LISCA Array Performance
DTSystems has installed many LISCA systems in public halls, so how do they perform and how easy are they to fit? Well here, warts and all, are the answers we found.
It has to be said that removing a standard 600mm x 600mm ceiling tile and replacing it with a drop in replacement speaker module and grille underneath is really easy. Wiring up the arrays poses some challenges but no more than with any typical sound installation. Wiring the arrays at low impedance using thick wire, twisted to reduce inductance, is not a great deal harder than any other wiring method. Indeed we made some custom connector boxes with rows of rising clamp connections which made the whole wiring loom very simple. Possibly the biggest problem was trying to avoid lighting and ventilation fixtures, so the secret was always to get in early before the other trades on new builds!
We tried two types of grilles to finish the systems off: a standard open cell Paracube and a finely perforated painted steel panel, which always looked by far the best.
In general terms, the concept worked very well. It certainly did make a difference with the sound source coming from above and in front of you rather then from above (the ceiling speaker might be in front, to one side or the other, or even behind you) as with a standard ceiling speaker. It sounded and felt much more natural. Given the correct spacing between arrays, there was little drop off as you moved down the length of the hall (a few dB variation), though for longer halls, three or more arrays were often necessary. This is where some of the disadvantages crept in. Sound levels were invariably poor at the extreme back of the hall and this, in most cases, was plum where the sound operator sat! Being unable to hear properly, he/she tended to crank up the volumes and this of course exacerbated feedback issues. The solution was a single LISCA speaker fitted above and in front of the sound console.
Floor to ceiling height has a large influence on final performance. Invariably the best systems were those fitted in higher ceilings. Obvious really as low ceilings tend to 'trap' the plane sound wave rather than allowing it to propagate down the length of the hall. Indeed, really low ceilings were a problem in this regard and sometimes required a third array to maintain uniform SPLs (Sound Pressure Levels). After much experimenting, it became obvious that for halls with really low ceilings, LISCA was probably the wrong solution. Having said that, the was increased clarity under the arrays and this led to the concept of a distributed array but using LISCA type housings. In other words, less speakers in a single array, but many more arrays down the length of the hall. Obviously really long halls will need some delays fitting.
This is analageous to a traditional ceiling speaker layout but using LISCA housings. Doing this maintained the improved clarity and evened out the SPL differences. Without a doubt, the Tannoy CMS401 produced by far the best quality sound and is ideal for this application as it can be angled just like a LISCA speaker is. Having a wide frequency response and dual cone speaker maximises intelligeability and they look very good too. Having a 100V line transformer included makes connections really easy too.
So, given the right circumstances, LISCA worked really well. Using Tannoy CMS401s has the potential to make it much better.