Infrasonic Microphone Part I

Another sensor planed for my atmospheric observatory is an infrasonic microphone. I have a design sketched out and have started on fabricating the mechanical bits needed.

So what is an infrasonic microphone  good for? The recent Chelyabinsk meteor was detected by the infrasonic microphone array set up around the world to detect cheaters on the nuclear test ban treaty.  Some work has been done on using a infrasonic microphone array to detect and track tornadoes. So far tracking has proved difficult but detecting a nearby tornado should be possible. From the data I have seen there are lots of other sources of infra-sound some of them so far unidentified. Infrasound propagates a bit like radio waves with the sound bouncing off different layers in the atmosphere and the ground. There is a wealth of things to look at.

There are a number of approaches to an infrasonic microphone. Some use a commercial off the shelf pressure sensor. Others particularly for volcano research use an array of electret microphone elements. Recently I have seen some built with piezoceramic  beeper disks. Since I like making things in my machine shop I decided to build most everything including the pressure sensor.

My sensor design is a differential pressure sensor that consists of a thin conductive diaphragm supported between two capacitor plates. If there is a difference in pressure on one side of the diaphragm it will move toward the low pressure side. Since I am measuring atmospheric pressure one side of the diaphragm needs to be exposed to the atmosphere and the other side needs to be at a reference pressure. This creates a couple of problems.

One is that the sensor needs to be very sensitive and normal variations in atmospheric pressure would easily overload the sensor. This problem can be solved by having the reference side of the diaphragm slowly leak to the atmosphere. This establishes a minimum frequency to which the sensor will respond. The sensors used in the CTBT (Comprehensive Test Ban Treaty) network are set to 0.025 Hz. This is where I will set my sensor. There does not seem to be much of interest below this frequency except the normal barometric pressure variations which I can record with a normal barometric pressure sensor.

The other problem is PV = nRT. The pressure in the reference volume will change with any temperature change of the reference volume. Again this is a very sensitive instrument so it is very sensitive even small changes in temperature. Some designs use a thermos bottle for the reference volume. The sensor needs to be outside away from human activity so it needs to be rugged and compact and low powered. My solution to the temperature problem is to bury the sensor. This will also protect it from other environmental effects. About 1 meter underground the temperature changes very slowly, well below the low frequency limit of the sensor.

sensor1

In the picture above I have laid out the mechanical bits of the infrasonic microphone. The metalized plastic survival blanket diaphragm is clamped between the input housing and the reference volume housing. The O-ring seals the assembly and makes sure the diaphragm is held tightly. The capacitor plates are made from copper clad printed circuit board material. They sit on the lip inside the housings. The plates have 4 holes in them to allow the air pressure to actually get to the diaphragm.  The depth of the lip determines the spacing between the diaphragm and the plates. In this case I have made the spacing about 0.02″. Four Allen head cap screws hold the housings together. There are two electrical standoffs which support the wires to the capacitor plates. I made two air fittings. One connects the  input housing to the hose that will eventually go to the input spatial filter. The other will connect the reference volume to the “leak”. In the final version the leak will be a length of stainless steel capillary tubing coiled up inside the reference volume but for now I need to experiment with the size of the leak during testing and calibration.

sensor2

In this picture you can see how I have stretched the diaphragm using an embroidery hoop. This will hold the diaphragm while I assemble the two halves on either side of it. I deliberately made the groove for the O-ring  bigger than the ring because I could not measure a rubber rings diameter very accurately and it is better to stretch the ring a bit than to compress it. That made assembly a bit difficult. I should have gone back and made the groove a bit smaller in diameter. Also the groove should have had a square bottom rather than being rounded.

sensor3

Above is the assembled sensor.  I simply sealed the wires to the capacitor plates with some RTV. The small hole on the other side of the air fittings will be the opening for the leak in the final version.  Then I will seal the tapped air fitting hole with a screw and some RTV. There are 4 tapped holes on the back to mount the circuit card with the electronics. By tapering the edges the sensor will neatly slide into a length of 4 inch PVC drain pipe which will house it when it is buried.  The discoloration of the aluminum is due to my method of de-greasing the metal after machining. I just run it through the dishwasher.

In my next article I will discuss the design and construction of the electronic portion of the sensor.

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