https://en.m.wikipedia.org/wiki/Cetacean_stranding
I read somewhere it's about 240db. For reference, a jackhammer at about 1m (3.3ft away) is 130db and is classified as Painful for humans,
http://www.asha.org/public/hearing/Noise/
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'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.
[0]: http://news.nationalgeographic.com/news/2003/11/1103_031103_...
So 240dB is 10^(2.4-1.3) times as much sound power as 130dB.
y = 10 * log(x)
and the conversion back is
x = 10 ^ (y/10)
Every change of 3dB is a doubling or halving of power. Every change of 10dB is an order-of-magnitude change in power.