Over the course of several years that culminated in 2022, Amsterdam-based academician Dr. Jules Ambroisine employed weather balloons in his doctoral research into soundscapes at 30 to 33 kilometers (approximately 20 miles) above the Earth, with each balloon lifting a pair of small Sanken microphones and a Lectrosonics SPDR two-channel stereo digital recorder to help capture sounds from ascent to descent and touchdown.

Ambroisine, a native of Australia, developed his curiosity about soundscapes early in his career as a location sound recordist. He started recording and collecting room tone and ambient background sounds, in particular seeking to capture soundscapes that were free of geophony (sounds of the natural world, such as wind, thunder, waves, etc.), anthrophony (man-made sounds, including sounds from man-made objects and devices), and biophony (sounds from living organisms, such as plants and animals).

After graduating from the University of Western Sydney, Nepean with a BA Music (Hons) degree, followed by his first teaching position, he got into film and television, doing audio postproduction and sound design for some animations.

He eventually landed his first full-time job in production audio at an environmental NGO in Beijing, continuing in audio post production and getting into location sound recording for documentaries in various global locations. The documentaries explored areas where damage to the local ecosystems and over farming affected local living conditions primarily in China and African regions. Other projects included “chasing K-POP boy bands around in Seoul” for Channel V, behind the scenes at a Michael Bolton tour of China, and location sound work for the first Chinese zombie film.

He notes that he preferred working solo on documentary style shoots, a “one-man band” as he describes it, and what caught his interest was working in some buildings, rooms and locations that not only looked interesting but could also have interesting basic sounds if you could strip back all the primary sound layers to the room tone alone – i.e., room tones that he could further explore buy possibly building musique concrète compositions from, as composer Pierre Schaeffer had many decades before. He says he wondered, “Is there enough difference in sonic character among room tones to create a composition? And even more basic: why is it so hard to record clean room tone on a film set?”

Ambroisine thus began his quest for recording room tone. He often recorded late on still nights when no one else was around, thereby eliminating both anthrophony and geophony, trying to capture the differences among different types of rooms, like a bedroom full of soft furnishings versus a bathroom with hard surfaces. He soon turned his focus toward the natural world as well, venturing into remote places to get away from human and animal and weather sounds. He cites the Australian desert and Grasslands National Park in Saskatchewan, Canada as places that are “wonderfully quiet.”

The Next Level

Seeking even more quiet, Ambroisine looked up. To the stratosphere. He turned it into a doctorate project at University of Technology Sydney from 2017 to 2022. Because he’d been using Lectrosonics wireless microphone systems in his location sound days, he now looked to the company for a recording platform that would work with what he had in mind.

The answer he came to turned out to be the Lectrosonics SPDR stereo personal digital recorder, which he pairs with two Sanken COS11 microphones, and records in 24-bit 96 kHz stereo. In addition to its quality recording capability, the SPDR weighs only 162 grams (5.7 ounces), which Ambroisine found advantageous because each weather balloon also carries a GoPro camera to document the flight and a GPS locator to track the position and allow for recovery; air safety laws in Australia allow no more than 4 kilograms (8.8 pounds) on a weather balloon payload.

The SPDR is engineered to work reliably and maintain clock stability even in extreme cold. The ambient temperatures in the stratosphere get down to -40 to -50 degrees (Celsius), but the camera, mics, and recorder worked well. What can particularly suffer in such cold, though, is battery life, so Ambroisine opted to use lithium cells in anything battery powered.

The helium weather balloon hoists the payload aloft, along with a parachute to protect the electronics. For each mission, Ambroisine looked for favorable weather conditions – no winds and an unintrusive jet stream. Then he would take the balloon and payload to be released in an area about six hours west of Sydney, in West Wyalong, NSW.

The balloon ascends quite rapidly; each flight takes only a few hours despite having a trajectory several times higher than the ceiling of commercial air travel. At some point the pressure differential between the inside of the balloon envelope and the outside atmosphere becomes so great that the balloon bursts and it all comes falling down, soon to be slowed by the parachute as the air gets denser.

The maximum ground distance a balloon has traveled is only about 25 kilometers. Ambroisine says a lingering question he had before the first flight was, “Would the SPDR survive the bitter cold, high altitude, free fall, parachute landing and all? I am very happy to say it functioned exceptionally well! It survived the impact of landing and the extreme cold perfectly well. There was no need for any external batteries … there was still plenty of charge in them after the three-hour recording. Yes, they’re built nice … the design is sturdy … ergonomic. It’s very strong!”

The GPS tracker signal tells Ambroisine where to recover the equipment. Typically, the kit landed on farmland, so he’d just go knock on a farmhouse door. “’Hey mate, ah, look, I’m just doing this doctorate research, and my balloon came down on your land – do you mind if I go and get it?’ and they’re always like, ‘What mate? Yeah all right, do you mind if I come along?’ and then someone else comes along saying ‘Yeah, I reckon it came down over there!’ I met a different farmer every time.”

On one flight, a second GoPro camera ran out of battery just after burst, and the camera’s beeps picked up in the mics about a meter or so away despite the near-vacuum air density as low as about 10 millibars (sea level average is 1013 millibars). Ambroisine therefore found that sound still propagates well in low-density air and a sound will sound the same as it does on the ground, simply a small but notable reference in the recording to help understand the sonic environment of the stratosphere, and measurements on the surface of Mars bear that out as well.

Now residing in Amsterdam, Ambroisine’s plans are to continue his research and focus on sound outside of film and television and go back to teaching.

Editor’s Note: When we followed up with Dr. Ambroisine for photos to accompany this article, he offered some additional thoughts on his work and motivations. He’s allowed us to share them here:

“As far as I know, I am still the only one to use a weather balloon flight as a sound-centric field recording technique, and the only person to have recorded the troposphere and stratosphere in high-quality stereo audio. There have previously been infrasound recordings captured via balloons dating back to the late 1940s, but none I know of within the human hearing range.

“Also, to further that, it is interesting that until the recent sound recordings captured on Mars, the only example of a sound recording outside of the stratosphere was a mono recording recording captured on Venus by the 1982 Russian Venera 14 probe that captured audio before it was crushed in the atmosphere. Most recordings that are touted as the ‘sounds of space’ are sonifications of data, not what we could hear if we could actually stand there.

“On the Moon or Mars, for instance, we have so many incredible pictures but no (or not good) sound. Sound is so important to our understanding of a place and therefore important to all exploration, especially remote. Of course the Moon is a vacuum and in theory silent, but I’d love to hear it even if it is truly silent – Oh, to have a sound recording of the Moon!”