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Dr. Jules Ambroisine Captures “Room Tone” in the Stratosphere Using Weather Balloons and Lectrosonics SPDR Stereo Digital Recorder

Amsterdam, the Netherlands (February 14, 2023) — Helium balloons in the stratosphere are a hot topic these days. Academician Dr. Jules Ambroisine used regular weather balloons in his doctoral research into soundscapes 30 to 33 kilometers above the Earth. Each balloon lifted a pair of small high quality microphones and a Lectrosonics SPDR two-channel stereo digital recorder to record 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 got his first full time job in production audio at an environmental NGO in Beijing, continuing in audio postproduction 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 Michael Bolton’s tour of China and location sound work for the first Chinese zombie film.

He preferred working solo on documentary style shoots, a “one-man band” as he describes it, and what captivated his interest was working in some incredible buildings and rooms and locations that not only looked interesting but could 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 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 inquisitiveness 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.

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. As he had been using Lectrosonics wireless mic systems in his location sound days, he now looked to the company for a recording solution that would work with what he had in mind. He’d always had great results with Lectrosonics gear, which he describes as “super reliable, sounds great. All the Lectrosonics gear I’ve used.”

The solution 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 high-quality recording capability, the SPDR weighs only 162 grams, which Ambroisine found ideal 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 kg on a weather balloon payload.

Lectrosonics engineered the SPDR to work reliably and maintain clock stability even in extreme cold. The ambient temperatures in the stratosphere get down to -40 to -50° C, but the camera, mics, and recorder worked perfectly well. What suffers 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; indeed, 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. It all takes place in a few hours, and the maximum ground distance a balloon has traveled is only about 25 km. A lingering question before the first flight was, “Would the SPDR survive — 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 3-hour recording,” Ambroisine reports. “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 lands in some 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, ‘Yeah 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 important reference in the recording to help understand the sonic environment of the stratosphere, measurements on the surface of Mars bear that out as well.

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

About Lectrosonics

Well respected within the film, broadcast, and theatre technical communities since 1971, Lectrosonics wireless microphone systems and audio processing products are used daily in mission-critical applications by audio engineers familiar with the company’s dedication to quality, customer service, and innovation. Lectrosonics received an Academy Scientific and Technical Award for its Digital Hybrid Wireless® technology and is a US manufacturer based in Rio Rancho, New Mexico. Visit the company online at For spare parts and a full line of accessories, visit the company’s online store: Lectrosonics U.S. Online Accessory Store.