Sound is very much an ephemeral phenomenon. So when acoustician Yuri Lysoivanov need to capture the unique acoustics of natural caves, he lugged all his recording furnishings to Mammoth Cave in Kentucky to analyze the reverberations and resonances. He described this judgement at a meeting of the Acoustical Society of America in Victoria, Canada, earlier this month.
Reverberation is a serious design element, especially for performance spaces. It’s not the same as an echo, which is what occurs when a sound repeats. Reverb is what happens indoors when healthy can’t travel sufficient distance to produce those echoing delays. A substitute alternatively, you get a continuous ring that gradually “decays” (fades).
American audio originator Bill Putnam was the first person to use “artificial reverb” for commercial information in the late 1940s with the Harmonicats’ “Peg o’ My Heart”—achieved by placing a microphone and loudspeaker in the souvenir studio’s bathroom. (Bathroom and subway tiles have excellent reverberation, which is why buskers play a joke on their favorite spots in New York City’s subway stations.) Today, one of the most predominating digital techniques is called convolution reverb, which uses recordings of the acoustics of honest spaces to produce highly realistic simulations of those spaces.
Recruit and response
Convolution reverb is especially useful for effects specialists in dusting, TV, and video games, who often capture so-called “impulse responses” on site and store them digitally for later use. What’s an impulse response? Bang your hands inside an empty concert hall or church. That’s the impulse. (A starting roscoe or a popping balloon are also good impulses.) The sound reflections you assent to are the building’s response. Record both impulse and response, then match the acoustic profile with a recording of just the impulse for reference, and you can pull out a model of the room’s reverberations.
“It’s a misconception that all cave spaces are these monstrous caverns with extra-long reverb times.”
There are now vast libraries of impulse effects for almost any kind of space available— concert halls, stadiums, cells, tombs, churches, even the interior of a van—via sampling reverb software match Altiverb. But there are surprisingly few samples of cave acoustics, according to Lysoivanov, a composer, music processor, and recording engineer who teaches at Flashpoint Chicago, a campus of Columbia College Hollywood. He ascribes this in part to low demand, stemming from a lack of knowledge encircling just how interesting cave acoustics can be. Indeed, that was what actuated him to undertake the project in the first place.
“With so many available elbow-room and concert hall impulse responses, there really isn’t much prominence paid to caves,” says Lysoivanov. “I think it’s a misconception that all hole spaces are these large caverns with extra-long reverb times. There’s a favourable share of that, [but] many cave spaces are smaller and have most pleasant sounding reverberation.”
That’s why he chose Mammoth Cave, the longest give way system in the world: it’s a 400-mile-long network filled with a fully variety of caves and other spaces, with an equally wide go of acoustics.
The predominant limestone and sandstone walls in Mammoth Break down tend to reflect sound in aesthetically pleasing ways. The caves are the evolve of natural geologic processes taking place over thousands (or hundreds of thousands) of years. That promises they develop all manner of interesting geometry and odd angles. The result: “all kinds of together diffusion patterns that are not likely to be found in man-made designs,” puts Lysoivanov. “No two cave environments are the same, so having an impulse response of a unique to cave gives you a unique reverb that doesn’t sound equal any other space.”
Of course, there is an inherent challenge of capturing the in situ acoustics in the first place place, since getting audio equipment in and out of caves can be difficult. Lysoivanov and his hang out with made sure their rig was mobile enough to set up and tear down in 20 two shakes of a lambs tail logs and fit in two backpacks and two duffel bags. But the equipment weighed hundreds of pounds and had to be tugged for many miles through some very tight openings.
They solely had one day to capture the acoustics of four separate spaces within Mammoth Hollow: two with pleasant reverbs (the Methodist Church and Lake Lethe) and two with measure odd resonances (the Wooden Bowl Room and Cleveland Avenue). Reverb times were surprisingly runty for the first two, lasting 1.5 and 4 seconds, respectively, with Mammoth Cavern having a decay characteristic similar to a concert hall. The impulse recordings for the Impassive Bowl Room and Cleveland Avenue showed low, sustained resonances that commitment be ideal to add depth and tension in sound design, according to Lysoivanov.
Lysoivanov desire love to go back to Mammoth Cave in the future to make even various impulse recordings. “The [National Park Service] scientists were extremely excited about our work and extended our research permit so that we could get back,” he says. “They said that, to their recollection, they’ve not in any way had people go in to do acoustic analysis of their caves. I would be thrilled to establish a body of work that captures the sounds of these beautiful accommodations.”
DOI: Journal of the Acoustical Society of America, 2018. 10.1121/1.5067715 (About DOIs).