Build an Infrasonic Microphone

10/01/2015

( 903) 383- 7047 bui ld an infr asonic m i cr ophone Home

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Detect slow sound waves and pressure waves in the range of 0.1 Hz to 10 Hz.

HOW TO USE THE ANTENNA (see below on this page). One use of this antenna is to detect infrasonic sound waves. There is no technical help with the project. See photos and instructions below on this page. Part Part # AF4T-0.1H-2K-BC-EB

Radio adio Band and Out Output put Imped Impedan ance ce DC Low Low Freq Freq Hi Freq Freq

ELF

1160 ohms

__ Price

0.01 Hz 2800 Hz

$ 125.00

Sales to USA only.

Using the Antenna to detect infrasound

What is infrasound? Infrasound waves are sound waves that are too low for human hearing which starts at 20 Hz on the low end and ends at 20 KHz on the high end. Infrasound starts at 0.1 Hz (infrasound waves of 1 cycle per 10 seconds) and ends at 20 http://ww w.stor m w ise.com /page31.htm

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(903) 383-7047 build an infrasonic microphone

Hz (sound waves of 20 cycles per second). There are both natural and human-made sources of infrasound waves. Storms, winds, tornadoes, earthquakes, volcanoes, and large meteors entering in the atmosphere are some examples of natural sources of infrasound waves. Rockets, supersonic aircraft (sonic booms), and the space shuttle are some examples of human-made sources of infrasound.

How it works: The antenna is sensitive down to 0.01 Hz (one cycle per 100 seconds), and up to 2.8 KHz, (2800 cycles per second). A pressure sensor is made from a balloon cut and stretched over a small glass jar. This makes an air-filled diaphragm. It expands and contracts with changes in local air pressure, exactly like the air pressure sensor found inside a barometer. Infrasound waves affect the local air pressure as they pass by. The balloon diaphragm expands and contracts very slightly along with the infrasonic pressure waves. The antenna detects the tiny motion of the magnet glued to the diaphragm.

How to build it: Use a 12 inch type party balloon. Cut the inflate part off of the balloon, keeping more than half of the balloon's big end. Stretch it tight enough over the glass jar mouth so that all the wrinkles are removed, but don't over do it. There must be room for it to expand and contract. Tape around the balloon on the sides of the jar with electrical tape to prevent any loss of sealed air pressure. A rubber band won't do it. You WILL need to tape it up good.

http://www.stormwise.com/page31.htm

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Tape a NdFeB type magnet of 1/2 inch to 1 inch diameter magnet of 5000 guass or greater (www.digikey.com) to the center of the balloon diaphragm. (We tried a smaller Wal-mart magnet shown in the photo, and later changed it to the 1 inch diameter 10800 gauss magnet from digikey). NOTE: www.digikey.com has 10800 gauss NdFeB (part # 469-1000-ND) 1 inch diameter x 1/8th inch thick round magnets which work very well. The magnet should be positioned with its NORTH pole facing up so the output voltage goes toward negative ( "-") when the diaphragm expands (air pressure dropping), and goes toward positive ("+") when the balloon contracts (air pressure increasing). The NORTH end of a compass points toward the NORTH POLE of a magnet. A wooden support can be made to hold the antenna and jar stable so they don't move. With the design shown, either the antenna or the jar can be moved closer or farther apart to control the sensitivity level.


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Testing: Allow the 2 transistor amplifier (see circuit below) to stabilize for 1 minute. You should see a wiggly trace on your scope. Open and close your room door or any door in your house and you should see a strong response. Shaking a 14 inch square piece of cardboard box in your room's doorway should cause a strong response at 2 to 3 Hz. You've just made infrasonic sound waves! You should easily be able to see your building's air conditioning system's pressure waves caused by the blower running. You may want to mount your sensor outdoors on your porch to get rid of the interference and to maximize sensitivity to long distance detection. On a windy day you should be able to see the pressure changes due to local wind speed changes. NOTE: This project will detect the relatively quick changes in air pressure due to sudden storms, winds, and weather that produce infrasonic waves. It will not detect the extremely slow changes that normally occur during fair weather. (The antenna is not sensitive to DC magnetic field level). NOTE: If you mount this outdoors you may want to use a hard plastic cup instead of the glass jar, having the same diameter ( 2.25 inches ) as the jar. Drill a 1/4 inch hole in the cup and place a stack of 10 to 20 sheets of 1 inch square pieces of paper to create a very slow air leak ( about 3 minutes decay time ) to prevent pressure build up due to temperature changes.

Some pre-amplifier designs:

If your chart recorder or oscilloscope is not sensitive enough on its own, then a simple single 2N3904 transistor preamplifier ( pdf file ) can be easily built from a few common parts from Radio Shack stores. This amplifier works from http://www.stormwise.com/page31.htm

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0.1 Hz to 10 Hz, gain rapidly rolls off above 15 Hz. Power drain is less than 1 milliamp at 9 volts for long battery life.

Or if you need some extra gain a simple two transistor pre amplifier can be used. This circuit works very well and is very sensitive and will detect a 10800 gauss magnet (of 1 inch diameter, 0.125 inch thick) at 10 feet away. If your chart recorder or oscilloscope is not sensitive enough on its own, then a simple two 2N3904 transistor pre-amplifier ( pdf file ) can be easily built from a few common parts from Radio Shack stores. This amplifier works from 0.01 Hz to 10 Hz, gain rapidly rolls off above 15 Hz. Power drain is about 1 milliamp at 9 volts for long battery life. Connect any logging millivolt meter or oscilloscope to the output of this pre-amplifier to see extremely slow waveforms. Use a data logging milliamp meter or data logging PC chart millivolt meter to record waveforms. The sample rate should be set at 30 samples per second if you want to see waveforms up to 15 Hz. See also www.dataq.com for data logging instruments. The 2 transistor amplifier shown in the above pdf file allows enough gain to additionally detect geomagnetic storms. NOTE: The 10 uF capacitor across the antenna terminals determines the antenna's tuned frequency. SEE ANTENNA TUNING DATA GRAPHS HERE ( pdf file ). You may want to change the capacitor value to peak the frequency of what you're listening for. The WinDaq Software from DATAQ www.dataq.com will allow you to perform a frequency scan measurement of received infrasonic waves. FFT transforms analyse the recording instantly (you don't have to stop the recording to do this), and find the frequency peaks. Shown above our seismograph has a resonant period of 0.6976 Hz. This resonant period is set by how long of a rope the antenna is suspended by. The antenna here was suspended by about 20 inches of rope. Much lower frequency periods can be explored by using a longer rope. Vertical scale of above graph shows levels in dB, Horizontal scale is frequency in Hz.


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The DATAQ software will scan down to 0.00027 Hz.


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Build an Infrasonic Microphone

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