Infrasound Technical
Workshop 2008
Session 1 Summary – Sensors
and Calibration
Chaparral Research
• Linearity testing of Chaparral sensor as the signal level
increases to large levels. Results show how the
diaphragm distorts with large signals
– Use the same sensor technology for characterizing the source
signal as well as signal detection at remote locations simplifies
analysis of the effects of atmospheric propagation.
– Signals over 20 Pa p-p will need correction if small scale
structure is important
• Measured frequency response of soaker hose
– Showed that foam has a flat frequency response but doesn’t
have an effect on wind noise
• New low cost sensor 30-40% less then M25
NCPA Infrasound Sensor
• Developing a ruggedized, low cost sensor with similar
characteristics to existing sensors used in the IMS arrays
– Intended for use in temporary deployments in hostile
environments
– Augment existing arrays to improve understanding of local /
regional sound sources (clutter reduction)
• Uses piezoceramic sensing elements
– Good calibration aging, insensitive to rough handling
– No change with altitude
– More vibrationaly sensitive then diaphragm
DASE MB2007
• Miniaturization of the MB2005 sensing technology
– Base sensor is passive. Various amplifiers can be used to
optimize for power or noise/response
– Much smaller form factor with reduced weight
– No adjustment necessary with altitude changes
– Currently working on prototypes
MilTEC Distributed Array
Gave an update on their distributed array, including their recent
studies with a 96-element “wagon wheel” configuration, co-located
on one of the elements of the Piñon Flat Observatory, which was
attached to a wind-screen filter.
Sensor use a piezoceramic sensing element, no active components
in current design
Similar performance to the Piñon Flat element were achieved by
using a “straight” average over the 96-element arrays. Because
each of the 96 elements were simultaneously digitized, time-shifted
averages that have less effect on the measured frequency of
infrasound signals could also be achieved for signals of interest.
How this would be utilized in an IMS station without the long-term
storage of all 96-channels has yet to be explored.
Finally, more sophisticated averaging techniques holds the promise
of achieving superior rejection of wind-noise over mechanical filters.
PTS Portable Array
Gave a report on the status of the PTS portable array,
which is near the completion of the testing phase, and will
be deployed soon near an active volcano in Chile.
Portable infrasound arrays can be used to:
•Augment existing IMS stations to improve the
understanding of regional sources of infrasound (“reduce
signal clutter”).
• Participate in “ground truth” experiments involving the
controlled demolitions of explosives as well as placement
near known infrasound sources such as active volcanoes.
• Evaluate site locations for proposed new infrasound
arrays by measuring the local/regional infrasound sources
at proposed site locations. The planned deployment of the
PTS array is expected to aid in the position of the future
I40PG infrasound station.
Development of a Portable, Controlled
Infrasound Source
One challenge in the performance evaluation of an infrasound array
is the prior absence of controlled infrasound sources which were
sufficiently powerful enough to be detected by all elements of an
array.
The University of Hawaii portable sound source is aimed to address
this problem, by utilizing a rotary subwoofer to produce infrasound as
low as 5 Hz. The current design of this system involved the
installation of the sound source in the back of a box truck.
Detections by infrasound sensors were obtained with this system for
distances as far away as 5 km.
This source would allow “in situ” calibration of a complete IMS array,
including the frequency-response of the wind-screen filters for the
microphones, something that is currently difficult to achieve by other
means.
UCSD M-Sequence Infrasound Calibrator
An alternative solution is being studied by the UCSD group.
This involves an array of eight subwoofers + associated power
amplifier(s) that allowed the generation of signals as low in frequency
as 8 Hz. This approach gives a linear sound source, which means
that signal generation/detection schemes such as M-sequences can
be efficiently used.
The UCSD group studied the use of M-sequence phase-modulated
signals. These have the advantage of producing a controlled broadband signal. Because different M-sequences are orthogonal to each
other, multiple sources can in principle be used simultaneously to
characterize the performance of an array or array element from
different directions of arrival.
The main limitation of this approach is the the lowest frequency
achieved is only 8-Hz (compared to 5-Hz for the Hawaii speaker), and
the maximum detectable range was only 240m; sufficient for singlesensor characterizations, but not for full arrays simultaneously.
End
Overview Summary
• Continued Infrasound Sensor Development
– Reduced cost, power, weight, size
– Improved characterization of existing sensors
• Various sensing technologies
– Piezoelectric, diaphragm, bellows, optical
• Development and use of infrasound sources
– Conventional speakers
– Rotory speaker
– Use of signals and processing techniques to improve
the usefulness of these sources with low S/R
Overview Summary
• Advantages of distributed arrays (sensor
carpet)
– Able to reduce wind noise as much as a
conventional hose/pipe array but without
resonances, and an ability to still work with
higher frequencies
• PTS portable array
– Development of portable array packages
– Many applications