L1® and Feedback
Here's the official explanation from the main Bose FAQ. (no longer available since the March 2007 reorganization of the site).
"The Personalized Amplification System™ approach is resistant to, but not immune from feedback. It's performance in this critical dimension has been shown under the vast majority of cases to be significantly better than the traditional "triple system" approach of backline instrument amplifiers, monitor system and PA system. Here are some of the reasons why:
- There are far fewer feedback loops because a microphone is connected to only one Cylindrical Radiator® speaker, not many as in the case of monitors and PA speakers. Fewer loops that can feedback mean less feedback.
- The radiation characteristics of the Cylindrical Radiator speaker, whereby the sound levels changes little with distance, means that a good level can be set for the audience without it being blasting loud near the stage. Less level at the stage means less level bleeding back into microphones that can cause feedback.
- Because of the unique properties of the Cylindrical Radiator speaker (little level change with distance) you can move a microphone around with much less fear of triggering feedback.
- In a conventional triple system, the sound operator makes multitudes of adjustments for one reason or another not related to feedback. And yet, these changes can all conspire to produce conditions which upon the next adjustment feedback is triggered. The sound operator (I have been one for twenty five years) never knows which adjustments contributed negatively to conditions for feedback. When it occurs, he/she must quickly make major adjustments to stop the squealing, then more or less start again to try and create higher gain without feedback. In the new approach, the musicians, and no one else, control the sound. They have far fewer controls to keep track of and thus if they happen to trigger feedback, they make a quick adjustment.
With all of this said, all of the normal practices that control feedback in a traditional system should still be employed here, with signal processing as a last resort. We have had the best performance from standard dynamic stage microphones. We do not use feedback suppressors. We were able to get the mics loud enough without the suppressors. Most important of all techniques is to get as close to the microphone as possible. The difference in gain before feedback between an inch from the capsule and 1/10" is 20 dB."