How can we work with the atmosphere as a modulation source in modular synthesizer system? How can we understand the significance of working with the atmosphere in this way?
Both questions have many answers. The first set of answers are more technical the second more scientific, artistic, and philosophical. Here I touch on both questions in the context of the work that I have been doing with a synthesizer module that I have created called El Duende. The module is built around a barometric pressure sensor.
El Duende creates control voltages (CVs) that correspond to fluctuations in barometric pressure. When pressure rises the voltage goes up, when pressure falls the voltage goes down.
Patch this CV into a voltage-controlled oscillator (VCO) and you now have an audio barometer that will track pressure changes relative to the moment that the module is turned on. If the pressure shoots up you hear a corresponding rise in the pitch of the oscillator.
What are we hearing when we listen to air pressure changes?
A barometer weighs the atmosphere. When the pressure changes, we hear the changing weight of the atmosphere above us.
What are the factors that influence the weight of the atmosphere?
Global Factors
The spinning of the earth on its axis drives atmospheric dynamics in two ways, the atmospheric tide and the Coriolis effect. The atmospheric tide is a diurnal cycle of pressure changes that correspond to the heating and cooling of the atmosphere over the 24 hours that it takes the earth to rotate on its axis. The sun warming the atmosphere causes air to rise, lifting its weight causing a corresponding drop in pressure. As the sun sets the air cools, its weight and mass fall back to the earth causing pressure to rise. The daily cycles of rising and falling air are the atmospheric tide.
The Coriolis effect refers to the way that the spinning earth deflects the mass of the air in opposite directions north and south of the equator and in so doing determines the direction of the prevailing winds and the associated patterns of air pressure fluctuations. The strength of this effect is greater towards the poles.
Continental and Regional Factors
Air-flows across oceans and continents also influence the pressure changes in any given location. Land absorbs and releases heat energy from the sun faster than the ocean leading to pressure differences that shape air-flows depending on your proximity to a significant land mass or body of water. The topography and vegetation of your surroundings also influences the patterns of air pressure changes that you might observe in any give location.
A large tropical forest creates its own atmospheric dynamics that are related to the way it dissipates the incoming solar radiation, cooling the atmosphere through the transpiration of trees and plants, and dissipating the sun’s energy further through the metabolic processes of all the organisms that are part of the forest.
The connection between air pressure and biology has another dimension when we consider what the air is made of (particularly its oxygen, nitrogen and carbon content) and the relationship of these constituents to the metabolic processes of life.
Local factors
Depending on the conditions, a plane on its final approach to land, a truck, car, bicycle or person passing in the street, movements of body and breath, can all influence our air pressure observations.
Under the right conditions, when a bird, bat, or butterfly flaps its wings it might deflect the air in a way that influences what you hear when you listen to pressure changes through your synthesizer.
To better understand how small movements might affect pressure fluctuations it helps me to think of the air as analogous to water. When an insect flaps its wings, it creates ripples that propagate through the fluid medium that is the air. Whether or not these pressure waves can register in a sensing system depends on the background state of the air and the proximity of the movement to the sensor.
These local, transient pressure changes occur within the larger context of seasonal and diurnal barometric pressure dynamics, operating on a global scale, related to the daily cycle of the earth spinning on its axis and its yearly journey around the sun.
Techniques
How can we work with these barometric pressure fluctuations as a modulation source in a modular synthesizer system?
There are three broad categories of modulation that I have been exploring; frequency modulation (FM), timbral modulation, and event generation.
When air pressure changes produce audible shifts in the frequency of an oscillator we are doing a form of FM. The oscillator is the carrier wave for air pressure information. Like FM radio, a higher frequency carrier wave is modulated by lower frequency oscillations.
Low frequency oscillators (LFO’s) can also be frequency modulated by the air pressure information. This form of FM with a lower frequency carrier wave can help us to appreciate air pressure changes across longer time scales.
Air pressure information can be used to modulate timbre directly, (for example, via a filter cutoff, wave-folder, or oscillator sync) or indirectly via audio-rate FM or LFOs that are responding to the pressure changes patched to timbral parameters.
On the air pressure module there are three gates that fire when pressure rises, is steady, or falls. These gates reflect short term trend lines and give us an indication of what the pressure is doing moment to moment. These gates can be used for event generation. For example, I have often patched the steady gate to trigger of an envelope so that a synth voice is activated whenever the pressure levels out enough for the gate to fire. A rise gate, patched to a slew limiter creates and envelope that can be used for volume swells when pressure is rising. The sensitivity of these gates can be adjusted on the module.
These are just a few more general technical answers to the question of how we can work with barometric pressure fluctuations in a modular system. Specific approaches to patching pressure information come down to the sensibility of the artist and their relationship to composing with the air as a subject that is both specific to their time and place and connected to global dynamics.
Expressive and Informational Dimensions
There is also an expressive dimension to this work. The tuning of audible and low frequency oscillators, the selection and shaping of timbres, the use of noise, the level of gain applied to the pressure information and gate outputs, all these things can be tweaked in a way that might allow us to express through sound what we are experiencing of the air through our other senses.
The informational and expressive layers of working with barometric pressure changes as a modulation source in a synthesizer system can exist in tension with one another. The tension comes from the fact that expressive approaches are related to affect in a way that might not be entirely compatible with processing the air as information. However, this tension is far greater when we listen to recordings of air pressure changes than when we experience sound and moving air together in a live context. Feeling the moving air can bring information and affect into a mutually reinforcing feedback loop.
The instrument
This brings us back to consider the nature of the instrument that allows us to work with barometric pressure as a modulation source. The module, El Duende, has both scientific and artistic dimensions. The scientific aspect depends on cross referencing the relative pressure changes that the instrument tracks with the absolute readings of weather observations.
El Duende’s status as an art/musical instrument is related to the expressive and affective dimensions of playing the instrument that I discuss above and to its relationship to other instruments that might do similar things or perform analogous functions within a synthesizer system.
El Duende is a chaotic modulation source. Chaos can be created in analogue circuitry. Andrew Fitch (Non Linear Circuits) and others have produced modules along these lines including chaotic oscillators, gate, and modulation sources.
These modules are part of the larger synth context for El Duende as a chaotic gate and voltage source. The relationship between patterns and unpredictability is one of the defining features of a chaotic system.
The CV and gates from El Duende are tied to the patterns of daily fluctuations in air pressure, and the patterns of pressure change that occur around storms, fronts, and significant areas of high and low pressure. The gates and CV from El Duende are chaotic because they are tied to the paradigmatically chaotic weather system.
This relationship between chaos theory and the weather comes from Edward Lorenz’s work that demonstrated the exquisite sensitivity to initial conditions that is inherent in weather forecasting (‘initial conditions’ are the weather observations that go into the forecasting model). A tiny variation in observed conditions can lead to dramatic differences in the forecast conditions days or weeks out from the measurements. While weather forecasting now incorporates ‘exquisite sensitivity to initial conditions’ into its models through ‘ensemble forecasting’, the chaotic nature of the weather system means that forecasting beyond a 2-week time-frame is impossible.
My experience listening to local, transitory pressure changes suggests that they fit with the broader definitions of chaos that I have outlined here. Which is to say that while there are broader daily and seasonal patterns, there is an inherent unpredictability to working with the atmosphere as a modulation source in a synthesizer system.