Sound masking comes in several varieties. The biggest difference in systems is the speaker location. Some systems use speakers that point down into the workplace (direct-field). These systems will provide improved privacy in the area but they will not provide uniform sound coverage. They are also very noticeable (especially to tall people) as one walks around the space. Downward facing speakers also must be placed around lights, HVAC components and other ceiling features.
Plenum speakers are located above the acoustical ceiling and are pointed up. The sound bounces around above the ceiling and filters down through the ceiling uniformly.
We think this works better and is more acceptable to the occupants of the office. We have removed several direct-field systems and replaced them with plenum based systems.
Of course in areas without acoustical ceilings, the sound can be delivered from attractive speakers (also pointed up) suspended from the structure. We also frequently locate speakers under raised access floor.
There are other key differences in systems. Give us call and we can make it easy to understand.