Field Note: Sound travels through time. The number of times a molecule compresses/rarefacts over time determines its frequency or pitch.
Field Note: A piezo disc. The top surface is coated with piezoelectric material which has a naturally occuring electronic charge.
Field Note: Wires are stripped and soldered on to piezo discs. We use create two contact mics so we can record in stereo, simulating human hearing.
Brooklyn Bound MTA B Train
Contact mics are attached to interior of the subway car near the doorway and on the window.
NOTE: This article corresponds to a sound installation for the exhibition, Geologic City, that opened on September 8th at Studio-X, a global initiative of Columbia's Graduate School for Architecture, Preservation and Planning.

Can material objects hear?

Conceptualizing sound in geologic terms is a daunting task. Both the geologic and the sonic are time-based media, defined by duration. Yet sound speaks in milliseconds, the geologic in eons. They exist in different time zones. So, given this conundrum, instead of considering how to measure sound within a geologic scale, perhaps we are better served to ask: What do geologic materials sound like or, more precisely, what do geologic materials hear? This latter question might seem ridiculously anthropomorphic, given that we are talking about material objects. So let us first consider what exactly sound and hearing are.

Sound, as we commonly know it, is the compression and rarefaction of air molecules. This compression/rarefaction is the result of some sort of physical movement, which creates oscillation or sound pressure in the atmosphere. But such oscillation can occur in any solid, liquid or gas. Therefore sound exists on many plateaus, many of which we have no inherent means of interpreting as humans.

Hearing, in its most basic form, is the translation of sound into something else or transduction. As humans, we hear sound through our ears. Sounds resonates our eardrum and enters our inner ear quivering hair cells in sympathetic vibration. This is not so different than how a microphone works. The main difference being that a microphone transduces sound into electricity whereas the ear transduces sound into neural activity.

On that note, it’s important to point out that just as a microphone is a device designed for a particular task (some designed for voice-over, some for musical instruments, others for field-recording), so too is the ear a designed apparatus. It delivers only a particular spectrum of sound that evolution has deemed useful to us as humans. That is to say, hearing is always an interpretation of sound. In short, the sound that is out in the world is not always the sound that is heard in our head.

To traverse just a bit in the realm of psychoacoustics, it is also worth mentioning that human hearing is not fixed but an evolving infrastructure. Not only are there physiological characteristics/limitations in human hearing, but also when we humans listen, when we cognitively process and thereby translate what we hear, we meet sounds with a complex cultural history and personal disposition. Although it would take some creative arguing to say that material objects listen per se, we can very easily make the case that not only do these objects hear, but they also do so in a way that is distinct to their material infrastructure.

All this is to say that sound and hearing are inherently physical and material acts.  It’s of little matter whether a sound resonates an eardrum or a piece of metal; they are both forms of hearing. The distinction lays more in how and what is heard. A piece of glass, rock, metal, they all have their own unique form of hearing. However, there is one particular type of geologic material that most closely resembles human hearing in that it not only hears sound, but also transforms it into something else: electricity.

Piezoelectric Time

Piezoelectricity describes the naturally occurring electric charge found in certain geologic materials. This property was first discovered in crystals in 1880 by Pierre Currie, who along with his wife, Marie Currie, famously went on to discover radioactivity. He observed that when physical pressure is applied to certain crystals they emit electricity. Thus he named this property after piezein, the Ancient Greek work for “squeeze.”

As many scientists in the late 19th century grappling with the many known unknowns that new scientific methods had unearthed, Currie was initially unsure why piezoelectricity existed or of its potential applications. His first instincts were spiritual. Like other scientists of his day, Alexander Graham Bell included, Currie was briefly involved in the Spiritualist movement and even sought the counsel famous medium Eusapian Pallidin. Today many practitioners of New Age mysticism hold piezoelectric crystals in high regard for their supposed spiritual quality. It is unclear what Currie’s final conclusions were on the spirituality of piezoelectricity, but his discovery had a dramatic impact upon the evolution of technology in the century to come.

Perhaps the most notable consequence of Currie’s discovery was a transformation of time. Piezoelectric crystals, such as quartz, oscillate at such consistent frequencies that, when integrated into mechanical clocks, tracking time became accurate as never before. This new technology gave us the aptly named “quartz clock.” Later it led to what became known in cinema as “crystal sync.” A quartz crystal was installed separately on both tape recorder and 16mm camera. The reliable oscillation from the crystals allowed the machinery to run at identical speeds, thus making sync sound filmmaking possible. Intrepid filmmakers, such as Jean Rouch, used this technology to invent cinema vérité. Lastly, even today, crystals regulate the circuit operations hidden beneath the computers and digital devices we use everyday. These piezoelectric materials, products themselves of an inconceivable geologic time and transformation, transform daily how we measure and regulate time.

Can we listen to what material objects hear?

However, of particular interest to this article is the relationship of piezoelectric materials to sound. Because geologic materials such as quartz crystals have an intrinsic electrical charge, they are, like the human ear, natural transducers. This allows them to have some very useful sonic applications. In one scenario, a crystal can translate voltage into sound, effectively becoming a speaker. Electricity causes the crystal to physically vibrate creating sound waves. Yet, conversely, when physical/sound pressure is applied to a crystal, it translates this occurrence into electric current. This is the basic premise behind contact microphones. A microphone made of piezoelectric material is placed in physical contact with an object. It allows us not to hear the sound around the object, but rather the resonance and vibration of that object itself as sound vibrates through the object. Metaphorically we could say this, in effect, allows us to listen to what material objects hear.

With this metaphor in mind, I’m tasked with bringing the conceptual into practice; to bring forth concrete examples of listening to what material objects hear through the use of geologic tools. To this end, I’ve constructed a simple set of simple binaural contact microphones with which to conduct fieldwork. For the record, these are likely the most affordable microphones one could possibly build. Piezo discs are available at most electronic stores for about a dollar apiece. The discs are made of a conductive metal with an incredibly thin layer of piezoelectric material across the top. Solder a couple wires onto the disc, one on the piezoelectric material and the other onto the metal. When physical pressure is applied to the disc, it is transduced into electric current. Plug the microphones into an audio recorder and, voilà, the current is converted into sound.

However, in order for these hand-made contact microphones to work effectively, thought needs to be put into where to place them. As mentioned before, unlike conventional microphones, they will not pick up sound travelling through the air. They must be in direct contact with a material object in order pick up the object’s physical vibrations, what we are calling its hearing. They also are not suited for recording especially subtle or nuanced sounds.  The sound pressure must be forceful enough to physically engage with the material object. In addition, an object that more easily vibrates and resonates, such as a pane of glass, is a better candidate than a rigid and inflexible object, such as a brick. To this end, two sites were selected for fieldwork in New York City that have particularly resonate material infrastructures and an aggressive surrounding soundscapes: the MTA subway system and the Brooklyn Bridge. These two sites also possess iconic sounds that will be at once familiar and alien to listeners as they are channeled through geologic ears.

For my decent into the subterranean I chose to record the beast of burden itself, a subway car. During my normal commute on the downtown B train, I placed my contact mics on the surface of the train in two distinct places. In the first instance, the mics are taped to the inside of the subway car near the doors. If you listen closely you can not only hear the vibrations of the train in transit, but also an electronic underworld as the doors open and close. In the second instance, mics were placed on the inside of a glass window. This material yields a brighter more reflective tone and we hear an amalgamation of exterior and interior soundscapes resonating in the glass.        

The Brooklyn Bridge was chosen not only because it is a favorite location of mine for purely aesthetic reasons and also part of my daily commute, but also because it is a suspension bridge. The suspension cables of the bridge make excellent conduits for sound, picking up moving traffic, bicycles, pedestrians, wind and the general resonance of the bridge itself. Considering the surprising amount of movement of the cables, it was especially difficult to get these microphones to make contact. In the end, best results came from using a set of wood vices. For variation I also attached mics to the steel gate at the center of the bridge. This placement brought a more metallic, tonal quality to the sound recording.

Obviously this is just a small sampling of the potential uses of piezoelectric contact mics. I would not argue that this field study offers any concrete conclusions about the New York City soundscape nor necessarily brings us closer to understanding the inconceivable reach the geologic has on our daily life. However, as a concept, it is interesting tool for reconsidering what exactly hearing is and how it can manifest in material form. Perhaps we can even listen to this concept differently and imagine ourselves wearing non-anthropomorphic ears. And if we begin shift our understanding of how we interpret sound, a time-based medium, maybe we can also begin to acknowledge that our understanding of how we move through time is also a human perspective that has hidden material consequences. The geologic moves beneath our feet, we simply pass by too quickly to hear it.

Field Note: Air molecules collide with each other, creating a chain reaction of compression/rarefaction oscillation that becomes sound pressure. This is not the same a molecules individually moving across space, that would be wind.
Field Note: The frequency response of human hearing. Notice that there is a slight peak at around 2-5kHz. This is frequency zone of intelligibity for speech. Clearly our ears were designed for a particular use.
Field Note: When pressure from vibrations is applied to a piezo disc, the current is modified between the conductant metal base and piezoelectric material.
Field Note: With a 9v battery and a simple hand-made circuit, we can turn a piezo disc into an oscillator and also produce sound.
Brooklyn Bridge
Contact mics are attached with a vice to the suspension cables and frame of the bridge.

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Field Note: As you can see from this video, even visably the bridge is quite a resonate instrument.