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    Naval conflict has always been a concern for militaries around the world. Since World War II, submarines have been a particularly effective weapon against surface vessels carrying supplies, cargo, soldiers, and civilians. A great deal of effort has been expended in recent years to create new ways to accurately track enemy submarines for the purpose of avoiding or eliminating the threat that they pose. Unlike on land and in the air, where radar is used to track enemy units, seafaring craft track underwater submarines using sonar. There are effectively two types of sonar that are in use:

    • Passive Sonar: Passive sonar is essentially careful 'listening' to the sounds that are made by submarines and other underwater entities. Experienced sonar technicians, along with the aid of sophisticated computers, can identify individual submarines simply by listening to the sounds that their engines make. From the 1940's to the 1970's, passive sonar was used on a short-range scale by surface vessels and friendly submarines to detect the noise made by enemy submarines, usually powered by diesel engines. This lessened the threat of enemy attack, but only allowed a few minutes to respond to a submarine detection. In the 1970's, more sensitive passive sonar devices were developed that allowed sonar technicians to hear submarines at much greater distances, giving friendly ships much more time to react.

    • Active Sonar: Active sonar is the process of sending out a sound wave and listening to the echo it makes when it reflects off underwater objects such as enemy submarines. This is similar to the echolocation used by many animals, including some marine mammals and cave-dwelling bats. In submarine warfare, a submarine can send out an active sonar pulse called a 'ping' and listen for the echo, therefore detecting (at short range) other enemy submarines - even those who are making no noise. Sending out a sonar 'ping' has one severe disadvantage: the submarine that sends out the 'ping' instantly reveals its own location to all nearby submarines and surface vessels when it does so. This puts the 'pinging' submarine at great risk.

    The Need for Low Frequency Active Sonar

    Unfortunately, the world's oceans are not entirely safe. While incidents of high-seas attacks on surface ships are rare, major trade routes could be left open to attack by submarines should a crisis occur. This threat is compounded by a new generation of 'ultra-quiet' submarines which are difficult if not impossible to detect with existing passive sonar devices. Of course, these submarines can be detected with active sonar, but not without revealing the source of the active sonar 'ping' first.

    Low Frequency Active Sonar (LFAS) attempts to change this by sending out relatively loud sounds into the ocean at low frequencies. In water, low frequency sounds travel farther than high frequency sounds. Furthermore, the louder the initial transmission of sound in an active sonar 'ping,' the further it will travel. By combining high volume with low frequencies, land-based or surface-ship based LFAS transmission stations can send out 'pings' throughout the ocean and listen to the reponses, just as a submarine might in close-quarter combat. However, because of the range of these 'pings,' the LFAS stations will be able to detect even ultra-quiet submarines hundreds of miles away, giving friendly vessels hours to avoid danger. Furthermore, because the LFAS transmission stations would be well inside friendly waters, the fact that they reveal their location through active sonar is somewhat irrelevant. Friendly vessels could use data gathered from LFAS stations to track enemy submarines, while keeping their own positions secret.

    The Physics of Sound and Sound Intensity

    As stated above, low frequency, high-volume sounds travel farther than high frequency or low volume sounds. However, very few people are concerned with the frequency of the sound being used in LFAS. Most environmental groups are highly concerned with the intensity of sound, as measured in decibels (dB), used by LFAS transmission stations. There are two relevant equations to determining the intensity, in dB, of a given sound:

    1. I = Pav / A
      where I is the sound intensity, Pav is the average power (in watts) of the sound, and A is the perpendicular area over which it is transported.
    2. B = 10 log (I / I0)
      where B is the sound's level in dB, I is the intensity as calculated in equation 1, and I0 is a constant, 10-12 W/m2 in air.

    As you can see, the intensity of a sound in dB is a logarithmic function of a ratio of intensities measured in W/m2. These intensities increase linearly with sound output power and decrease linearly with area over which the sound has dissipated. Simply put, the further away a listener is from the source of the sound, the quieter it is. A sound which has been dispersed over 100m2 will be twice as intense as a sound dispersed over 200m2. Sound intensity does not necessarily relate to 'loudness,' however. 'Loudness' is a function of the listener, and can be interpreted somewhat figuratively. The human ear, for instance, hears on a somewhat logarithmic scale, making decibels an accurate measure of 'loudness' for humans. As such, if sound A is 10 times more intense than sound B, it will seem about twice as loud to a human ear.

    A special note to add to the above equations is that the base constant, I0 is different depending on the media through which the sound travels. In air, as noted above, this quantity is 10-12 W/m2. However, in water, where LFAS sound is broadcast, this "reference quantity" is twenty times as high (20 x 10-12 W/m2). Therefore, sound levels, in decibels, are lower in water than in air.

    How Intense is the LFAS Sound?

    According to the United States Navy, the sounds experienced by whales involved in LFAS testing will never exceed 155 dB, partly due to the fact that no testing will occur when whales are within 1000m of the LFAS transmitting ship. Intensity of sound, the I quantities referenced in equation 1 above, can also be affected by the pressure of a sound wave, which is somewhat different in water than in air due to the different bulk modulus (a quantity related to density) of water. As such, it is difficult to compare a 200 dB sound in air, which is extremely loud and potentially damaging, to a 200 dB sound in water, other than to work from the energy used to create both sounds. In air, about 1000 kW of power is required to generate a 200 dB sound, whereas only 1 kW of power is required to generate a 200 dB sound in water. Unfortunately, the Navy's claim that a 200 dB sound in air is 100 times louder than a 200 dB sound in water is not entirely accurate due to their use of the word 'louder.' A 200 dB sound in air is 100 times more intense than a 200 dB sound in water, but only about 3 times louder assuming that underwater creatures like whales have a logarithmic ear similar to that of a human's.

    The U.S. Navy claims that whalesong is in the 170 dB (water at source of sound) range, and therefore the LFAS transmissions, at only 155 dB (water at 1000m), which is roughly equivalent to 195 dB (water at source), would not be unreasonably loud. Assuming that the Navy's claim about the intensity of whalesong is accurate, this would seem to be a logical argument. However, only the LFAS tests were being run at 155 dB (water at 1000m); environmental groups are concerned that the source intensity of LFAS in practice might be as high as 235 dB (water at source). If the source sounds were broadcast at 235 dB then the sound would be drop to 155 dB (water) at approximately 100,000m, or about 57 miles. The Navy does not comment on their web site about testing sound levels vs. actual sound levels.

    The Effects of Sound Levels on Whales and Other Marine Life

    It must be noted that the communications and hearing systems used by whales and other marine life are not totally understood; this is one of the key causes for the debate surrounding LFAS. It may turn out that whales and other marine life forms have an incredible tolerance for loud sounds, or it may turn out that their hearing is much more easily damaged than we have anticipated. Only through testing and research can these questions be answered. Of course, eliminating the use of LFAS altogether would eliminate the need to ask these questions, at least in the immediate future. It seems that the Navy and several prominent environmental scientists and biologists have attempted to answer this question through testing in the Pacific Ocean. Their testing has been controlled to help protect the whales and other marine animals involved, as noted above. The LFAS sounds used in the tests, again, should not exceed average levels of sound heard in whalesong, but it must be remembered that our lack of understanding about whale hearing and behavior prohibits us from making an entirely accurate guess regarding LFAS sound and its effects on marine life. The Navy's reports on LFAS sounds and their effects on the behavior of marine animals have yet to be released.
    Eric M. Dashofy, emdashof@uci.edu
    Last Updated 4/22/98