Things That Go Boom

In this episode, we explore things that go boom: from volcanic eruptions to underwater ordinances to the (relatively) tiny explosions of gunshots. Thomas Blanford (University of New Hampshire) joins us as a cohost as we discuss the use of high-amplitude acoustic sources in research with three members of a special session on the topic from the Ottawa ASA meeting: Steve Beck (Beck Audio Forensics), Daniel Bowman (Pacific Northwest National Laboratories), and Andrew McNeese (University of Texas at Austin). 

Associated paper: Thomas E. Branford. "Summary of “Things that go boom: High amplitude acoustic sources." Proc. Mtgs. Acoust. 54, 002002 (2024) https://doi.org/10.1121/2.0001991.

Read more from Proceedings of Meetings on Acoustics (POMA).

Learn more about Acoustical Society of America Publications.

 
Music Credit: Min 2019 by minwbu from Pixabay. 

Transcript

ASA Publications (00:26)

Today we're taking a bit of a different focus than we usually do. Recently, Thomas Blanford published his session summary of “Things That Go Boom: High Amplitude Acoustic Sources” in POMA. He gave an overview of the various presentations from the Ottawa session in the article. However, rather than just interview Thomas, he'll act as a co-host. We have three presenters from that session with us as well who will talk about their research into things that go boom: Steve Beck, Daniel Bowman, and Andrew McNeese. Thank you all for being here today.

Thomas, first, can you tell us a bit about the special session? What was your goal for it and how did it come about?

Tom Blanford (01:00)

Yeah, we hosted the session in engineering acoustics and we were really interested in the kind of proliferation of these high amplitude sources that are getting used underwater, in air. And sometimes they're sources that scientists and engineers designed and other times they're sources of opportunity. And one of the things that we were hoping is that people would come with their different ideas, the way they characterize these sources, the way they design these sources, and then ultimately the way they get used, and talk about the overlap between all of these seemingly different types of sources and how these really complicated devices in the end actually work and get used.

ASA Publications (01:45)

It's very cool. It’s very interdisciplinary and such, yeah. Okay, so for everybody, can you first tell us about your research backgrounds?

Tom Blanford (01:55)

Yeah, I'm Tom Blanford at the University of New Hampshire. I do work generally in underwater acoustics, sometimes designing transducers and the sonar systems as well as the signal processing for finding objects underwater.

Danny Bowman (02:09)

Hi, my name is Danny Bowman and I work at Pacific Northwest National Laboratories. And I work in geophysical acoustics, so using very low-frequency sounds to events far away from human and natural sources. And I specialize mostly in using high-altitude platforms to record distant sounds.

Steven Beck (02:32)

Hi, my name is Steven Beck and I work for Beck Audio Forensics in Austin, Texas. I have degrees in electrical engineering, but I also studied acoustics while I was in school. Early in my career, I was working with short duration, what we call transient signals, and that worked into a 16-year consulting job with the FBI as their acoustics expert. For them I did a lot of experimental design with gunshots, which are explosions in air, and I got to analyze those results and now I'm using that in my forensic work.

Andrew McNeese (03:17)

I'm Andrew McNeese. I’m with the Applied Research Laboratories at the University of Texas. My background's in mechanical engineering and acoustics, and we primarily focus on underwater acoustics. In terms of things that go boom, we've kind of done some things on maybe both sides of the coin in intentionally making things go boom. These are generally alternatives to underwater explosions. And we've also done some work more on the kind of,  think oil and gas energy industry on offshore construction, like things that go boom that you don't necessarily want to go boom. So how do you handle that and maybe provide kind of abatement solutions to kind of meet environmental regulations and that type of thing. So been in this field for maybe 15, 20 years or so.

ASA Publications (04:00)

Cool, okay. So what do you all mean by “things that go boom”? How does this type of sound show up in each of your fields of research?

Steven Beck (04:09)

Well, gunshot muzzle blasts are explosions in air that are created when compressed gas is suddenly released, like when the bullet leaves the muzzle. My research involves recording these gunshot sounds under a variety of conditions and then analyzing how it affects those sounds. Today I use this knowledge in my forensic work as an expert.

Danny Bowman (04:34)

So, I usually think of explosions as originating on Earth's surface from volcanoes or industrial accidents, or in the air from the reentry of high velocity objects like meteors or the breakup of rockets. And we use those known sources as a template to better understand unknown energetic events, both of human and natural origin from their acoustic signature.

ASA Publications (05:01)

Okay, okay.

Andrew McNeese (05:03)

So in underwater acoustics, there's kind of a couple of players and things that go boom that you might think of. So in more Navy applications for many decades, they have deployed explosive charges. You'll hear them called things like SUS charges, so signal underwater sound. It's kind of an odd acronym for what it is, but those are deployed. They sink to a depth and they explode, which obviously creates a high-amplitude broadband pulse.

And the more kind of oil and gas industry, they do some similar things, but they use devices known as air gun arrays. But in both of these applications, you're ultimately trying to learn about either your water column, the wave guide, or the seabed and properties, you know, they're about. You know, the Navy's interested in, you know, transmission loss and propagation effects,  and oil and gas, you know, they're trying to penetrate the sea bottom to figure out what's down there. You know, they're looking for geophysical exploration type things.

Some of our research has been to create alternatives to some of these sound sources. They all work, but there's pros and cons to each of them. And then in the offshore construction world that we kind of mentioned, think one of the big fields is, say, like offshore pile driving.

In things such as like offshore wind farm and the construction of that, one of the first steps is to drive very large steel pipes into the ground. That process is very high-amplitude booms. Nobody really wants that to occur, but it is a nature of these events. And so if you can do things to help modify that or reduce that sound, that opens a lot up to you in terms of like permitting and regulations and just your basic operations.

ASA Publications (06:35)

Okay, so you actually sort of started to touch on this, Andrew, but why is understanding high amplitude acoustic sources important?

Andrew McNeese (06:44)

Well, so in the kind of propagation standpoint, you know, if you know your source signature, and that's for these type of events, they're generally very-high amplitude broadband events. So there's a lot of frequency content in them. And then if you can record this event at some known distance, and in underwater acoustics, this can be very far away, you can learn about how that sound propagates.

Many times in this field, your propagation distance is much further than your water depth. So you have a lot of interaction with kind of the air water interface and then the seabed. And not only the seabed, but any layering that happens within the seabed. So a lot of these events kind of penetrate deeply into the seabed and then they come back out. You can record these events and you can learn a lot about what's down there, what the properties are. Is that oil and gas type stuff? Is it mud? Is it sand? And also just more of kind of your oceanography type things of what your properties of the water column is through over pretty great distances.

Danny Bowman (07:42)

I think using, kind of to go back to what Andrew said, using a known source, a known size, a known location allows us to understand phenomena that have more complicated event histories. So for example, if I want to understand how the noises a volcano makes corresponds to the level of activity it's experiencing, one way I can do that is by setting off a chemical explosion in a rugged topographical area that's an equivalent of the volcano's topography, or at least an order of magnitude similar, and seeing how that sound travels across the landscape. If I can then go back to the volcano of interest and remove the influence of the landscape and the atmosphere, that gets me the source processes of the volcano's vent, which then can correlate to hazards not only to people on the ground, but, for example, ash ingestion in the commercial aircraft engines. So I think by using known sources and known locations, it allows us to understand anything unplanned, whether natural or human, typically is more complicated.

ASA Publications (08:56)

Right.

Steven Beck (08:57)

Most people have heard gunshots and they're familiar with them. But what most people don't realize is that there's a lot more going on than what they just hear. There are many sources of variation in recorded gunshots and understanding this phenomenology is important in my work as a forensic expert.

This work requires understanding everything that affects these sounds, including source characteristics, the propagation environment, and recording system effects. One of the interesting effects I've discovered is just how directional the muzzle blast is. That means that recordings that are made in front of the line of fire can sound very different than those from behind. Also, recordings from behind often get multiple sounds besides the muzzle blast, whereas in front, most of those sounds are covered up by the main explosion.

ASA Publications (09:54)

Okay, okay. So how can things that go boom be used in research?

Andrew McNeese (10:00)

So kind of like we discussed before, I think, in underwater acoustics, the goal of these things that go boom when you're doing propagation experiments is kind of generally the same. You want a high-amplitude broadband pulse that can propagate great distances and penetrate the seabed and that kind of thing. But there are some limitations when you want to do this in research. For example, using explosive charges and big air gun arrays be kind costly.

There's a lot of other considerations you have to take. So some of the work we've done is to create some alternatives to these kind of general use sound sources. And some of those kind of open doors to maybe basic research applications or things where you don't the logistical support to do explosives that you can imagine. So we've created some sources.

One’s known as the combustive sound source, it's not truly an explosion, but a combustion process where a chamber can be filled with different combustible gases and ignited to create a pulse. So it gives you a similar phenomenon, but you don't need all the support that you need when handling explosive charges. Then we've developed another device known as the rupture-induced underwater sound source. And this one becomes a bit clever, in that you have a chamber whereby a vacuum's pulled on that chamber and on the top of it is some sort of expendable disk or diaphragm. That gets lowered through the water column. It ultimately breaks and you get more of an implosion as opposed to an explosion. But this device is nice in that it's more passive. There is no explosive material. There’s no combustive gas. There's no high pressure air hoses running to it. And so it kind of opens this up to those that maybe have fewer resources or are in a more environmentally sensitive area. And another part of these is, you know, there's times when you're doing this research that you don't need the amplitude of maybe a full on explosive charge. So some of these alternative sources provide a mechanism for you to create, you know, a similar phenomenon. But you know, maybe not quite as high of an amplitude.

ASA Publications (11:57)

Very cool, very cool. 

Tom Blanford (11:58)

Andrew, could you talk about why you'd use one of these explosive sources rather than just transmit a short pulse on an echo sounder or something like that?

Andrew McNeese (12:10)

So at times you can get a much higher amplitude from an explosive charge than you can maybe your kind of standard more electromechanical source or even piezoelectrical. But with these, you can also have some depth limitations at times. With the explosive charges, you drop those down, you can reach greater depths that are harder to achieve than some other sound sources.

But really just the energy density that’s found in some of these sources is much greater. It also gives you a very broadband event. And some of these have a lot of low frequency energy in. The low frequency energy is harder to produce with kind of your more standard underwater acoustic sources. And this low-frequency energy is really what can penetrate the seabed and propagate to these greater distances, more so than the energy in other frequency bands. And yeah, the easiest way to do that is these big bang boom broadband sources.

ASA Publications (12:59)

Hmm, very cool.

Danny Bowman (13:02)

One of the great ways that we can have explosions on demand tell us more about how the atmosphere transmits sound is because you can also send off these explosions in rapid sequence. So we've had some experiments where we set up up to a ton of chemical explosives in a series with a gap of only a minute or two between them. And when you do that and you record the sounds that travels across the landscape, you'll find that the sound properties change even though only a minute has passed, and the explosions were located in the same place. So what that means is that the atmosphere itself varies enough on a seconds to minutes time scale to actually change the sound quite a bit. And the reason this matters is that you can actually use the properties of the sound to, in principle, determine the size of explosion. So if you set off some in series, you're able to say, if we were to have an unknown event that sounded like this, how certain would we be on the size and location of that unknown event, given all these changes that can happen on a minute time scale in the atmosphere?

ASA Publications (14:12)

Hmm, okay.

And Steve?

Steven Beck (14:18)

Well, with gunshots, we are still discovering different aspects of these blast waveforms and the additional sounds that are related to the main blast. We still do not have complete mathematical models to account for all of these waveform variations.

ASA Publications (14:35)

Okay, So this session discussed booms both in the air and underwater. How do booms vary depending on the setting and are there any special considerations that need to be made for your specific setting?

Andrew McNeese (14:50)

So one interesting aspect of working in underwater acoustics is you start to have very significant dependence on the depth you are within the water column. So about every 10 meters you go down, the pressure increases by about one atmosphere, or 14-15 PSI or so. And so as you start going down hundreds or thousands of meters, the hydrostatic pressure is very great.

And you can kind of take advantage of this in a number of ways.  I mean, the deeper you go, the more energy you need to kind of displace some of that water and create a boom. Or you can start taking advantage of the kind of potential energy that that provides as you go down to deeper and deeper depths. And you have some volume as it collapses will create a very high-amplitude pulse. And the amplitude of that signal is kind of directly dependent of the depth in the water column in which you are.

I do think that's one kind of big difference in working underwater versus in air.

ASA Publications (15:48)

Okay, okay.

Steven Beck (15:51)

So gunshots range from very small pops to some very large booms. And most of that is dependent on the caliber of that firearm. Gunshot blasts, as I mentioned, are also highly directional. And they do depend on a number of factors. Like I said, the source and also some of the propagation characteristics.

For instance, one of the things affecting the size of the blast is the barrel length. And most people don't realize that can have a very wide range of effect on how large the blast is.

ASA Publications (16:28)

Interesting. How would the barrel affect the size of the blast?

Steven Beck (16:28)

So that's very interesting. Most people are aware that the longer the barrel, typically the faster the bullet, the higher the muzzle velocity. But if you have a longer barrel, that means that more of that energy is expended, accelerating that bullet through the barrel. And when it finally exits, there's less compressed gas in there, and it makes a smaller muzzle blast.

ASA Publications (17:00)

Okay, so it's faster but quieter. Okay, okay.

Steven Beck (17:03)

Yes, exactly.

Tom Blanford (17:06)

Steve, Danny was describing how repeated explosions can sound really different. Would two shots in, you know, succession from the same gun sound very similar or would they sound different?

Steven Beck (17:19)

So typically in air, they're going to sound very similar, as long as everything remains the same. So there’s typically very small variation in the ammunition, but also the barrel heats up. So those two things can create small changes in the sound, but most of that is not really distinguishable. I can see these effects when I analyze a recording, but the ear typically doesn't hear it. Now, one of the things, you know, if there's wind or some other type of environmental effect that might create some changes. But, you know, two shots fired in quick succession, I think, are going to sound very similar.

Danny Bowman (18:04)

And I should follow up on that actually. He's absolutely right. Where we start to see these kind of dramatic tens of seconds to minute variations, is at ranges beyond about 10 kilometers. So I assume, that the effects you're looking at are probably at most a kilometer or two away where your recordings are. And ours are up to even hundreds of thousands of kilometers when you've gone through the stratified atmosphere for such a long distance that those variations start to add up quite a bit.

ASA Publications (18:39)

Oh, okay. So it's just like such a different scale between the two.

Danny Bowman (18:45)

Yeah, I really think the interaction of sound with Earth's atmosphere is fascinating. I mean, the one continuum you have is a solid Earth where seismic waves travel the same way almost every time. There's very few places on Earth where the structure of the Earth itself changes on a human time scale. Then you move to the ocean, which moves a lot faster than the solid Earth, but still dynamically compared to the speed of sound

doesn't have a huge amount of variation. And then you have the atmosphere, which is changing all the time. And some of those acoustic effects that can happen are extremely bizarre. For example, you can have shadow zones where sounds from an explosion would be expected to occur, but don't. And I'm aware, for example, of civil war battles that I believe have been lost because people were told, listen for the cannon shots and the cannons were firing, but they couldn't hear them. And then there's other effects where you can hear things at much greater distances than you'd expect. The Hungatanga volcanic eruption in, I believe it was the South Pacific, was audible in Alaska, which is just kind of like mind-boggling to me. And so you've got all these dynamics at play, and really one of the best ways I've found to probe them is by setting off these big chemical shots and listening for the changes.

ASA Publications (20:12)

Okay. So why is it important to understand the effects of propagation in characterizing these types of sources?

Steven Beck (20:21)

In my work I found that gunshot sounds are greatly affected by propagation and by the environment. So in my forensic work I typically have to contend with all of these propagation effects, you know, including the spherical spreading, where it gets quieter, you know, the farther it propagates, with molecular absorption because I lose high frequencies at long distances. And then we have the effects of diffraction, refraction, and reflections. I use reflections a lot, you know, and you get different reflecting characteristics as the distance increases. The other thing is that a lot of times you think you'll receive a sound, but you don't because of the temperatures causing refraction or other times you'll get sounds that travel great distances. So sometimes someone will give me a recording and I won't find it in there and then when I analyze the temperature and the distance away where we think a gunshot occurred it won't even be in the recording because the sound has refracted up and it's just completely gone.

ASA Publications (21:35)

Very cool.

Danny Bowman (21:39)

I mean, I think from a kind of understanding the source standpoint, how big was this object that entered Earth's atmosphere or what's going on at the volcano, really understanding propagation is critical. And it goes down to how much of the source's information is transmitted to great distances. And as I mentioned earlier, one of the very basic metrics we use is energy release, typically stated in kilograms TNT equivalent or tons TNT equivalent. So you need to understand how that information travels through the atmosphere in order to derive it. And then of course like the sequence of events, if we see a meteor enter earth's atmosphere and we record three pulses of sound, does that mean that fragments broke off the meteor three times or does that mean that there were three different paths through the atmosphere through which that sound could travel, and we're hearing the same event three different ways. So that's why, I know the situation is even more extreme in the ocean when you have many multi-paths that you can take, so that's why understanding long-distance sound propagation is absolutely critical for the sort of work I do.

ASA Publications (22:56)

Yeah, yeah. Andrew, can you speak to underwater sound propagation at all?

Andrew McNeese (23:02)

Yeah, so I mean, for the Navy application, in some ways, it's a complicated process that boils down to something fairly simple of, you know, you use these sources to fully understand the propagation effects in different areas. And in many ways, that kind of boils down to, you don't want to be found and you want to find the other people. So from kind of a submarine aspect, knowing the propagation you know, effects in different areas is exactly what you're ultimately after. Like it was talked about before, you kind of have some of these shadow zones and, you know, paths that, you know, where rays get refracted. And as you can kind of use these sources to help characterize an environment. I mean, that's ultimately why the Navy wants to know. And then you have oil and gas industry who, you know, they need to know propagation effects, as they're looking deep into the seabed to know where different things are and how they're characterized. You can output a relatively simple signal, but everything you receive is going to be a fairly complicated receive signal. And this is highly dependent on all the propagation effects and things that the sound encounters.

ASA Publications (24:06)

Okay, okay.

Tom Blanford (24:07)

Andrew, another side to that is when you're designing your source, how do you back out what the source is actually doing and make sure that it's working the way you hoped, versus what the environment's contributing to an individual measurement?

Andrew McNeese (24:23)

So a lot of alternatives to explosive sources that we've developed, the amplitude of those is not quite as high as an explosive source, so we are able to have fairly low sensitivity hydrophones that we can put somewhat near the source, and we don't fear that they'll just get completely demolished. As you can imagine, that is harder to do with a real explosive source. The nature of those is it's pretty difficult to get a sensor very near the source. But there are low sensitivity hydrophones and other receivers you can use to kind of help monitor things. And if you do that close enough, you can take out some of these propagation effects.

ASA Publications (24:58)

Okay, okay. So we've sort of touched on this a little bit already, but how does the nature of what you hear change as your distance from the source changes?

Danny Bowman (25:08)

Yeah, in the simplest possible case, building on some of what Stephen's been talking about, the higher frequencies attenuate more rapidly than the lower frequencies. So the kind of real-life analog of that is when you have a car with a big loud sound system, you tend to hear the bass before you hear the treble as the car approaches you, because the base is able to travel through the car and through the atmosphere more efficiently than the treble. And the same is true of large geophysical events in general. So the same way that distant thunder sounds rumbly but close thunder sounds like a gunshot applies to volcanic eruptions or meteor entries or what have you, except in certain edge cases, which I touched on earlier, where the sound can actually get steepened and shocked back up into the audio range in really unexpected ways. So I think that, as usual in science, the answer is it depends, but in general, the further the sound travels, the deeper elements are amplified compared to the sharper elements.

ASA Publications (26:19)

Steve, since you're also air propagation, do you have anything you want to add?

Steven Beck (26:25)

Okay, so I think we've addressed this before large amplitude waves traveling in the air are greatly affected by spherical spreading and also by the molecular absorption. The things that tend to affect me more in my work are the diffraction and also the reflections. I want to give a real quick example of a case that I recently worked on where there were two firearms that were separated. They were shooting at a different object, but there was a wall off to the side, you know, between the gunshots and the receiver. And the way I was able to separate out those different gunshots was by the reflection. In one case I got a very sharp reflection and in the other case it was very diffuse. And this follows the theory very well, but it enabled me to differentiate those two different shots.

ASA Publications (27:26)

Okay. Andrew, do you have anything you could add for underwater?

Andrew McNeese (27:30)

I will say, at least in my experience, in most of the underwater applications, we kind of touched on this a little before, but your propagation distance is many times much greater than the actual water depth. And so what generally happens is that you have a lot of interaction with this kind of pulse with the seabed and the air-water interface. So as you're near the source, you might get a relatively quick, short-duration pulse. But as you get further and further away, you have all of this sound interacting with the environment, bouncing off the surface, off the seabed, and back and forth. And so your signal actually becomes, in a time domain, a fairly long signal, actually, when you have this reverberation and all these reflections. I would say that's generally what happens. There are some kind of unique cases when maybe you create a pulse in what's known as the SOFAR channel. In some areas of the ocean, you can create a pulse in this channel, and it's kind of trapped in a duct where it essentially never makes any contact with the surface or the sea bottom. And so you get great propagation to long distances, and the signal has changed very little. So in this kind of unique example, since you're not reflecting off anything, your source signal remains fairly unchanged over pretty great distances.

ASA Publications (28:47)

That’s cool. Why isn't it reflecting off of the top of the water or the ground?

Andrew McNeese (28:51)

It all has to do with the sound speed profile and how the refraction occurs. That you have a sound speed minimum, and so as you begin to kind of bend these rays up or down, they're just eventually bent back into this channel. And so the sound kind of never escapes the channel due to the refraction and the sound speed profile in the water column.

ASA Publications (29:06)

Okay.

Danny Bowman (29:14)

And actually something similar exists in the atmosphere right around the tropopause, which is the temperature minimum between the troposphere and the stratosphere. So if you have a sensor floating on a balloon near the tropopause, and there's some sort of energetic event like a rocket launch or a meteor incoming, far away on the earth, the sound just travels sideways just like it does in the ocean. Never touches the ground, never touches the upper atmosphere. And it can be transmitted, as far as we can tell, quite efficiently. So my colleague Sarah Albert took a page out of oceanography and we call it the Atmos Sofar Channel.

ASA Publications (29:54)

That's hilarious.

Andrew McNeese (29:56)

Yeah, Danny, I've kind been struck by a lot of the things that you've said that I didn't really appreciate some of this upper atmosphere acoustic propagation. But yeah, as you've talked about some of this. Yeah, I think there's a lot of similarities between that and underwater acoustics.

ASA Publications (30:09)

Yeah, yeah. So a theme from this session was that these sound sources seem fairly simple since they're short, sharp, acoustic events. But they're actually pretty complicated and contain several components. How are these sounds more complicated than one would expect, and why is that complexity important?

Steven Beck (30:27)

As I mentioned, most people are familiar with gunshots. What they don't realize is that there are actually multiple sounds that are being generated with every gunshot. Now some of those are mechanical, but there are actually a couple other sources of explosions in there besides the main muzzle blast. One of those is the primer detonation. And the other one that some people are familiar with is the ballistic shockwave. Now the bullet has to be going supersonic in order to create the shockwave, but that's also a very important piece of knowledge if you're trying to understand all the aspects of this. For instance, if you receive a ballistic shockwave and then a muzzle blast, you can actually calculate the distance to the shooter. And I've done that in several cases. The other thing of interest, and I've used this in a case, is being able to find that little primer detonation. Now you're to receive that primer detonation if in front of the main blast, but if you're back behind and fairly close in, I can actually find each one of these separate events and use that to help distinguish, say for instance, who fired first, or get the sequence of firing.

ASA Publications (31:47)

Okay, okay.

Andrew McNeese (31:48)

I can say maybe one example for how you can take some of these relatively simple seeming sources and maybe configure them into what ultimately becomes a much more complicated sound source is how oil and gas industry uses air gun arrays. In many ways, an air gun array is relatively simple, or an air gun is relatively simple, in that you fill a chamber with high-pressure air and you release it, and it creates a pulse. Many times that's kind of an omnidirectional pulse. But as you start getting more and more of these sources tied together and deployed at the same time in a certain geometry and you start changing the time at which each of them go off, you can kind of start steering some of these sources and making them into a bigger directional source. You may not want all the sound to propagate horizontally through the water column if you're looking at the seabed. So you can take some of these simple sources and configure them in a much more complicated way. And I think you see that a lot in air gun arrays.

ASA Publications (32:50)

Okay.

Tom Blanford (32:51)

Andrew, with underwater sources too, bubbles end up being a big factor in the sound that you hear. What role do they play?

Andrew McNeese (32:51)

Yeah, so that's a great question. And so an explosive charge, you see this exactly. You think of the time series that's created from this as kind of just being this kind of shock front from the explosion. But the reality is, you know, after you have your kind of very high-rise shock front, you're then left with kind of explosive byproducts, some cavitation where you ultimately have a bubble cloud. And this bubble cloud has been excited and so it continues to oscillate. So an explosive charge doesn't really just have a clean impulse. It's kind of this very high shock front followed by a decaying oscillating signal. And that's your byproducts and bubble cloud that continue to oscillate after the fact.

ASA Publications (33:40)

Danny, did you have anything you wanted to add?

Danny Bowman (33:43)

Yeah, I kind of approached this field naively in the beginning, and I thought, well, I want to study propagation effects, so I want to set off two explosions that are exactly the same and do them close together and have them be really big. So I went to the test range and I said, hey, I want two explosions that are the same in the same place. And it was then that I really understood the complexity of translating theory into experiment, because you can't put two one-ton TNT equivalent piles of ammonium nitrate and fuel oil at the exact same place and detonate them a minute apart. You have to separate them. You have to decide how to initiate them. You have to decide where you're going to put them and under what weather conditions that you don't enrage the nearby town by waking them all up on a Sunday morning. And so it's kind of like the idea is always perfect until it's put into execution and it never really gets back to that state of perfection you had in your mind. It's like anything in science. It seems simple from the outside and then you start looking into it and it's really not simple, even from an experimental standpoint. And I think that's one of the reasons why geophysical acoustics to me is more interesting perhaps than seismology. I don't want to get seismologists calling the podcast annoyed with you, but everything is changing in acoustics all the time. The medium is changing, the source is changing, and it's just a lot to sort out and it's endlessly fascinating.

ASA Publications (35:19)

Yeah, yeah, it sounds like there are a lot of layers that you have to think about when you're trying to implement your experiments or just trying to understand the results from the experiments.

Tom Blanford (35:33)

Danny, when I think of an explosion on the ground, I think of the shock wave and then rocks and dirt getting thrown into the air. Can you hear all of those different things in the signals?

Danny Bowman (35:47)

Well, I can attest in my youth when we thought in a small town that, you know, setting off things that were equivalent to fireworks out in the desert was a jolly good time, that you can in fact occasionally hear fragments traveling overhead. In terms of long-distance acoustic propagation of that sort of thing, I'm not sure. I think... A lot of where I would go would be to follow up on Steven's discussion where we've recorded a big artillery cannon firing shells over and over. And sometimes you would hear the shell hit the pile of dirt before you heard the muzzle blast because the shell was traveling supersonically and the pile of dirt was closer to you. But yeah, in terms of ground explosions, the interaction with the medium really starts to matter when the explosion is buried. When it's on the surface or the air, I don't know if there's like a far-field imprint of like fragments and such flying around, but that's a really good question.

ASA Publications (36:54)

Okay, so speaking of explosions, it sounds like these sounds are frequently tied to explosions or always tied to explosions, whether they're really big explosions or really small explosions. That seems pretty dangerous. So how do you study these sources without endangering the researcher?

Andrew McNeese (37:14)

I'll say in underwater acoustics, yeah, logistically, it can be difficult. I mean, although these sources are used on a fairly regular basis by the Navy and more military applications, they can be used in the basic research world. But as you can imagine, these aren't things that you can just go on the internet and just buy. So in order to take some of these out on a cruise, you have to, first of all, have a ship that's willing to allow ordinance on board. You have to go to special piers. There are special handlers that might come with you. There’s a magazine that only they can access and deploy the sound sources. So although it can be done, for this exact reason, there is an inherent danger in some of this. And so a lot of care and efforts taken to make sure that certified and qualified personnel handle all of the charges.

ASA Publications (38:01)

Okay, so now just anybody gets to blow things up. Okay. What about with guns, Steve?

Steven Beck (38:06)

Oh, great. So with firearms, there are a lot of things that are dangerous. So whenever we're setting up experiments, I always have to generate a safety plan. And everybody has to read it and follow it. We have to make sure that everybody is behind the firing line, because we don't want anybody to get hit by any bullets. We have to make sure that they wear ear protection because these are very loud. We also want them to wear eye protection if possible. And we have to make sure that they're far enough away from the shooter that they don't get hit by any expended shells. The other thing is that sometimes there can be ricochets. You know, I've been in situations where they were shooting at a metal target and you can hear that ricochet, that zing sound. But there's been cases where they've come back and almost hit the shooter. So we do have to be careful about the targets. Sometimes I have to create loud impulsive sounds inside the city. Say an event occurred in a neighborhood and I need to go and see if I can determine maybe who fired when or something. I need to create a loud impulsive sound, and I've typically had to hire a security expert or a police officer to come in and maybe shoot not a live round but a blank. So we always have to take not only the researchers' safety into account, but also people in the neighborhood. We don't want them thinking there's something going on that might frighten them or something like that. But having a safety plan is always the first thing that I put together.

ASA Publications (39:55)

Okay, yeah, well that's good to know.

Steven Beck (39:57)

Hahaha

ASA Publications (40:00)

Danny, is there anything else for big above ground explosions or in-air explosions?

Danny Bowman (40:07)

I mean, it echoes, so to speak, what everyone else has been saying. Our job is to deploy and retrieve instruments and do data analysis. We're not trained in the logistics of setting up or executing the sources. In my professional career, every institution I've worked for has a very clear safety plan that clearly stipulates what you can and cannot do, and that is consistent with federal law and safety regulations, both that stem from the organization all the way up to the federal level. In general, just having a safety forward attitude and thinking through what you do, making sure that access control is always in place. Because you're right, I mean, whether it be a natural source or an artificial source that you can control, the sudden release of energy in any situation is dangerous. And as a scientist, I know my place, and I stay out of places I'm told to stay out of, and I think about what I'm doing, and I make sure that I understand what's going on around me just like you would in any, know, just like you do when you're driving home from work.

ASA Publications (41:23)

That's a good perspective. How does boom noise affect hearing versus other types of noise? Is it more or less harmful?

Steven Beck (41:31)

So in my work, I sometimes have to investigate hearing damage due to gunfire. So both the Occupational Safety and Hazard Association and the National Institute of Health have defined safe noise exposure levels and durations. And they've done that for both continuous and for impulsive sounds. The sound pressure level that you should never be exposed to for any duration, as defined by these organizations is 140 dB. Okay, so as an example in my work I've had to compute the 140 dB contour around a source and I've had to determine if someone was inside or outside of that distance. I want to point out that this is not trivial because of the directional aspect of gunshots.

ASA Publications (42:23)

Okay, okay. I've also heard about, you know, there's hearing issues that have to be kept in mind with underwater Like we always hear about what's going to happen to the animals underwater. Andrew, can you speak to that at all?

Andrew McNeese (42:38)

Yeah, that's a pretty major consideration and a lot of time and effort is taken before we can do any of the experiments that we do and any of these sources are deployed. Yeah, the fear is not so much for the hearing of, you know, humans that are on a ship or that kind of thing. But obviously, this is an environment where at times there may be marine mammals and fish and other critters around. And so yeah, in the same way that anybody can't just go get these sources and deploy them, there's a lot of work taken to understand where any migratory patterns may be happening, how you may affect different animals in the region. And there's certain areas and times you just can't perform these type of experiments because you know there's going to be things in the area. Pretty much always when you're on board a vessel, deploying any of sources like this, there'll be marine mammal observers on board. Their whole job is to just kind of keep a lookout for things. There's a protocol that must be followed when any of them come into the area. But yeah, you're making very high amplitude events in their world, and so you do have to keep that in mind, and you do have to take pretty extensive steps to help ensure their safety. And as I mentioned kind of earlier, some of this offshore construction where you have some of these events, say offshore pile driving, this has kind of been exactly our role of when you have some of these events that nobody really wants to be loud, but just the inherent nature of hammering piles into the seabed are loud. The regulations are different throughout the world, but generally there's some standoff distance where you can't exceed some level, and it's primarily based on not harming any of the marine life that's in that area.

ASA Publications (44:18)

Got it, got it. So help protect them since they can't really protect themselves in this matter. 

Andrew McNeese (44:20)

Exactly.

ASA Publications (44:21)

So are there any alternatives for creating similar sounds?

Danny Bowman (44:32)

You know, the surface chemical explosions are really about the only way to create acoustic waves powerful enough to travel hundreds of kilometers on command. And given that this is a time-tested sort of technique that goes back, you know, probably over a century, I think it's pretty well established. There are test ranges that do this regularly. So it's kind of, practically no, but practically also it's a reasonable way to be able to do these tests. And then, you know, if you can't do that or you want signals that are even more powerful, I've seen studies that do look at volcanoes that erupt at a regular schedule to scales that we wouldn't be able to replicate as humans or that rely on natural meteor entries into the Earth's atmosphere to create signals. So, you know, it's kind of a spectrum, but in terms of things that can be done on command, there's not much more we can do.

ASA Publications (45:41)

Right, right, just because of the scale or how large you need your explosions to be. What about for smaller explosions like yours, Steve?

Steven Beck (45:50)

Well that really depends on the application, what you're trying to do. So if I'm trying to do environmental acoustic fingerprints, in other words, you know, trying to find a reflection pattern, I need a loud, impulsive sound, but I can do something simple like slap two two-by-fours together. It makes a really loud, sharp sound, but it's not an explosion. But sometimes that's good enough to mimic what a gunshot would do.

I'm often asked about the differences in sound among different gunshots, because they do have different loudness and also different sounds that get generated, and also the difference between someone firing a live round versus a blank.

I also have to deal with lot of false alarm sounds like car backfiring, firecrackers, and door slams. And we have to be able to go out and collect these data and analyze them to know what differences to look for.

ASA Publications (46:59)

Okay, okay. And then Andrew, what about underwater?

Andrew McNeese (47:03)

So I think yes and no. I mean, I think if you're going for purely the highest amplitude signal you can kind of create in underwater acoustic experiments, it's hard to beat the energy density of an explosive charge.

However, the Navy's been interested in alternative sound sources for various reasons for many years. And some of that is just the logistical nature of things, but also improvements have been made to sensors and signal processing techniques. So in some applications, you don't necessarily need things to be as absolutely loud as possible. And so with that, yes, there's a lot of the research we've done is developing some of these alternative sources, such as the combustive sound source or these implodable volumes that have been used successfully in the field. And there are some commercially available sources, some of these air guns you can buy, there's devices known as boomers and that kind of thing that you can buy and use to create a similar phenomenon that many times can get the job done. But ultimately, if you're going for the highest amplitude signal, it's pretty well accepted that, like Danny said, it's hard to beat that of an explosion.

ASA Publications (48:12)

Right, right, okay. So do you guys have any closing thoughts?

Andrew McNeese (48:18)

So I was gonna bring up something that's kind of been a headline in the news recently, that just kind of shows some of this phenomenon that here in the past few days, there's been some headlines on the Titan submersible and some audio recordings have been released of that. Let me be clear, we've had nothing to do with that professionally, just kind of looking at these on my own time. But yeah, they're releasing audio recordings of some of these signals that they say happened about 900 miles away from the actual event. So it's just kind of this example of, you know, how high of amplitude these signals can be and how far they can actually propagate in some of these environments. But that's kind of an event that happened, I think a couple of years ago that kind of caught everybody's attention and it's been resurfaced, but it's very similar to what we're talking about here. It's just a big volume that collapsed underwater and obviously it was tragic for many people, but these acoustic recordings are kind of making a recurrence in current headlines.

ASA Publications (49:15)

Yeah.

Steve or Danny, do you have anything?

Steven Beck (49:19)

Well, I've described how gunshots are actually complex sounds. Typically, they're composed of multiple acoustic events that are happening. And it's important for me to be able to sort of decompose these and try to recognize what those sounds are. In terms of research, there are a number of groups now that are using AI and deep learning to try and do this and try and help discriminate among different gunshots or discriminate from other environmental sounds. So that's one of the areas of research that is pretty active right now.

ASA Publications (50:01)

Danny, did you want to say anything?

Danny Bowman (50:03)

Yeah, I mean, just say that, you know, studies of explosive sources are just another tool in our toolbox for understanding the natural world and keeping human lives and property safe, I think, ultimately, from hazards like volcanoes or re-entering meteors or the like. So they're a tool. They should be used appropriately and understood how best to use them. But they're just another tool in our scientific toolbox.

ASA Publications (50:33)

Tom, do you have anything overall you want to say?

Tom Blanford (50:37)

Yeah, I think it's kind of remarkable how there's all these different sources, know, small, relatively small gunshots to large explosions that propagate through the air, through the ground, through the water, through the seafloor. And

the methods that these different researchers are using are all actually fairly similar, and they're considering these similar effects in their different domains to really understand these complicated sources.

ASA Publications (51:06)

Yeah, yeah. It's interesting too, as Danny and Andrew were saying earlier, that there's actually more overlap between in the air and underwater depending on where you are, or how up you are.

Well, thank you all again the time to speak with me. It was fun learning about some of the ways impulsive sounds are both studied and used in research. 

For our listeners, if you liked this episode, please take a moment to share it with someone else who you think may enjoy it. And thank you all. Have a great day.