Sonic Deception in World War II

You may have heard of the "ghost army" of World War II—a set of rubber tanks used by the US Army to mimic active battlefields in Europe. What you may not know was the tanks were accompanied by an equally fake soundtrack of battle noises. Even more surprising? This "sonic deception" was developed by a team of the ASA's founding members whose contributions were almost lost to history. In this episode, we talk with Walter Montano (ARQUICUST Argentina Peru) about the innovations that helped save thousands of lives during the war.

Read the associated article: Walter A. Montano and Gary W. Elko. (2025) “Sonic Deception During the Second World War,” Acoustics Today 21(2). https://doi.org/10.1121/AT.2025.21.2.38

Learn more about the ASA Committee on Archives and History.

Read more from Acoustics Today.

Learn more about Acoustical Society of America Publications.

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

Transcript

ASA Publications (00:26)

Have you ever heard of the “ghost army” employed by the US Army during World War II? If you have, you may know about the rubber tanks used to mimic active battlefields. However, this ghost army actually used sounds to deceive the enemy as well. Today, we're talking to Walter Montano, who published the article, “Sonic Deception During the Second World War” in the most recent issue of Acoustics Today. Thanks for taking the time to speak with me today, Walter. How are you?

Walter Montano (00:49)

 I'm fine. I'm here, early in the morning. I'm in Lima, Peru.

ASA Publications (00:56)

Yeah, very nice. I like to have a worldwide representation, or international representation. So first, tell us a bit about your research background.

Walter Montano (01:01)

Yes. I’ve always been interested in history and understanding the past. However, it was until I was 50 years old and had spent years working in the technology and acoustics and I came back to college to study and got a degree in social science and humanity. It was then that I realized the importance of the history of science as a tool for understanding the past, which led me to focus my research on the history of acoustics. So among my first researches, one of them was finding out that urban noise measurements had been taken in Melbourne, Australia, was in 1928. This fact was completely unknown for our Australian colleagues because it wasn't included in any acoustics textbooks. Through this research I realized the importance of reviewing media publication because they provide direct insight into the social context of the acoustic pioneer's experience. So in 2022, out of my curiosity, I started reading about the people who founded the ASA in 1929. I discovered that the ASA was actually established in December 1928. Aside from eight renowned acousticians, very little was known about the other founders. This led me to join the ASA Committee on Archives and History where I have worked as a historian and promoted several projects that I will explain later.

ASA Publications (02:45)

Awesome, yeah, I'm excited to hear more about that committee. So how did you first learn about the Ghost Army from World War II?

Walter Montano (02:53)

As most people know, as you told first, the Ghost Army was an army brigade that manufactured rubber airplanes, trucks, and tanks to deceive the enemy when they took aerial photographs of military camps in England. While searching the internet for information about Harry Fletcher, I found scattered information that he and a team of scientists from Bell Labs recorded the sounds of this vehicles to reproduce the European battlefields. I believe that the lack of information was the fact that some official documents were classified until the 1990s. This discovery led me to conduct an in-depth search of library digital repositories that block Google's and other search engines. Additionally, thanks to help of Gary Elko, my co-author, who contacted the Library of Congress to share with us a copy of a document that was declassified in 1962 and remained there stored until 2025 when we found it, because there are only two or three survival copies of that report.

ASA Publications (04:08)

Very cool, very fun. So what is sonic deception and how did this idea come about?

Walter Montano (04:14)

Well, it is impossible to pinpoint when the idea came about. I think the need to make the rubber vehicles seem real was more important. The idea of sonic deception was likely perfected by electroacoustic specialists and the Harvard Psychoacoustic team. One of these acousticians had experience using sound effects to create physiological sensations among movie and theater audiences. I suppose that's where the idea came from, to reproduce recordings of real battlefield sounds using high quality electroacoustic equipment. Furthermore, since sound techniques for recreating sonic scenario in movies and animated films were already well developed by 1942, all that learning was used to create the sonic deception.

ASA Publications (05:07)

Oh, very cool, okay. So you already mentioned this a little bit, but it turns out that the work to create the Ghost Army was largely done by some early members of the ASA. A lot of names that folks have probably heard, actually. Who was initially involved, and how did the research get started?

Walter Montano (05:23)

Well, the Army and Navy contacted Bell Labs, Bell Telephone Laboratory, because the company had created the multi-channel electroacoustic installation for the movie Fantasia, named Fantasound. All people already know that important movie, which was recorded on audio multitracks. They recreated and invented the sounds for it. I mean the technicians at Bell Labs, which is why the Army and Navy needed to involve acoustic experts in that new film field. Given the high level of secrecy that this project was supposed to have, Fletcher, as director of one section at Bell Labs, organized a small team of scientists to work on the sonic deception specifically.

Besides Harvey Fletcher, three of them were founders of the ASA also, Vern Oliver Knudsen, Leon John Sivian, and Edward Christopher Wente. Other four were past president or important fellows of the ASA: Hallowell Davis, Floyd Alburn Firestone, Philip McCord Morse, Stanley “Smitty” Smith-Stevens.

And I must mention other acousticians who were fundamental in the development of technology for sonic deception, like Leo Beranek, Harold Burris-Meyer, Marvin Camras, and Carl Ferdinand Eyring. The work began with significant challenges because at first they assumed they could adapt to existing technology. But no, they realized that they would have to invent completely new and innovative technology. Hence the importance of the work of these acousticians.

ASA Publications (07:14)

It’s like a real Who's Who of the Acoustical Society. It's kind of crazy to think about Leo Beranek working on this. 

Walter Montano (07:20)

Mm-hmm. Secret.

ASA Publications (07:21)

Yeah, secretly. Okay, so what sort of equipment and techniques had to be developed to create this ghost army?

Walter Montano (07:30)

First they had to develop technology to make accurate measurements because they needed to record high-quality audio signals and sounds with the lowest possible distortion. Then they had to investigate the physical properties of different and new materials to develop, for example, high-power, low-distortion speaker, audio amplifiers and so many other electroacoustic equipments. And this led Leo Beranek to build the world's first anechoic chamber, because those speakers had to be tested at their maximum performance in an open field, a situation impossible to practice given the secrecy of the project. From there, Leo, he invented a totally absorbent room without the sound propagating to the outside.

ASA Publications (08:22)

Oh! That's so cool. So the start of the anechoic chamber. So how did they make the recordings that would eventually be used for sonic deception?

Walter Montano (08:31)

Hmm, well, this is a good question, because the big challenge was not only inventing unknown technology, but recording high-quality sounds not inside recording studios. They had to recreate real military actions in different military camps around the US, like fake natural battlefields. This was not only to preserve secrecy and keep people's attention away, but also because they had to record hundreds of sounds of different weapons, different vehicles, sounds of soldiers marching or shouting, sounds of tanks moving. This is the enormous amount of sounds that could be recreated from military action. The sounds then were recorded on portable magnetic wires because tapes didn't exist yet, whichwere specifically for use in moving vehicles. These sounds were then recorded on special glass master discs to conserve the sound's quality at each specific military station. It was a very long and challenging task.

ASA Publications (09:41)

Very interesting, okay. So as you mentioned, there were quite a lot of audio tracks that they were using. And so mixing the audio tracks they would eventually use to deceive the enemies wasn't a simple feat. How did the researchers create audio that would sound authentic to an outside observer?

Walter Montano (09:57)

Well, they had to make a large archive of sounds recorded on those glass master disks, which were patiently mixed by trained soldiers. There is no information about this, but I believe that these people were sounds or recording technicians, because they had to make precise mixes of different sounds.

In the pictures of the article, one can see that they use at least six wire players, thus recreating, for example, soldiers marching on foot next to a truck crossing a wooden bridge. Something like that.

This is recreating all the possible situations that could arise on the European battlefields. It wasn't an easy task, because the mixes were made from wires on those glass master disks, which were then re-recorded on magnetic wire to save playback. Since the disks are fragile and couldn't be used in moving vehicles, because of the needles could moving or something like that. And this was part of making the sonic deception believable. It not only the one task. In parallel, the first in-depth investigation were conducted into how the atmosphere influenced the propagation of sound waves, how air humidity and temperature affect their propagation and so on.

ASA Publications (11:23)

Okay, okay. Then how was the Ghost Army actually implemented on the battlefield?

Walter Montano (11:28)

Hmm. Another big question, because the sound technician responsible for reproducing those records of European battlefields had to adjust the power of the amplifiers and the angle of the speaker according to temperature, humidity, wind speed, etc., to make the deception believable at the distance where the enemies were located. The document explained that they had charts specifying the sound levels to be used depending on the weather condition and distance, so that the sounds would reach the enemy with the best possible impression. So, to enhance the deception, the sounds were played at night or in low-visibility conditions, because what is heard but not seen makes the sound source more believable.

ASA Publications (12:20)

Interesting, interesting. That makes a lot of sense.

Walter Montano (12:22)

It was early psychoacoustic techniques.

ASA Publications (12:26)

Yeah, right, right. And you actually go into that more in your article. Do you have any closing thoughts?

Walter Montano (12:33)

Yes, we have only the legacy left by the research of Fletcher, Wente, and the rest of ASA members, namely the great advances that were achieved in the fields of electroacoustic and communication. Gary and I are convinced that this group of acousticians served the progress of science and human life, saving thousands of lives in Europe. We must point out that some of the acousticians who worked for sonic deception lived until the late 1980s and kept this secret throughout their lives. The article, we think, fulfills an outstanding debt to history by naming all these persons as essential parts in their research and development of technology that made sonic deception possible for European battlefields.

ASA Publications (13:25)

Right, absolutely. Yeah, give them credit even though, you know, they couldn't receive the credit for how many people they helped during their lives, but now we can know. Yeah, that makes sense. To pivot a little, how can our listeners get involved with the ASA Committee on Archives and History?

Walter Montano (13:41)

Well,  our colleagues could participate with us at the Committee on Archives and History or in the future committee that will be created to celebrate the ASA centenary. Here I want to invite our colleagues to take an interest in preparing for the ASA centennial or to be part of it. It is still a draft, but the project that will be presented to the ASA board aims to publicize the lives of the lesser-known founders of the ASA, many of whom were important acousticians, too. One initiative we have is that it would be interesting if ASA have its own wiki with the biography of those acousticians.

Other preliminary idea that came out was reaching out to the municipalities, schools, or colleges where many of the ASA founders were born, raised, and studied, to involve them in the ASA Centennial. For this task we'll need all possible help. Therefore, we have a lot of work ahead of us throughout the Centennial.

ASA Publications (14:51)

Well, I look forward to hearing about all these new projects and seeing them unfold. Thank you again for sharing this fascinating bit ASA history with us. I encourage all of our listeners to go read the article in Acoustics Today. It's absolutely fascinating, and there's some really cool images with it And as I mentioned, I'm really excited to see what the ASA Committee on Archives and History comes up with and encourage our listeners to get involved. Have a great day. Thank you!

Walter Montano (15:16)

Have a great day, too, and thank you for allowing me to share my additional thoughts on the article. Have a good day. Bye

ASA Publications (15:24)

Absolutely.