Active sonar is a technology used to detect objects underwater by transmitting powerful sound pulses and then analyzing the returning echoes. It is employed by military and commercial vessels for navigation, mapping, and target detection. When a human is exposed to the high-intensity sound pulses from active sonar, particularly in the water, the resulting pressure waves can cause immediate physiological trauma. The severity of the damage depends directly on the sound intensity, the frequency of the pulse, and the proximity to the source.
Understanding Sonar’s Physical Impact
The danger posed by high-intensity active sonar originates from the sheer power of the pressure waves it generates. Water is an extremely efficient medium for sound transmission, allowing these waves to travel great distances with less energy loss than in air. Military-grade sonar can produce sound pressure levels that exceed 235 decibels (dB) at the source.
Sound intensity is measured on a logarithmic scale, meaning a small numerical increase in decibels represents a massive exponential increase in physical pressure. For instance, a 10 dB increase signifies a roughly tenfold increase in sound power. At close range, this intense acoustic energy acts less like typical sound and more like a shockwave, imparting mechanical force directly to the body’s tissues.
Sonar systems utilize a broad spectrum of frequencies, ranging from very low (infrasonic) to extremely high (ultrasonic). Low-frequency sonar travels farther and is more likely to create large-scale pressure fluctuations that impact the body’s gas-filled spaces. High-frequency sonar delivers energy more rapidly and can cause localized damage to sensitive structures.
Immediate Effects on Hearing and Balance
The human ear and vestibular system are the most common and immediate sites of injury from sonar exposure. The intense pressure waves from a sonar pulse can easily exceed the threshold for pain, which is typically around 120 dB in the water. Exposure to levels above 100 dB can initiate damage to the delicate structures of the inner ear.
A common initial effect is a Temporary Threshold Shift (TTS), where hearing acuity is diminished but recovers over time. However, exposure to intense, close-range sonar can lead to a Permanent Threshold Shift (PTS), resulting in irreversible hearing loss. This occurs when the mechanical force destroys the hair cells within the cochlea, which are responsible for translating sound vibrations into neural signals.
Damage to these sensory cells often leads to chronic tinnitus, perceived as a persistent ringing or buzzing sound. Beyond the cochlea, the sonar pulse also affects the adjacent vestibular system, which controls balance and spatial orientation. Disruption of the fluid dynamics within the semi-circular canals can cause severe vertigo, dizziness, and profound disorientation.
Such vestibular trauma is particularly dangerous for a person underwater. The loss of balance underwater, known as the Tullio phenomenon, can be triggered by intense sound stimulation, leading to nausea, vomiting, and a complete inability to maintain orientation.
Potential for Severe Internal Injury
In cases of acute, very close-range exposure to the most powerful military or research sonar, the mechanical force can translate into life-threatening internal trauma. The body is largely composed of incompressible fluid and soft tissue, but gas-filled organs are particularly vulnerable to rapid, extreme pressure changes. This effect is a form of barotrauma, or pressure injury.
At sound pressure levels around 200 dB, the vibrations become strong enough to cause structural damage to the lungs. The acoustic energy can rupture the delicate tissue of the air sacs, leading to pulmonary hemorrhage or lung collapse. These injuries are associated with the sound wave acting as a physical shockwave moving through the chest cavity.
A second, more localized mechanism of injury is acoustic cavitation. This involves the formation and violent collapse of microscopic gas bubbles within the body’s tissues and fluids. The intense negative pressure phase of the sound wave causes these bubbles to form, and their subsequent collapse releases a burst of energy, creating micro-jets and shockwaves.
Cavitation can tear tissue at a cellular level, causing localized bleeding and tissue destruction. It is especially damaging at the interface between tissue and gas, such as the lining of the gastrointestinal tract and the lungs. Exposure to levels approaching 210 dB has been theorized to cause hemorrhaging in the brain tissue.