To accomplish that, Lewitz started with the intelligibility factor, reducing the noise and reverberation to minimize their impact on spoken word audio by applying various types of absorptive materials to the walls and ceiling after modeling the space in EASE and applying the Common Intelligibility Scale (CIS). For Maples Pavilion they targeted a CIS rating in the 0.80 to 0.90 range without unduly attenuating the reflections of crowd noise by reducing other types of noise that are commonly induced into large reflective spaces; in this case, the dampening and isolation of vibrations from the center’s HVAC systems were able to be toned down thus reducing the noise floor and eliminating other sources of sonic energy that would combine with the crowd energy and overwhelm the space. (By comparison, a CIS of 0.70 has been selected by the National Fire Protection Association as the break-point for intelligibility of emergency systems; this corresponds to comprehension of approximately 80 percent of words and 95 percent of sentences.)
He found that they also had to make the house announcer aware that he needs to constrain himself when the room gets noisy since there is a limit to the maximum volume of the house announcers into the PA – as the crowd noise and excitement grow higher, so generally does the tenor of the (understandably partisan) announcer.
“It’s all about finding compromises,” says Lewitz. “It’s the same even in churches – the organist or the choir director wants a three-second RT for the music, but the pastor wants it under one second to maximize speech intelligibility. The need is always to find common ground.”
Variable Acoustics
If most music and other audio program content could project itself sufficiently unamplified, the acoustician’s work would be done when the last algorithm was crunched and reconciled with the architect’s CAD renderings. But the practical pressures on new construction and renovations of existing music and performance venues almost always mandate that some level of amplification is needed. At that point, acoustical and electrical power must find a way to coexist.
That’s where the high steerability of line array types of PA systems come in to place, putting sound where it’s intended to go and keeping it out of places where it’s not wanted, particularly empty spaces that will become reverberant and intrude back on the covered spaces.
“Ideally, you want the sound system to essentially act as if it were a symphony orchestra,” observes Tom Clark, a principal in Acme Professional, a New York-based audio systems design company. In fact, he says, sometimes the best way to achieve that is to put the sound system components on the same plane as the sound generators. But that’s rarely possible since it would create sightline issues; thus, more reverberant spaces have their steerable line arrays flown, suspended from the ceiling, with carefully positioned fill speakers to cover close-in sections of audience, with the intent of introducing as little energy as possible into non-audience areas to minimize reflections and reverberation.
But making that transition from the mathematically pristine vision of the architect to the pragmatic execution of the electro-acoustical systems designer has not always been smooth. Clark says there have been come epic battles, such as that between noted architect I.M. Pei and Artec founder Riddick Johnson over the outcome of the sound for the Myerson Concert Hall in Dallas. It was a back and forth that ultimately produced one of the best-sounding concert spaces in the country, as acoustics and electronics found a compromise that both could live with. Not all such contests end as well, says Clark.
One technique that’s been successful at taming reflective spaces without undercutting the purpose of their reverberation – and which helped produce the winning compromise at the Meyerson – is the notion of variable acoustics, which can range from anything from adding (automatically or manually operated) curtains along walls to damp reflections, to rotating panels covered with a hard reflective surface on one side and an absorptive on one the other side, such as was done at the Carmel Symphony Hall. The Meyerson Hall solution was even more complex: entire rooms were built on either side of the hall that add as much as 30 percent more cubic volume to the building and act as holding tanks to store excess sonic energy generated in the room by amplified music.
The inside of these chambers is filled with sound-absorbent materials (such as banners or curtains lowered along the walls from above) whose volume can be varied, thus making the absorbance adjustable and, in instances such as unamplified choral music being performed, can actually be used to increase the amount of reverb in the main room, a technique provided for at the Meyerson and at the new Miami Arts Center.
The greater diversity of music genres that appear in all types of spaces has led to more low-frequency energy being part of the frequency spectrum. Low frequencies tend to be omnidirectional and are harder to steer and control, and their implicit long wavelengths can produce difficult reflections. One technique that’s been developed to address this is the cardioid subwoofer, whose drivers are faced in opposite directions in such a way that they can cancel part of the direct signal at the source. “That improves the directionality of the low-frequency information without reducing the presence of the lows, especially on stage, where this technique also reduces the potential for feedback,” Clark explains.
The techniques and technologies for conquering sonic reflections have become pretty sophisticated, in the process making the job of the A/V systems integrator substantially easier and more precise. Nonetheless, it’s far from plug and play for them. As WJHW’s Mark Graham puts it, “The A/V integrator still has to execute, and they also run into the occasional issue that just can’t be predicted in a CAD model or on software,” he says. “Keeping sound tame is a process that involves everyone.”
