Had the same test a few times Louis the same as a lot on here. There is a certain standard for normal hearing ,sonar operators are a bit above that level it’s the ones with exceptional hearing and can point the finger what the noise is probably between different whale and dolphin species and other sea clutter . I thought underwater telephones was a joke at one time until had to use one and it’s surprising how clear it can be , noise can travel through different density’s of water and can be quite clear and amplified considerably .JS