to the logarithmic nature of sound levels, 243 dB at 200 miles is quite preposterous, but that's Robin White for you. 240 dB at 250 ft would require at least 100 tons of TNT (if it's physically possible at all). (10 dB more=10x the power required).

 

[Graham1973]

Sound levels are limited by the air or water pressure at the transmitter. A sound wave is a series of high and low pressure areas that travel. In air at 1 bar, the low pressure areas cannot achieve less than 0 bar. A conventional sound producing mechanism (loudspeaker, sonar transducer etc) will produce pressure peaks and troughs at the same delta relative to the ambient pressure, so it'll never produce peaks of more than 2 bar. That's still quite a lot (something like 196 dB according to a quick search).

It's possible to create 180 dB in a small volume (a car interior) using 5-10 kW of conventional amplifiers and speakers.

It is possible to produce louder sounds than 196 dB if you only produce a peak (via an explosion). I found one quote for 210 dB from 1 ton of TNT measured at 250 ft.

Due to the logarithmic nature of sound levels, 243 dB at 200 miles is quite preposterous, but that's Robin White for you. 240 dB at 250 ft would require at least 100 tons of TNT (if it's physically possible at all). (10 dB more=10x the power required).

Thank you for your reply.

I'd figured something was not right when the book describes the system making orange juice explode out of it's cup, but I didn't realise just how much power that would need at the distance.

 

[Sea Skimmer]

US Navy Low Frequency Active Array produces sound at 215 decibels at the source. Systems exist for underground seismic exploration that are much louder then this using arrays of air guns, they can hit 255 decibel. You need some rather large arrays to do this, LFA uses a giant underwater mast for example. Exploding TNT would produce sound in all directions, which vastly increases energy requirements compared to a directional system like sonar. An earthquake might be able to make 243 dB at 200 mile range, but I doubt anything else other then nukes could.

 

[sferrin]

One can't help but wonder how loud Tsar Bomb was.

 

[Lauge]

One can't help but wonder how loud Tsar Bomb was.

If a 50 MT thermonuclear device detonates, but no one is there to hear it, does it then in fact make a noise? :

Regards & all,

Thomas L. Nielsen
Luxembourg