If you ever took a trip to the beach as a child, it's possible you will have been encouraged to hold a shell to your ear so you can "hear" the ocean. But why is it possible to hear sounds resembling the sea inside a shell? Are we somehow listening to noises from the shell's past, or is it something more easily explained?
"It isn't the sound of the sea," Trevor Cox, a professor of acoustic engineering at the University of Salford in the United Kingdom, told Live Science in an email. "But, as you're holding a seashell to your ear, it makes sense that people would think it might be."
So, if it isn't the sound of the sea you're hearing, what exactly is it?
"You are hearing ambient or background noise that has been increased in amplitude by the physical properties of the seashell," said Andrew King, director of the University of Oxford's Centre for Integrative Neuroscience and head of the Oxford Auditory Neuroscience Group.
Related: Why are there so many giants in the deep sea?
King explained that the "hard, curved surfaces" inside shells reflect soundwaves, causing the waves to "bounce around" inside the shell. Consequently, the shell "acts as a resonator, boosting certain sound frequencies, so that they are louder than they would be without the seashell placed next to your ear," King told Live Science in an email.
The frequencies you hear will depend on the size and shape of the seashell. If the seashell has an irregular shape, it will likely resonate at multiple frequencies, King said.
"The seashell is like a wind instrument," Cox said. "It has a set of resonant frequencies where the air inside the shell will vibrate more strongly. Hold the shell to your ear, and it is those frequencies in the ambient sound that get amplified. Because the sound changes, your brain pays attention to it."
According to both Cox and King, you don't actually need a seashell to hear a sound that replicates that of the ocean; you can get a similar experience at home simply by using a cup or bowl.
"The same effect is produced by placing other objects — or even, to a small extent, your cupped hand — next to your ear," King said. "What you will hear is, again, determined by the size and shape of the object."
However, King noted that "background noise must be present" for anything to be heard. "You won't hear anything in a completely soundproofed room," King said.
"If I go into Salford University's anechoic chamber, which is a completely silent room, I'd hear nothing, because there is no ambient sound," Cox said.
An anechoic chamber is a room specifically designed to achieve complete silence by preventing "the reflection of sound from the room boundaries," according to the University of Southampton (opens in new tab). These rooms, according to a 2018 CNN report (opens in new tab), are so quiet that, after a short period of time, an inhabitant would be able to hear their heartbeat and also their bones grinding or creaking, and would eventually lose their balance "because the absolute lack of reverberation sabotages your spatial awareness."
So, it is essential for background noise to be present to hear sounds within seashells, but this does raise a question: Given that the sound you hear when listening to a seashell is simply amplified background noise, when listening to a shell while beside the sea, are you actually hearing the sound of the sea?
"If you use a seashell at a beach, the ambient sound being altered by the shell is the sound of the sea. So, I guess you are listening to the sea indirectly," Cox said.
Originally published on Live Science on Feb. 16, 2011 and rewritten on July 22, 2022.