Voices of Earth
John Holland and Josh Caswell

Voices of Earth: A Global Symphony

A Musical Simulation of Acoustic Phenomena

Computer-controlled in Real Time from Sunset to Sunset

Programming: Josh Caswell
Sound Design: John Holland

Acoustic Phenomena

There are four categories of acoustic phenomena that are generated within the various
layers or spheres of earth. These sounds vibrate and propagate through the air
(atmosphere), liquid (hydrosphere), solid (lithosphere), and organic substance
(biosphere). Within each category of phenomena, there are a variety of sounds that
vibrate within the range of human hearing. When these sounds enter the ear and brain, we experience the magnificence and wonder of speech and music.

Of course, we think of sound as something we hear or listen to. Yet, there are many
sounds that occur on earth that exist outside the range of human hearing. Many acoustic
phenomena, such as high and low atmospheric pressure systems, tidal motions,
earthquakes, etc. may be perceived if the frequencies and amplitudes of a sound are
transposed to within the range of human audibility.

Sounds on earth that are undetectable by human ears extend from tiny microacoustic
waves produced by fluctuations of the trapped particles in a normal sound field in air, to
macroacoustic wave disturbances, including seismic waves and global waves. These
sounds may oscillate at frequencies ranging from millions of cycles per second to a single
cycle within a period of days, months, or years. Sounds may travel at speeds ranging
from several feet per second, to speeds approaching the speed of light.


For the presentation of Voices of Earth, frequencies of about thirty different acoustic
phenomena – excluding sampled sounds - were converted into audible hearing range.
Sounds were scaled up, as needed, by the proportion of eleven octaves. Some adjustment was made for the few sounds that were outside the eleven-octave boundary.

Many phenomena vibrate within a range of frequencies, similar to a musical instrument
that can produce a variety of tones from low to high. For each of these sounds, a
fundamental frequency was established, as well as a low to high range. Finally, the
fundamentals were converted into musical tones.


Amplitudes of the various phenomena were translated into dynamic intensities, expressed in degrees of loud and soft. The greater the amplitude of the phenomena, the louder the tone, the lesser the amplitude, the softer the tone. Overall, the dynamic range of the phenomena vary from softest for the smallest vibrations, such as stress waves in crystals or acoustic cavitation (bubbles), to loudest for the larger rhythms. Ocean and land tides are represented by a dynamic crescendo (flood- 6 hr.) and decrescendo
(ebb - 6 hr.) applied to harmonics of the earth’s rotation.


Voice of the Earth plays for a duration of 23.7 hours, extending from sunset to sunset.
Since the earth has been rotating for 4.6 billion years, one period of rotation, or day,
represents a tiny fraction, or sample, of the earth’s lifespan. Similarly, this period
represents an infinitesimal sample of the earth’s acoustic phenomena as they occur in

Comparatively large-scale phenomena, such as the earth’s rotation, tides, light-dark
periods oscillate continuously throughout the life of the planet. These oscillations are
represented by continuous sounds that persist in the music from beginning to end.

Other phenomena vibrate in repeated segments of similar or varied durations, separated
by periods of inactivity. These oscillations – referred to as pulses – which include, but
are not limited to, ocean waves, brain waves, circadian rhythms, are represented in the
music as sustained sounds, separated by varied durations of silence. These ‘pulses’ tend
to occur periodically, but are not necessarily regular, within the course of a day.

Finally, there are sounds dubbed cycles that occur for a finite duration, typically once
only, on a particular region of the earth, within a 24 hr. period. These include phenomena such as high or low pressure fronts, atmospheric waves, seismic waves, cyclones, tidal waves.


Each of the four categories of acoustic phenomena – air, liquid, solid, organic substance –has been assigned a unique quality of sound, or timbre. These four ‘voices’ serve the
music much like different instrumental groups in a symphony orchestra. As a general
rule, tones associated with air tend to sound light, transparent; liquid tones, buoyant,
flowing; solid tones, dense, dark; organic tones, lively, animated.

Timbres for individual sounds within a category were selected and modified according to aesthetic qualities that, in our musical judgement, best suited both the unique sound, and the category to which it belonged.

Computer Program

After gathering and categorizing acoustic phenomena, converting the frequencies to
musical tones, and assigning appropriate dynamics and timbre, a computer program was
created to coordinate the musical events in time.

The goal of the computer program is to activate various algorithms, or self-contained
mini programs, that simulate the flow of acoustic phenomena in nature The computer is
programmed to organize continuous tones, pulses, and cycles within a period of a single
rotation of the earth, from sunset to sunset.

Some sounds are continuous over the course of a day, such as the Earth’s spin, or a
hurricane. In many cases, dynamic intensity of these sounds is varied, referencing
changes in amplitude.

To find the start-stop times for cycles, the program calculates the probability of
occurrence of a particular cycle. Then it plays that sound for its given duration. The
program divides the period of the earth’s rotation by the cycle duration to find the
maximum number of times a cycle might occur within a day. For example, a cyclone has
an average duration of about 8 hours, with an initial probability of occurring on a given
day. As the time of day increases, if a cyclone hasn’t occurred, the program increments
the chance of occurrence every hour.

Pulses are programmed to repeatedly start and stop, with intermittent silences (periods of inactivity) separating the sounds. The program generates a phase offset number from 0% to 100% of the sound’s duration, which gives the new start time for each pulse. The
duration of the silence between each pulse is determined by generating an upper fraction
(50% to 100%) of the total duration. A simulated Delta brain wave, for instance, lasting
for an hour, will play again following an intermittent silence of 30 to 60 minutes.

In addition to controlling start and stop times for the various phenomena tones, the
computer program automatically incorporates recorded sampled sounds into the music.

Sampled Sounds

There are a variety of sampled sounds that are integrated into the music. These range
from environmental noises, such as rain, wind, and thunder, to human speech sounds
representing more than 25 different languages. The music also includes various animal,
bird, and insect sounds that are representative of diverse regions and environments
throughout the world.

The computer program controls the selection of individual sounds, and their respective
start times.


The names of the various phenomena are displayed on a video monitor located near the
audience. Names of phenomena appear and disappear on the screen in real time as a
visual analog to the sounds that are being played. Different groups of phenomena – air,
liquid, solid, organic – appear in different colors. The name of each phenomenon is
accompanied by numbers indicating its duration, and the time elapsed.

In addition, a number of screen images of the various acoustic phenomena are projected
on the wall. Many of the images are low-orbit satellite photographs.

Satellite Images: NASA Visible Earth