Although it's commonly used in front of a stage to pickup concerts or theater events, the MXL FR-400M boundary microphone has a wide cardioid pattern and can be used in other applications like under the lid of a piano. The switch located on the bottom of the mic can be used to boost or cut low frequencies by 6 dB in order to balance the frequency response.
Boundary mic with 3-capsule array
120 degree cardioid
50Hz - 16kHz
48V Phantom Power
Bass Cut/Boost switch