Acoustic Bubble Pulse Energy
By Bruce Rule - August 28, 2013
The following, quoted from Chapter TWO of WHY THE USS SCORPION (SSN-589) WAS LOST, is provided for background information on the the phenomenon of bubble-pulse produced acoustic energy.
A bubble-pulse frequency can be created either by an explosion or by an implosion. In the case of SCORPION, a bubble-pulse frequency (4.46-Hz) detectable at long-range was created by the implosion (collapse) of the submarine’s pressure hull at 1530-feet. The energy associated with a collapse event (implosion) is produced by the essentially instantaneous conversion of potential energy (external sea pressure) to kinetic energy as water expands at supersonic velocity into the relative vacuum of a submarine pressure-hull. The intruding water “meets” at a “focal point” within the collapsed structure where it creates - in the case of SCORPION - a highly compressed (680 psi) “bubble” of super-heated air with sufficient energy (literally steam pressure) to force the water to ‘”rebound” until the falling pressure of the surface of the expanding “bubble” is overcome by ambient sea pressure. The bubble then collapses to rebound and collapse again multiple times until the energy is dissipated by friction with the water and, only in the case of an implosion, by the distortion (weakening) of the expanding-contracting pressure front when it encounters what remains of the collapsed structure.
Note that, contrary to the conclusion by the SCORPION Court of Inquiry, the acoustic signal produced by a collapse event bubble-pulse cannot be "swallowed" within the structure of a submarine pressure-hull because that structure is destroyed by the compression phase of the event and thus there is no structure left intact to contain (swallow) the expansion phase of the event and its acoustic signal.
The reciprocal of the length of time that is required for the first contraction-expansion cycle of the bubble is the frequency of the bubble-pulse. Example: in the case of SCORPION, the first contraction-expansion cycle required 0.224 seconds. A cycle of that length would repeat 4.46 times (1/0.224) in one second which was the frequency of the SCORPION pressure-hull collapse buble-pulse. A collapse event begins with a negative motion of the water inward followed by a positive motion of the rebounding water outward.
The primary source of noise from an underwater explosion or implosion is the pulsing (oscillation) of the bubble created by the energy release; hence, the term “bubble-pulse.”
Collapse event bubble-pulse frequencies can be as extremely low if they result from the release of great energy, e.g., the circa 3-Hz Russian submarine KURSK bubble-pulse (seismically detected at a range of 3100 statute miles) was the product of an energy release equal to about 10,000-lbs of TNT at a depth of about 320-feet. Note that in the case of the KURSK, the bubble-pulse was produced by the actual explosion of the entire weapons (torpedo) load and not by the collapse of the KURSK pressure-hull. Also note that the KURSK probably was in contact with bottom at the time of the explosion which enhanced the seismic transmission of the signal.
Is it possible to distinguish between bubble-pulse energy produced by an explosion and an implosion? Theoretically, if an event - or multiple events of both types - could be analyzed in ultra high temporal resolution (circa 0.001 seconds), characteristics that could permit this discrimination might be identified however, to the best of the writer's knowledge, this has never been done successfully.