Floor microphone pickup of stage vibrations

Some sound people like to put a foam pad under a floor mic to isolate it from floor vibrations. That creates a problem, though. As you raise a boundary microphone off the stage floor, the frequency response changes drastically. It may be a high-frequency rolloff or an upper-midrange dip.

Floor mics are not very sensitive to mechanical floor vibrations because their mic diaphragm is vertical, so vertical vibrations of the stage floor do not move the diaphragm in and out. The mics still pick up footsteps acoustically, as your ears do.

If you stand in front of the stage, and you can hear footsteps, the microphones will too. Some wardrobe people put rubber soles on the actors' hard shoes to reduce foot noise.

To test how much mechanical floor vibrations your floor mic is picking up, have someone walk around on stage in rubber-soled shoes. Pick up the mic 1/4" off the stage floor as the person is walking, and see if you hear any change in volume over the house speakers. If not, the mic is insensitive to mechanical floor vibrations.

What has been your experience?

Bruce Bartlett, www.bartlettmics.com