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.
 
    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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