I am wondering what the upper limit for frequency is currently as far as audio signal generators go. Basically I want to know how high of a frequency I can get on a compression wave. I don't mean the theoretical limit based on the atomic spacing in the medium, but rather if I want to buy a signal generator (speaker) how high I will be able to get. Ultimately I am interested in creating a standing wave with very small widths for the compression region. I am thinking in the 10's to 100's of GHz but I don't know if such frequencies are realizable. I am suspecting that if it is going to be possible that piezoelectrics are going to be required. Can someone help me out? Thanks.