The fundamental choice of which frequency to use for an active sonar is a trade-off:

• lower frequencies allow detection at longer ranges
• higher frequencies allow better resolution.

Modern ship sonars (such as the SQS-53 on Arleigh Burke class destroyers) generally operate at ~3kHz. This provides detection ranges of approximately 20,000 yards (for a large target - such as a submarine). This frequency is within the audible range for humans (20 Hz - 20 kHz).

The wavelength (which is a function of the frequency) determines how often we get a “look” at our target image during a certain time period. The distance between each crest of the waves is the period. Imagine, instead of a wave, that the crest of each wave is a short pulse, or a flash of light. For a low frequency, the flashes might appear to us like a strobe light, and the image we see would be “jerky”. If the frequency is really low, the image could completely jump from one place to another. In fact, if the subject is moving quickly, we may never even see it.

As we speed up the flashes of light, the apparent movement of our target image will get less “jerky”, and will appear more normal. If we made the flashes of light really fast, we could get fabulous resolution. This is the principle of high speed cameras.