Values for heavy shipping and sea state level 6 are of interest, since an operational sonar must be designed to perform against expected worst-case conditions. The lowest point on each curve corresponds to the optimum frequency, in the sense that it is the least amount of source power required for the system to achieve detection. Size and weight constraints for the source are a major factor in determining sonar system feasibility. For example, the experimental Low Frequency Array system weighed approximately 100,000 pounds, and required one megawatt of power-generation capacity. For every 3-dB increase in source level required, these numbers double.
Optimum frequency becomes lower as the detection range increases.
The upper-frequency cutoff is driven mainly by the absorption term in transmission loss. The engineering design of the array becomes more difficult at very low frequencies. For example, at 1000 Hz, a 20-dB array is approximately 250 feet long. The same gain array at 10 Hz would be 25,000 feet in length. Very short range acoustic sensors with detection ranges of a few kiloyards, such as mines, torpedoes, and air-deployed sensors, are typically utilized for final localization and prosecution.