Now that the DVLA is all tucked away and listening under the ice, we need to play it some music! …well, we need to play it something anyway. We have two sound sources, the J-15 and the HLF-5, and we are lowering one down in the water and transmitting sound on and off for about a day and then lowering the other one down into the water and transmitting. We did this first at 50 km from the DVLA and then 100 km from the DVLA and now we are on our way to a spot 200 km away. We are going to see how far we can get before our time is up on this research cruise.
One reason that we are doing this is because we want to know how far sound travels in the Arctic Ocean and how what we receive on the hydrophones changes depending on the distance that we are transmitting over. Because the sound travels from the source through a bunch of water to the hydrophone receiver, the sound that we receive on the hydrophone can tell us a lot about the water through which the sound traveled.
Sound travels a lot farther and faster underwater than it does in air, but we also have some tricks up our sleeve to actually “hear” the sound at these long ranges. The J-15 and the HLF-5 sources are sending coded signals called m-sequences over and over again. The J-15 sends signals at around 75 Hz and 125 Hz and the HLF-5 sends signals at 250 Hz, which is about a middle C on the piano – actually a pretty flat middle C for the musicians out there! After we receive the sound, we decode the signals and separate all of the different repetitions of the signal. We then add all the different repetitions of the signal together to get a bigger signal. This makes it easier to pick the signal out from the background noise. This way we can get a signal to appear louder without actually playing the source louder. We call this signal processing gain.