Recording the strange sounds of Ringing Rocks:

The trailer for the Ringing Rocks SFX library

My name is Thomas Rex Beverly and I am a field recordist based in Philadelphia. I love to travel the world searching for new sounds and experiences to explore. One of the places I was lucky to visit this past year was Ringing Rocks County Park in Pennsylvania. The park contains a field of ancient boulders that produce a uniquely resonant, chiming tone when struck.

A few years ago, my sister Emily, who now has a Ph.D. in geology, visited the park as part of a geology class field trip. She invited me to explore the park with her, but under one condition: I needed to bring my Zoom portable digital recorder along. As we walked around the field, I noticed little worn spots on the rocks where previous visitors had hit them. So, I took Emily’s rock hammer and hit one of these spots, not expecting much more than a dull thud. What I got was a pure chime-like tone that resonated with the rocks around it. I was captivated for hours – I went around to each boulder to find the rock’s sweet spot and made hundreds of recordings.

Ringing Rocks has the chiming, beautiful tones of an ancient boulder field

According to Emily, the rocks are a particular type of igneous rock called diabase, which formed as molten rock (magma) intruded between other sedimentary rocks and then cooled during the Early Jurassic epoch (~200 million years ago). This created a tabular body known as a sill, which was tilted slightly and exposed at the surface millions of years later. These unique boulder fields form because diabase is very resistant to erosion and on a slight slope. Repeated freezing and thawing during the ice ages that our planet has experienced over the past million years broke the diabase sill into large boulders through a process known as frost wedging. Water gets into cracks, freezes, and expands, and over time, this wedges the rock further and further apart and breaks it into boulders. The slope is very important though because too shallow of a slope and the boulders will be become part of the soil, rather than forming these ringing boulder fields.

The size, shape, and the boulder’s contact with the other rocks affect the type of sound generated, but something about the rocks is also unique. There are several theories about what causes the rocks to ring. One is that the iron content gives the rock its metallic resonance, but the diabase does not have a unique chemistry. If this were true, ringing rocks would be much more common. Many other theories exist, but none have been sufficiently tested to know for sure. The ringing rocks remain a mystery to geologists.

After my initial visit with Emily 4 years ago, I did a scouting trip to find the time of day with the lowest ambient noise levels and least airplane traffic. There was a lot of air traffic from New York City and Philadelphia but I was still able find many 5 – 10 minute gaps between planes where I could bang on the rocks to my heart’s content. I decided I wanted to go with a Mid-Side microphone setup and record on early winter mornings to avoid tourists, insects, and birds.

The ringing rocks remain a mystery to geologists

I recorded the library with a Sennheiser MKH 50 and 30 Mid-Side pair and a Sound Devices 702. I was thrilled to hear these rocks through these amazing microphones for the first time. I did my first recording session right after I purchased the Sennheiser microphones and was very pleased when these microphones sounded better than real life and captured the wild frequencies that are produced by striking these rocks.

I must have hit most of the thousands of rocks in the field trying to find the best ones. I ended up finding about 30 rocks that were particularly interesting. I proceeded to experiment by hitting each of those rocks in every way I could. I hit them on all sides with many different small hammers, crowbars, and sledgehammers. I found the rocks also make resonant sounds when you drag the head of the hammer or crowbar across its surface. This scraping brings out rich shimmering and sparkling harmonics and incredible textures of metal clattering on rock.

I was surprised at how the resonance and harmonics of certain rocks fluctuates with the amount of snow in the boulder field. On my second trip, there was more snow around the rocks so the pitch of certain boulders was significantly different.
The whole rock field tends to have similar resonant frequencies. There is a lot of pitch variety in the recordings but there are definitely certain pitches (lots of D♭ and G♭) that resonate the same way in multiple rocks. Some rocks have a very bright quality, while others have a distinctly dark sound, and occasionally I would find a wild inharmonic clang unlike anything I’d heard before.

There was a wonderful little moment during one of my winter recording sessions. As I was recording some repetitive hits on a beautiful minor-toned rock, a distant woodpecker joined in from the nearby forest. I would bang away at the rock for a few seconds and then I would hear the woodpecker. He would start his tapping, I would pause until he stopped, and then I would start banging again. We kept this little rhythmic counterpoint going back and forth for about 15 minutes.

As hammers strike stone, the powerful clangs resonate like church bells and bring to life a natural sonic wonder

By far, my favorite sounds were created by scraping my hammers and crowbars against the stone. I already knew that the single strikes were stunning but once I started experimenting with other ways to play the rocks I was captivated by the richness and variety of the drones brought out by scraping.
Sometimes I wish I was born in an earlier time when there were still uncharted places to explore. One way we can still explore in our completely mapped world is to find and preserve stunning natural sounds. I believe the best sounds are the ones you can’t imagine and don’t yet know exist, like the ringing rocks in Pennsylvania. I encourage you to go exploring to find some new sounds too!

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