assumptions. still guesswork in there. >> a lot of guesswork. changes with heat, cool, warm. >> once you figure that out, you deal with the question what are the things we're hearing? >> let's listen to the sountd first so everyone knows what the sound is we're talking about. so very faint. you can hear that. that's displayed at a sound we can audibly hear. this is supposed to be at 37.5 kilohertz. keep in mind this sound may not be as strong. they may not be able to hear this sound. of course, a lot of things change sounds under the sea. most importantly, this sound under the water is emitted by a lot of equipment. it's not just the black boxes that emit the sound. we've been talking to a lot of experts saying there's a lot of things the militaries use, the divers use that have this exact frequency at this exact same