In fact, shell is the resonator, like any other closed air cavity. Therefore, "sea noise" can be heard not only in the sink, but in a simple mug, Cup, glass and even folded in a shell of the palm. In any such cavity focus external sounds. The world is not in absolute silence, there are always noises of different volume. These sounds are reflected by the walls of the shell.The volume and variety of "sea songs" depends on several factors. If you zoom out or Vice versa to bring the shell to your ear, the noise will change. This depends on the size and shapes of shells. This kind of resonator enhances all inaccessible to the human ear sounds. If the shell is tightly pressed to the head, one hears not external noise, while circulating in the head with blood.To the ear when nothing is attached, one hears a variety of external sounds. If your ear something prevents to capture the noise, the eardrum begins to perceive the internal sounds, ie circulating blood, which acts on the membrane from the inside of the ear. If the human brain was wired differently, we could hear much more sounds, and the shell would not be our helper in this. Best of all, hear the splashing of the waves" in a large spiral shells.If you hold a seashell to your ear is not close, and somewhat remote from it, the sound will be louder. Noise will also be more intense, if the outside of many different sounds. In any case, the splash that is heard in the sink is irrelevant to the sea. Theories related to the nature of these noises a lot, but the most reliable is the theory about the reflection of the walls of the shell external sounds.This theory is easy to check. If you hold a shell to your ear in a soundproof room, in the sink there will be no noise. Despite the fact that the head continues to circulate the blood, and the room has air flow.