A sound wave representing acoustic metamaterials

What Are Acoustic Metamaterials? Sentient Digital Engineer Adam Kingsley Explains

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Acoustic metamaterials have a lot of exciting potential for naval applications. Sentient Digital, Inc. and its acoustical engineering subsidiary, RDA, work on the cutting edge of technology solutions. In this “Technical Takeover” of the blog, RDA Research Scientist Adam Kingsley, who recently presented his research on acoustic focusing with metamaterials at the 184th Meeting of the Acoustical Society of America in Chicago, explains his research and its applications. 

About Adam Kingsley and The Acoustical Society of America Meeting

Adam Kingsley is a research scientist who has worked for RDA since 2022. He recently received a doctorate in Physics from Brigham Young University, graduating alongside several RDA colleagues who were also studying at the University.

The Acoustical Society of America’s 184th Meeting took place in Chicago from May 812, 2023. The meeting topics included research in the areas of acoustic signal processing, acoustic engineering, and underwater acoustics. Hundreds of presentations were made by members of the society as well as invited speakers and around 1000 acousticians attended this semiannual meeting.

Adam Kingsley Explains Acoustic Metamaterials

To get started, here are a couple of vocabulary words that I’m going to use in this post.

Frequency: The frequency of the sound is the note of the sound you hear. High frequency would sound like a high note. A single frequency is “clean” like the sound of a whistle. Note that all instruments playing the same note are all playing the same fundamental frequency. They all sound different because of the other frequencies that the instrument makes on top of the fundamental.

Resonance: Acoustic resonance is when something rings because some acoustic waves are taking a longer time to die out. Hitting a bell will cause it to ring at a lot of frequencies, but only the resonance frequency sticks around and that is the note we hear ringing. Resonance is determined by what type of material and the shape. Usually there are many resonances that will all be ringing. I believe all instruments use a resonance to make the individual notes. Violins, guitars, pianos have strings that resonate. Xylophones, bells, chimes have solid structures that resonate. Organs, flutes, trumpets have air in pipes that resonate.

Acoustic Metamaterial: An acoustic metamaterial is a repeating pattern of elements. It isn’t a real material in the sense that you can’t have as much or as little as you would like. The metamaterial is the collection of the item and its properties as a whole – like a steel spring. A steel spring isn’t a material, it’s made up of spring steel. Spring steel is the material. The steel spring is an arrangement of steel that gives the spring special properties.

A Simple Example

Let’s talk about perhaps the simplest example of an acoustic metamaterial, open bottles. You know that when you blow over the top of a bottle, you hear a single note. The flow of air over the edge of the bottle makes a lot of sound that goes into the bottle. However, because of the shape of the bottle, it prefers a single frequency. This resonance of the bottle is special because, like a whistle, it is a single frequency.

By the way, if you want to know what it would sound like inside a human-sized bottle, just roll the car window down a little while you are traveling on the highway. The note is much lower because the car is much bigger, but you will hear it. Actually, it is so low you might feel it more than you hear it!

More Specific Examples

Here is an animation of a single bottle (granted, a very ball-shaped bottle) attached to a section of a pipe. It is resonating in the same way that a bottle resonates when someone blows over the top. The changing colors represent the sound waves as they move into and out of the resonator.
Animation of a bottle connected to a pipe, resonating

The red is high pressure, and the dark blue is low pressure. The surfaces show the boundary between levels. You can see the surfaces move out the ends of the pipe. Note, you don’t see the surfaces come back because of an asymmetry to the pushing and pulling. This means you can move the resonator with just sound. You can watch a video of acoustics professor Dan Russell demonstrating this, but I recommend turning down your headphones when he turns on the speaker.

The above illustration is showing what would happen if the resonator had sound coming from inside. If the sound is coming from down the pipe you get a different effect.

Animation of resonance

This illustration shows the same colors as before. The sound waves are coming from the left. Here you can see that because of the resonator, the waves are almost entirely reflected back upstream. This resonance frequency is indeed a special property. When you have a lot of resonators, other special frequencies show up. When you know where these other special frequencies are, you can change the sound waves in special ways.

Now, if you put a bunch of bottles side-by-side, this wall of bottles would very much prefer that frequency. This simple arrangement constitutes an acoustic metamaterial. The individual elements are the bottles but the whole surface is the material. The resonance frequency of a single bottle would be a key property of the whole surface. Weird things happen above and below that frequency. 

Soda Can Experiment

Here is an experiment we did with an array of soda cans. Cans resonate even better than a bottle if you have a speaker making sound at the resonance frequency. However, you can’t get a can to resonate by blowing over the top, like you would with a bottle.

This is a wall of soda cans. There are magnets glued to the bottom so that they stick to the metal plate.This wall of cans was placed inside a large room and sound was played out of 8 speakers. The frequency of the sound was a little below the resonance frequency. We moved a microphone around and measured the sound over the whole wall of cans. Then we removed the cans and repeated the same process.

Here you can see sound waves making patterns above the cans (left) and without the cans (right).  You can see that the wall of cans makes the sound waves look very different compared to when there are no cans. Specifically, the difference is that the waves are tighter together.

Summary of Research and Applications

When sound waves are measured underwater, a computer works on the signal to make it more useful. What if your metamaterial had already changed the sound in a way that made it more useful? For example, what if the resonance frequency of your metamaterial was the same frequency that is sung by an interesting “whale”? Well, you would hear that whale a lot better with a metamaterial. 

Learn About Cutting-Edge Technology Like Acoustic Metamaterials with Sentient Digital and RDA

From acoustic metamaterials to artificial intelligence to a low-cost advanced processor for acoustic testing, Sentient Digital and RDA stay at the forefront of technology solutions for our military, government, and private sector clients. To keep up with these exciting developments, check out our blog. If you are interested in our services, reach out to us today!