Choosing Microphones

Before you can do cool things such as beamforming by processing signals from microphones, you need a way to gather the signals. A few essentials components in this are the microphones themselves, the analog-to-digital converter, and in many cases a preamplifier for the microphones.

The type of microphones we used in our project were Electret Microphone Elements. We chose these because they:

Data Acquisition

The analog-to-digital converter we used was a DAQPad device generously lent to us by National Instruments.

PreAmps

The DAQ device requires an input signal of 50 mV to 10V, and the Microphone Elements output signals in the 20-200 μV range. So we built preamplifiers that took in the signal from the microphones and amplified it 3158.44x. We used LM324 quad-operational amplifiers, with 56.2 kOhm and 1 kOhm resistors. At the inputs of each amplifier is a 2.2 uF capacitor designed to eliminate a slight DC offset produced by each microphone.

Figure 1: The configuration for preamplifiers with a LM324 opamp.

PreAmplifier Schematic with LM324

Putting it all Together

The next step is putting everything together. We built the perimeter array of microphone in a shallow box. To make a perimeter array we put 8 microphones in a pattern around the edge of a square 5.5 centimeters apart. The preamps and batteries were housed under the box, with the outputs coming from the side to connect to the DAQ card.

Figure 2: The box housing the perimeter microphone array and preamps.

Microphone Perimeter Array