Aloha! This is the first video in a series of 9 on surf infrasound.
Enjoy, and have an excellent weekend!
Welcome to the world of infrasound! Volcanoes, severe weather, tsunamis, meteors, earthquakes, and all large things that blow up can produce inaudible atmospheric sound, which can be recorded by a global microphone network. In this clip, Dr. Milton Garces introduces low-frequency acoustics as background to surf infrasound studies.
I am very excited to share the following with you! At 9:30 PM on the night of February 3, 2012 two rockfalls occurred into the Halema'uma'u Vent at Kilauea Volcano. These events were reported by HVO and recorded at our station (MENE) in Volcano, HI. This event has now been given new life as the inspiration for a percussion sextet. I am very happy to share with you "Volcano Music Mvt. II" by Jason
Thompson for percussion sextet. Please go to his site and listen to the piece and read his program
notes. UPDATE: unfortunately this piece is no longer available online. The following information and sound files are what was sent to Jason Thompson, and were used to create this wonderful artistic interpretation of a real event.
RAW SOUND:
POST PROCESSED SOUND:
We generally try to clean our tracks up a bit, and the post
processing procedure used here can be found in the following report.
(This report is also available in pdf format on Jason Thompson's website and upon request. Just shoot me an email!)
Volcanic Infrasound Event VIE120203 Halemaumau Crater, Kilauea, Hawaii
Source: Halemaumau crater, Kilauea Volcano,
Hawaii
Location: Halemaumau Crater
Origin
time: 03 February 2012 09:30:00 PM HST, 03
February 2012 07:30:00 GMT. All times in narrative are GMT. Description: Two rock falls into the Halemaumau Crater were
reported by the Hawaiian Volcano Observatory (Appendix A) on the night of 02
February 2012 (HST). Associated infrasonic signals were recorded at the local
network. IS Array: UH ISLA - MENE
Data
Quality: Collapses occurred in the quiet of
the night, with very little wind and surf noise.
Method Summary: The
initial signal characterization used automated analysis tools implemented at ISLA.
These include array processing and spectral plots. Further characterization was
completed with the aid of logarithmic scale spectrograms.
1.
Instrumentation
The MENE infrasound array has a
24-bit Reftek 130 digitizer recording four Chaparral 2.2s and one Chaparral 50
infrasonic microphones at 40 samples/second. The Chaparral 2.2s are located at elements
1-4 and have a flat response down to 0.1Hz, and the Chaparral 50 is collated
with MENE1 at element 5 and has a flat response down to 0.02Hz. Array
processing returns the direction of arrival of infrasonic signals as a function
of frequency, which allows a clear azimuthal and spectral separation of source
regions.
2.Chronology
The sequence of events for the Signals of Interest
(SOI1 and SOI2) can be seen in Figure 1, and are described below. Figure 2
shows the signals recorded by the array spanning the times for both signals of
interest. Figures 3 and 4 are
logarithmic spectrograms, and figures 6 and 7 show a four minute window around
the signals of interest.
1)Between 1800 and 2200 GMT of 2 Feb 2012 we have a partition of energy
between the Pu’u O’o Crater complex and the Halemaumau vent, with a clear
frequency and azimuth separation.
2)From around 2100 of 2 Feb 2012 to around 0315 of 3 Feb 2012 we see the
emergence of several harmonics above 1 Hz. Array detections are predominantly
from Halemaumau.
3)From 0315 GMT to ~0615 there is a drop in the overtones, ~1.5 Hz
signal stays, there is temporary drop in the total number of Halemauamau
detections, then the ~1.5 Hz signal strengthens and detections pick back up
4)After ~0615 GMT even the ~1.5Hz signal shuts off, and there is a transition
to deeper frequencies that lasts around 10 minutes. The soundscape becomes
ominously quiet above 1Hz, while there is an increase in observed detections
from the Pu’u O’o azimuth.
5)First collapse signal is at 07:29:20, and the second at 10:06:15. Both
of these signals are broadband and extend down to .02Hz (Figure 3), there is a
change in harmonic structure, with harmonics present below the ~1.5 Hz band and
above the microbarom. Pu’u O’o detections continue and are joined by
Halemaumau.
6)There is a small event at 1500 which may be seismic or volcanic in
origin.
7)After 1600 to the end of the study period, Halemaumau quiets down
while Pu’u O’o resumes radiation, but drops to a lower frequency.
3. Sound
Editing
I select the 24h time period of 3 February,
with Halemaumau as the initial prevailing volcanic source. We preserve all
features in the original data by assigning it different sample rate of 8 kHz
and 44kHz, essentially speeding up the raw data by a factor of 200 and 1102.5
times the original time. We use channel MENE5 because of its better performance
down to 0.02 Hz (50 second periods). The resulting waveforms are shown in
Figure 7.
A time compression of 200x maps 0.02 Hz to 4
Hz, which is below the response of most commercial speakers. It will map the 0.4 - 5 Hz volcanic bandpass
typical of Kilauea to 80 - 1000 Hz. For this event, there are very interesting
features beyond this bandpass that we would like to preserve.
A time compression of 1102.5 maps 0.02 Hz into
22 Hz, which good subwoofers can reproduce. It also maps 5 Hz to 5.512 kHz, well within
the range of most speakers.
Master 8 kHz wave file, MENE5BDF2012020300_8khz.
3.1. Remove data gaps, clean attack and fade,
save as esound_vie120203_8khz.wav.
3.2. ID neat signal at ~4:30 from origin,
corresponding to small event at 1500 GMT. Sounds like a pop and a woosh.
3.3. Lowpass below 1500 Hz up to dusk (0-1:52)
and leading up to the first small burst, and after 3:40 when helicopters start
up again. Save as esound_vie120203_8khz_lp.wav.
3.4. Highpass above 80 Hz, which removes the
microbarom band. Rescale by -12dB. Save as esound_vie120203_8khz_bp_sc.wav
Keeps only the volcanic signal and removed
ocean and wind noise in the low register and aircraft noise in the high
registers.
3.5. Now that I’ve deconstructed the sound,
pick a dull noise segment and deconvolve it from the original esound_vie120203_8khz.wav.
Rescale, clean up, save as esound_vie120203_8khz_decon.wav. Remove aircraft by
lowpass filtering below 1500 Hz, and save as esound_vie120203_8khz_decon_lp.wav
Figure 1. Results from the automatic processing. Top panel
is a linear-frequency spectrogram of MENE1, and lower panel is the array
processing detections using PMCC3. Events 1-4 are illustrated with colored
boxes. (1) Green: detections from Halemaumau, harmonics above 1Hz. (2) Red:
decrease in harmonics above 1Hz besides the ~1.5Hz band and initial decrease in
Halemaumau detections with a pickup of Pu’u O’o detections. (3) Yellow: drop in
~1.5Hz band and VLP/ULP event over a
10minute span, Pu’u O’o detections and break in Halemaumau detections. (4) Blue:
SOI-1 with associated coda, and harmonics, SOI-2 and stronger coda in both low
and high frequency, detections from Pu’u O’o and Halemaumau.
Figure 2. Time series around the two signals of interest.
Note the difference between the Chaparral 2.2s (1-4) and the Chaparral 50 (5).
Next figures use only MENE1 and MENE5.
Figure 3. Log-frequency spectrogram from MENE5 over .02Hz to
10hz. Note that SOI-1 and SOI-2 extend down to the lower bound at .02Hz. Also
note the changes in harmonic structures over the time period.
Figure 4. Log-frequency spectrogram from MENE1 over .1 to
10Hz. Also note the change of harmonic structures.
Figure 5. Four
minutes surrounding the first signal of interest with initial downward
deflection and longer low-frequency coda.
Figure 6. Four minutes surrounding the second signal of
interest with a compressional onset and a higher frequency coda.
Figure
7. Original waveform and time-compressed .wav files.
Appendix A
HAWAIIAN VOLCANO OBSERVATORY DAILY UPDATE
Friday, February 3, 2012 7:02 AM HST (Friday, February 3,
2012 17:02 UTC)
KILAUEA VOLCANO (CAVW #1302-01-)
19°25'16" N 155°17'13" W, Summit Elevation 4091 ft (1247 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE
Past 24 hours at Kilauea summit: The summit tilt network continued
to record DI deflation punctuated with two abrupt positive offsets due
to large rockfalls from the vent rim (Halema`uma`u Crater floor) into the lake
at 9:30 pm and midnight; the first collapse involved a portion of the north rim
while the second took a long sliver of the northeast rim; the first collapse
apparently induced secondary collapses of the inner ledge and ejected hot
spatter onto the nearby portions of the Halema`uma`u Crater floor; the second
collapse deposited a large amount of debris into the northeast side of the lava
lake. Both collapses severely disrupted the lava lake with the second
significantly dropping the level which was slowly recovering lost elevation
this morning. The most recent (preliminary) sulfur dioxide emission rate
measurement was 600 tonnes/day on January 30, 2012.
Seismic tremor levels dropped when a small spattering source appeared on the
north rim of the lake at 8:20 pm last night and remained low with the
two large rockfall seismic signals superimposed. Ten earthquakes were strong
enough to be located beneath Kilauea volcano: one north of and one beneath the
summit caldera, one within the upper east rift zone, and seven on south flank
faults.
Background: The summit lava lake is deep within a ~150 m (500 ft)
diameter cylindrical vent with nearly vertical sides inset within the east wall
and floor of Halema`uma`u Crater. Its level fluctuates from about 70 m to more
than 150 m (out of sight) below the floor of Halema`uma`u Crater. The vent has
been mostly active since opening with a small explosive event on March 19,
2008. Most recently, the lava level of the lake has remained below an inner
ledge (75 m or 250 ft below the floor of Halema`uma`u Crater) and responded to
summit tilt changes with the lake receding during deflation and rising during
inflation.
Here is a video clip from the lab vaults. This one is from the PBS show Kilauea: Mountain of Fire that aired on PBS Sunday, May 2, 2010
Pele, the Hawaiian volcano goddess, sings a continuous chorus beneath the surface of the Earth. Geophysicist Milton Garces uses infrasonic technology to listen in on whats happening in Kilauea's lava tubes.
(Note, this will also be posted under the "What is infrasound?" tab for the blog for quick and easy reference!)
What is infrasound? An ongoing, never ending, quest to
explain what we do….
"In the grand scheme of things we're all pretty much
blind and deaf" by Abstruse Goose
This comic from Abstruse Goose is a good place to start.
Take a second to acclimate yourself to the ranges of light and sound that we
cannot perceive. That low end of sound is the area we are going to explore.
Infrasound is technically any “sound” below 20Hz. I don’t
blame you if that makes zero sense. So let’s start with what sound is. Sound is
an oscillation of pressure. Our ears detect these vibrations and our brains
translate these signals into what we hear. Sound is a wave.
There are several different parameters we use to describe
waves. The most important (for reading this blog) is frequency. Frequency can
also be seen as the x axis on the above figure, using the unit Hz. Hertz (Hz) is
cycles per second. The following figure will help illustrate how waves of similar
amplitude look when they differ in frequency.
There are several sine waves in the above diagram with one
thing different about them. Let’s say that these waves are all 1 second long.
That means the X axis is time and the Y is amplitude. We can now talk about my
favorite word: frequency. Let’s start with the red line. Focus on the blue dots
at the trough or lowest point of the wave. Every time the wave returns to that
trough, it goes through one cycle. If we count those troughs we can see that it
goes through its oscillation 4 times. That is 4 cycles per our defined time
scale and since our time scale is one second that is 4 cycles per second. So
the red line is 4Hz. The orange, green, blue, and purple have more cycles in our
defined time scale and therefore they have a higher frequency. Let’s skip down
to the purple line and look at the frequency in Hz again. Let’s look at the
blue dots in the troughs again. There are 15 troughs in this wave, plus about
half a cycle left over, so 15.5Hz.
Now just for fun imagine if our time scale was 2 seconds
instead of 1, then the red line would be 2Hz and the purple line would be 7.75Hz.
Hopefully that made some semblance of sense, because now we
are going to use that to explore sound and infrasound. Frequency and amplitude
are two of the main variables that we use to describe a signal. We are again
going to start with the audible range.
The human ear can, in general, hear from 20Hz to 20,000Hz.
Above 20,000Hz is ultrasound (bats use it and dogs can hear it). Below 20Hz is
infrasound (whales, giraffes, and elephants use it to communicate).
Now, to put some of these numbers in context, let’s move to
the piano. In the middle of the keyboard is the “middle C”, which is blue in
the above figure. This musical note has a frequency of 261.626 Hz. In that
range is also the “A” (in yellow), that you always hear when an orchestra tunes
before a concert. This note is at 440Hz.
The lowest note on the piano is 27.5 Hz and the highest is 4186.01 Hz.
Now for some fun, here are some links so you can hear the different pitches.
First is concert A (440Hz)
In the Baroque period an A was 415 Hz
If we jump down an octave (440/2 = 220) this is what it
sounds like
… and if we go down another octave (110 Hz) this is what it
sounds like
In the sound range, we perceive amplitude as “loudness.” In
Infrasound we use Pascals (Pa) or a measure of pressure to quantify amplitude
of a signal.
Ok, enough of audible sound. That is not what this blog is
about, it is about INFRASOUND!
Infrasound, like I said, is anything below 20Hz (the value
for the base of human hearing). There are a few more variables I would like to
define before we go any farther. The speed of sound in air is one of them. In
general the work in this blog is done on signals that travel through the
atmosphere. Because the temperature and density of the atmosphere affect the
speed of sound, we are going to just use the speed of sound at sea level
throughout this blog. That value is ~340 m/sec or ~1116 ft/sec. You will
generally see us using the metric units.
Now that we have the speed and frequency of a wave, we can
talk about how long it takes to pass a single point, and how long the
wavelength is. Let’s go back to our sine wave shall we?
The
wavelength is a measure of length. In order to calculate the wavelength for a
wave you need the velocity, the frequency, and this equation: where is velocity, is wavelength, and is frequency.
Let’s plug in some numbers! Let’s take our 4Hz signal from
before and 340 (m/s). If we rearrange the above equation to velocity/frequency
= wavelength we get 340(m/s) /4 (/sec) = 85 meters. The length of a sound wave that is 4Hz is 85
m. We have the velocity so we know that each second 4 waves can pass by a
single point.
Alright, now that we have all of that out of the way….
Infrasound is any sound below 20Hz. That makes more sense
now right? Good.
I have a quick question. This blog is meant to share results from chasing down different infrasound signals. I am currently almost done with a post on the landslide in Alaska, and came up against a very serious question. Who will be reading this? What level should I write for? What level do you want? So audience... please tell me what you would like to see!
