I was wondering if anyone knows of a fairly inexpensive way to build a silent device that will cause a microphone/PA system to feedback/disrupt. I know that feedback is generally caused by the speaker output being recorded back into the microphone and then back into the amplifier, creating a loop of increasing noise. Although sometimes I have noticed a PA system starting to feedback when their is no initial noise into the microphone. Could this feedback be triggered by some type of (probably electronic) wireless device or circuit? Maybe through the use of ultrasound, radio waves, or some other inaudible signal? An ideal case would be to have a silent device that when activated and pointed (or held close) to an on microphone would cause disruption in the audio signal. Would it be easier to disrupt the signal created by a wireless microphone? I do not know if this is possible, and had trouble finding information elsewhere. Any information regarding the possibility of this would be greatly appreciated. Thanks!