"I am your wall breaker!" Air & Water: ...

Did you know that there is a "wall" between air and water?

Have you ever heard the sound of fish on the shore? (Don’t doubt it, fish really do make sounds.)

Also, did you know that during synchronized swimming competitions, there are sound equipment both above and below the water?

Image source: veer gallery

This experience seems to tell us that it is difficult for the air and underwater to share the same sound world . Sound seems to hit a "solid wall" at the junction of air and water. Why is this? How can this "wall" be broken?

The energy intensity of sound is reduced by 1000 times when it passes through the water-air interface.

This starts with the propagation characteristics of sound in different media.

As we all know, sound waves are a kind of mechanical waves, and the propagation of sound waves depends on the transmission of mechanical vibrations in the medium . In different media, the transmission capacity of sound vibrations will be different, and this difference in transmission capacity can be reflected by the acoustic impedance of the medium.

The acoustic impedance of a medium is represented by the product of the density of the medium (unit: kg/m3) and the speed of sound (unit: m/s). The difference in acoustic impedance between different media is called impedance mismatch. The greater the difference in acoustic impedance between two media, the greater the difference in the ability of sound waves to propagate in the two media.

Therefore, when a sound wave is incident on the interface of two media, the greater the difference in acoustic impedance between the two media, the lower the energy transmitted by the sound wave at the interface. The relationship between the acoustic energy transmission coefficient and the acoustic impedance of the two media can be expressed as:

Where R1 and R2 are the acoustic impedances of the two media respectively.

The sound energy intensity is reduced by 1000 times before and after passing through the water-air interface (Image source: drawn by the author)

For water and air, at room temperature, the acoustic impedance of air is 415 kg/m2·s, while the acoustic impedance of water is 1480000 kg/m2·s, and the acoustic impedance between the two differs by about 3600 times. Therefore, when sound waves are incident on the interface between water and air, only 0.1% of the sound energy can pass through the water-air interface. In other words, the sound energy intensity is weakened by 1000 times before and after passing through the water-air interface. The extremely low transmission of sound between water and air makes it difficult for synchronized swimmers who dive into the water to hear the music played on the shore. This is why synchronized swimming competitions require the installation of audio equipment both above and below the water.

With this device, water-air sound transmission is no longer a dream~

How can we achieve cross-medium transmission of sound between water and air?

Since the low transmission of sound at the interface between water and air comes from the huge impedance difference between water and air, we just need to build an acoustic impedance "bridge" between water and air (as shown by the green curve in the figure below) to make up for this impedance difference!

Schematic diagram (Image source: drawn by the author)

With this "bridge", sound can "flow" between water and air along the gradual impedance gradient as shown by the yellow signal in the figure.

Doesn't it sound so easy? That being said, it is not so easy to realize, because it is still difficult for us to find materials in nature that can cover the acoustic impedance range from water to air. Existing metamaterials based on air background medium or metamaterials based on water background medium are also powerless in the face of such a large impedance difference. Therefore, past research on water-air sound transmission mainly focused on narrow-band sound transmission based on resonance, which greatly limited the application prospects of water-air sound transmission.

Recently, Yang Jun, a researcher at the Institute of Acoustics, Chinese Academy of Sciences, has innovatively proposed underwater hollow-structured acoustic metamaterials , which introduce the air component and the geometric shape of the unit as a new degree of adjustment freedom, greatly expanding the range of acoustic parameters that can be achieved by underwater acoustic metamaterials. Furthermore, by combining the acoustic metamaterials composed of periodically arranged square columns in the air, they have successfully filled the acoustic impedance gap from water to air and designed a water-air gradient acoustic impedance matching layer, thereby achieving broadband acoustic energy transmission from water to air. The relevant research results were published in the Applied Physics Journal Applied Physics Letter and were exclusively reported by Scilight Weekly.

Underwater hollow acoustic metamaterial (Image source: Reference 1)

So what kind of effect can this matching layer achieve? The researchers provided an example in the article. The experimental test results of the water-air gradient acoustic impedance matching layer designed in the example can achieve an average of 16.7 decibels and a maximum of 25.5 decibels of transmitted sound energy enhancement in the range of 880 Hz to 1760 Hz. What is the level of 25.5 decibels? This is equivalent to installing this matching layer. Compared with the absence of the matching layer, the transmitted sound energy is enhanced by 350 times, which can be said to be a successful "wall breaking"!

Water-air "breaking the wall" has a greater effect than you think

The circular part in the picture represents the metamaterial, through which water-air sound transmission can be achieved (Image source: Institute of Acoustics, Chinese Academy of Sciences)

Why do we need to develop this material that can achieve water-air sound transmission?

If water-air sound transmission can be achieved, it will not only allow synchronized swimmers submerged in the water to hear music played in the air, but it can also play a role in broader application scenarios such as ocean exploration.

Existing ocean exploration mainly relies on sonar to scan and detect in the ocean, and transmit the collected information back to the mother ship. The detection means and transmission paths are complicated, and the information detection cycle is long. If water-air sound transmission can be achieved, we can directly use the airborne sound sensor system to detect the underwater world, which can greatly shorten the detection cycle, streamline the detection and information transmission process, and improve the efficiency of ocean exploration.

In addition to bringing about innovations in ocean exploration methods, achieving effective sound transmission between water and air also plays an important role in alleviating ocean noise pollution.

A joint study by the University of Hawaii and Curtin University in Australia showed that the noise from just one mine in marine mining can travel about 500 kilometers, which is enough to cause severe damage to the sensory organs of marine life within the range. The sound of the exploration air guns used in marine oil and gas exploration is even higher than 200 decibels. These noises are constantly reflected between the water-air interface and the seabed, and spread thousands of kilometers in the ocean. A single air gun can cause a mortality rate of up to 40%-60% for zooplankton.

If we can effectively integrate water-air sound-permeable devices into mining and sailing ships in the future, we will be able to open an outlet for these ocean noises at the water-air interface, and transmit these noises that are difficult to attenuate in the ocean into the air for diffusion and attenuation, thus providing a peaceful home for marine life.

References:

[1] Ping Zhou, Han Jia, Yafeng Bi, Yunhan Yang, Yuzhen Yang, Peng Zhang, Jun Yang. Water-air acoustic communication based on broadband impedance matching. Applied Physics Letter,123,191701 (2023).

Author: Zhou Ping

This article is from the "Science Academy" public account. Please indicate the source of the public account when reprinting.