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REVEIL 2021: Listening to Bats, 1-2 May

A Little Brown Bat at Arcadia, in a binaural decode from an ambisonic recording.
A Big Brown Bat, recorded in my back yard using an A/B stereo array.

Last year, as part of Soundcamp’s REVEIL festival, I had the great pleasure of moderating a discussion about the sound world of bats, and how listening to bats can help us better understand and protect them. This coming Saturday 1 May I’m looking forward to revisiting the topic as part of REVEIL 2021. Joining our discussion as last time will be researcher Erin Ruggiano, and this time Erin and I will be able to meet in person at the Massachusetts Audubon Arcadia Wildlife Preserve to hunt for bat sounds. Using sensors that can modulate a bat’s calls into the human hearing range, we hope to give REVEIL’s global audience an audible glimpse of the Big Brown Bat (Eptesicus fuscus), the Eastern Red (Lasiurus borealis), and with luck, the endangered Little Brown Bat (Myotis lucifugus). The discussion starts 7:30 PM Massachusetts time, UTC-4. For our UK and Europe friends that’s the wee small hours!

On a recent survey at Arcadia, I was able to pick up this recording of a Little Brown Bat using my Sennheiser ambisonic microphone. I wasn’t sure I’d get anything on that mic, which is only rated up to 20 kHz, but I was able to get some decent (if a bit noisy) recordings of this bat hunting at around 50 kHz.

Last night I set up a pair of LOM Uši Pro mics in an AB array in my backyard, and got some really clear recordings of a Large Brown Bat.

In both cases I recorded to a Sound Devices MixPre6 operating at 192 kHz sampling rate. I decoded the ambisonic recording using the Harpex-X plugin in Reaper, and did the pitch-shifting and filtering in Izotope RX. I generally prefer to pitch-shift bat recordings by changing the sampling rate and simply reading the file more slowly; that preserves the original data and doesn’t introduce artifacts. Also, it gives an interesting insight into how the bat might be listening, by letting you hear the reverb tails as the calls bounce off of objects (and, ideally, moths).

I plan to spend more time this spring testing different mic arrays for bats; my intention is to get bat recordings that are not only as clean as possible, but can also give us spatial information about the bats movements. I suspect that some of the quick echos we get from these recordings are the bat’s calls bouncing off of buildings.

Bats already had an undeserved bad reputation before the coronavirus pandemic, but since then, it’s been the easy – but unproven – assumption that humans got SARS-CoV-2 from bats. In fact, bat rescuers take precautious to avoid giving the virus *to* bats, though it’s not known whether bats can become ill from the virus.

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Wood Frogs in a vernal pool at Audubon Arcadia, Easthampton MA

Wood frogs in a vernal pool

Wood frogs are starting to wake up and call to each other in vernal pools around here. I visited a large pool just behind the visitor’s center at Mass Audubon’s Arcadia Wildlife Refuge. I got some great recordings of frogs there exactly a year ago, working with Maine-based sound artist Steve Norton. I’m pretty sure that’s the last time I did anything in person with a friend with no mask and no distancing.

If you listen with headphones, be aware that some of these frogs get very loud. There’s one in the left channel who sounds like he’s croaking right into the mic – which is possible, since I recorded with a pair of LOM Uši Pro omnis dangled maybe an inch above the surface of the water. In between this frog’s sharp, high barks, you can hear a quiet sort of moan, perhaps the air he’s building up to make the next loud call. The land edge of the pool is just under the left mic, so he may be sitting on that, making his calls closer, and perhaps even louder for bouncing off of both the surface of the water and the underside of the wooden walkway.

After this flurry, the frog calls died down more suddenly than they started. The clip ends with a rustling sound, perhaps our soloist hopping away.

In the photo you can see two neon-red hairy puffs – those are the Uši Pro in a pair of the windshields we make here at EIS HQ.

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Window pass-through for microphone cables

I cut down a pool noodle (a long foam floatation toy for swimming pools) to serve as a window pass-through for the birdfeeder microphones. The foam cuts easily with a pocket knife and does a decent job of keeping out the last cold drafts of late winter, though I still take it out at night. A 2-inch deep slice at the halfway point is enough to pass the microphone cable through without compromising the window seal.

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Eponymous microphones, part 1 / Chewtoy windscreens, part 3

We get a lot of birds at our Ears In Space HQ birdfeeders starting in the late winter: black capped chickadee (the state bird), tufted titmouse, dark-eyed junco, woodpeckers (downy, hairy, red-breasted, and, rarely, pileated), white-breasted nuthatch, cardinal, house finch, and a few others. We have to take down most of the feeders when spring arrives and the local black bears wake up very, very hungry. But in the meantime, it’s busy – and noisy – at the feeders outside our kitchen window.

I built a stereo pair of small omnidirectional mics with a 40-ft. cable. I then attached it to one of the birdfeeder poles and ran the cable inside, to a mini mixer connected to a powered speaker and small recorder. Linda made fake-fur windscreens, smaller versions of the one she made for my Sony D-100 but this time with velcro closures, wrapped around smaller versions of the rubber chew toy from the previous post.

I’ve started building small microphones from electret condenser capsules by Primo as well as anonymous ones I’ve picked up over the years. I was going to call the Mike’s Mics, but that seemed too obvious; my friend Liz suggested I call them “Eponymous.” Liz is a poet and editor, and I trust her advice on words.

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Maple sap sounds on our live stream

EDIT 3/17/21: It’s been cold again here and the sap has temporarily slowed/stopped. Also, we’ve been noticing more bird activity in the trees a little bit behind this tree. So we’ve moved the streambox back there for the time being. The best time to listen is probably early in the morning our time, EST in the US (recently UTC-4 instead of UTC-5, thanks to the antiquated foolishness called Daylight Savings Time).

The sounds of running maple sap are now on our live stream on the Locus Sonus soundmap.

The sap is running in New England and beyond, and is harvested through the late winter/spring to be boiled down into the gold of the north, maple syrup. You’ll also hear locals like black-capped chickadee, tufted titmouse, and downy woodpecker.

We connected a pair of LOM Uši Pro and stuck them under the lid with magnetic clips, then ran them into our SoundcampStreambox.(More information on the Streambox, including how you can make one yourself, is here.)

Video by orangecookie
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Lyman Loop, Westhampton MA

Ambisonic recording decoded to Blumlein stereo

A windy day in the Lynes Woods Wildlife Sanctuary, on the Lyman loop. Nearly still under the trees, with no other people and no virtually wildlife except for some distant crows, the sky above the trees was on the move. I love the full sound of the rush of wind high up in the sky, and the Sennheiser Ambeo microphone seems to enjoy it too. Here I have decoded the recording in Bluemlein stereo; I often find Blumlein gives a rich rendering of the sense of space, with detail and room.

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Chew toy windscreens, pt. 2

Artist and fellow ear-in-space Linda Aubry designed and made this faux-fur windscreen for my rubber chew toy basket described in part 1. So far it fits better and more easily than the Rycote fur, with some minor mods to the chew toy. It’s blocking the wind about equally as well, with no appreciable increase in high-frequency attenuation. Also, it makes the D100 look like The Heatmiser.

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Chew toy windscreens, pt. 1

The best way to keep wind noise from overwhelming your recordings is to keep a bubble of empty space between the inside of your wind screen and the capsule of your microphone.

As Chris Watson puts it, wind makes noise when it stops. With only a foam or furry windscreen, the wind stops when it hits your microphone capsule and creates noise. This is why you see sound recordists carrying around mics that look like blimps: the microphone sits inside a metal or plastic frame, which is covered with soft materials such as felt, fake fur, or other fabrics. The soft materials slow the wind as it passes through the frame, but it’s the empty space inside that makes the biggest difference with cutting wind speed and preventing the microphone from being blasted directly with wind.

All-in-one recorders may come with a small furry or foam windscreen, which do OK with keeping out low-speed puffs of air from a speaking person or a curious animal’s sniffing snout. They can’t really keep out air moving faster than a light breeze. Even third-party furries like the Rycote Windjammer made for the Sony PCM-D100 can’t slow down the wind much. But you like your all-in-one recorder for its portability and quick deployment, right? Mounting it in a blimp windscreen the size of a rugby ball will make it hard to keep the recorder in your coat pocket or handbag.

In this project, I set out to build a windscreen basket for my Sony PCM-D100, using a dog’s rubber chew toy. Don’t worry, I didn’t take a toy away from a dog, and my cats have no interest. This is another project based on an idea I got from the ever-resourceful Michael Rosenstein.

This particular brand mesh-ball dog toy has one solid hexagon, so I cut that one out to make the opening. The ball was a good size to fit over the built-in mics of the Sony, though it partially covers the signal overload lights on the front as well as some of the sockets on the sides. This is a problem even when using the Sony’s furry windscreen alone; it’s down to the design of the Sony, a company famous for well-designed products but also for sometimes sticking with poor choices like allowing the Walkman brand to be the Wile E Coyote to Apple’s Road Runner.

Next I stuffed the rubber ball up inside the Rycote Windjammer. This can take a few tries to get it in straight.

Fitting this new combination over the Sony again is a bit of a challenge. The rubber “legs” of the inner structure want to fold up inside as you are trying to make the basket go over the top completely. Using a thin piece of wood or stiff plastic (like a picnic knife) might help at this point. The Rycote as a foam skirt that normally goes up inside; I rolled that down and put a rubber band around the bottom of it. This helps keep out breezes coming from behind.

Overall, there’s definitely an improvement in how my Sony experiences wind noise; it’s by no means perfect, and I’m looking at other possibilities like lining the inside of the rubber ball with a thin layer of felt or foam.

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“Stereo” contact microphone

A two-channel “stereo” contact mic, based on a design by Boston-area sound artist Michael Rosenstein. Made by modifying a 1/8″ stereo plug => 2x 1/4″ plug cable.

I cut off the two larger jacks and soldered large piezo discs onto each lead, then sealed the connections with shrink tubing and plasti-dip. I used red dip for the piezo disc that connects to the ring of the tip-ring-sleeve (TRS) mini-plug, which would be the right channel in a stereo configuration. I put “stereo” in quotes for this mic because, while it’s a two-channel mic that works great with a mini-plug input on any audio recorder, the recordings don’t attempt to depict a sound field like we expect from a stereo microphone array.

This means you can arrange the two elements in any relation to each other and it’s never “wrong.” In the above image, I clipped the two elements to each end of a strip of heavy watercolor paper, then left it out in the gently falling Western Massachusetts snow plugged into my Sony PCM D-100 digital recorder. The paper was heavy enough that it never became waterlogged, and was stiff enough that the snow made a satisfying pop upon impact. The result was a “stereo” recording of a tiny paper landscape.

In the below photo I’ve attached them to a wire shirt hanger, an adaptation of “The Watson,” a technique invented by environmental recordist Chris Watson for a quick, cheap, and effect stereo mount. (I’m not sure Watson himself invented the name for it.)

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Ultrasonisphere

Ultrasonisphere is the first piece conceived for the Ears In Space wave field synthesis array. It is composed primarily of the ultrasonic sounds used by bats for hunting and communication, along with other environmental and synthesized sounds. The bat calls were recorded using various means to translate them into the human hearing range. 

The Ears In Space wave field synthesis array is a linear array of 24 speakers coordinated by a Max patch, which creates sonic point sources in a two-dimensional space. These virtual sources are physically accurate and retain their position regardless of the position of the listener in the room. 

Wave field synthesis is best experienced in person; however, until we can all get together in physical reality, this version of Ultrasonisphere is a binaural rendering for headphone listening, attempting to recreate the experience of the Ears In Space array. I recommend listening on headphones with the volume at a moderate level. 

Thanks to Jo Kennedy for providing some of the bat sounds. 

The idea of the Ears In Space array was born from a series of spatial audio workshops held at EMPAC at RPI in Troy, NY; deep gratitude to many people for hours of inspiration, consultation, and discussion, both during and after the workshops. 

Recorded by Mike Bullock: early evening, front yard in Florence, MA USA; early morning, village of Aulus-les-Bains, France; and Jo Kennedy: Cemetery, Basque country, Spain.

credits

released November 13, 2020 
Sounds recorded by Mike Bullock and Jo Kennedy