Underwater explosions can create large sound pressures. With an explosion, there is an extremely rapid conversion of a solid or liquid into gases with high temperatures, pressures, and volumes. This conversion creates a “shock wave” that is characterized by an exceptionally high pressure that is almost instantaneous (rise time on the order of microseconds), followed by a slower decay to a pressure below ambient. The speed at which the shock wave travels initially (detonation velocity) depends upon the explosive used and the medium through which it is traveling but, as an example, for TNT the velocity in seawater is approximately 6900 meters per second (versus approximately 1500 meters per sec for sound underwater). A sound wave does not have sufficient energy to produce the blast injuries described below.
The figure below shows an underwater explosion in which an oscillating gas bubble is created (bottom half of the figure) with the associated quick rise time in pressure (top half of figure), creating a shock wave, followed by the decay to a pressure below ambient (the horizontal line in the top half of the figure). This sequence of events can also be seen in the video that shows an explosion and two subsequent bubble pulses.
The rapid changes of extremely high pressures followed by lower than ambient pressures can produce multiple types of physical injury to animals and destruction of inanimate objects. The effects are called blast injuries and are commonly referred to as:
- Primary: direct effects from the shock wave itself on body tissues;
- Secondary: body injuries from objects blown apart by the shock wave; and
- Tertiary: injuries from bodies being thrown into stationary objects.
Depending upon several factors, like distance from the blast or animal size, the injuries can range from lethal to non-lethal, such as breakage of parts of the ear (auditory trauma) in mammals.
Blast injury occurs in the immediate vicinity of a high-pressure sound source. The ability of the shock wave to shatter objects, including animals, is referred to as brisance which is related to the massive detonation pressures. This is one type of blast injury. Like the shock wave velocity, brisance varies with the type of explosive. Unable to respond mechanically to the sudden, extreme pressure, stiff tissues like bones may fracture. Another type of injury is related to the drop in pressure that follows the peak of the shock wave. This drop in pressure can cause gas-containing structures to rapidly expand (e.g., lungs, swim bladders, and middle ear air spaces in some species). This rapid expansion can exceed the ability of surrounding tissues to adapt, resulting in tears and bleeding.
Blast injuries to ears and hearing in mammals can range from mild to severe. In addition to these direct shock wave blast injuries, animals can also have harm to their auditory structures from the high sound pressure levels of the associated sound wave (acoustic trauma). These injuries may produce temporary or permanent hearing loss.
From experiments with mammals and explosive events on land, scientists know that the ears are among the most sensitive tissues for pressure-induced damage. Eardrum (tympanic membrane) rupture is common to all stages of blast injury; i.e., the eardrum is typically the most sensitive tissue to blast pressures and eardrum rupture can occur at great distances from an explosion. Blast damage to larger, stronger tissues, like lungs and bones, generally occurs closer to the blast source. It has also been found that injuries are inversely proportional to body mass; that is, smaller animals sustain more severe blast injuries than larger animals at equivalent received pressures.
All marine mammals have special adaptations to protect against the pressure changes associated with diving. Despite these adaptations to minimize damage from the pressure changes in a typical dive, marine mammals are not immune to blast injury, in large part because blast pressures far exceed the pressures of a dive and the time course of a shock wave is much faster than pressure changes during a dive to which the animals are adapted. All marine mammals have retained air-filled middle ears. Therefore, just like humans and other land mammals, the ears of marine mammals are the most sensitive structures to both blast injuries and acoustic trauma.
Blast injuries that have been observed in marine mammals range from minor (such as surface contusions and lacerations) to severe (broken bones and extensive hemorrhages) to lethal (concussions, lung collapse, and major organ disruption)Ketten, D. R. (1995). Estimates of blast injury and acoustic trauma zones for marine mammals from underwater explosions. In R. A. Kastelein, J. A. Thomas, & P. E. Nachtigall (Eds.), Sensory Systems of Aquatic Mammals. De Spil Publishers.. Some severe effects are the result of rapid, extensive gas bubble oscillations causing breaks in capillaries and even larger vessels as well as creating tears in the membranes of gas filled spaces. In mammals, bubble or gas formation related injuries occur typically in the lungs and airways but can occur catastrophically in the brain.
Like mammals, fishes may also be subject to damages. Swim bladders in fishes are the most sensitive structures to blast injuries. It is commonly observed during blast fishing that fish will float to the surface immediately following exposures to even relatively small explosive charges, having lost their buoyancy control with damage to the swim bladder. This is due to a rapid expansion of the swim bladder in the low-pressure phase of the shock wave.
Another form of injury related to gas filled spaces is barotrauma. Barotrauma is not a direct blast injury nor acoustic in origin. Instead, it is the result of an inability to equalize pressures between the environment and an air-filled area of the body. This can happen in air (such as in planes descending or ascending) or in water (such as in lungs or middle ears when scuba diving), but it is usually a gradual, slower event that can sometimes be prevented by equilibrating pressures as needed. This is a far slower process and at much lower pressures than a blast related shock wave.
Additional Links on DOSITS
- Science > Sound > What happens when sound pressures are large?
- Animals > Effects > Marine Mammals > Hearing Loss
- Animals > Effects > Fishes > Hearing Loss
- Animals > Sound Reception > Marine Mammals > Hearing in Cetaceans
- Audio Gallery > Explosive Sound Sources
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|⇡1||Ketten, D. R. (1995). Estimates of blast injury and acoustic trauma zones for marine mammals from underwater explosions. In R. A. Kastelein, J. A. Thomas, & P. E. Nachtigall (Eds.), Sensory Systems of Aquatic Mammals. De Spil Publishers.|