Maybe you can hear sounds in space after all

However, as the researchers illustrate in their study, “at the atomic scale,” the vibrations of the nuclei can spread through the vacuum by interacting electrically. 

What the scientists suggest is that there are situations when a sound wave can jump or “tunnel” trough a vacuum gap in between two solid piezoelectric materials. These kinds of materials (i.e. quartz crystals, ceramics, or even bones) allow for vibrations like sound waves to produce electrical responses. Because electric fields can function in a vacuum, they can as such transmit sound waves. 

The researchers found that in order for this phenomenon to work, the size of the vacuum gap has to be smaller than the wavelength of the sound wave. They determined the effect could work in a range of audio frequencies (Hz to kHz), as well as in ultrasound (MHz) and hyper sound (GHz) frequencies if the vacuum gap shrinks as the frequencies get larger. 

In a press release, Professor Maasilta pointed out that while in most cases the effect achieved is small, “we also found situations where the full energy of the wave jumps across the vacuum with 100% efficiency, without any reflections.”

Potential practical applications of this effect could be seen in microelectromechanical components (MEMS, smartphone tech) and in controlling heat, he shared.

A researchers' insight

Interesting Engineering reached out to Professor Ilari Maasilta for more insight on the paper. Maasilita mentioned that he actually found it surprising that "100 % of the power could be transmitted (with the right conditions).”

He explained the situations in which a sound wave can jump or ‘tunnel’ across a vacuum gap, sharing that there are instances when “an incoming sound wave can be converted to a special sound wave that is localized [not] only on the surfaces of the solids, but on both sides of the gap (coupled surface waves).” 

It is this localization that makes it possible for the sound to transmit fully. “The coupling between the two surfaces is facilitated by electric fields, which can exist in vacuum, unlike the actual vibrations of the sound wave (which generate the fields through piezoelectricity),” he explained further.

So, can anyone hear you scream in space?

Utilizing such piezoelectric materials can ("in principle") make it possible to hear a scream in space, confirmed the researcher. It could work “across a gap below 1 m (for a sound frequency  around 1 kHz), but details would have to worked out how loud it would be,” he wrote in our email exchange.

One complication would be how well the sound produced inside the air-containing spacesuit could be converted to the right type of input mode in the piezoelectric material. Maasilita also mentioned that this kind of speculation is not “technologically relevant for communication” since superior systems, like microphones and radio, are already in existence that utilize conversion to electromagnetic waves.

What's the significance of this discovery?

Professor Maasilta said it’s too early to evaluate the importance of the discovery as it will be judged by its impact on the future of science and technology and “not by the discoverer.” Still, he proposed that one implication of their find is that “sound can be manipulated (coupled without physical contact to nearby structures) using a new mechanism.”

He elaborated on the possible applications of their study, saying that microelectromechanical components often use piezoelectric materials to convert electrical signals to sound waves or use sound waves to generate electrical signals. “Tunneling of these signals offers a new pathway for transmitting them between different components, “ he stated.

The scientist also noted that heat was, “basically a random collection of sound waves at higher frequencies,” noting that the phenomenon they uncovered could be used to create additional cooling of devices, as one example. His team is currently researching the “heat aspect” of the tunneling mechanism in greater depth.

Check out the full article “Complete tunneling of acoustic waves between piezoelectric crystals” in Communications Physics.

Abstract

The mechanical displacements in piezoelectric materials carry along macroscopic electric fields, allowing tunneling of acoustic waves across a vacuum gap beyond the charge-charge interaction distance. However, no rigorous proof of complete acoustic wave tunneling has been presented, and the conditions to achieve complete tunneling have not been identified. Here, we demonstrate analytically the condition for such phenomenon for arbitrary anisotropic crystal symmetries and orientations, and that complete transmission of the incoming wave occurs at the excitation frequency of leaky surface waves. We also show that the complete transmission condition can be related to the surface electric impedance and the effective surface permittivity of the piezoelectric material, relevant to realize the complete tunneling experimentally. We support our findings with numerical results for the maximum power transmittance of a slow transverse wave tunneling between identical ZnO crystals. The results show that complete tunneling can be achieved for a large range of orientations.