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Complete tunneling of sound waves: physicists demonstrate sound through a vacuum

2025-01-18 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > IT Information >

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As we all know, acoustic waves are mechanical waves that require a material medium to propagate. Without a medium, sound waves cannot exist and cannot transmit energy. It's like we can't hear sound in a vacuum. So, if there is a small vacuum gap between two objects, can sound waves travel from one object to the other? This seems impossible because sound waves decay to zero in a vacuum. But there is a special substance called piezoelectricity that can make this seemingly impossible possible.

Piezoelectric material is a material capable of converting mechanical stress into an electric field, or an electric field into mechanical deformation. When an acoustic wave propagates through a piezoelectric material, it generates an electric field associated with it. This electric field is not confined to the interior of the material, but extends beyond the surface and decays exponentially with increasing distance. As we have learned in electromagnetism, when a plane electromagnetic wave reflects off a conductive surface, it produces a surface current and a surface charge density. These surface charges produce a decaying electric field that extends outward.

If we place two piezoelectric materials so close together that the vacuum gap between them is smaller than the wavelength of the sound wave, then it is possible that the attenuating electric field generated by the sound wave emitted from one material is strong enough to affect the surface charge of the other material and excite the other sound wave. This is equivalent to the sound wave passing through the vacuum gap, realizing a tunneling effect. This effect does not depend on any principles of quantum mechanics, but is described purely by classical mechanical and electromagnetic theory.

Perfect Tunneling Conditions Under what conditions, then, can acoustic waves travel completely from one piezoelectric material to another? In other words, under what conditions does the energy of sound waves emitted from one material equal the energy of sound waves entering another? This requires us to consider the matching problem between two piezoelectric materials.

First of all, we should note that not all types and directions of sound waves can produce significant electric field effects in piezoelectric materials. Only those sound waves that are compatible with crystal symmetry and have a transverse component (i.e. perpendicular to the propagation direction) of deformation or stress can be efficiently coupled into the electric field. These sound waves are called piezoelectrically active sound waves. For example, in piezoelectric materials with hexagonal symmetry (e.g., zincite), only shear waves propagating along the hexagonal axis are piezoelectrically active. Therefore, if we want to realize the tunneling effect of acoustic waves, we must choose suitable piezoelectric materials and polarization and propagation direction of acoustic waves.

Second, we should note that not all frequencies of acoustic waves can achieve complete tunneling between two piezoelectric materials. Only when the acoustic impedance of the two materials (that is, the resistance to acoustic wave propagation in the material) is equal or similar, can the reflection of acoustic waves at the interface be avoided and the maximum transmission of acoustic energy be guaranteed. As we have learned in optics, when light passes from one medium to another, if the refractive indices of the two media are equal or similar, then the light is not reflected, but transmitted completely. Therefore, if we want to achieve complete tunneling of acoustic waves, we must choose appropriate piezoelectric materials and acoustic frequencies.

If these conditions are met, sound waves can be transmitted completely from one piezoelectric material to another without any reflection or attenuation. This is the sound wave complete tunneling effect.

So, does this effect really exist? Has anyone done an experiment to test it? The answer is yes. In a newly published paper, the authors use two different types of piezoelectric materials: lead zirconate titanate (PZT) and strontium zirconate titanate (PST), both of which have tetragonal symmetry. The author selects the transverse wave propagating along the tetragonal axis as the piezoactive acoustic wave, and adjusts the frequency of the acoustic wave and the vacuum gap between the two materials so that they satisfy the complete tunneling condition.

The authors used an ultrasonic transducer to generate and receive sound waves, and measured the transmission coefficient of sound waves between two materials (i.e., the ratio of the energy of sound waves entering the other material to the energy of sound waves emitted from one material). The authors found that at certain frequencies the transmission coefficient approaches unity, indicating that sound waves are transmitted almost completely from one material to another without any loss. This is the complete tunneling effect of acoustic waves observed experimentally.

What's the use of applying the foreground effect? The authors point out that this effect can be used to fabricate a new type of ultrasonic transducer that can achieve efficient acoustic transmission between different types and shapes of piezoelectric materials, thus improving the sensitivity and flexibility of the transducer. In addition, this effect can also be used to create a new type of acoustic isolator, which can achieve selective transmission or reflection between sound waves of different frequencies, thus achieving modulation and filtering of sound waves.

This article comes from Weixin Official Accounts: Vientiane Experience (ID: UR4351), by Eugene Wang

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