Ideal HOW Acoustics
Feb 2, 2011 3:49 PM, By Bob McCarthy
How variable acoustics can create a continuously adaptive sound environment for contemporary houses of worship.
Simulating the acoustic space (simplified)
An acoustic space creates a very complex layering of reflections. Every location has a unique series of individual arrivals, yet taken as a whole the number of reflections and decay time are largely uniform over the space. A well-behaved decay structure will have sufficient density and variability so that individual reflections cannot be localized. The level of the decay tail should fall steadily over time. One of the critical failures in physical acoustics is when reflections are heard with evenly spaced intervals, known as flutter echoes, which are easily detected by our ears. Another failure is when a single reflection stands out above the crowd, a specular reflection, which draws the listener's attention to a single spot in the room.
The job of the reverberation enhancement system is also to increase the density at all locations evenly, just as would occur with reduced absorption and added volume without allowing for evenly spaced reflection intervals or having any single speaker stand out on its own. We have the additional constraint of having to ensure that our system does not do any of the other things that would blow our cover such as feedback, hum, noise, rattles, distortion, or impossible events such as a reflection arriving ahead of the direct sound. A key factor that makes it possible and practical to simulate a reflective wall is the use of large numbers of separate signal channels.
Whereas your usual surround speakers might all run on a single feed from the console, the lateral speakers that create our side wall "reflections" will each be driven with unique channels of processing. The input into each speaker comes from the mics, but each speaker gets a different mix of the multiple mics. This creates a unique semi-correlated signal for each speaker feeding back into the space. It might not be intuitive that this simulates a physical wall, but consider this example: When there are 20 violins on stage, the signals all originate from unique locations. You will hear each of their reflections arriving from different locations off the walls and ceilings. No two sets of reflections follow the same paths, although neighboring instruments will track closely. To our ears, the violin reflections are spread along the wall rather than concentrated to a single spot. Our signal processing will create the same effect by uniquely blending the arrivals of the different mics with randomized timing and level differences. So now we get to the heart of the matter.
How does this really work? The first thing to understand is that the input signal is acoustic. It begins with sound arriving at our mics in the room (not close mics on stage). Maybe the sound came direct from a performer, from the PA speakers, or even from the audience. The electro-acoustic architecture re-circulates every sound source just as a wall would. So let's take a very basic system with 16 mics, 24 speakers along the walls (laterals), and eight overhead speakers. The original sound goes into the room and arrives at all 16 mics but each at slightly different times (due to path length) and with varying frequency responses (due to directionality of the source and mics).
The 16 signals are then routed into the room acoustic signal processor(s) and mixed together in a complex matrix which then is sent to the speaker outputs. Each output then has a unique blend of the mics and this can be sent to the speakers. Since the sound leaving each speaker is a mix of multiple timings, the source appears much more diffuse (and therefore like a wall) than a speaker reproducing the sound from a single mic. Because the neighboring speakers are reproducing non-identical signals, the blend between the adjacent speakers is much more gradual than would be the case with correlated signals (like your traditional surrounds). Round one is complete. The next thing that occurs is that the sound from the speakers hits the mics. Now each mic hears every single speaker, each of which are sending a diffuse mix of the 16 mics. Then back to the variable room processor we go, this time with signals more dense than last time, and the cycle continues. In some systems, the processor can optionally add internally generated reverberation to the matrix mix. In some systems the outputs have varying transit time through the device to help suppress feedback.
In practice, I have seen these systems put to use in houses of worship on a cue-by-cue basis through the course of the service. The changes are as simple as clicking the mouse to select one of the settings from the library of configurations. These are operated by mix engineers, not rocket scientists or acoustic consultants. When done well, the audience simply accepts that the sonic experience seems appropriately adapted to the music or speech and becomes very involved in the service. And if you want the congregation to sing along, for God's sake surround them with a lot of reverb.
This is just the tip of the iceberg in terms of the complexity and capability of these rapidly evolving systems. The capabilities described here are not necessarily universal to all of the systems available on the market. A feature by feature comparison of the different systems is not possible without getting much deeper into the math, and therefore will be saved for another article (and another author).
These are the main systems out there that I know of and you can get lots of information about them online: Meyer Sound's Constellation, The System for Improved Acoustic Performance (SIAP), Lexicon Acoustic Reinforcement and Enhancement System (LARES), and Yamaha's Active Field Control (AFC).
Bob McCarthy is president of Alignment and Design. McCarthy specializes in the design and tuning of sound reinforcement systems and conducts trainings around the world. His book, Sound Systems: Design and Optimization, was named "2007 Sound Product of the Year" by Live Design. Visit his blog at bobmccarthy.wordpress.com.
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