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MillimeterA TOUCH OF CLASS
Mar 1, 2000 12:00 PM, Bruce Borgerson
Meeting the sound reinforcement needs of the Sydney Opera House's Opera Theatre.
The Sydney Opera House is certainly one of the busiest, if not the busiest, performing arts centers in the world. On average, the facility's five theaters host more than six shows a day, every day of the year, requiring a crew of 20 sound technicians just to keep up with the multitude of audio requirements for each show. Until recently, David Connor headed this staff, serving as the facility's sound and A-V technical manager before departing to serve as a consultant in sound system design.
Prior to leaving, however, he oversaw the design and installation of an effective new sound reinforcement system in the Opera Theatre, the venue's second-largest performance space. This was just one of many A-V projects at the time, and Connor pointed out that the system upgrade process at the facility is virtually never-ending.
"For example, concurrent to the Opera Theatre project, we were outfitting a new theatre while we were also in the planning stage for a self-contained recording studio that will allow us to record performances being held anywhere in the building," he said.
The new system for the Opera Theatre, a 1,547-seat, two-tiered room that primarily accommodates opera and repertory theater, replaced the original system installed 25 years ago when the facility was new.
"In those days, sound was the little brother of the total production," Connor said. "You'd run a few rain and thunder cues through a Revox, mixing at the stage manager's desk, feeding a small, inconspicuous house system, and that was pretty much it. Obviously, times have changed with sound becoming more and more important to performance. Now, almost every performance involves some aspect of amplified sound; from subtle enhancement of opera to substantial amplification of a rock band for modern dance. This was the primary impetus behind the new system effort.
"We had reached the point where we were being asked to do much more than was originally conceived. We needed more of everything - more level, more dynamics and higher fidelity. Over the past five years or so, this need was met with temporary systems, but the results were often less than satisfactory. Not only was sound quality less than optimal, but the theatre aesthetics were also negatively impacted.
"System design is becoming more complex and thankfully, more effective. In the old days, designers would just point a bunch of boxes at the audience, and the resultant sound quality was a bit of a lottery; it often varied too much with where you sat, and many times, the acoustics took the blame for poor system design," Connor said. "I also don't believe that running a few modeling programs is really the definition of sound design either. Although modeling programs are a real asset to the sound system designer, there is a lot more going on that needs to be addressed. The main issue is how the sound system interacts with the auditorium and just as importantly, with itself and its immediate surroundings. As the arrays get larger, and the impact of the auditorium increases with size, the sound you hear is characterized by the whole system design rather than the loudspeaker product you are using. Getting appropriate, consistent off-axis loudspeaker behavior is as important in a liv! e auditorium as getting the dire ct field right."
With this in mind, Connor developed a specification for a new system quite pointed about performance expectations but not specific regarding the actual product to be used and then floated it to see what designs qualified systems designers would propose. Over the years, he has done quite a lot of work as a consultant in electroacoustics, contemplating how to specify systems and what makes them sound the way they do, particularly in large auditoriums, and this experience manifested itself in this facet of the project.
"In my view, one of the important factors that's been missing is real specifications that effectively define what we hear in auditoriums," Connor said. "Typically, a system is specified in terms of frequency response, say 20 Hz to 20 kHz, with a tolerance, and away you go, but that is only valid for one seat. It came to me that there's technology and there's what you hear, and until recently, the two weren't always related. Specifications were often rather loose or irrelevant, particularly with regard to coverage in three dimensions. An installed system could end up meeting the spec but end up sounding horrible."
At the same time, Connor recognized that these two facets are starting to move toward each other, especially in light of more widespread use of Fast Fourier Transform (FFT) measurements and a concerted effort to control what he calls, the "direct-to-everything-else ratio".
"We are now starting to be able to measure roughly what we hear," he said.
In the simplest of terms, the main goal for this system (and most others, for that matter) was to ensure that the sound impinging on the audience areas had technical integrity and that the sound hitting other parts of the auditorium and stage were minimal and even over the entire frequency spectrum. This required a highly specific and controlled loudspeaker coverage pattern, particularly to get acoustic gain off the stage and to help stop reflections from bouncing around the room.
"My specification had several levels," Connor said. "Coverage was one, and by coverage, I mean that you can't just specify a certain sound pressure level. You need to specify a tolerance and the area in which that tolerance occurs. Fidelity was another. In these terms, I'm talking about frequency response, which should be flat and true. Then there's clarity or intelligibility, which is effectively a ratio of the direct sound from the loudspeakers to the reverberant sound arriving at the listener's ears. Sound pressure level requirements were also important to make sure that thunder and rock bands could truly frighten the audience."
One other aspect that Connor emphasized as crucial is where sound energy from the system goes within the room other than the audience; what spectrum and level of sound feeds back to the stage, for example. This sets the available acoustic gain when using mics, which is particularly important when omnidirectional radio mics are placed in an opera singer's wig. Such aberrations as narrow band energy directed at the ceiling or walls and then reflected back to the audience area also demanded attention.
"It's OK to get some bounce," said Connor. "I mean, that's the sound of an auditorium, but the problem is bad bounce. For example, with stacked systems it's very possible that you will get 10 dB lobes in small bands, say at 1 kHz, heading up towards the ceiling and bouncing back down, and thus, your reverberant field is clouded. In other words, even though your direct sound may be quite good, the reverberant field combines with it, and what you end up with is a nasty sound.
"It was a fairly intensive performance specification. It all came down to a system meeting these factors while being loud enough to handle a rock band, down to quite subtle amplification or enhancement of classical music. Enhancement is a much more acceptable term for people in the classical genre; they're sensitive to amplification issues, and this must be taken into account."
In collaboration, Glenn Leembruggen of Elecoustics and Jands Electronics supplied a proposal best meeting Connor's expectations. Leembruggen, now with Arup Acoustics, is active in systems design as well as acoustic and electronic measurement both in the lab and on site. Much of his work involves the important interface between architectural acoustics and electroacoustics. Jands is the Australian agent for JBL and a leading Australian systems contracting firm. Its focus was loudspeakers and their specific performance parameters and locations with the addition of tailored processing. Remaining system elements would be largely retained with the exception of additional power amplification.
Leembruggen offered some specific views on system design that in many ways match up with Connor's vision, which he distilled to a focus on loudspeaker directivity.
"This can be achieved through two methods," he said. "The first is to constrain the radiation using the angled faces of a structure, which is how a horn radiator operates at frequencies that are well above the horn's lower cut-off frequency. Another way is to form a line array of sound sources and harness and control the constructive and destructive interference that always exists when sources are separated.
"Two problems with line arrays are that they become too directional when the array becomes acoustically long, and they exhibit strong lobing effects off-axis when the interdriver spacing array exceeds one quarter of a wavelength. These problems can be prevented using a combination of three techniques. The first is to keep the array's acoustical length constant with frequency by using low-pass filters (called tapering filters) to remove the outer drivers from operation progressively as the frequency rises. The second is to make the spacing between drivers operating at these frequencies less than one quarter wavelength. The third is to adjust the relative broadband level of each drive unit.
"Currently, good control of radiation patterns is readily available above 600 Hz using off-the-shelf horn devices, but if the direct-to-reflected ratio or acoustic gain is to be constant with frequency, then the same directional behavior must be provided below 600 Hz. This also ensures a truly flat frequency response at each listening position without the off-axis boost in the low mids that often occurs with constant directivity systems.
The proposal offered a main loudspeaker system largely located around the stage proscenium with five line arrays consisting of JBL Professional HLA series loudspeakers and JBL midrange horn and low-frequency drivers. These arrays are divided into left, center and right zones, upper and lower.
About the crucial aspects of this approach, Leembruggen said, "There are two main electrically tapered arrays in the Opera Theatre that provide both directional control and extremely high SPLs at the lower frequencies. A low-frequency subwoofer array operates up to 160 Hz and is crossed over to a midrange array that operates up to 1 kHz. Each of these arrays is unusual. The subwoofer array is 16.5 feet (5m) long and consists of six JBL 18 inch (450 mm) 2242H subwoofers spaced asymmetrically. The midrange array consists of five JBL HLA 4895 low/mid horn elements and is 12.5 feet (3.8 m) feet long. Both arrays use tapering filters set up in the JBL DSC260 processor.
"By making the spacing between the drivers in the subwoofer array asymmetrical relative to the center of the array, the radiation pattern is made to compensate for the different distance loss between close and far seats. It also provides attenuation of the energy that is radiated towards the ceiling by some 15 dB above 63Hz.
"In the HLA array, the low/mid elements are turned 90 degree, allowing the formation of two side by side individual arrays, one of the low-mid horns (with the 14 inch or 357 mm driver) and another of the mid-range horns (with the 10 inch or 254 mm driver). Turning the elements 90 degree also allows the narrower vertical pattern of the horn above 600 Hz to minimize irradiation of the side-walls of the theatre. The low-mid array is crossed over to the high-mid array at 320 Hz. At frequencies above 600 Hz, most of the signal is fed to the innermost mid-horn, and the directivity of the array is controlled by the directivity of the horn itself rather than the array.
"The off-axis behavior of the HLA array was designed to match that of the associated high-frequency horn, a JBL2352 fitted with a 2447J driver. In this way, compensation of the distance loss from near to far seats by the loudspeaker directivity was achieved over the full frequency range. The HLA array could have been made even more directional, but at the expense of consistent frequency response across all seats."
Leembruggen suggested that distance loss compensation is not usually undertaken in the low and mid frequencies and is the key to obtaining a flat frequency response at each listener.
"The subwoofer and HLA arrays are located side by side so that lobing in the crossover region is prevented," he said. "A JBL 2352 high-frequency horn is also located right next to the innermost mid horn of the HLA array, again to minimize crossover lobing.
"To confirm that our mathematical model was reasonably accurate, JBL Professional made acoustical measurements of the electrically tapered HLA array at Summit Labs. Because the array was to be flush mounted in the proscenium wall, we did not attempt to predict the radiation behind the array."
Figure 1 shows Leembruggen's prediction of the array's polar pattern and the measured pattern.
"Because the measured and predicted performances are close," Leembruggen said, "the mathematical model was judged to be pretty good, and a further refinement was made. The innermost horns were raised 6 dB in level, resulting in a smoother and more controlled polar pattern while slightly decreasing the directivity."
Figure 2 shows the predicted polar patterns for the array with and without the 6 dB increased drive level to the inner horn pair.
Leembruggen said, "Another way of showing the improvement in directivity below 600 Hz yielded by the array is to look at the off-axis frequency responses. Figure 3 shows the measured off-axis frequency response of a single 4895 HLA low/mid element relative to the on-axis response. Normally this plot is shown for horizontal angles, but with each element turned 90 degree, it is now the vertical response. Figure 4 shows the predicted off axis response for the HLA array with the 6 dB higher drive level to the inner horns. The increase in directivity up to 600 Hz over the single horn element is quite evident with substantial directivity improvements below 300 Hz".
Extremely limited space was available with all loudspeakers being housed in cavities carved out of the walls. Jands undertook substantial mechanical design to allow the loudspeakers to fit the cavities while providing facilities to allow fine-tuning the loudspeakers' orientations. Key to Jands' work were Peter Grisard, who handled the project management and the mechanical design, and Peter Twartz and Kim Hasanic who oversaw the difficult electrical and mechanical installation. From the Sydney Opera House side of things, the project was handled by Tony Lawrence. The whole system was commissioned by Leembruggen and Jands using an MLSSA analyzer and much listening.
Amplification for the main system is provided by a complement of Australian Monitor AM1600, 1K2 and AM1200 amps as well as the onstage foldback loudspeaker system consisting of JBL 4892s and ServoDrive Tech 7 subwoofers. A 32infinity16infinity8 Amek/TAC SR9000 console is installed in the audio control room. Recording of an event in the Opera Theatre can be accomplished from the control room or the recording studio via a full complement of tie lines and video links. A wide range of wireless, dynamic and condenser mics are available from Shure, B&K, Neumann, Sennheiser and AKG. Outboard processors and effects are also available with EQs from BSS, Klark-Teknik and Amek along with such effects generators as TC Electronics M5000, M3000 and M2000, the Lexicon PCM-80 and the Yamaha SPX990.
Leembruggen charactarized the sound as "really lovely."
Connor said, "Even at the very back of the theatre, the sound is extremely present and intimate and the tonal balance is excellent. The system is so well balanced that during commissioning, we were able to hear changes to the equalization of only 0.5 dB in most third-octave bands. Leembruggen and Jands did a superior job in meeting what I had envisioned in the specification. JBL has extremely good components, and the HLA system is a very responsive horn-loaded system. This became even better with the expertise applied in the design and installation. I'm comfortable that the system is capable of meeting the needs of the room for the next 20 years or more."
