Acousto-Optics: Sensing Sound with Light

In this episode, we dive into the world of acousto-optics, where light is used to visualize and measure sound-- particularly acoustic phenomena that are difficult to observe. Samuel Verburg (Technical University of Denmark) and Kenji Ishikawa (NTT Communication) share the history of this field of research, as well as discuss modern day applications and potential uses for acousto-optic sensing in the future.

Read the associated article: Samuel A. Verburg, Kenji Ishikawa, Efren Fernandez-Grande, and Yasuhiro Oikawa. (2023) “A Century of Acousto-Optics: From Early Discoveries to Modern Sensing of Sound with Light,” Acoustics Today 19(3). https://doi.org/10.1121/AT.2023.19.3.54

Read more from Acoustics Today.

Learn more about Acoustical Society of America Publications.

Intro/Outro Music Credit: Min 2019 by minwbu from Pixabay. 

Transcript

Kat Setzer  00:06

Welcome to Across Acoustics, the official podcast of the Acoustical Society of America's publications office. On this podcast, we will highlight research from our four publications. I'm your host, Kat Setzer, Editorial Associate for the ASA.

Kat Setzer  00:25

Did you know that light can be used to see sound? Today I'm talking to Samuel Verburg and Kenji Ishikawa about their article that discusses this very concept, "A century of Acousto-Optics: From Early Discoveries to Modern Sensing of Sound with Light," which appeared in the fall 2023 issue of Acoustics Today. Thanks for taking the time to speak with me today. How are you two doing?

Samuel Verburg  00:44

Thank you. Good. Thank you for having us. It's going well.

Kat Setzer  00:47

Good.

Kenji Ishikawa  00:47

Yeah. Great to be here. 

Kat Setzer  00:49

Yeah, very excited to have you. This is a very highly downloaded article. A lot of people liked reading this article. So very exciting. So first, tell us a bit about your research background. 

Samuel Verburg  00:58

Thanks again. So I'm an assistant professor at the Acoustic Technology Group in the Technical University of Denmark, and my research focuses primarily on sensing and processing of acoustic signals-- so I investigate new ways to capture and measure acoustic information. So I work with advanced microphone array techniques, automated measurements using robots, and maybe what's more relevant for the topic of today, I also work with optical sensing of acoustic waves, so I use lasers and photodetectors in order to capture acoustic information. Apart from that, I also teach in the Masters of Engineering Acoustics here at DTU. 

Kenji Ishikawa  01:40

So my name is Kenji Ishikawa. I'm a research scientist at NTT communications science laboratories in Japan. NTT is a Japanese telephone and communication company and I belong to its fundamental research lab. I have been working on the research in acoustic optics for almost 10 years, since I was a student of Oikawa Laboratory in Waseda University. My research interests are in acoustics, and optics, of course, and especially exploring how optical technologies can bring innovations to acoustics.

Kat Setzer  02:14

So what is acousto-optics?

Samuel Verburg  02:17

So acousto-optics is a general term that we use to describe the different ways in which acoustic waves and lights interact. So imagine that you have a transparent medium, for example, could be air or water. And so also imagine that we shine a beam of light, we normally use a laser, so that this beam of light travels through the medium. Now, what acousto-optics describes is the different ways in which the propagation of this light beam is going to be affected when there is an acoustic wave present in the medium. In the case of air, for example, that could be the acoustic pressure generated by a loudspeaker.

Kat Setzer  02:57

Interesting, okay, so what are some situations where using light to sense sound would be helpful?

Samuel Verburg  03:03

One of the biggest advantages of acoust- optic sensing is that it is non-invasive. And I can give you an example of this. So if you were to measure the acoustic pressure at a certain location inside some sound field, you would most likely use a microphone. But as you place the microphone, you're introducing a physical object into the sound field that you want to measure. And that is going to have an influence; you're introducing some kind of bias. While this is normally not the problem, as microphones are relatively small, so their influence on the sound field can be neglected in most cases. However, say that you want to know the pressure in many places at the same time, for example, you're interested in, like, knowing how the sound field looks like over space. So, you would need to place many, many microphones and now the influence of these many microphones cannot be neglected anymore. And this starts to be a problem. And this is a problem that we can actually overcome using acousto-optic sensors. Since as the light is the sensing element, we are not introducing any physical device in the measured sound field. So that's why we say that these type of measurements are non-invasive or non-contact. And you can imagine there is a long list of situations and applications in which having this type of non-contact measurements is advantageous. So, for example, if we want to study how sound generated by a source that is moving, or the relationship between acoustics and fluid dynamics, for example, the noise generated by wind, also when we want to look at high-resolution measurements of high-frequency sound fields, or the sound generated by very small devices like hearing aids or micro speakers.

Kat Setzer  04:46

Okay, that makes a lot of sense. So it's kind of like, if you have a bunch of microphones there, then the sound waves bounce off of a bunch of them at the same time, kind of, and throw off the measurements.

Samuel Verburg  04:56

Exactly. That is something that is called sound scattering. And these microphones, when we have many, many of them, are going to introduce a lot of scattering, which we cannot neglect in our measurements anymore.

Kat Setzer  05:08

Okay, okay. So what are the ways that light and sound can interact?

Kenji Ishikawa  05:13

Here in acousto-optics, the interaction between sound and light can appear in different ways depending on the properties of acoustic and electromagnetic waves. So to make things clearer, let's think about how acousto-optic interaction occurs from perspective of the speed of light. As you might know, light travels fastest in a vacuum. And when it's in a medium like air or water, its speed slows down compared to a vacuum. This happens because light interacts with the atoms and molecules in the medium, which causes its speed to decrease. The same thing happens with sound; I mean, if the sound pressure is positive, the light becomes slightly slower because the density increases. And if the pressure is negative, the light travels faster. So, of the light passing through a sound field, it gets modulated by the sound. So, this is what we call the acousto-optic effect. 

Kenji Ishikawa  06:12

And there are two different ways to use the acousto-optic effect: One is to control light using sound and the other is to use light to measure sound. For the first one, the controlling light with sound, this is achieved by generating vsery strong ultrasonic waves in a material like crystal, and by propagating laser beams through a specific acoustic pattern in a material, we can, for example, scan the direction of the laser beam or shift its frequency. And these devices are called the acousto-optic modulators, and which are essential in many kinds of optical experiments and measurement. And the second method, which is the actually the focus of our article in Acoustics Today, is using light to measure sound. And as I explained, light passing through our sound field gets modulated and the degree of its modulation conveys the information of sound. So, there are two types of changes that occur. With larger amplitude the sounds, like ultrasound above 120 DB, the light's refracted. So the amount of bending can be detected using methods like shadowgraphy or schlieren method, and for smaller sound pressures like audible sound, there is no apparent bending but still the phase of light is changed and this phase variation can be detected using optical interferometer, which can achieve very high-sensitivity measurement, so that most of recent measurement techniques for audible sounds are based on the optical interferometry.

Kat Setzer  07:52

Hmm, interesting, interesting. Okay, so how can light be used to visualize sound?

Kenji Ishikawa  07:57

Okay, so, as Samuel explained, whether we measure sound using light or microphones, the important thing when visualizing the sound is the number of points, and the spacing between them, that we sample. So the more points we can measure at finer intervals, the richer the spatial information we can capture. However, using microphones to measure at many points has its limitations, as you can imagine. So if you wanted to measure sound on a grid of 100 points, horizontally and vertically, you would need 10,000 microphones as well as a 10,000-channel AD converter. So you can see how challenging this would be. 

Kat Setzer  08:41

Yeah, right. 

Kenji Ishikawa  08:42

Yeah, and in addition, such a huge, complex microphone array can significantly alter the sound field we want to measure. So in contrast, with light, achieving massive multi-channel measurement is quite simple. Expanding the laser beam with a lens to illuminate a specific area of the sound field, and use a high-speed camera as a sensor. For instance, if the camera has a resolution of 1000 by 1000,  which is actually quite standard for an image sensor, you're observing the sound at 1 million points simultaneously. And it's impossible to place one million microphones, but with the optical measurement, you can just need one high-speed camera. So this allows us to literally visualize the spatial-temporal variations of sound pressure in a medium. Also apart from using cameras, an approach of optical visualization is to rapidly scan the directions of laser beam in a sequential manner. So this is scanning visualization is often used with a laser doppler vibrometer. In any case, optical measurements are very effective for visualizing sound fields, because there are tons of powerful devices for detecting and the controlling optical waves. 

Kat Setzer  10:02

Yeah, it sounds like a much more practical solutions in some cases. So in your article, you mentioned that the visualization of sound fields dates as far back as the Greek and Romans. Can you tell us a bit about the history of the field?

Samuel Verburg  10:14

Yes. So, just to know, that the understanding of how sound propagates has been kept us busy since antiquity and still does. So as you said, there were, or there are, early records of ancient Greek and Roman times, on theories about how sound propagates. Of course, they did not have all the technology that we have nowadays. So these theories mostly relied on analogies with how waves propagate in water, because that's something that they could actually visualize, they could see it with their naked eyes. 

Samuel Verburg  10:44

Now, moving forward, it wouldn't be until the 19th century that techniques to visualize sound were developed. Curiously, at the beginning of the 20th century, the American physicist Wallace Clement Sabine pioneered the visualization of pressure waves using optical means. So Sabine, as you may know, is widely regarded as one of the fathers of modern acoustics, and he was interested in knowing how pressure waves propagate inside auditoria, or theatres, opera houses, concert halls, and so on. So what he did was, he built a number of scale models of different auditoria. And then he generated very high-amplitude pressure waves using an electrical spark or small firecracker. And with an optical system, he was able to capture very detailed photographs of those high-amplitude pressure waves propagating inside the scale models. And by the way, those pictures are very beautiful. So I encourage all the listeners to go check them out. You can find them online. And it's quite impressive that Sabine was actually able to take such photographs with the equipment that he had at the time. Just to give you an idea, this was around the same time as the invention of the condenser microphone. And despite the pioneering work of Sabine and others, the technology was not there yet, because these optical systems were not that sensitive; you needed to generate very high-amplitude pressure waves in order to take these type of photographs. And maybe that's perhaps the reason why the use of optical methods was quite limited in the study of acoustics in favor of the microphone, which is what we use normally nowadays. 

Samuel Verburg  12:27

Now, a definite landmark in the fields was the work of French physicist Leon Brillouin, who in 1922 predicted the diffraction of light by acoustic waves. This is nowadays normally regarded as the birth of the field of acousto-optics. And in the following decades, there was a lot of both experimental and theoretical work, but we don't see the first, or we have to wait until the 1960s to see the first applications of acoustic optics, since this coincided with the laser technology becoming more available to researchers and scientists. And as Kenji mentioned, it's not only about observing sound using light, but also optical engineers have come up with a lot of different devices that utilize the acousto-optic effect, to actually modulate or change the propagation of light using sound waves. 

Samuel Verburg  13:23

Moving on, in the 80s and 90s, there was a development but more in terms of digital recording devices that allowed to record and process those acoustic signals. And since the beginning of the 21st century, that technology has advanced massively, and we have also super-precise optical devices, which are much less costly than what they used to be. And that's, that has made it possible to measure and visualize a huge range of sound fields. 

Kat Setzer  13:52

Very cool. So how is acousto-optic sensing being used currently?

Samuel Verburg  13:56

In recent years, and this is part of my own research, acousto-optic sensing has been used in combination with state-of-the-art signal processing, acoustic modeling and also computational methods, so that we are actually able to recover the acoustic fields in three dimensions. As we explained before, measuring the sound pressure at every point of space using microphones is quite difficult, especially if we want to go 3d. However, using acousto-optics, we're actually able to capture and visualize sound fields over large volumes, for example inside the room, or also complex radiation phenomena, in for example, in three dimensions around the sound source. 

Kenji Ishikawa  14:37

And from the application side, acousto-optic imaging is used to understand the transient physical phenomena of sound, through visualization. So for example, in 2020, we published a paper in JASA titled, "Seeing the sound of castanets." The castanet's a Spanish percussion instrument that produce the sound by clapping to wooden shells together, and in our paper, we observed the sound radiation from a castanet  using the acousto-optic imaging with a high-speed camera. What was particularly interesting was that when viewed from the side of the instrument, there's a small gap of a few millimeter between the two shells. And we were able to pass the laser beam through this tiny gap and observe the sound field inside. And by aligning the sound field within this tiny gap, we discovered that acoustic modes were generated inside the castenet's shell. So this was possible due to the ability of light to enter even very small gaps and its capability to capture transient acoustic field with high-spatial resolution.

Kat Setzer  15:49

Okay. It's funny, I actually remember when we published that article, I was like, "Oh, yeah. Castenets!"

Kenji Ishikawa  15:53

Thank you very much.

Kat Setzer  15:58

Where do you see this field going in the future?

Samuel Verburg  16:00

In my case, I... My hope is to see a more widespread understanding and adoption of this type of technology in the acoustics community, because, like, even if we acousticians are very comfortable with our microphones, which is because it's what we know, and optical setups may look a little bit intimidating for us, I strongly believe that the many fields within acoustics could greatly benefit from these type of non-invasive measurements and visualization. So that's my hope for the future.

Kenji Ishikawa  16:32

Yeah, I agree with Samuel. And also from my side, I'd like to provide a two specific examples of future developments. So, if we think of industrial applications, for example, acousto-optic sensing may be used for the transducer development, like loud speakers and headphones. So, by measuring and evaluating the spatial radiation characteristics of these devices, one can verify whether their prototype can produce sound as intended. So this can then be used to refine the device design. So this kind of feedback process may enhance the quality of products, especially in terms of the spatial characteristics, which is quite important for applications such as spatial audio production or 3d-audio experiences. Another potential future is in metrology, which is the science of measurement, focusing on how accurately we can measure physical quantities. You know, historically, the most accurate measurement methods of physical quantities, like time, length, mass, have shifted from classical type of measurement to modern optical and quantum measurement methods. And, for example, the definition of the kilogram, the unit of mass, was redefined in 2019, from a physical artifact, the kilogram prototype, to a definition based on the Planck's constant. So, for me, it's really fascinating to think that a similar paradigm shift could happen in the field of acoustics, I mean, from microphone to light. And anyways, the acoustic measurement gives us a completely different way of observing the sound from the microphones. So I believe it can be applied to a wide range of acoustic problems. And I hope that many researchers engineers will join our community and benefit from this technology.

Kat Setzer  18:29

Yeah, it sounds very, very useful... I'm on the team. Well, thank you both for enlightening me about this fascinating field of research. (Pun absolutely intended.) This was such a fun article to read, and it is really exciting to see how acousto-optic sensing may be used in the future. Thank you again for taking the time to speak with me today, and have a great day.

Samuel Verburg  18:52

Thank you so much, Kat. It was a pleasure to be here.

Kenji Ishikawa  18:55

Yeah, thank you so much.

Kat Setzer  18:59

Thank you for tuning into Across Acoustics. If you would like to hear more interviews from our authors about their research, please subscribe and find us on your preferred podcast platform.