Microphone Sensitivity and Feedback

This Page Reviewed By Bose

It is a common misconception that a microphone that has lower sensitivity (in other words, a sound of a given intensity at the microphone produces a lower electrical signal than a microphone of higher sensitivity) is somehow more susceptible to microphone feedback.

In the argument that follows, the assumption is that other variables in comparing two microphones are equal. In other words, only the sensitivity differs.

Feedback occurs when the loop gain of the whole system exceeds one. If you just decrease the microphone sensitivity, the sound in the room is less loud and the loop gain goes down by the amount of the sensitivity reduction. If you restore the level in the room by adding more gain at any trim or volume control, we increase the loop gain again. When the added gain exactly compensates for the reduced mic sensitivity we have 1) the same level in the room as before and 2) the same loop gain as before. If the microphone fed back at a certain level in the room before, it will do so again at the same level in the room. This is obscured by the other variables that tend to change when we pick a mic with a different sensitivity: different mic frequency response and polar pattern details, different placement, etc.

The only way to get more gain before feedback is to lower the strength of the feedback path (between speaker and mic) while the feed-forward path has the same gain. The mic has to be in a quieter part of the sound system’s coverage or the mic has to be directionally resistant to the speaker’s sound field compared to how well it picks up the desired input.

So it all comes down to one thing: when there is enough loudness in the room, how much better can the mic hear the desired source than it hears the sound system? Sensitivity doesn't enter into this equation.