PSN-L Email List Message
Subject: Re: GULF OF CALIFORNIA INFRASOUND DETECTION
From: ChrisAtUpw@.......
Date: Wed, 4 Jan 2006 22:10:18 EST
In a message dated 05/01/2006 00:40:06 GMT Standard Time, AHrubetz@.......
writes:
Hi Chris,
The detector is a Validyne pressure transducer modified by Dr. Chris
Haywood at SMU. I don't know what modifications were made, and have borrowed the
detector to see if I could record anything of interest with it. There are 8
inlets into a small volume inlet box. A metal diaphram measures pressure
variations differentially with respect to a reference pressure in a larger
volume. The bridge circuit electronics is contained within the detector and is
powered by 12 volts furnished by an external power supply located about 50' from
the detector. It has an internal 1 Hz low gain cutoff. I have no specs on
the unit. I am only using two of the inlets, connected to two 50' porous hoses
extended inline from opposite sides of the detector. Because of space
restrictions, the hoses are doubled back on each other at a length of 25'.
What type instrument are you using, and have you recorded any
quake-related events?
Hi Al,
Lately, I have been mostly occupied with a seismometers for schools
project. One of the sensor types that I have tried uses piezo electric disks.
They have a high voltage sensitivity.
The disks also seem to be quite good at detecting low frequency sound /
air pressure changes; low flying helicopters in particular! I have a twin
flask arrangement which I have been doing some initial experiments with. One
flask acts as a stable air pressure reservoir and the other as a low pass
acoustic filter. I prefer to remove ambient noise as far as possible, rather than
detecting it and then filtering the signal. This also stabilises the
temperature of the detector disk reasonably well. Reducing ambient wind and other
noises is reported to be a major consideration.
One of the older types of infrasound array used a large circular pipe
connected to a sensor at the centre by several hard radial pipes, typically six
to eight. You should be able to use porous hose for the circle and maybe PVC
hose / water pipe for the radial arms? This might give better noise
cancellation and a higher sensitivity than two folded pipes? A circle of 25 ft radius
has a perimeter of about 157 ft. This could give you a collection area of
nearly 2,000 sq ft. Air pressure eddies less than this size will tend to cancel
out. You might get a hf noise cut-off at about 20 Hz. I note that some of
the current CTBT arrays are using solid pipe at the centre of the spokes and
porous pipe for remainder. Porous hose is reported to change it's acoustic
characteristics with time and it is also sensitive to rain.
Is your 1 Hz figure a high frequency cut-off? I have got some
characteristics of Validyne sensors somewhere. The low range sensors are from 0.1"
water FS upwards, but I don't remember any frequency or noise specifications. I
don't know how much they cost, but a Ball Glider Variometer might well offer
both price and sensitivity advantages. They use the same principle of pressure
sensor, but with a mylar membrane. You can also get low pressure range
silicon sensors.
Regards,
Chris Chapman
In a message dated 05/01/2006 00:40:06 GMT Standard Time, AHrubetz@.....
com=20
writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000>
Hi Chris,
The detector is a Validyne pressure transducer=20
modified by Dr. Chris Haywood at SMU. I don't know what modifications=
=20
were made, and have borrowed the detector to see if I could record anythin=
g of=20
interest with it. There are 8 inlets into a small volume inlet=20
box. A metal diaphram measures pressure variations differentially wit=
h=20
respect to a reference pressure in a larger volume. The bridge=20
circuit electronics is contained within the detector and is powered b=
y 12=20
volts furnished by an external power supply located about 50' from the=20
detector. It has an internal 1 Hz low gain cutoff. I have no spe=
cs=20
on the unit. I am only using two of the inlets, connected to two 50'=20
porous hoses extended inline from opposite sides of the detecto=
r.=20
Because of space restrictions, the hoses are doubled back on each oth=
er=20
at a length of 25'.
What type instrument are you using, and have you=20
recorded any quake-related events?
Hi Al,
Lately, I have been mostly occupied with a=20
seismometers for schools project. One of the sensor types that I have tried=20
uses piezo electric disks. They have a high voltage sensitivity.=
DIV>
The disks also seem to be quite good at detecti=
ng=20
low frequency sound / air pressure changes; low flying helicopters in=20
particular! I have a twin flask arrangement which I have been doing som=
e=20
initial experiments with. One flask acts as a stable air pressure reservoir=20=
and=20
the other as a low pass acoustic filter. I prefer to remove ambient noise as=
far=20
as possible, rather than detecting it and then filtering the signal. This al=
so=20
stabilises the temperature of the detector disk reasonably well. Reducing=20
ambient wind and other noises is reported to be a major=20
consideration.
One of the older types of infrasound array used=
a=20
large circular pipe connected to a sensor at the centre by several hard radi=
al=20
pipes, typically six to eight. You should be able to use porous hose fo=
r=20
the circle and maybe PVC hose / water pipe for the radial arms? This might g=
ive=20
better noise cancellation and a higher sensitivity than two folded pipes? A=20
circle of 25 ft radius has a perimeter of about 157 ft. This could give=
you=20
a collection area of nearly 2,000 sq ft. Air pressure eddies less than this=20=
size=20
will tend to cancel out. You might get a hf noise cut-off at about 20 H=
z. I=20
note that some of the current CTBT arrays are using solid pipe at the centre=
of=20
the spokes and porous pipe for remainder. Porous hose is reported to change=20=
it's=20
acoustic characteristics with time and it is also sensitive to rain.
Is your 1 Hz figure a high frequency cut-off? I=
=20
have got some characteristics of Validyne sensors somewhere. The low range=20
sensors are from 0.1" water FS upwards, but I don't remember=20=
any=20
frequency or noise specifications. I don't know how much they cost, but a Ba=
ll=20
Glider Variometer might well offer both price and sensitivity advantages. Th=
ey=20
use the same principle of pressure sensor, but with a mylar membrane. You ca=
n=20
also get low pressure range silicon sensors.
Regards,
Chris Chapman
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