Across Acoustics

How loud is X-59’s shaped sonic boom?

February 26, 2021 ASA Publications' Office Episode 2
Across Acoustics
How loud is X-59’s shaped sonic boom?
Show Notes Transcript

“How loud is X-59’s shaped sonic boom?”
Proc. Mtgs. Acoust. 36, 040005 (2019);
Authors: William Jeffrey Doebler and Jonathan Rathsam
Published Online: 17 June 2020

 How loud is X-59’s shaped sonic boom? In this episode, we speak with co-authors from NASA Langley Research Center, Will Doebler, and Jonathan Rathsam. We will discuss NASA's X-59 Quiet SuperSonic Technology low boom flight demonstrator aircraft, built to generate a 75 dB Perceived Level (PL) shaped sonic boom or "sonic thump". As most members of the public have never heard a sonic boom, NASA realized a communication challenge in relaying how to describe the unique sound of a “sonic thump”. Due to this fact, the co-authors created a thermometer of acoustic levels or an “Impulsive Noise Perceived Level thermometer” to assist the public and media in understanding what the X-59 will sound like. We will examine the goals and mission of the X-59, and how the co-authors created and are using the “Impulsive Noise Perceived Level thermometer.”

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

 X-59 webpage link

Updated PL scale graphic

Impulsive noise database

Music: Min 2019 by minwbu from Pixabay.



Welcome to Across Acoustics, the official podcast of the Acoustical Society of America's publications office. On this podcast, we will highlight office research from our four publications. The Journal of the Acoustical Society of America, also known as JASA. JASA Express Letters, proceedings of meetings on acoustics, also known as Poma and acoustics today, I'm your host Malene, Walters. publications, business manager ASA. Today I will be speaking with co-authors of how loud is X- 59 shaped sonic boom, published online June 2020 in the proceedings of meetings on acoustics, also known as Poma. Our two authors are both research aerospace engineers from NASA Langley Research Center and include Will Doebler. And Jonathan Rathsam. Welcome to the program. Thank you for joining. How are you both doing today?



Doing great, thank you.



Doing well, thanks.



Perfect. Now I'm sure our listeners are eager to learn more about your backgrounds. So Will can we start with you? Could you tell us a little bit more about your background?



Yeah, sure. I'm originally from Valparaiso, Indiana. I got my undergrad in physics from Gustavus Adolphus College in Minnesota and then went on to do a master's in acoustics from Penn State. And I'm currently working remotely on a Ph.D. with Vic Sparrow, Dr. Sparrows a past ASA president, and I'm a third-year research engineer at NASA Langley.



Very nice. And a nice shout out to Dr. Sparrow as well. And Jonathan, can you tell us a little bit more about your background?



Certainly, I am originally from California. I have an undergraduate degree in physics from Grinnell College in Iowa, a Ph.D. in acoustics with another ASA past President Dr. Lilly Wong at the University of Nebraska, and a postdoctoral fellowship in acoustics with Dr. Boaz at Ben-Gurion University in Israel. And I've been at NASA Langley for 11 years. Just as a side note, I spent my first several years at NASA conducting sonic boom studies in a laboratory with human subjects in a simulator that was set up to look like a living room.



Oh, wow. That's amazing. And also another past ASA president, Dr. Lily Wong. I'm sure Dr. Wong and Dr. Sparrow will be particularly interested in this podcast. Very nice. Now, Jonathan, before we jump into your research, I'd like to take a step back so our listeners can understand how and why your research came to fruition. Can you tell us a little bit more about how NASA is using its X-59 aircraft to lift current bands on supersonic flight overland?



NASA's goal overall is to improve air travel for the public by drastically reducing travel times. So the X 59 aircraft is going to demonstrate that supersonic flight is possible without loud sonic booms. And just as a side note, the X 59 is for research purposes only, it will never carry any passengers. NASA is planning a nationwide campaign of community overflight tests with the X 59 aircraft. And we will survey the communities in that survey data is going to help enable changes in aircraft noise regulations, changing the supersonic speed limit that exists right now into a noise limit. This mission with the X 59 aircraft is comprised of multiple phases. Phase One is building and testing the X 59 aircraft. That's what we're in right now. Phase Two is acoustic validation to make sure the aircraft is as quiet as it was designed to be. And phase three is community response testing with the X 59 aircraft. And then there's Uh, excuse me a fourth phase afterwards, which is delivering the final data to the noise regulators. And for a little bit more background, NASA has been involved in SuperSonics research for 70 years. This low-boom aircraft design technology was enabled by improvements in computer simulation tools over the past few decades. About 10 years ago, there was some wind tunnel test results that showed that new approaches to aircraft shaping could have a really dramatic effect on the sounds of supersonic flight. And that success is what set the stage to begin planning for x 59 and experimental aircraft.



I see,  and so now, how does the X 59 create a sonic thump instead of a sonic boom? And just out of curiosity,  what is the difference between a sonic thump and a sonic boom?



Well, that's, that's a great question. And maybe the best way to answer that is to, to refer to our scale that we developed. And so let me if it's alright with you, I'll answer the first question first, and then maybe return. So these are two very different sounds, and one is much quieter than the other a sonic thump. Is, is much quieter than a sonic boom. And I think we'll maybe we'll get more into that a little bit later. How does x 59 create a sonic thump instead of a sonic boom, it's all about the unique shape of the aircraft. all existing supersonic aircraft produced shockwaves that pile on top of one another as they travel to the ground. And that's what creates the big boom in the ground. By contrast, x 59 has this long nose, highly swept wings and integrated engine, and a very unique external vision system. So there's no forward-facing window on the aircraft. The special design prevents the shockwaves from piling up as they travel to the ground. So the result is a sonic thump. And, you know, just for the purpose of comparison, a sonic thump sounds more like distant thunder, or a car door slam, whereas a sonic boom, sounds more like thunder from a cloud and a lightning strike that's very nearby.



I see that's really interesting. Now I'd like to turn to Will, due in part to the 1973 regulation that prohibits commercial supersonic flight overland, the X 59, quiet sonic boom is going to be tested in US communities to determine if the regulation can be overturned. Is that correct?



Yes. So NASA's job is to provide data to regulators to help them decide how to proceed with the future supersonic aircraft noise regulations. And the data that will provide includes objective measurements. So that's like recordings of the loudness of the X 59 shaped Sonic Boom. And we're also going to provide estimates of the loudness experienced by community members who complete surveys, and also will provide the responses to the community members or the responses that the community members made to these surveys. And then NASA will also be doing an analysis of this objective and subjective data to get to a summary data which which we call a dose-response curves where the dose is the estimated sonic boom, loudness or Sonic thump loudness that people experienced and response is their perception of the boom. And there's also other regulations, or well, other aspects of this regulation, including the landing and takeoff noise with aircraft and emissions, but the X 59 mission focuses just on the on root noise. So that's like the sonic boom during cruise. And the actual metric to describe the future certification standard hasn't been chosen yet. But one metric that's commonly used to describe the blooms is this Stephens perceived level metric. And to better understand this PL perceived level of scale, we recorded and computed the perceived level of various common sounds.



And I have to say, when I read your paper, that was the first time that I heard of Stephens perceived level metric. So Will, can you talk about using the Stephens perceived level scale to measure sounds? And can you expand on what the Stephens perceived level scale is?



Yes, so, yeah, like you before going to grad school and studying sonic booms, I hadn't heard of the perceived level metric either. And I didn't really know what to make of NASA's plans for 75 decibel perceived level of Sonic Boom. But the perceived level scale and other acoustic metrics just allow us to assign a single number to an acoustic event. In this case, it's the X 59, low boom-shaped Sonic Boom. And so these metrics allow us to put a sound that no one has ever heard the X 59 Sonic thump in context with other more familiar sounds



And the target loudness for the X 59 is 75 decibels as you mentioned on the Stephens perceived level scale. Could you describe the perceived, could you go into a little bit more detail about the Stephens perceived level metric?



Yeah, the sapio perceive level is a single number noise metric. And it's calculated using the one-third octave band sound pressure level spectrum of the sound. And that corresponds to a sound spectrum. And it's sort of similar to the A or C weighting curves. But it's more complex because it depends on the level of the sound and also these frequency bands. So after you compute the one-third octave, band sound pressure levels to get the song spectrum you do sort of a summation of the sounds, and it also depends on the maximum sound level.



And again, just go back to the 75 decibels. Is there a justification for 75 decibels as the target loudness?



Yeah. past lab studies showed that the onset of annoyance begins around 75 decibels on this perceived level of scale.



Okay, so just for instance, if you're sleeping on a Saturday and a leaf blower comes in, and kinda of jolts, you out sleep is that kind of annoyance? Would that fall somewhere in that 75 decibels?



I would say leaf blowers probably a little bit. Yeah, maybe if you are a super light sleeper or something. And you could sense a storm was brewing nearby, you might wake up from this distant thunder type 75 pl DB sound.



Ah, okay. And now, Jonathan, will this scale be used as part of the public survey during a community response testing?



This scale is a it's more of a tool for communicating with the public about X59 unique sound it's not really intended for the public survey itself. But since before construction began on the X 59, members of the public and the media have been asking NASA how quiet x 59 will be. And it's it's hard to describe x 59 unique sound. And it's hard to play that sound over common devices like phones or computers. Because there's so much low-frequency audio content in those signals. It can't be reproduced well, to the scale that we developed quantifies the sound level at 75 decibels and it tries to make that number meaningful via quantitative comparisons with other common impulsive sounds. Most existing loudness scales use vacuum cleaners and rock concerts or like you said leaf blowers for context. But it's also very hard to compare continuous sounds like those with impulsive sounds like a sonic thump. And so that's why we specifically chose other impulsive sounds like a basketball balance in a hand clap for comparison. And before we go on, I'd like to just return to something you asked before about a difference between a sonic boom and a sonic thump. You know, up until the construction of X 59 and recent NASA NASA testing, there's really only ever been a sonic boom. And that's the sound of any object traveling faster than the speed of sound. You know you get a sonic boom from supersonic aircraft, you get a sonic boom from a spacecraft that re-enters the atmosphere. You get a sonic boom from a bullet that travels supersonic Li You even get a small sonic boom from from a whip. When you crack a bullwhip. That's a small sonic boom you know from the tip of the bullet that's traveling faster than the speed of sound. There was no word to describe a low sonic boom or a quiet sonic boom or as we're describing as a sonic thump of the sound of a an object that's been specially designed to to minimize the the resulting the sound of the shockwave and so that's what a sonic thump is. Oh,



okay, that's interesting. That makes that makes a lot more sense. I get it now the sonic thump versus the sonic boom. And Will to go back to you for a moment. You developed a thermometer of acoustic levels to help someone such as myself. I understand what the X 59 will sound like. Could you explain your impulsive noise perceived level thermometer and how it works?



Sure. In the early stages of this work, we had a really fun brainstorming session where we sat down and listed as many impulsive noise sources that we could think of that were fairly common and that we could potentially record or simulate. And then we set out to measure each of these noise sources. And we also reached out to several colleagues outside of NASA for measurements of noise sources that we didn't have available to us. And so the Pl thermometer allows us to put these sounds in context with the X 59 shaped Sonic Boom.



And what types of impulse noises were chosen for the research and why?



Yeah, we collected noises that most people would probably heard, of course, except for the sonic booms. And we wanted a range of levels that could span this scale from soft to loud, so our noises included car door slams handclaps, as Jonathan said, nails being hammered into wood, basketballs, bouncing thunder, balloon pops, fireworks, and gunshots. Oh,



 Wow. Okay. And how were the impulse noises created or added to the database?



We had a few different ways of recording or simulating them. When we could we recorded the sounds in an anechoic chamber at NASA with a recording system that we have. And some of the other sounds were recorded in the usual environment. So the car door slams for example, or just outdoors or inside the car. The basketball bounces that we recorded were actually at NASA's fitness center, which is a large hard floor, Fieldhouse style gym. And the hardest to record for us was the thunder recordings. So the first attempt I made I recorded the sounds I left my office window open overnight, I placed a microphone near the window so it hopefully survive any rain. And when I arrived at work, the next morning, the whole office building was super humid. And it didn't even storm overnight. Oops. But for the next several attempts we made I get a thunder recordings, I would be like constantly watching this lightning website, which shows lightning strikes in real-time. And we will be watching these storms. They look like they're coming right towards NASA. And at the last second they would veer off or they would rain themselves out. So we wouldn't get those recordings, but we set up the measurement equipment anyway. And then one day we finally got a storm that passed directly overhead and we placed a microphone outside under an overhang so it wouldn't get wet. And we finally got the recordings and I was so happy. We also got a couple of the sounds from our some external partners. The gunshot and the firework recordings were provided to us and the X 59 Sonic thump and the Concorde sonic booms were both simulated. And we also scaled the car door slams to be at different distances. So those were scaled versions of real recordings. And so for each of these sounds when we could we made several recordings and weird calculate the perceived level of each of the recordings and we placed the median perceived level we measured and the range of the perceived level we measured on this PL thermometer so we could present these things graphically.




I see, just again out of curiosity, how long did it take to get the thunder sounds? How many weeks or months did it take?



Well, luckily these thunderstorms are pretty common in the fall and summer in Virginia, which is where NASA Langley is and I would say it took at least a month I think to get to get the recording. I would be watching this website every day I would have it on one of my monitors like okay, is it gonna be today? We finally got it.



Very nice. And again, Will in your Poma paper, I noticed that you showcase two versions of the Impulse Noise PL thermometer chart. Can you explain the difference between the two charts? 



Sure, the one of the thermometer figures has Yeah, like I said the median perceive level the range of perceived level and it also shows the distance from the source to the receiver so sound source to the microphone and all that That's included on the on the chart. And this is for a more technical audience who, you know, understands that sound. Noise noises sound different as you move further away from the source. And not every hand clap sounds exactly the same, for example. And, yeah, this is for a more technical audience. But we simplified the scale, also and included just the median perceived level on version of the scale. That's for a more general audience. Okay, so



I would probably understand the scale for the general audience more than the first chart?



Sure. But we can, we could certainly get the understanding from every sound sounds a little bit different. And depending on how far away you are, things can be quieter louder. It just takes up a little bit less space, I guess, to the simple thermometers a little bit cleaner and easier to explain, like right offhand.





I see. Okay. And, Jonathan, I want to ask, what were your lasting conclusions? And how could this thermometer be used in other areas?



We were using Car door slams and distant thunder to describe what a sonic thump sounds like, long before we made these recordings. So I was kind of relieved when the when we finished these measurements, and it showed that the levels of the sonic thump really are comparable to you know, distant thunder and car door slams. The database of high-quality waveforms that we recorded and analyzed is available online. So anyone else who may need to communicate with the public about the loudness of unique impulsive sounds relative to more common sounds? can use it. And the levels can be expressed in terms of any relevant noise metric, not just Pl.



Oh, very good. And so anyone can use this database of sounds. And we'll add that, will actually add that website to the details of the podcast, so everyone can take a listen. Thank you.






Will do you have any ongoing next steps for this research?



Well, the X 59 is currently under construction at the Lockheed Martin skunkworks in California. And it's scheduled to fly for the first time in the summer of 2022. So we're quite busy preparing and planning for the X 59 completion, and its acoustic validation. And then finally, the community response test flights. But specifically for the PL thermometer, we have dreams of filling out the lower end of the scale with even softer impulsive noises, such as keyboard clicks or pen clicks. And maybe once the extra 59 begins flying, we could add some potentially super fun impulsive noises like the pop of a champagne bottle, for example. And if listeners have ideas of other impulsive noises that they'd like to see or include on the scale of us analyze, I hope they'll reach out to us.




Very good. I'm sure I'm definitely sure you'll get varying ideas. And lastly, Will are there any closing details you would like to share anything else you would like to relay to our listeners about the research?



I guess supersonic flight is going to be coming to you in the next couple years. So keep your ears open.



And Jonathan, 



I think the only thing I would say is I would direct listeners to the other website where there's a lot more information and you can you can take a look at x 59 getting constructed in a you know time-lapse video. So that's slash capital X five, nine, for more information about the mission.



Very nice. I will definitely take a look myself. So I want to say thank you so much for joining us today. It was a pleasure. And good luck with all of the research and the X 59. I'm excited and I will keep my ears open for supersonic flight. That's very, very exciting. So thank you again for joining.



Thank you Malene’. This was very fun.



Yeah, thank you so much.



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