Acoustical requirements for performance halls can vary dramatically depending on their usage - be it a classical concert, a theatre performance, an opera or a pop music concert. Each of these performances need different acoustic environments. One of the most common descriptors of a hall's characteristic is the Reverberation Time (RT). This descriptor provides an indication of the time it takes for sound to decay. The ideal RT for a space varies with the proposed usage:
| Presentation | Ideal RT (sec@500Hz) |
| Cinema | 0.7 - 1.0 |
| Pop Music | 1.0 - 1.2 |
| Speech | 1.0 - 1.2 |
| Drama | 1.0 - 1.4 |
| Opera/Musical | 1.4 - 1.6 |
| Symphonic | 1.8 - 2.5 |
If a space is designed for a given type of presentation, then the acoustics of the venue will not be ideal for another type of presentation. Thus, if the RT could be altered to suit, the usability and revenue potential for a given hall could be significantly improved.
Two options are available for altering the Reverberation Time (RT) of an auditorium:
Variable Passive Acoustics -
RT is dependent on both the volume of the auditorium and the absorption characteristics of the surfaces in the auditorium. A passive enhancement system is one where panels change their face to alter the absorption characteristics or where hanging panels are lowered into the auditorium to introduce more absorption. Movable ceilings, adjustable reflectors, retractable banner curtains are expensive and have huge consequences in terms of architecture and flexibility. The change in RT achievable using this approach is also only of the order of 0.3 to 0.5 sec. This is a small change and still limits the range of venue uses.
Electronic Reverberation Enhancement System (ERES) -
An ERES is a recent innovation which allows the acoustics of the auditorium to be varied by electronically changing the reverberation using digital signal processing. There are two basic requirements: - Change the RT of the space in such a way that it is indistinguishable from the result that could have been achieved by architectural means. -Not adversely affect the structure of the sound (music) nor introduce audible distortion or colouration. The objective is therefore to provide a natural sounding reflection pattern, by adjusting reflection sequences of the sound to suit various reverberation requirements.
In one form of implementation, multiple inputs from microphones located adjacent to the proscenium are fed into a digital signal processing unit (DSP). These inputs are then processed digitally through the DSP
processor to provide a multi-channel digital frequency shift. This prevents feedback and provides the necessary time and magnitude shift which determines the variation of reverberation within the auditorium. This is done without any physical change to the location of or type of absorption already in the space. The result is the ideal RT to make any performance sound right. Different RT requirements become achievable with the "flick of a switch".
Enhancement Systems are capable of filling in reflection sequences in venues where "something is missing". The sound quality and naturalness of an ERES is mainly achieved by reflection density and the extent to which the reverberation sound reaches the listener's ears. Reverberation level and energy ratios of early to late and of lateral to total early reflections can easily be optimised to provide a more balanced acoustic experience. With ERES, a performance hall can be used for many different events with the ideal reverberation time to suit each performance.
Experience
Vipac has a number of successful installations using ERES. Vipac has for example specified, supervised and commissioned an ERES for Shatin Town Hall in Hong Kong and is currently assisting the Regional Services Department with the installation of an ERES in the Kwai Tsing Theatre, Hong Kong.
Vipac's services include client demonstration, initial feasibility studies, preparation of technical specifications, tender return reviews, factory acceptance and commissioning.
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