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The audio recordings featured in this program
are from Steve Rowell's ongoing Sonic Boom Archive project,
incorporating autonomous (programmed and unmanned) sound monitoring
equipment installed in the Mojave desert of Southern California,
near Harper Dry Lake and beneath the restricted airspace known
as the R-2508 Special Use Airspace Complex. Rowell is an artist
living in Los Angeles, where he is an associate director at
the Center for Land Use Interpretation.
Sonic booms are the audible effects of shock
waves caused by objects moving faster than the speed of sound
- approximately 761 MPH at sea level
at 59 °F - through the atmosphere. In this case, the
objects are various supersonic aircraft that may include the
US Air force F-16, F/A-18, F-22, and F-35 fighter jets and
NASA's F-5A and F-15B Shaped Sonic Boom Demonstrator jets.
Such exotic aircraft are common to the airspace above the Mojave
Desert.
The captured booms that you will hear in this
program are selections from an 18 month-long field recording
and data logging session spanning February 2006 to September
2007. The equipment array monitors and records acoustic signatures
above a decibel threshold, filtering out the atmospheric silence
and or low volume sounds, natural and man-made. The result
is a reverse noise gate, allowing only abrupt and loud sounds
to be recorded, thereby capturing the elusive soundscape of
a militarized airspace. A software-based buffer allows for
a few seconds to be recorded before and after the booms, thereby
providing another kind of archive of the ambient desert. This
incidental sound track includes birds' reactions to sonic booms,
weather sounds, and passing vehicles on a nearby road. The
metal roof above the equipment array room sometimes reverberates
from the severe air pressure impact of the more powerful sonic
booms.
The airspace above the equipment array is known
as the R-2508
Special Use Airspace Complex and is the most effectively
integrated multiple service Special Use Airspace in the National
Airspace System, indeed it is the most active peacetime military
air space in the world. Managed by a group representing the
complex's three primary user organizations - the Naval
Air Warfare Center Weapons Division at China Lake, the Air
Force Flight Test Center at Edwards Air Force Base, and the National
Training Center at Fort Irwin. It provides the largest single
area of overland Special Use Airspace within the United States.
The complex includes the R-2515 High Altitude Supersonic
Corridor (directly overhead) and the Black Mountain Supersonic
Corridor beginning just a few miles north of the equipment
array.
The public relations division at Edwards Air
Force Base describes the R-2508 and R-2515 as airspaces capable
of almost anything and that have "intense test and test
support traffic. Operations may include live bombings and airdrops...
lifting body experiments (like the Space Shuttle), all imaginable
types of flight testing to include supersonic flight on a daily
basis, 4 spin areas, and flight training for the USAF Test
Pilot School. This area is active at all times." The Air
Force does not provide schedules of supersonic training and
testing to members of the public, including those residents
living beneath the R-2508.
These recordings were made
by machines in an unmanned building on a remote and barren
plain in the high desert of California. The microphones used
were two unidirectional boundary mics, one weather-conditioned
for outdoor placement, the other installed in a room with an
acoustic tiled ceiling, but with some tiles removed directly
beneath the metal roof of the building. Due to the nature of
these recordings playback levels will vary considerably. Sonic
booms generate violent air pressure changes that produce bass
frequencies below the threshold of human hearing and
below what most loud-speakers can reproduce. Because of this,
frequencies below 20 Hz have been trimmed for these edits.
Headphones or high quality speakers are recommended to hear
subtletes in the ambient sounds. Maintain resonable levels
to avoid damage to speakers or hearing.
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