What! The magical conch is actually a "live broadcast expert"?

Take a walk along the beach, pick up a conch carried by the waves to the beach, and put it next to your ear to hear the sound of the sea. If someone gives you a horn-like conch, it is like giving you a piece of blue sea. You can listen to the sea breeze and the surging sea tide anytime and anywhere, and hum "You like the salty breath of the sea breeze, stepping on the wet gravel" anytime and anywhere, and go to "face the sea, spring flowers bloom". Just thinking about it is a super romantic thing!

So, is the conch the phonograph of the sea? Is the sound we hear through the conch really the sound of the sea?

Image source: Institute of Acoustics, Chinese Academy of Sciences

Practice is the only criterion for testing truth.

We can also hear the sound of the "sea" by putting our fingers together and bending them into a semi-closed concave shape, or by putting a cup, a small bowl or other container close to our ears and listening carefully . These containers are not from the sea, so could the sound coming out of them be the sound of the sea? Of course not. Then, the sound heard from the conch is naturally not the sound of the sea.

01

Is the sound you hear from the conch the sound of blood flowing?

Some people say that the sound heard in the conch is the sound of blood flowing, because when the conch is placed against the ear, it amplifies the sound of blood flowing.

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Let's do another small experiment. In the same sound field environment, let the same person listen to the conch in a normal state and after strenuous exercise. There is no obvious difference between the two sounds. The sound does not become sharper or louder due to strenuous exercise. This shows that the sound in the conch is not the sound of blood flowing in the human body .

02

Where does the sound in the conch come from?

So where do the sounds heard in conches and similar small containers come from?

We can also do a small experiment to verify this. Find a few cups of different sizes and slowly bring them close to your ears. When the mouth of the cup is just about to touch your ear, you can hear a sound similar to "the sea". The sounds you hear are slightly different when using cups of different sizes.

The bigger the cup, the deeper the sound; the smaller the cup, the clearer the sound. The sound we hear is the cup filtering the ambient noise . At this time, the cup can be regarded as a sound filter, which is professionally called a filter.

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In summary, the sound in the conch is not the sound of the sea, but the noise of the sound field environment filtered by the conch . If you listen to the sound in the conch at the seaside, what you hear is mainly the sound of the sea after filtering. If you are in a sound field environment other than the sea, what you hear in the conch is not the sound of the sea.

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03

Conch is a live streaming expert

The sound of the sea in the conch does not come from its original ecological memory, but a live broadcast of the current sound environment. The live broadcast of the conch relies on a common phenomenon in physics - resonance . Resonance refers to two objects with the same vibration frequency. When one object vibrates, it causes the other object to vibrate. In the field of acoustics, resonance is called "resonance", which refers to the phenomenon that an object makes a sound due to resonance.

Conch shells, small bowls, cups, etc. are physical systems that easily cause resonance. When the sound frequency is consistent with the intrinsic frequency of the cavity, resonance will occur, amplifying the sound . At this time, if you put it next to your ear, you can hear the sound of the "sea".

When the frequency of the actor's voice is the same as the natural frequency of the glass, the glass will break due to resonance. GIF source: "Come on, Future"

A tuning fork used to demonstrate the resonance phenomenon. Image source: Institute of Acoustics, Chinese Academy of Sciences

The Helmholtz resonator , designed and named by German physicist and biologist Hermann von Helmholtz in 1850, is similar to the principle of listening to the sound of the "sea" through a conch shell. The design concept of the Helmholtz resonator is to distinguish sounds of different frequencies or tones in noise or music through the principle of resonance.

The original shape of the Helmholtz resonator is a container with a thin neck or a small mouth. The sound in the environment causes air disturbances, and the elastic deformation of the air causes pressure changes, which pushes the air column at the thin neck or small mouth of the container to move back and forth. This continuous oscillation process will produce sound. Both the conch and the Helmholtz resonator have a small mouth and a large cavity, and their sound-generating principles are the same. In a sense, the conch can be regarded as a live broadcast "expert" in nature. It not only filters environmental noise, but also has the effect of amplifying sound.

Schematic diagram of the Helmholtz resonator. Image source: Institute of Acoustics, Chinese Academy of Sciences

04

From listening to the "sea" from conch shells to ancient acoustic applications

The principle of sound resonance has been applied in ancient China.

As early as the early Warring States period, "Mozi·Bei Xue" recorded the resonator used for military defense: dig a deep well at a certain distance under the city wall, bury a jar with a large belly and a small mouth, cover the mouth of the jar with thin leather, and then put it into the well. Send people with sensitive hearing to lie on the mouth of the jar to listen to the sound, so as to determine the direction of the enemy's tunnel and then counterattack. This device suitable for defending the city is called "Wang Ting", also known as "Di Ting".

Regarding "setting up urns to aid sound", the ancient stage of Longtian Temple in Shita Village, Fenyang City, Shanxi Province is a good example. On the gables on both sides, three types of large, medium and small pottery urns are symmetrically embedded, totaling 14, with the mouths of the urns facing each other to achieve the effect of amplifying sound.

Fourteen pottery urns embedded on the gable of the ancient Longtian Temple stage in Shita Village, Fenyang, Shanxi. Image source: Document 3

In ancient times, in order to facilitate carrying during marches and battles, the design of "ground listening" tended to be light and practical. In the "Taibai Yinjing·Youyi Ground Listening" of the Tang Dynasty, Li Quan recorded a hollow pillow called "empty Hulu". He wrote: "Choose those who sleep less and let them sleep with an empty Hulu. When a person and his horse are traveling thirty miles away, there will be sounds in the Hulu from the east, west, south, and north. This is called 'ground listening'."

In the Northern Song Dynasty Shen Kuo's "Mengxi Bitan·Instructions", he described the materials and general shape of this kind of warning pillow: "According to the ancient method, cowhide was used for quivers, and then used as a pillow when lying down. The hollow part inside was used to make a quiver, and it was placed on the ground as a pillow. If there were any sounds of people or horses within a few miles, they would all be heard. This is because the hollow part can absorb the sound." This kind of "ground listening" is a quiver made of cowhide. When the army rests during a march, it is placed on the ground as a pillow. Because it is hollow, it can "absorb the sound".

Just imagine, when the ancient soldiers were camping and resting, they could use these magical gadgets to hear the enemy’s marching movements from dozens of miles away, and make corresponding countermeasures. The ancients were really smart!

The ingenuity of the application of ancient acoustic knowledge is admirable. The seeds of science were sown in people's hearts as early as in ancient times, and the flowers of science bloomed in ancient history books. In modern times, the resonance phenomenon is widely used in all aspects of social life, such as medical stethoscopes, resonance boxes of pianos, violins, guitars and other musical instruments, microwave ovens, oscillators for construction, etc.

When we put a conch next to our ear, we hear the noise in the environment, and the conch acts as a filter and amplifier of the environmental noise.

So, what we hear from the conch is not just the sound of the sea, but the sound of nature!

References:

[1] Ma Dayou. The development of Helmholtz resonator[J]. Physics, 1993(08):452-456.

[2] Yang Yang, Gao Ce. The dispute over “setting up a jar to help with sound”: a case in the history of science [N]. Guangming Daily, 2016-04-06(T14)

[3] Yang Yang, Gao Ce, Ding Hong. Acoustic technology of pottery urns installed on the ancient stage of Longtian Temple in Shita Village, Shanxi Province[J]. Journal of History of Natural Sciences, 2014, 33(01): 70-82.

Produced by | Science Popularization China

Author: Wang Lina and Yang Fujin (Underwater Environment Information Perception Laboratory, Institute of Acoustics, Chinese Academy of Sciences)

Producer｜China Science Expo

Submitted by: Computer Information Network Center, Chinese Academy of Sciences

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