With Wave Field Synthesis you are not limited to placing sources in depth and width, but you can also work with height. We’ll discuss in this article the possible consequences on a PA with speakers on the floor usually for the front rows in the audience and a flown system for the audience sitting further from the stage.
At first there is not distinction in how to work with height compared to width. Whether a sources moves left to right or upwards or downwards doesn’t change the way we calculate the delays.
But most of the time in a PA design there are speakers on the stage level and some flown at the proscenium. And this needs some tuning since the flown array is made of larger more powerful speakers than the ones in front of the first rows. If you don’t want to have the audience feel too much the sound as coming from above you have to delay the flown array more than the front array. Roughly around 10ms to 20ms more delay, in other words 3m to 7m.
If there is a physical source on stage that is reinforced a listener that would walk away from the stage should still have the impression the sound comes from on stage even-though he’s going from the lower array’s coverage to the flown array’s. If there is no physical source on stage, as for a sound track or off stage voice, there are no visual cues helping the sound not rise.
This way of tuning is not limited to WFS, but common to regular panned mono sound reinforcement. What’s different here is that there is much more play on the placement.
If we move sources in width and depth without taking account of height in the calculations then the relative time difference between low and high speakers will stay constant. The impression of height is not changed. The source is alway percieved in the same horizontal plane.
If you wish to work with height then all you have to do is take it into account in the calculations of distance between a source and the speakers. It’s nothing more than mathematically.
This is what happens when a source moves up or down. As expected the time difference changes with height.
The second sketch shows what happens when a sources move in depth (upstage<>downstage). The time difference changes too. For a source close to the ground there is more delay difference downstage than upstage. As a consequence the listener will have the impression to hear the source rise the further upstage it goes.
Should you wish to work on height, you should start by compensating the delays due to changes in depth. A possible way to deal with this is to have a coefficient assigned to the height in the distance calculations to limit this rising impression. It’s also possible to lower the source according to the position in depth on stage.
A note, this impression of hearing the sound rise should not affect the people who only hear the eventual on-stage source and the lower speakers.