Editing Sonar (section)

== Frequencies and resolutions ==
The frequencies of sonars range from infrasonic to above a megahertz. Generally, the lower frequencies have longer range, while the higher frequencies offer better resolution, and smaller size for a given directionality.

To achieve reasonable directionality, frequencies below 1&nbsp;kHz generally require large size, usually achieved as towed arrays.<ref name="navalweapsyst">{{Cite book|url=https://books.google.com/books?id=l-DzknmTgDUC&q=sonar+frequency+kHz&pg=PR25|title=The Naval Institute Guide to World Naval Weapons Systems, 1997–1998|url-status=live|archive-url=https://web.archive.org/web/20180426001103/https://books.google.com/books?id=l-DzknmTgDUC&pg=PR25&dq=sonar+frequency+kHz&hl=en&sa=X&ei=m74xVcqEEMrraK3dgPgJ&ved=0CC4Q6AEwAw#v=onepage&q=sonar%20frequency%20kHz&f=false|archive-date=2018-04-26|isbn=9781557502681|last1=Friedman|first1=Norman|year=1997|publisher=Naval Institute Press }}</ref>

Low frequency sonars are loosely defined as 1–5&nbsp;kHz, albeit some navies regard 5–7&nbsp;kHz also as low frequency. Medium frequency is defined as 5–15&nbsp;kHz. Another style of division considers low frequency to be under 1&nbsp;kHz, and medium frequency at between 1–10&nbsp;kHz.<ref name="navalweapsyst"/>

American World War II era sonars operated at a relatively high frequency of 20–30&nbsp;kHz, to achieve directionality with reasonably small transducers, with typical maximum operational range of 2500 yd. Postwar sonars used lower frequencies to achieve longer range; e.g. SQS-4 operated at 10&nbsp;kHz with range up to 5000 yd. SQS-26 and SQS-53 operated at 3&nbsp;kHz with range up to 20,000 yd; their domes had size of approx. a 60-ft personnel boat, an upper size limit for conventional hull sonars. Achieving larger sizes by conformal sonar array spread over the hull has not been effective so far, for lower frequencies linear or towed arrays are therefore used.<ref name="navalweapsyst"/>

Japanese WW2 sonars operated at a range of frequencies. The Type 91, with 30 inch quartz projector, worked at 9&nbsp;kHz. The Type 93, with smaller quartz projectors, operated at 17.5&nbsp;kHz (model 5 at 16 or 19&nbsp;kHz magnetostrictive) at powers between 1.7 and 2.5 kilowatts, with range of up to 6&nbsp;km. The later Type 3, with German-design magnetostrictive transducers, operated at 13, 14.5, 16, or 20&nbsp;kHz (by model), using twin transducers (except model 1 which had three single ones), at 0.2 to 2.5 kilowatts. The simple type used 14.5&nbsp;kHz magnetostrictive transducers at 0.25&nbsp;kW, driven by capacitive discharge instead of oscillators, with range up to 2.5&nbsp;km.<ref name="USNTMJ-200B-0343-0412"/>

The sonar's resolution is angular; objects further apart are imaged with lower resolutions than nearby ones.

Another source lists ranges and resolutions vs frequencies for sidescan sonars. 30&nbsp;kHz provides low resolution with range of 1000–6000 m, 100&nbsp;kHz gives medium resolution at 500–1000 m, 300&nbsp;kHz gives high resolution at 150–500 m, and 600&nbsp;kHz gives high resolution at 75–150 m. Longer range sonars are more adversely affected by nonhomogenities of water. Some environments, typically shallow waters near the coasts, have complicated terrain with many features; higher frequencies become necessary there.<ref>{{Cite book|url=https://books.google.com/books?id=swDJtOj6yRIC&q=sonar+frequency+kHz&pg=PA105|title=Ships from the Depths|url-status=live|archive-url=https://web.archive.org/web/20180426001103/https://books.google.com/books?id=swDJtOj6yRIC&pg=PA105&dq=sonar+frequency+kHz&hl=en&sa=X&ei=m74xVcqEEMrraK3dgPgJ&ved=0CEkQ6AEwCA#v=onepage&q=sonar%20frequency%20kHz&f=false|archive-date=2018-04-26|isbn=9781603442183|last1=Søreide|first1=Fredrik|date=2011-04-28|publisher=Texas A&M University Press }}</ref>