Sound Image Costs and Benefits, Part 2
Apr 4, 2013 1:38 PM, By Bob McCarthy
Practical sound system design for the best preservation of imaging.
Since we want things to image to the stage source it will be a good idea to find out how long it takes to get from the stage to various locations in the room. The point of temporal origin is found by having a loudspeaker stand-in for the actor/musician. The placement can be somewhat subjective in the case of an actor, unless they spend the entire show in the same spot. In the case of a drum kit, unless otherwise specified, we can assume it is staying put. This temporary source speaker is termed the “fictitious source.” In the case of a moving actor, the placement should be approximately mid-depth of the actor(s) front/back range. If the source is too close, the delay timing sequence will start too low and the image control will be needlessly compromised. If the source is placed too far upstage, then the stage source will lead by too much time and a new set of problems such as intelligibility loss arise.
Now that the fictitious source is in place let’s recheck the signal path to make sure that there is no digital latency, since current day actors are still running analog. If a digital console, signal processor, amp, or all of the above is driving the source loudspeaker, then there is a latency error that will cause you to read a lower delay setting than that required to accurately sync to an analog actor. Either go analog or measure the digital path and factor the latency into your calculations. From here the procedure is straightforward: measure the time it takes to arrive from the fictitious source, then measure the time it takes to arrive from the local speaker, subtract the difference, and put that time into the local speaker’s signal path. Done.
If that sounds too simple, it is. There are a few details to consider such as where to locate the mic to make the timing calculations and exactly who do we delay to whom. Let’s cover mic placement first: In general we place the mic in the middle of the particular speaker’s coverage, the middle horizontally and the middle depth. This way the timing and level errors accumulate most evenly in all directions from the central zone of coverage.
Now on to who do we delay to whom. This is more challenging. It is tempting to try delaying every speaker in the building back to the fictitious stage source. This would seem to ensure the best timing sequence for minimal image distortion. This is nice in theory but rarely in practice. For this to work in practice we will need to have very strong level from the live stage sources to give us enough level to minimally reinforce at the far reaches of the room. This could be the case for a very small room or with a very loud stage source, in which case the sound system is providing very light reinforcement. In practice we will usually find that the stage sound provides strong enough levels only in the closer areas of the room and most likely not in remote areas covered by our fill systems such as the extreme sides, under, and overbalcony areas. Think about it: If our sound system is having trouble reaching these areas, how well do you think our stage source is doing there? While it may seem desirable to sync these remote areas to the stage source, to do so will degrade the intelligibility in those areas. Why? Because you are in sync to an inaudible source (the stage) and out of sync to an audible one (the mains). In practice, the timing cannot be initiated until we have discerned the level relationships between the stage sources, and each main and subsystem to each other.
The easiest of all to begin with is the frontfill. Let’s begin with the left and right main. In their coverage area, there are likely to be arrivals from the stage, the frontfills, and the center. The center is late and high. The frontfill and stage source come from the same place. Who do we sync to? The stage is fine or the frontfill; they will be about the same. How about the frontfill? Listeners there hear the stage and a very late center and left or right. This is a no-brainer: the stage. The center speaker is the ultimate engineering challenge because we would really like to insert an accelerator on it rather than a delay. It is high and late, and it has digital processing on it. Typical desired setting is -15 milliseconds to -30 milliseconds (negative delay). Typical actual setting: 0 milliseconds. The extreme sides covered by our sidefills will hear an extremely low level stage source and the off axis edge of the left or right main (which was already delayed to the stage). The dominant source here will be the L/R main. If you get tricky and sync to the stage source, you are syncing to a ghost but will have a very real fight with the main speaker in your neighborhood. This is not advised; sync to the main. For the infill, it is again advised to sync to the sidelobe of your (already delayed) main neighbor and not to the stage source. Your inward angle means that you will arrive later than the stage source in your coverage area, which is to everyone’s advantage.
Finally, on to the delay speakers. It is tempting again to want to join the stage source, but once again you are chasing ghosts. The main systems (provided we can see them) are the dominant players under the balcony. If you are not in time with them, it is the same as adding reflections under/over the balcony.
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