Jan 22, 2013 2:30 PM, By Bob McCarthy
Sound systems and the rooms they live in
Used by permission from Sound Systems: Design and Optimization by Bob McCarthy, Focal Press
The biggest divergence between our approaches is in the category of early reflections. In the world of unamplified acoustics, these reflections are the “sound reinforcement system.” Their job is to provide strong copies of the direct sound that arrive early enough to be integrated by our ears as signal strengtheners and extenders. These are quite distinct from the dense reverberation decay tail. The power game is played by a small number of arrivals in a few milliseconds rather than the hundreds of arrivals over a few seconds that comprise the decay tail. These strong early reflections are vital to unamplified acoustics to “get the party started.” As we move back in the hall, the proportions of direct sound and early reflections changes, which helps to keep the loudness relatively even over distance. From an amplified sound system perspective, these are largely unneeded and potentially troublesome. We have our own tool for creating even levels from front to back: directionally controlled speakers and arrays. From an acoustic power point of view, we don’t need the walls. Although, our subwoofers will happily accept the help of the floor and nearby walls as long as they are close enough to add constructively. But for the high frequencies, there is absolutely no good outcome for the addition of strong early room reflections to our speaker output. The reason is simple—they will never be early enough to add to our direct sound in a controlled, constructive manner. And remember, we are control freaks.
It may help to visualize a typical horn or waveguide attached to a high frequency driver for the strong early reflection device that it is. Even as small and close to the source as these reflective devices are, they present an incredible engineering challenge because the wavelengths involved vary from small to extremely small. The room? Way too late to ever provide constructive addition at the high end without also creating cancellations and gross non-uniformity.
Differences in Perspective
Let’s step back for a moment and consider a critical difference in perspective between architectural acoustics and sound system engineering: direct sound. The evaluation of room acoustics for an unamplified acoustic space begins with the assumption that direct sound from the stage will have an uninterrupted path to all seats. All of the seats can see the point of sonic origin: the performer onstage. It would be fair to say that the acoustician’s prime focus begins only after he has gotten the direct sound path over with. Now the fun begins, orchestrating the reflected paths into the desired timing and level arrangement of early and late reflections.
On the other side is the sound system engineer. We do not have the luxury of assuming that just because people see the stage, they can see our speakers. Our speakers are placed on the sides of the stage or above the stage, locations that architects may have considered unimportant to keep in the sightline of the seats. But let’s say we do have a line of sound to the seats. What then? The sound system engineer creates a plan to reach each seat with direct sound at close to the same level. This consideration preempts all others regarding the room. Once we see how we can reach every seat, we look at the walls, not for help, but rather to see how much damage will occur from strong early reflections. This may cause us to revise the design by partitioning coverage into segments such as sidefills, underbalcony, and overbalcony delays. In essence, the more damage we foresee from the room reflections, the more we can tighten the coverage and/or subdivide the coverage into smaller parts that can more precisely avoid the room.
But if the walls are not prone to sending us strong early reflections, our design work is essentially done. We don’t need to follow hundreds more reflection paths around the room because we accept the reality that there will be a reverberation tail and that there is nothing more we can do about it. If we have covered the seats with an even quilt of direct sound and minimized spill onto to strong reflective surfaces, we will now move on to taking advantage of the richness that a reverberant decay field has to offer.
For us, the game has already been won or lost at the point where most acoustical analysis metrics have scarcely begun to gather data. In the high frequencies, we have about a 5-millisecond window in which to complete our work. That’s not much room is it? The amount of time we have to work with increases proportionally as we go down in frequency, because the decisive factor is how the reflections reshape the frequency response. By the time we reach our subwoofer range, the time has stretched to several hundred milliseconds so there is plenty of room for the room. In short, we are much more aware of the frequency response effects of the room on the low end than the highs.
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