We get a lot of calls from fitness instructors and group exercise directors complaining about feedback on their wireless mic systems.  No problem, we love getting the calls, but we find that there is a lot of confusion about what feedback means in a sound system.
First of all, 'feedback' usually refers to the squealing or whale-like sound you get when a microphone picks itself up through the speakers, thus feeding the sound back into the speakers and making a loop.  Nobody likes to hear that right at the peak of an intense aerobics (or cycling (or any)) class.  There are a number of reasons that a microphone headset might feed back during use.  
The main remedies are to turn down the trim or gain control on your wireless microphone transmitter, rearrange the speakers in your group ex room so that they are not pointing back at the instructor (please don't put them in the four corners!), and adjust your equalizer controls to minimize the group of sounds (bass or treble) that is giving you issues.
Sound confusing?  It's really not that bad.  Give us a call at 800-491-6874 and we'll walk you through it step by step.  

P.S. if your wireless mic system is cutting in and out, making popping sounds, or you hear static, it's probably not 'feedback', but we can usually help with that, too.