Sound is very much an ephemeral phenomenon. So when acoustician Yuri Lysoivanov wanted to capture the unique acoustics of natural caves, he lugged all his recording equipment to Mammoth Cave in Kentucky to analyze the reverberations and resonances. He described this experience at a meeting of the Acoustical Society of America in Victoria, Canada, earlier this month.

Reverberation is a critical design element, especially for performance spaces. It's not the same as an echo, which is what happens when a sound repeats. Reverb is what happens indoors when sound can't travel sufficient distance to produce those echoing delays. Instead, you get a continuous ring that gradually "decays" (fades).

American audio engineer Bill Putnam was the first person to use "artificial reverb" for commercial recording in the late 1940s with the Harmonicats' "Peg o' My Heart"—achieved by placing a microphone and loudspeaker in the recording studio's bathroom. (Bathroom and subway tiles have excellent reverberation, which is why buskers have their favorite spots in New York City's subway stations.) Today, one of the most popular digital techniques is called convolution reverb, which uses recordings of the acoustics of real spaces to produce highly realistic simulations of those spaces.

Call and response

Convolution reverb is especially useful for effects specialists in film, TV, and video games, who often capture so-called "impulse responses" on location and store them digitally for later use. What's an impulse response? Clap your hands inside an empty concert hall or church. That's the impulse. (A starting pistol or a popping balloon are also good impulses.) The sound reflections you hear are the building's response. Record both impulse and response, then compare the acoustic profile with a recording of just the impulse for reference, and you can extract a model of the room's reverberations.

"It's a misconception that all cave spaces are these large caverns with extra-long reverb times."

There are now vast libraries of impulse responses for almost any kind of space available— concert halls, stadiums, rooms, tombs, churches, even the interior of a van—via sampling reverb software like Altiverb. But there are surprisingly few samples of cave acoustics, according to Lysoivanov, a composer, music producer, and recording engineer who teaches at Flashpoint Chicago, a campus of Columbia College Hollywood. He attributes this in part to low demand, stemming from a lack of knowledge about just how interesting cave acoustics can be. Indeed, that was what inspired him to undertake the project in the first place.

"With so many available room and concert hall impulse responses, there really isn't much attention paid to caves," says Lysoivanov. "I think it's a misconception that all cave spaces are these large caverns with extra-long reverb times. There's a fair share of that, [but] many cave spaces are smaller and have very pleasant sounding reverberation."

That's why he chose Mammoth Cave, the longest cave system in the world: it's a 400-mile-long network filled with a wide variety of caves and other spaces, with an equally wide range of acoustics.

The predominant limestone and sandstone walls in Mammoth Cave tend to reflect sound in aesthetically pleasing ways. The caves are the result of natural geologic processes taking place over thousands (or hundreds of thousands) of years. That means they develop all manner of interesting geometry and odd angles. The result: "all kinds of unique diffusion patterns that are not likely to be found in man-made designs," says Lysoivanov. "No two cave environments are the same, so having an impulse response of a specific cave gives you a unique reverb that doesn't sound like any other space."

Of course, there is an inherent challenge of capturing the in situ acoustics in the first place, since getting audio equipment in and out of caves can be difficult. Lysoivanov and his assistant made sure their rig was mobile enough to set up and tear down in 20 minutes and fit in two backpacks and two duffel bags. But the equipment weighed hundreds of pounds and had to be lugged for many miles through some very tight openings.

They only had one day to capture the acoustics of four separate spaces within Mammoth Cave: two with pleasant reverbs (the Methodist Church and Lake Lethe) and two with rather odd resonances (the Wooden Bowl Room and Cleveland Avenue). Reverb times were surprisingly short for the first two, lasting 1.5 and 4 seconds, respectively, with Mammoth Cave having a decay characteristic similar to a concert hall. The impulse recordings for the Wooden Bowl Room and Cleveland Avenue showed low, sustained resonances that would be ideal to add depth and tension in sound design, according to Lysoivanov.

Lysoivanov would love to go back to Mammoth Cave in the future to make even more impulse recordings. "The [National Park Service] scientists were very excited about our work and extended our research permit so that we could come back," he says. "They said that, to their recollection, they've never had people go in to do acoustic analysis of their caves. I would be thrilled to build a body of work that captures the sounds of these beautiful spaces."

DOI: Journal of the Acoustical Society of America, 2018. 10.1121/1.5067715 (About DOIs).