PSN-L Email List Message

Subject: Re: Infrasound
From: ChrisAtUpw@.......
Date: Sun, 25 Aug 2002 19:42:27 EDT


In a message dated 25/08/02, ajbv@............ writes:

> What I would like to do is detect a quake and its associated Rayleigh waves, 
> a tall order. But I can wait a few years. 

Hi Arie,

       You can make a sensitive Infrasonic system using a 50 mm piezo speaker 
as the sensor element and a TLC2201 low noise opamp + some filtering. You 
need a very high input impedance and you need to control the temperature of 
the PZT element. It can be a help to use a pneumatic low pass filter on the 
input.

       The original infrasound sensors used a brass bellows about 2.5" dia 
with a free armature LVDT sensor on the closed end. One of the Schaevitz HR 
050 LVDT sensors would be suitable, coupled up to a NE5521 LVDT chip.

       It is also possible to use a NE5521 chip with an additional CMOS input 
amplifier with a moving diaphragm, either of aluminised mylar or of Al, SS or 
Brass foil. The twin input electrodes on either side of the diaphragm may 
need to be made of mesh or have a lot of holes in them, so as not to severely 
air damp the motion of the diaphragm.
 
       Any suggestions on the best frequency bands to detect micro pressure 
waves 
> associated from quakes? 

       How about the same frequency as the quakes? You might consider 
surround lengths of plastic tube to provide some directional sensitivity.

> 
> windy. Wind and its turbulence is a big problem.

       The American systems seem to use either micro porous irrigation tube 
as shown in the photos or rigid plastic tube with restricting plugs every few 
feet. The fine tubes in the restricting plugs need to be protected from 
insects and from dust. As far as I know, elliptical section silicone rubber 
tubes have not been tried. The published layouts show radiating tubes. 
Hexagonal or circular plan arrays with connecting tubes to a central sensor 
have also been used successfully. These may be over 100 ft in dia and average 
out small scale turbulence quite well. Tubing may be installed in shallow 
covered trenches just below soil level, to reduce wind noise.

       Let me know if you want any further info or references.

       Regards,

       Chris Chapman
In a message dated 25/08/02, ajbv@............ writes:


What I would like to do is detect a quake and its associated Rayleigh waves, a tall order. But I can wait a few years.


Hi Arie,

      You can make a sensitive Infrasonic system using a 50 mm piezo speaker as the sensor e lement and a TLC2201 low noise opamp + some filtering. You need a very high input impedance and you need to control the tempera ture of the PZT element. It can be a help to use a pneumatic low pass filter on the input.

      The original infrasound sensors used a brass bellows about 2.5" dia with a free armatu re LVDT sensor on the closed end. One of the Schaevitz HR 050 LVDT sensors would be suitable, coupled up to a NE5521 LVDT chip.

      It is also possible to use a NE5521 chip with an additional CMOS input amplifier with a moving diaphragm, either of aluminised mylar or of Al, SS or Brass foil. The twin input electrodes on either side of the diap hragm may need to be made of mesh or have a lot of holes in them, so as not to severely air damp the motion of the diaphragm.

      Any suggestion