Macau COD Tuning: Day 4 (Work in progress)
Note: I have completed day 5 at the time of this writing – I am posting this work in progrees so that the posts will come out in the right order – I will fill this one in ASAP
Bus level adjustments for equal level
One of the major components of an LCS-based system design is the managment of buses. Buses are they way signals are routed to provide a particular effect. In this case there is a desire to sometimes operate the mains with a high vertical image, sometimes low and sometimes in the middle. In this case this can be achieved be relative levels between the Melodie Mains (high) and Soundbeam Mains (low). To get the middle vert positon we use both. But we don’t want it to get LOUDER just because we move into both speakers – we only want to move the image – if we simply combine then it will get louder.
A dedicated bus (when properly calibrated) can provide a means to send signal up or down without level change. In this case three buses – Hi – Mid – Low are set to different level combinations of the two main outputs – to achieve the same COMBINED response. Sounds easy enough with a 2-part combination but it can get tricky when multiple parts are in play.
A secondary set of buses was created for the surrounds. These moved the sound outward AND control the vertical – in 3 parts. High overhead surrounds for the middle and lower areas, low overhead surrounds for the outer perimeter , and lateral surrounds that cover everybody. This makes for a tricky set of crossovers and bus level combiniations. In the end, matched acoustic levels were created in buses that allow the surround signal to move out, and up as needed. Signals assigned to the high bus go up and out, the low bus go out, and the “all” stretches out AND up, but keeps the same level as it doubles its quantity of devices. To do this we do a series of measurements and adjust the drive levels to compensate for the actual combined acoustic levels in the house – it is NOT a simple matter like – take away 6 dB and done. The fact that the different devices overlap their patterns in the seating area from DIFFERENT directions means that simple addition does NOT apply.
expand constell zone parts – the constellation zones were touched up and we got the speakers needed to measure them.
Subwoofer Delay Steering
The subwoofer system is a hybrid of two distinct array types: coupled point source and quasi-coupled line source. I call the latter “quasi-coupled” because the 3m spacing leaves them coupled down at their low range (30-60 Hz) and stretching toward uncoupled at the top end (80-120 Hz.) The fact that they are all above the pool in the central fly space means that we have no worries about the near-field response before the line pyramid and fully combined. As it turns out, all audience members will see fully finished array performance.
Our seating area is 270 degrees of a circle. The center +/- degrees we will call “front” and the side 90 degrees “side”. The design intent is for the coupled array to concentrate on the middle zone and the line source to take the sides. Since the front would be well covered by the coupled array it was hoped that we could steer the line asymmetrically toward the rear part of the sides.
We went to MAPP and ran a series of calculations. The 3m spacing (dictated by structural beam spacing in the grid) is pretty wide to work with delay steering. The 100 Hz range breaks up pretty badly before we get much of an effect on the bottom range. The biggest challenge was the extreme narrowing in the center. Even if we got it to bend toward the back, it was WAY too narrow. Instead we focused on a symmetric spreading strategy which combined delay and level tapering to give a very even spread over the range of 30-80 Hz. The outers were turned down in level and delayed the most. Config was -4,-3,0,-3,-4 dB (from outer to center to outer ) and 3,2.25,0,2.25,3 ms.
We measured across the bowl and the results were sufficiently uniform – and similar tp the prediction that we moved on to the coupled cluster.
Coupled Cardioid Sub Array
Cardioid steering investigation for the coupled array 3 forward – 2 rear firing (pol reverse, 4 ms, -2.5 dB)