• Microphone feedback. 10 causes y 10 ways to avoid it

    In this article, we discuss the origin of the dreaded feedback and how to avoid it.

    1. Definition

    Feedback (also referred to as howling or, in the old days, Larsen effect) happens at resonance frequencies of the sound systems and the room. When the sound from the loudspeakers (either the main system or the stage monitors) exceeds the sound that originally enters the microphones, the system goes into oscillation feedback (that annoying squealing sound) occurs.

    2. What does it depend on?

    1. Acoustic gain level. The higher the level we send to our system, the higher the probability of feedback.

    2. Source level. The louder the singer sings or the saxophone (say) blows, the lower the gain needed and the lower the chance of feedback occurring. Conversely, it will be more difficult to main a soft-speaking presenter out of feedback.

    3. Reverberation. The greater the reverberation, the higher the pressure level, and therefore the greater the likelihood of feedback.

    4. Distance of the microphones from the loudspeakers. The further away, the better.

    5. Distance from sources to microphones. The closer the saxophone or singer/speaker is to the microphone, the higher the level recorded by the microphone, and therefore the lower the gain required and the lower the probability of feedback.

    6. Microphone directivity and aiming. The more directive the microphones, the less reverberation they will pick up, and therefore the lower the overall level picked up and the lower the probability of feedback. The more directive they are and the more their minimum pick-up angles are aimed at the loudspeakers, the lower the chance of feedback.

    7. Number of open microphones. The more open microphones, the lower the level at which feedback occurs.

    8. Directivity and aiming of the loudspeakers. The more directive the loudspeakers are, the less they will leak into the stage microphones, and therefore the lower the probability of feedback. The more directive they are and the less oriented they are to the microphones, the lower the chance of feedback.

    9. Frequency response of microphones and loudspeakers. If there are peaks in the frequency response of any of them, those frequencies will tend to show feedback (resonate) first. Similarly, if the response of the loudspeaker at different angles is highly inconsistent, it is possible that moving the microphone will change the frequencies that are howling, since the frequency response will be different.

    10. Position and arraying. The acoustic modes of a room, or interference between loudspeakers, can cause feedback in some positions and not in others.

    And another one. On cardioid microphones or their variants (hyper, sub, super), if the lower part of its grille is blocked with the hand (what's referred to as 'cupping' the mic), the microphone will change its frequency response and lose directivity, thus going into feedback more easily (Joe Brusi measured the changes in this LSI article)

    The parameter that defines the amount of sound reinforcement that we can apply is called 'gain before feedback' (GBF). The optimization of the points mentioned above will allow us to achieve the highest possible value of this gain.