This is a feedback eliminator. Works great. I used it when we had a small PA that we were pushing the gain on to be heard over loud instruments. No longer need it, as we invested in a larger PA. Note on the pics: the one in the rack is this actual unit. The ones on the table are taken from the internet to show you all the sides of it. I didn't pull mine from the rack yet. It's identical to these pic though.
From the internet:
This Sabine FBX-901 Feedback Eliminator is a simple addition to your PA, designed to automatically detect and eliminate feedback. Power adapter included.
From Sabine:
The FBX-901 incorporates the latest in design and digital signal processing technology. It automatically senses feedback in a sound system and determines its pitch. It then places one of nine micro-filters on the resonating frequency and eliminates the feedback in typically less than one second.
Virtually every sound system will be improved with the FBX. Auditoriums and churches of all sizes will benefit from automatic, real-time feedback control. Hotels and conference centers around the world can offer meeting rooms with sound systems that won't howl during presentations.
The simple beauty of the FBX is its ability to quickly and effectively eliminate feedback with narrower filters than ever before possible. With the FBX, your system will finally be loud enough and sound natural and transparent, and everyone in the audience will understand each word.