The placement of the microphone is critical in order to get a balanced sampling of the harp and not collect the ambient sounds around you as well. Once positioned, you must not move yourself or the microphone. Contact microphones or transducers. By sensing the vibrating of the body of your harp with a piezo transducer, you do not collect normal room noises. Placement of the sensor in or on the harp must be done with care, to obtain a good balance of sound, both high and low.
Transducers give a very true picture of what the soundbox of a harp is doing when played.
In general, they are a very successful means of moderately amplifying a harp. There are two potential problems, however. One, there is great possibility for feedback- if the note you pluck is strongly amplified, the sound coming from the loudspeaker will cause the soundbox of your harp to vibrate even more vigorously. This is in turn picked up and amplified, etc, until you have a deafening roar. Very bad, and very scary.