This question deals with the way sound waves move or more precisely bounce. Since the most common idea for this is a seashell we will work with this model. We are going to imagine we are in a normal room. There are other noises around.. maybe people talking in the other room or cars driving by outside.
Sound moves in waves. Let's imagine the sound of the car. Sound waves come toward us from that car. Some bounce off of our body, some enter our ear, some pass us by and some, a small few, enter the shell we are holding near our head. The sound waves then resonate within the shell. Resonance means that the air and the shell itself are now vibrating at the same frequency as the sound of the car. Those vibrations of the shell then make it to our ear and we hear sound coming from it.
The interesting thing is that different shells sizes, shapes and shell thicknesses will change the frequency of the sound waves and therefore change the sound we hear. The same experiment can be done with cups of different sizes and materials. The one trick is to find an area with a constant sound to ensure that the frequency of the item vibration is different and not the frequency of the incoming sound. You can also produce a similar effect by cupping your hands over your ear at different distances from your ear. Angle of the object (particularly cup shaped objects.. cups, glasses, coconuts, etc) to your ear can also alter the sound you hear.
There are apparently even relaxing earplugs you can purchase? that replicate this phenomenon.
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