THE National Ice Centre
Dec 1, 2000 12:00 PM, Peter Mapp
kating icons - and Olympic gold winners - Jayne Torvil and Christopher Dean hail from Nottingham, a town that's now home to a new state-of-the-art ice arena. When The National Ice Centre Nottingham opened in April, Torvil was active in the festivities. The stadium provides two Olympic-size ice pads that will host national and international ice-skating competitions and much more.
The $60 million project was conceived and designed by the Nottingham City Council but has also received major funding from the UK sports council, which recognized the need for such a venue. One of the ice pads will be permanently available for both public and professional ice-skating activities. The other pad, located in the main arena, can be covered and the arena used to seat audiences of up to 10,000 for concerts, performances, sporting events, or conventions. The Nottingham Panthers-the local ice hockey team-will also call this unique new center home.
With such a wide range of activities, and with national prestige at stake, considerable attention was given to the acoustics of the space and the sound systems' design, though within the constraints of a tight project budget. In order to provide the flexibility required by multipurpose usage and to meet the strict codes for voice and fire alarm installations applicable in the UK, two separate but inter-connected sound systems were designed and installed: one for performance and related aspects and one for voice and fire alarm.
The voice alarm system operates throughout the complex with loudspeakers located in every occupiable space - including plant rooms and catering facilities, as well as dressing rooms and backstage (over 1,000 loudspeakers in total). In the main arena, a dedicated set of CD horns provides paging and voice alarm, while a combination of a center cluster and a distributed ring of full-range, controlled dispersion loudspeakers provides high-quality performance sound reinforcement without the restrictions and compromises of the VA regulations. In the community pad, two clusters of full-range, controlled dispersion devices fulfill both voice-alarm and music/DJ requirements.
ARENA AND ICE PAD ACOUSTICSInternally, the volume of the arena is approximately 3,000,000 cubic feet (87,400 m superscript 3) with the ice pad/main floor measuring about 200 by 100 feet (60 by 30 m). A technical gantry/catwalk located 60 feet (18 m) above the floor provides a readily accessible structure for lighting and sound system installations.
I was appointed as sound system design consultant and acoustics advisor early on in the project. As such, I convinced the client and design team to apply acoustic treatment to the underside of the roof surface and to upgrade the seating to a semi-absorbent padded type that not only added considerable sound absorption but also had the advantage of being much more comfortable. Thus the acoustic needs also added to the ambience and quality of the space.
A target reverberation time (RT60) of 2 seconds (empty) was set for the arena in its concert/convention mode.
While still reasonably lively, it was anticipated that this would reduce to around 1.6 seconds when occupied, which when coupled with the large volume, potentially enables high speech intelligibility and clarity to be achieved. A services noise criterion of NR 40 (~ NC 40/45 dBA) was set for the arena and public rink.
Acoustically, arenas - and ice rinks in particular - are complex spaces, often exhibiting stratified or polarized sound fields. This is primarily due to uneven distribution of the sound absorbing and reflecting surfaces. For example, in its ice rink mode, the bottom section of the bowl becomes totally reflective due to the presence of the ice and dasher boards. The protective glass perimeter screens for ice hockey extend this local reflection zone up to a height of around 10 feet (3 m), thereby enveloping the skaters in a local reverberant field with reflective surfaces on five sides and an absorptive layer overhead. Furthermore, the size and shape of the space and the distribution of the absorption make it decidedly non-Sabine, which makes calculation and intelligibility prediction interesting, to put it mildly.
Figure 1 shows the measured reverberation times for the arena in its two main configurations - ice pad and concert hall. As the figure shows, in the concert condition (with 6,500 seats) the RT60 is close to the target specification of 2 seconds, with an average value of 2.2 seconds over the range 500 Hz to 4 kHz. With the ice pad uncovered and the dasher boards and protective glass screens in position, the average reverberation time noticeably increases to around 2.7 seconds. On the ice itself, the RT is a little longer due to the local enclosure, increasing the RT to just over 3 seconds. Figure 2 shows a set of typical impulse sound decay curves, made in the empty arena under concert conditions. Note the smooth decays and freedom from late specular energy and echoes.
The community pad (currently under construction) is predicted to have a reverberation time between 6 and 8 seconds without acoustic treatment and approximately 2.5 seconds with treatment added to the ceiling and upper wall areas. The acoustic design is a compromise between the need to reduce the RT in order to ensure adequate speech intelligibility and music clarity (less than 2.5 seconds) and providing a lively acoustic atmosphere to match that of the Nottingham's old rink (RT 5.5 seconds at mid-high frequencies) which the new community pad replaces. A degree of "tuning" will therefore take place once the structure is completed. Again, services noise is targeted at NR40.
The structural envelopes of both the main arena and community pad are designed to protect the locality against internal sound levels of up to 105 dBA. Fortuitously, there is little residential accommodation within the immediate vicinity, which also helps to reduce the potential problem of noise breakout.
ARENA PERFORMANCE SYSTEMDue to the multipurpose use of the arena, both a center cluster and a distributed sound system were needed in order to cover the ice (arena floor), and tiered seating was required. The center cluster was designed primarily to cover the ice for both normal and competition skating and for sporting events such as boxing or tennis. The distributed system, located around the technical gantry/walkway (see Figure 3) provides coverage to all the tiered and bleacher seating. Parts of the cluster can be switched in or out to partially cover the seating and to provide a "focus" for the sound when required.
The distributed system was originally designed so that each of the 26, 70 by 40 full-range cabinets (d+b Audio C7) would be wired back individually. In order to save costs, this ultimate flexibility was reduced slightly so that in the final installation the units are addressable in pairs - on opposite sides of the arena. This creates five "time delay" zones, enabling good synchronization with either the center cluster or a separate performance system located on and above the stage at the RHS end of the arena. I chose the d+b speakers not only for their specific sound quality and precise directional control, but also for their reputation and familiarity in the UK and potential acceptance by visiting users.
The performance system is controlled by a computer-based Peavey MediaMatrix DSP routing and mixing system, which also provides the individual channel delays and parametric equalization. A number of pre-set configurations have been programmed and calibrated for different arena uses and setups. A separate stage-monitoring system and basic flying/stage-stack system have also been provided.
Speech intelligibility of the main system is extremely good. Measurements made in the tiered seating, under empty ice pad conditions (RT = 2.7 seconds), average around 0.6 STI (6% Alcons). On the ice itself, readings between 0.55 and 0.6 were taken (8.5-6% Alcons equivalent). As would be expected from the RT data, both readings improve under concert conditions when the ice pad is covered. In the seating, the SPL variation is within 2 dB over the range 500 Hz to 15 kHz.
Installing the arena's systems had to be carried out in two phases - several months apart - due to the installation of the ice pad, which prevented the further use of any mobile platforms or "cherry pickers" once the freezing process began. All the loudspeakers had to be aimed far earlier than normal, with much of the seating inaccessible. System tuning took place during the week and weekend before the building's hand-over, with functional and operational tests being conducted during the day and equalization, delay setting and balancing at night. During these late-night sessions, we heard an undesirable low-frequency resonance that significantly affected the overall sound quality, giving it a woolly, poorly defined characteristic. (Strong resonance was found at 90, 140, 170 and 240 Hz). However, appropriate parametric equalization eliminated the problem and produced the sound quality expected from the system.
Thirty-six tie lines run from either side of the stage to the sound control room at the rear of the arena at VIP-box level. Tie lines and intercom points are also provided in several of the VIP boxes for press and broadcast use as well as on each side of the arena bowl and around the roof gantry. (A comprehensive selection of loudspeaker tie lines is also located at each position.) Tie lines also connect the community pad (which becomes the practice ice pad during competitions) with the main arena. A broadcast tie-line panel is provided backstage in the loading bay. Fiber optic tie lines are also provided between key positions to allow for system expansion and to provide a degree of "future proofing."
A 2-channel Sennheizer infrared transmission system is provided throughout the main arena for hard-of-hearing users and for event commentary or simultaneous interpretation when required.
A 36-channel Allen & Heath GL-4 mixer with an associated comprehensive outboard equipment rack is located externally to the level four sound and lighting control room, enabling the sound in the arena to be directly monitored. A portable 16-channel Allen & Heath GL Series mixer is available either to control smaller functions locally within the arena or to act as basic stage monitor desk.
An easy-to-use skater "practice system" has also been provided. This consists of a small, rack-mounted mixer with Denon CD and tape players and Shure wireless microphone. A special socket is provided for this on either side of the arena and in the community pad/practice rink. Plugging the system in automatically sets the performance system to "practice" mode and configures the system to route the signals to the appropriate sections of the center cluster - requiring no technical staff action - a useful facility for the 5 a.m. practice sessions!
Arena audio programs can be routed throughout the building on a zoned basis via the paging and voice alarm system. Figure 4 shows a simplified block schematic for the audio routing and control system. The facilities outlined in red are voice-alarm related, while those in blue relate to the performance systems. The tie lines are in green. As can be seen from the figure, two separate microprocessor-based control systems are employed in the system. The MediaMatrix deals with all the performance-related aspects, whereas a Baldwin Boxhall BVR-20 unit provides routing control and priority allocation for the voice alarm system. The split of the audio systems into performance and emergency PA works extremely well, particularly as the MediaMatrix and BVR-20 communicate well with each other. The split also enables a more versatile sound system for the arena to be configured while still achieving the stringent fire alarm standards and requirements, as discussed below.
VOICE (FIRE) ALARM SYSTEMThe voice alarm system is fully monitored from microphone capsule to loudspeaker line. Separate interleaved A and B amplifier circuits are installed in all areas to ensure at least 50% coverage of any area in the event of an amplifier or loudspeaker line fault. The arena, backstage and main public area systems are fed from an essential services UPS which maintains power in the event of a mains power failure. A standby emergency generator is also provided. All loudspeaker, emergency microphone and critical control cabling is in fireproof cable as is related to mains-supply wiring. The complex is divided into 16 paging zones that relate to staff or plant areas and different public spaces such as the main arena, public pad, concourses, etc. External areas are also covered in order to make announcements to waiting spectators or to address the public when the complex is evacuated for fire or security reasons. The VA system also incorporates general paging and background music facilities.
The arena is covered from the roof-level gantry by a ring of 42 Community PC264 60 by 40 CD horns. STI measurements in the empty arena in its ice-pad configuration show the main seating to a have potential intelligibility of between 0.55 and 0.6 STI (8.5 and 6% Alcons). The rink itself measured 0.5 to 0.55 STI (10 to 8.5% Alcons), excellent results for a 3-second RT environment (See Figure 5). A separate voice-alarm loudspeaker system is used within the arena, primarily for the flexibility of operation required from a multipurpose performance system that cannot guarantee all necessary loudspeakers will be available or operational - or even plugged in - at any given time. The voice alarm CD horns are permanently wired in fireproof cable and run from amplifiers that contain the necessary fault-monitoring and indication circuitry to conform to British fire and voice alarm standards. This circuitry is not typically found in professional audio power amplifiers. The amplifiers and control equipment are located in a half-hour fire-protected room. The equipment feeding the community pad is located in a separate room located closer to the pad. Here battery backed-up 24-V power supplies are set to operate in the event of a mains power failure.
Equalization and signal ("time") delay facilities for the VAS are provided by an Allen & Heath DSP unit capable of 24-volt operation.
All ceiling loudspeakers employ steel grilles and back cans for fire resistance. Individual loudspeakers also employ thermal fuses to help safeguard the integrity of the main feed cables, which are routed throughout the complex.
The main arena and adjacent concourse employ ambient noise sensing and automatic signal gain adjustment devices in order to maintain a consistent S/N ratio and hence intelligibility. As these devices form part of the critical path signal chain they are also monitored and configured to provide a "fail safe." Plant rooms with inherently high levels of noise (like the main ice plant, which when the compressors are operating, measures 95 to 100 dBA) also have visual warning beacons that operate in the event of a fire alarm. Separate PA systems are provided for the fitness and function suites. These are connected to the VA system and automatically mute during an alarm or high-priority announcement. Alert and evacuate messages are provided in non-volatile digital memory stores with 2-stage alarm operation.
The main arena opened in April with a concert by Simply Red. Their music complimented the arena and its acoustics - as have all the other bands and performers since. The facility is set to become not only one of the UK's primary venues but also an ice skating center of international renown.