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Acoustic Feedback

Unless you’re a fan of the electric guitar, the squealing and howling sound of acoustic feedback is an extremely uncomfortable sound, both for your audience and for your presenters.

The unfortunate reality is that any time you have open microphones and loudspeakers anywhere near each other, you’ve got the potential for ear splitting feedback to occur.

Simply put, when the sound entering a microphone, or group of microphones, is amplified, and that sound is picked up by those microphones and re-amplified, there is a critical system gain (overall system amplification level) above which feedback will occur.

The more microphones that are open, the lower each one’s volume must be in order to stay below that critical amplification level.

So, the more microphones you have open at any time, the more likely it is that acoustic feedback will occur before a participant’s voice is amplified enough to be heard by the audience.

Some of the things that make feedback worse:

Using The wrong type of microphone.
Microphones come in two basic flavors: Omnidirectional Microphones that are equally sensitive to sound from all directions, and Directional Microphones that are much more sensitive to sound from one direction. Using omnidirectional microphones will increase the potential for feedback.

Not having enough microphones for all participants.
This leads to microphones being passed around the table and usually results in them being too far from the presenter who is speaking. The farther a microphone is from a presenter, the higher the amplifier volume needs to be in order for the presenter to be heard.

Loudspeakers placed too close to, or on the wrong side of microphones.
In many multipurpose venues, an array of ceiling speakers is used to allow the audience to hear the proceedings. These loudspeakers are often placed directly above the podium or the dais. And, all too often, these loudspeakers are not, or can not be adjusted, or turned off.

Loudspeakers should never be placed behind presenters. If they are, even the most directional microphone will always pick up and re-amplify the sound coming from the loudspeakers and feedback will occur.

Not having an experienced person to operate a sound system when using multiple microphones.
Many venues will set up a series of microphones before the event and set the volume of each microphone evenly. The problem is that all participants do not speak with the same volume, and a participant’s volume will change at different times. An operator is needed to adjust the volume levels of the microphones accordingly, raising the volume of the ones that are in use and lowering those that are not. As the number of participants increases, it becomes harder to manually control individual microphone volumes, especially when there is a group discussion in progress. There are times when the operator will have to go up to the dais or podium and re-adjust the position of those microphones that are too far (or too close) to a presenter.

Some of the things that you can do to minimize the possibility that feedback will occur before a participant’s voice is amplified enough to be heard:

Tuning the room
Without getting too technical, each room has particular frequencies of sound that are heard first when feedback occurs. They might be very low bass frequencies in one room and higher squealing frequencies in another. If you can lower the volume of just those frequencies that appear first, you can effectively raise the critical system gain of the room’s amplification system. The technique, which uses an equalizer to lower the volume of those frequencies that produce feedback in a particular room is called “tuning the room”. If the frequency ranges are narrow enough, there is no apparent degradation of the sound quality. Fortunately, there is automated, easy to use, digital equipment available, like the Sabine FBX1200, the Peavey Feedback Ferret or the Behringer Feedback Destroyer available for this purpose. If you’ve got feedback issues, they can make a big difference in your next presentation.

Use enough microphones and use the right kind
Never use omnidirectional microphones. They’ll pick up sound from every direction and contribute to feedback as well as to an overall muddiness of the sound reaching your audience. Never place a microphone more than 1 to 1.5 feet from a speaker’s mouth, or you’ll have to “reach” for enough volume to make his or her voice heard, and you’ll be working very close to that critical system gain that produces feedback.

If you’re planning a conversation between 2 or 3 participants, you might want to consider using directional lavalier microphones, especially if the participants will be turning around and referring to a PowerPoint presentation or other image. These microphones can be pinned on to the participants’ clothing. Two words of caution here. Make sure that you pin the microphone at about the top of the breast bone level. Also make sure that you pin it on the side that the participant will be turning towards. Otherwise, every time they turn, they’ll be turning away from the microphone and their volume will be considerably lower. Try to set the stage so that they’ll usually only have to turn one way.

If some of your participants will be moving around the stage a great deal, you might consider headset mounted wireless microphones. These microphones are always a fixed distance from the participants’ mouths and they’ll always be audible, no matter how or where they move.

If you’re planning a table with many participants, you can use table microphones, one for each participant. If you’re going to have more than 4 participants, or, if the room you’re working in is very feedback prone, you might want to consider using automated microphones, like the Shure AMS systems that we use and highly recommend.

AMS microphones remain off or at a very reduced volume until a participant in front of one of the microphones begins to speak. Then that participant’s microphone instantly turns on while the others remain off. This automatically reduces the number of “live” microphones in the room at any given time, and allows you to raise the overall system amplification level without fear of feedback. These microphones won’t turn on if the participant is speaking more than 30 degrees off center, so the chance of off side comments being heard in the audience is virtually eliminated.

Loudspeaker positioning
If you have the option, keep all microphones at a distance from all loudspeakers. If you don’t have an option, at least try to point the microphones so that their sensitive or “live” side is not pointing at the loudspeakers.

As we mentioned before, it’s never a good idea to have your loudspeakers in back of the presenters. If you’re stuck in a room with ceiling speakers directly over the dais, try to raise the microphones and point them down towards the participants mouths.

General rules for a successful presentation.

Have backup equipment ready
If you’ve planned your event well, your audio and multimedia support should work flawlessly and any technology that you use should be transparent to the audience. However, a microphone may start acting up, a cable may develop a loose connection or a short circuit, a mixing console or other piece of electronic hardware may fail, your DVD player may begin to hiccup or your projector’s lamp may blow out, but the show must go on. Usually, there’s insufficient time to call someone, look for a replacement piece of gear, hook it up and then continue the presentation. Have spares ready to go at a moment’s notice, your audience and your participants will appreciate it.

Have an experienced operator actively controlling the audio and multimedia system
The only thing that is unchanging in an event is that things are constantly changing. It’s impossible to set up an audio or multimedia system before an event and expect that everything will automatically go smoothly. Microphone levels may need to be constantly monitored, microphone placement may need to be adjusted, some of that backup equipment may be needed and someone should be responsible for these areas. That way, if something does go wrong, you’ll know that there’s someone around with the necessary expertise to make it right again.

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