Why Can We Hear The Sound Of The Ocean When We Hold A Dry Seashell To Our Ear ?
Seashell
A seashell is a protective outer layer usually created by an animal or organism that lives in the sea.
They use mollusks to make shells to protect their soft insides. [14] You can see empty seashells washed up on beaches by beachcombers. The shells are empty because the animal has died, and the soft parts have decomposed or eaten by another organism.
A seashell is usually the exoskeleton of an invertebrate (an animal without a backbone) and is typically composed of calcium carbonate[14] or chitin. Most shells on beaches are the shells of marine mollusks, made of calcium carbonate, and endure better than shells made of chitin.
Apart from mollusk shells, other shells on beaches are those of barnacles, horseshoe crabs, and brachiopods. Marine annelid worms in the family Serpulidae create shells made of calcium carbonate cemented onto other surfaces. The shells of sea urchins are called tests, and the molted shells of crabs and lobsters are exuviae. While most seashells are external, some cephalopods have internal shells.
We use seashells for many different purposes throughout history and prehistory. However, seashells are not the only kind of shells; in various habitats, there are shells from freshwater animals such as freshwater mussels, freshwater snails, and shells of land snails. [5]
When I was young, it was fun and confusing for me. Somebody fooled me by saying that seashells are from the sea, so they produce sounds like this. Now, it is clear. There is no magic. It is the trick of the sound waves trapped inside the shell.
You can hear similar sounds if you hold any bowl or container up to your ear or try cupping your hand up to your ear, and you can experience it.
It is a popular folk myth that the sound of the ocean may be audible through seashells, particularly conch shells. It is seashell resonance.[5]
- Seashells have many hard, curved surfaces that are good sound reflectors.
- Trapped sound waves within the shell undergo multiple reflections, creating a jumble of echoes.
- These echoes create a roaring sound, similar to the ocean's roar.
- The hard, curved surfaces inside shells reflect sound waves, causing them to bounce around inside the shell.
The resonant sounds are created from the surrounding environment by reverberation (the persistence of sound due to multiple reflections from surfaces such as walls, furniture, etc)and acoustic amplification within the shell, which makes them louder than the normal sounds you hear without a seashell.
Significant factors:
- The size and shape of the shell have some effect on the sound you hear.
- Sound waves are different in various shells because they accentuate different frequencies.
- Each seashell has a unique shape.
- Hollow and curved ones can catch some of the sounds around you.
- This happens when the sound enters the opening of the shell.
The sound trapped inside the shell bounces around and becomes slightly louder (or amplified) before leaving the shell.
The sounds seashells catch are low-frequency sounds.
The sound of the ocean is also a low-frequency sound. That’s why it sounds similar to the sounds caught in a shell.
It is not the sound of the sea. But, since you are holding a seashell to your ear, it makes sense that people might think it could be.
Dry shell
So, if it is not the sound of the sea you hear, what do you hear?
You are hearing ambient or background noise that increases in amplitude due to the physical properties of the seashell.
- 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.
The seashell is like a wind instrument. It has a set of resonant frequencies where the air inside the shell will vibrate strongly. Hold the shell to your ear, and the frequencies in the ambient sound get amplified. Because the sound changes, your brain pays attention to it.
If you go into an anechoic chamber, a completely silent room, you will hear nothing because there is no ambient sound.
I will tell you briefly about seashell resonance
Seashell resonance
Occlusion effect
The occlusion effect occurs when an object fills the outer portion of the ear canal, causing that person to perceive echo-like hollow or booming sounds generated from their voice.
The bone-conducted sound travels to the cochlea through different pathways. The outer ear pathway corresponds to the sound pressure generated in the ear canal cavity due to the vibration of the ear canal wall, which constitutes the source of the occlusion effect. At low frequencies, the outer ear pathway is negligible when the ear canal is open but dominates when it is occluded. The occlusion effect is thus objectively characterized by an acoustic pressure increase in the occluded ear canal at low frequencies. We can measure it with a probe-tube microphone. [10]
Considering that the vibrating ear canal wall acts as an ideal source of volume velocity (also known as volumetric flow rate), the occlusion device increases the opposition of the ear canal cavity to the volume velocity imposed by its wall and thus, increases the amplitude of the acoustic pressure that is generated in reaction, leading to the occlusion effect. [6]
The acoustic impedance of the ear canal cavity represents its opposition to the volume velocity transfer and governs its reaction in terms of acoustic pressure. In other words, the occlusion effect is mainly due to the increase in the acoustic impedance of the ear canal cavity. [6][7][8][9]
A person with normal hearing can experience this by sticking their fingers into their ears and talking. Otherwise, this effect is often experienced by hearing aid users who only have mild to moderate high-frequency hearing loss but use hearing aids that block the entire ear canal. The occlusion effect is a notable discomfort to workers wearing shallowly inserted passive occlusion devices such as earplugs. [10][11]
Active occlusion algorithms are needed to help people with severe hearing loss adequately. If a person suffers from near-normal low-frequency hearing and mild to moderate hearing loss of up to 70 dB at mid and high frequencies, hearing aids with increased vent size or hollow ear-molds/domes are more suitable for them in lessening the extent of the occlusion effect. [12] In the latter case, the open-fitting decreases the ear canal acoustic impedance and the occlusion effect.
For earplug users, an incomplete seal has a similar effect at frequencies lower than the Helmholtz resonance formed by the system (the neck of the resonator corresponding to the incomplete seal at the earplug/ear canal wall interface and the resonator cavity being the partially occluded ear canal). In the general case, the deep-fitting reduces the occlusion effect because the volume velocity imposed by the ear canal wall to the occluded ear canal cavity decreases. [5]
Reference:
1 Gerard Cheshire (2006). Sound and Vibration. Black Rabbit Books. p. 25. ISBN 1-58340-997-1.
2 ^ Joseph P. Olive; Alice Greenwood & John Coleman (1993). Acoustics of American English Speech: A Dynamic Approach. Springer. p. 64. ISBN 0-387-97984-0.
3^ Dorita S. Berger (2002). Music Therapy, Sensory Integration, and the Autistic Child. Jessica Kingsley Publishers. pp. 86–87. ISBN 1-84310-700-7.
4^ John Watkinson (1998). The Art of Sound Reproduction. Focal Press. ISBN 0-240-51512-9
5 Wikipedia
7^ Jump up to: a b Carillo, Kévin; Doutres, Olivier; Sgard, Franck (May 2020). "Theoretical investigation of the low-frequency fundamental mechanism of the objective occlusion effect induced by bone-conducted stimulation" (PDF). The Journal of the Acoustical Society of America. 147 (5): 3476–3489. Bibcode:2020ASAJ..147.3476C. doi:10.1121/10.0001237. PMID 32486794
8 ^ Stenfelt, Stefan; Reinfeldt, Sabine (January 2007). "A model of the occlusion effect with bone-conducted stimulation". International Journal of Audiology. 46 (10): 595–608. doi:10.1080/14992020701545880. PMID 17922349. S2CID 39146020.
9 ^ Occlusion effects, Part II: A study of the occlusion effect mechanism and the influence of the earmould properties, Report by M. Ø. Hansen, T. Poulsen, and P. Lundh, Technical University Denmark, 1998.
10 ^ Zurbrügg, T.; Stirnemannn, A.; Kuster, M.; Lissek, H. (1 May 2014). "Investigations on the physical factors influencing the ear canal occlusion effect caused by hearing aids". Acta Acustica United with Acustica. 100 (3): 527–536. doi:10.3813/AAA.918732.
11 ^ Rating and ranking methods for hearing protector wearability. J.G. Casali, S.T. Lam, and B.W. Epps - Sound & Vibration, 1987.
12 ^ Doutres, Olivier; Sgard, Franck; Terroir, Jonathan; Perrin, Nellie; Jolly, Caroline; Gauvin, Chantal; Negrini, Alessia (2 December 2019). "A critical review of the literature on the comfort of hearing protection devices: definition of comfort and identification of its main attributes for earplug types" (PDF). International Journal of Audiology. 58 (12): 824–833. doi:10.1080/14992027.2019.1646930. PMID 31362514. S2CID 199000288.
13^ Winkler, Alexandra; Latzel, Matthias; Holube, Inga (1 January 2016). "Open Versus Closed Hearing-Aid Fittings: A Literature Review of Both Fitting Approaches". Trends in Hearing. 20. doi:10.1177/2331216516631741. PMC 4765810. PMID 26879562
14 "How are seashells made?". Woods Hole Oceanographic Institution. Archived from the original on 22 March 2022. Retrieved 14 May 2024.
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