With the signal amplified, this is then fed into a buffer (U1B) which is then coupled by a 100uF capacitor to a speaker or headphone set. One problem exists with the sound signal, it's AC by nature which means that there are negative portions of the input waveform. Therefore, our op-amp circuit needs a negative voltage and thus a negative voltage generator is used. This negative voltage is generated by the 555 IC (IC1), which is configured as an astable multi-vibrator. As the 555 output swings from power to ground (and vice versa), the diode and capacitor arrangement (C6, D2, D1, and C7) creates a negative voltage equal to the supply (for example, a 5V supply would produce -5V).

The sound amplification cannot just come from a gain circuit because increasing the microphone signal also amplifies noise. Therefore, to make the microphone more sensitive to distant sounds, the microphone is built into a parabolic reflector. The parabolic reflector focuses sounds waves (in a similar fashion to a radio dish) onto the electret microphone which helps the microphone to pick up weaker signals. Therefore, distant signals are more easily received with the help of the dish. To improve the sensitivity, a larger dish can be used but the microphone must be at the focus point.

Constructing the Acoustic Telescope

The circuit itself can be made using just about any construction technique. The circuit demonstrated here is made on a custom PCB which was fabricated on a CNC machine (you can find the CNC files in the provided project's zip file). The project is put into an open case mount to demonstrate the wiring and setup of the circuit and the potentiometer is put on the front panel for ease of use.