I have no idea what sonar frequencies are what, but to expand on your point, sonar is basically the only kind of sensor that works underwater. Radar and sight are basically useless.
Sonar is really all you have to see underwater, and is used heavily for scientific purposes, not only military purposes. Ocean-floor mapping is done with sonar. Active sonar of the kind that hurts whales is not in frequent use by deployed military vessels on military or surveillance missions because, while it gets you a good picture, it also announces "HERE I AM!!!" to the entire world.
I am not knowledgeable on acoustics, let alone acoustic dissipation or acoustics in water - but I can see logic behind the argument that LFA sonar causes a problem for whales.
I did a little digging, and it seems that it's not really appropriate to compare underwater decibel measurements with decibel measurements in air. For example, [0] discusses whales that generate clicks at 230 dB, and suggests that that is "equivalent to 170 decibels on land."
I'm not sure if it's even possible to get to 240 dB in air... Ah, an article on wikipedia sort of backs me up: [1] says that, at 1 atmosphere of pressure in air, the maximum loudness for undistorted sound is around 194 dB. Anything louder would be a shockwave.
> Sonar is not a weapon; it's a sensor system intended to discover enemy submarines.
When a friend was working on sonar software they had extremely rigorous safety controls in place when working in the same room as a test rig. Pings could kill you. At a minimum you would shatter your ear drum.
The medium is different. Sound travels 4 times further and faster in water than air. Dolphins even use sound to stun their prey.
Sound in water is a more effective weapon. The ping of a sonar is a weapon within the medium of water. A quick search:
"There are no noise-cancelling headphones to stop the U.S. Navy's 235-decibel pressure waves of unbearable pinging and metallic shrieking. At 200 Db, the vibrations can rupture your lungs, and above 210 Db, the lethal noise can bore straight through your brain until it haemorrhages that delicate tissue. If you're not deaf after this devastating sonar blast, you're dead."
Sonar is really all you have to see underwater, and is used heavily for scientific purposes, not only military purposes. Ocean-floor mapping is done with sonar. Active sonar of the kind that hurts whales is not in frequent use by deployed military vessels on military or surveillance missions because, while it gets you a good picture, it also announces "HERE I AM!!!" to the entire world.