Why do we hear the noise of the sea when we apply the sink to the ear? Well, let's start in order: it doesn't matter how much this sound is similar to the noise of rolling waves, this, of course, is not the noise of the sea.
But the question still remains: what exactly are we hearing in the shell? In a word - this is noise; The noise within us and the external noise, which we usually do not hear or do not pay attention to it, because it is too quiet.
To strengthen this noise, you will need a resonator. The easiest you can create yourself. Open the mouth in the form of the letter "O" and knock your finger on the throat or cheek. You will hear a note. If you increase or decrease the form "O" or change the form of the mouth, then you will receive different notes. In this case, your mouth acts as a Helmholtz resonator, in which the sound is created by air vibrating in the cavity with one hole. By changing the form of a resonant cavity, you can get different tones.
Perhaps, at this moment, you already think to close the article - after all, the question was about the sea and shell, and not about some kind of resonator Helmholtz. But in fact there is nothing complicated in it. The resonator is a spherical vessel with a hole in the neck. Impacting the resonator, we increase the pressure inside the cavity and force the air "shrink". Then the air begins to "flow" back, and the pressure in the cavity falls, forcing the air to "fly inside" again. The emerging oscillations have a much greater amplitude than fluctuations in the affecting field. Visptiously the work of the Helmholtz resonator is shown on the video below.
It is interesting: did you know that the resonators are sold in huge quantities in ordinary grocery stores? It is enough to buy any water bottle and, having freeing it from the liquid, pour perpendicular to its neck. Hear a buzz? It makes airbag in the neck.
With the marine sink, the same thing happens as with a resonator of the Helmholtz. The noise we mentioned above, in the form of air moving inside and outside the sink, the blood circulating in your head, the conversation in the next room - all this resonates inside the shell cavity, is enhanced and becomes loud so that we can hear it. Also, as different molds of the mouth create different tones, different sizes and shapes of the sink sounds differently, since different resonant chambers enhance different frequencies.
The fact that the sound of all seashells is a little resembles the noise of the sea, a pure randomness. If you bring to the ear any item working on the principle of the Helmholtz resonator, then hear similar sound, regardless of whether this object is connected with the sea or not. Put an empty glass to the ear or simply attach a palm to it, leaving the cavity between its surface and ear, and you will hear exactly the same sound.