Hear the sea in the shells

Not all noise is noise pollution.

The sea offers gentle and relatively constant auditory stimulation, which can help to soothe. This effect is enhanced if the sea is associated with pleasant memories, such as a holiday by the sea.

But why do we hear this soothing sound when we place a shell close to our ear?

This article explains the acoustic principles behind this phenomenon.

If you can hear the sound of the sea by placing a shell close to your ear in a calm environment, it's because the shell acts as an acoustic resonator that filters and amplifies ambient noise. It transforms ambient sounds into a melody that evokes the sound of the waves.

Shellfish: a natural acoustic resonator

The shell acts as a natural acoustic resonator.

When a sound penetrates the shell, some of its frequencies are amplified depending on the shape and size of the shell's internal cavity. The frequencies amplified by common shells are the medium and high frequencies. Exceptionally, if the shell is very large, it can also amplify the bass. Generally, however, the shell only amplifies certain frequencies in the midrange and treble range.

Depending on the shape of the shell, it behaves like a Helmholtz resonator, a simple reflecting wall, or a quarter-wave resonator. These behaviours can be reproduced by replacing the shell with a beer bottle, a flat hand or a glass.

The inevitable ambient noise

Ambient noise is omnipresent in our environment, even when we think we're in a quiet place. Whether it's a gentle breeze, the hum of electronic equipment, or even the sounds produced by our own bodies (the sound of blood flow or breathing), there is always some background noise.

This ambient noise is often ignored, but it can become perceptible if it is amplified by an active system (microphone + loudspeaker) or a passive system. In addition to shells, other well-known passive amplification systems are the sonotone or the body of a guitar.

Ambient noise filtered and interpreted

When you place a shell close to your ear, the ambient noise is filtered and amplified by the structure of the shell. If the ambient noise is, for example, the latest Céline Dion song, placing the shell close to your ear will modify the equalisation of the song, but you will still hear a song and not the sound of the sea.

In a calm environment, ambient noise is generally broadband, covering a wide range of frequencies. If it comes from the wind or from our own bodies, it presents slight variations at a slow rate. Once filtered and amplified by the shell, which amplifies mainly the medium and high frequencies, this noise resembles the sound of waves. In fact, wave noise is also made up of medium and high frequencies with slight variations.

Because of this similarity, we often interpret the noise filtered and amplified by a shell as that of the sea. This phenomenon is a form of auditory pareidolia, where the brain associates relatively random sounds with known patterns. Pareidolia is a psychological phenomenon in which the mind perceives shapes, sounds or meanings where none actually exist. In this case, our brain transforms the noise heard near the shell into a soothing, familiar sound: that of the waves of the sea.