[Nathan] sent in his speaker microphone project. Speakers and microphones are physically similar – usually mics are much smaller to allow decent high frequency response. In this case, [Nathan] wanted something to pick up kick drums or bass guitars, without the cost of a commercial version like the subkick. It’s build around a dual coil 6.5 inch subwoofer. The passive circuit design allows the coils in the speaker to be configured for differing impedance, phase and isolation.