Wednesday, April 12, 2023

Geophone as Infrasonic Microphone with Case and Magnet

An SM-24 geophone (Sparkfun part SEN-11744) with a rare earth magnet (Jaycar part LM1626) to attach to metal surfaces. 

The pin on the geophone with the notch is the positive terminal. This should be wired to XLR pin 2. The other pin of the geophone is the negative terminal. This should be wired to XLR pin 1 and 3. Additionally, a 10k resistor should be wired between pins 1 and 2 of the XLR. 

Note that phantom power (48V) should be turned off prior to connecting the XLR to an input and should never be turned on while connected. 

The case is 3D printed with a wall of 1.5mm, leaving plenty of strength of magnetic field of the magnet and more than enough contact with the geophone element to transmit through. 

The case assumes a magnet thickness of approximately 4.5mm, diameter of 25mm and a microphone cable thickness of 6mm. Download the STL file here: 

This setup has been tested with a 3m two conductor microphone cable (Jaycar part WB1530). 

The input device will need to support frequencies below 20 Hz in order to make full use of this setup. Examples include RME interfaces and Sound Devices recorders.