Harp microphones

A few definitions:
Microphone: a standard microphone will pick up sounds as they vibrate the air molecules. The closer the microphone is to the sound source the better the microphone will pick up that sound rather than any other background sound. Directional microphones will pickup sound predominantly in one direction, omni microphones pick up sounds in al directions equally.
Contact microphone: this type of microphone picks up vibrations in a structure as opposed to vibrations in the air. This type of microphone or pickup is good in reducing feedback (see below) as it requires more sound energy from a loudspeaker to vibrate the structure to which it is attached.
Feedback: this is the awful screeching noise heard when a microphone is placed too close to a loudspeaker. The microphone picks up sound in a room, it is amplified in an amplifier and then fed to loudspeakers. The sound from the loudspeakers then goes in to the same room, if this sound is then louder than the initial sound which arrived at the microphone then feedback starts and quickly turns into a loud howl. To prevent this, either reduce the level of the loudspeakers, or move the microphones further away from the speakers. Feedback can always be made to occur, it is not possible to eliminate it but it is possible to reduce it's likelihood to a level that it no longer affects your performance.
Nodes: When a structure vibrates due to energy being applied to the structure, it vibrates in particular patterns. Some points on the structure move more than others, these are called nodes, or nodes of vibration. If a pickup is placed on one of these nodes then it will pick up this particular frequency particularly well. It is important to use a pickup which covers as many of these nodes as possible or the pickup may be biased to particular frequencies/notes.