Arena Sound System Design Issues

Sporting arenas are highly reverberant environments, presenting a challenge to any audio designer. Concrete floors, plaster walls and plastic seating all contribute to a long reverberant decay times and multiple short reflections which degrade intelligibility.

Multiple short delays caused by path-length differences are particularly troublesome. For example, if a listener is 20 meters from one sound source and ten meters from another, the delay between the two signals would be in the order of 35 milliseconds yet there will only be a 6dB difference in intensity between the two signals. Such a delay is clearly audible to the listener, and when multiplied many times throughout the venue causes severe degradation of articulation.

This phenomena often occurs when multiple loudspeaker systems overlap such as found in some outdoor venues, and/or when sound reflects from a large planar surface, such as the back or side walls of an arena or grandstand.

Distributed Systems

The most effective means to achieve maximum articulation in highly reverberant areas involves multiple distributed clusters of loudspeakers, each servicing a dedicated section of the audience with minimal overlap with adjacent loudspeakers.

Each element of a distributed cluster would be activated only when that section of seating is occupied. The downside to this approach is the additional cost presented by large numbers of drivers, amplifiers and cable required to provide sufficient coverage.

An excellent implementation of this approach can be found at the Colonial Stadium in Melbourne, where Sydney contractor, PA People installed multiple clusters of JBL horn loaded cabinets, together with under-balcony fills to produce seamless coverage to every seat in the house.

Single Cluster Systems

A single cluster of loudspeakers arranged in a hemispherical or semi-circular array, provides a near "point source" of sound, virtually eliminating multi-path delays between adjacent loudspeakers.

A single cluster system is prone to short delay reflections from walls and seating tiers which arrive at a listeners ears in conflict with the direct sound. Before implementing a central cluster design, careful attention needs to be taken of potential delays due to multi-path reflections.

Central cluster systems need to produce considerable peak sound pressure levels, because of the relatively longer distances to be covered. To achieve 100dB at distance of 80 meters requires 138dB to be produced at the horn mouth. Furthermore, high frequencies above 5kHz are further reduced due to air absorption. There are very few driver/horn combinations which can produce such extreme sound pressure levels from a single cabinet. The JBL PD764 is one of them.

Description of Vodafone Arena

The floor plan of the arena forms a square approximately 100 meters x 100 meters with tiered seating on four sides of a center court. A retractable roof extends to within 20 metres of the stadium walls on all four sides.

The most centrally located position for a loudspeaker cluster is therefore 20 metres from one wall and eighty metres from the opposite wall. From this location the majority of seats are within 40 meters of the cluster location.

The arena converts to velodrome mode by raising two entire seating areas on opposite sides of the center court, exposing the cycle track underneath.

The walls exposed by the raising of the seats, and the underside of the seats themselves are treated with sound absorbent panels to reduce reflections.

The result in velodrome mode is a remarkable short reverb time with minimal flutter echoes, which increase marginally when the seats are down. A number of seating locations are identified as suffering from multi-path delays in the 30-50 milliseciond range, however they represent significantly less than 10% of the total seated areas at any time.

Purpose of Audio System

The audio system shall reproduce voice announcements, running commentary and background music for sporting events such as basketball, tennis and cycling. Voice announcements shall be clearly audible and intelligible in every seat of the house above the noise of enthusiastic spectators.

Sound pressure level shall meet or exceed 90dB(A) SPL when measured over 90% of the seated area.

Determining Factors

The decision to upgrade the existing central loudspeaker with the addition of JBL PD long-throw loudspeakers was determined after due consideration of the following factors.

• The reverberation and short-delay characteristics of the venue were considered acceptable for a central cluster design



• Only one half of the previous cluster required upgrade, the rearward facing loudspeakers performing to specification for their intended coverage pattern



• The loudspeakers proposed to be replaced could be redeployed to serve the seating areas below the cluster, which previously had no coverage



• The new JBL PD series stadium horns makes available a single, coaxial loudspeaker cabinet, rated to produce 104dB continuous at 50 meters in the critical 250-1500Hz "vocal" frequency range, which in this case exceeds the required SPL target of 90dB(A) at the furthermost seats in the house.