Let’s start by dispelling a popular myth: it’s not the sound of the blood in your ears. Try it and you’ll see that there’s no rhythm to the sound matching your pulse. The truth is that we are bathed in a shallow sea of sound all the time. Wind, electric hums, distant traffic, birds – all these things add to the mix and they all get combined into an ambient susurration with a fairly uniform distribution of frequencies called ‘white noise’. Because it is ubiquitous, our brains filter it out and we don’t normally hear it. But when you put a shell, or any other resonating chamber, over your ear, some frequencies get amplified and others get suppressed.
Given time, your brain would habituate to this new background mix and you’d stop hearing that too. But for a while it represents a novel sensation and so you can hear it. It sounds like the sea because that’s mostly white noise too.
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