US Navy LFAS Signal Test


The U.S. Navy was experimenting with a long range underwater sonar system which is to be used to detect the presence and position of submarines deep in the oceans of the world. It uses a low frequency signal that can travel great distances and presumeably bounce off targets that are hundreds of meters below the ocean surface.

Unfortunately the effects on ocean life are not known. Whales use similar low frequency sounds for communications and navigation. So a great controversy arose as to the risk vs benefits of such a system.

You can hear the LFAS (Low Frequency Active Sonar) test signal by clicking the play button on the audio bar. You’ll immediately notice the similarity of LFAS to whale sounds. To analyze the sound itself we have provided spectrograms which show sound intensity and frequency. This was produced using the software that is supplied with our DolphinEar hydrophone system.

The top chart shows how the strength or amplitude of the sonar signal varies over a 3 second period. It starts off at moderate levels, increases to a maximum, then tapers off again.

The bottom part of the chart shows how the frequency varies slightly. The test signal is contained in the range of about 160 Hz to 180 Hz. As you listen to it you will hear how it varies pitch slightly. The colors represent intensity. The RED lines are the strongest, then YELLOW, CYAN, BLUE, all the way to BLACK which represents no signal at all. The audio levels have a dynamic range of about 87 decibels in this example. The pattern seems to repeat in this example about every 3 seconds.

Presumeably, computers are able to analyze this signal and determine if there are ‘unidentified’ submarines in the area.

These transmissions are well within range of the DolphinEar hydrophone system and can easily be heard in the earphones, or recorded, if you are ‘lucky’, or ‘unlucky’, enough to be in the test area.