Due to the generousity of Gustavo Matamoros I was able to borrow a HSS device and see if it could really work in an underground sound intervention. This article gives a nice comparison of HSS and the Audio Spotlight. In the world of ultrasonic sound west-side Woody Norris (inventer of HSS) and east-coaster Joseph Pomei (audiospotlight) are presented almost as rivals. Before deciding to go with the HSS – I was also fortunate enough to be able to borrow an audio spot light for a short time. The Audio Spotlight definitely had a longer range but was awkward to hold (shaped like a very large flat plate) – it nearly projected sound halfway down the block. The HSS was more audibly more subtle especially with human voices but could be amplified to project a decent distance – as well. It all took some tinkering — I tested both speakers numerous times both indoors and outdoors finding that the HSS was more discreet – it could fit in my shoulder bag and was easily moveable once I attached a guitar strap to it!
These fact sheets do a good job at explaining technically how HSS works. Basically FM modulates with recorded sound – so that the two sounds mix acoustically at your ear …. in this way you hear the projected recorded sound in your head! While this all sounds very futuristic — this high frequency technology has been around since the 1960s — though not for commercial application. Like most technologies, a more powerful variation of this was originally developed by the military and more research in this area continues on a number of levels.