How to Choose an Office Sound Masking System – Part 2: Plenum system and Direct Field system
Initially, the sound masking systems were fitted on the grid over the suspended ceiling. Big loudspeakers were clamped to the concrete surface above, and suspended with a chain. The sound from the speaker would then be blasted upward towards the concrete surface which will rebound around the inner space between the ceiling tiles and the roof surface called the plenum. Therefore, these older type systems were named ‘plenum systems.’ It was intended that the sound would rebound around till the plenum is filled and then infiltrate into the office space below through the tiles. This kind of arrangement was a necessity 50 years back, considering the level of speaker technology which existed then and the existence of ceiling structures that were comparatively uniform and plenum spaces that were empty. This looks sensible: speakers of those days sounded loud with a very narrow angle of dispersion however, if you are able to rebound the sound around for quite some time this limitation can be overcome.
But, as plenum spaces started getting filled up with various fittings and ceiling assemblies turned more complex, the job of getting the plenum filled uniformly with sound became still more challenging, and sound could not pass down into the office rooms below with any kind of uniformity. Today, plenum spaces are fully occupied by ducts for air conditioning and heating, network and low-voltage cabling, and electrical conduits for lighting etc. Ceiling assemblies are turning out to be acoustical nightmares of a combination of absorbent and reflective materials, disturbing the spectrum as well as volume. To remedy this, several plenum systems devised more and more complex methods to improve their speakers (usually where there are 3-speakers). But – it was only just like a band-aid solution.
Direct Field system
Therefore acoustical engineers continued working, and developed the prototype that would eventually be called a ‘direct-field’ sound masking system. Recognizing the fact that the problem is too big to be overcome by continuing with the old methods of patchy solutions, they took out the speakers from the plenum and placed them outside. But just by using speakers made for bouncing the sound about inside the plenum the problem is not solved because they had a comparatively narrow angle of dispersion. Using the old-type speakers under the new configuration didn’t work either. There still existed the problem of ‘hot and cold’ points that were non-uniform.
At last engineers, taking the idea from the home theater business, developed a speaker with ultra-wide-angle dispersion, which could spread out the sound at an angle of 170-degree. This became the key solution to bring in the new level of sound masking in offices that was effective- and because the direct-field system that resulted is simpler, which removed the human errors in tuning the system and the intricate failure-prone technological systems, it had practically no failure.