August / September 1999

by John Matheson

Sound Systems: Through the 1980s and 1990s there has been an unprecedented advancement of computer and digital signal processing (DSP) technology. Spawned by the introduction of personal computers and the compact disk, the electronic tools now exist to recreate the quality of real acoustic spaces, to create virtual rooms if you like.

Acoustical engineers traditionally control the acoustics of spaces by adjusting the properties of boundary surfaces. To the extent that architects and interior designers will allow them, they control the sizes, angles, and absorptive and reflective properties of the walls and other surfaces in a space.

On a small scale, DSP has been used to create concert hall or ambience effects for home stereo and theatre systems and even in car entertainment systems. Now large scale electronic architecture systems are being installed in public venues such as drama playhouses, opera houses and concert halls, both as remedial acoustical treatment for existing under-performing venues and in new multipurpose venues to allow for variable acoustics to suit different performance styles.

Electronic architecture does not eliminate the need for good architectural acoustical design, but its application is very flexible and it can be used to overcome some of the physical limitations of traditional architectural acoustics.

The desirable acoustical behavior of a performance space depends very much upon the type of performance presented in that space. For example, there are fundamentally different acoustic requirements for speech, amplified performances, opera and orchestra.

The spoken word needs strong early frontal reflections to give adequate strength (ie loudness) to be heard, an absence of echoes and a short reverberation time to prevent loss of articulation.

Similarly, amplified performances need a freedom from echoes and a short reverberation time.

Symphony orchestra concerts need a longer reverberation time to blend the sounds from each part of the orchestra, with strong lateral, rather than frontal, reflections to create a sense of envelopment and spaciousness. Because early composers were constrained by the performance spaces available to them, they wrote music to sound good in those spaces. Thus music from the

The 2000 seat Adelaide Festival Theatre auditorium’s new electro-acoustic system greatly enhances the musical experience of the audience .baroque period is best performed in small auditoria, classical music in larger auditoria and late romantic music in very large auditoria.

Concert hall reverberation would, however, impair articulation of the human voice in an opera performance. Opera, like speech, needs strong early frontal reflections for the voices to be audible, but also needs the auditorium to blend orchestral sound from the pit harmoniously without overpowering the singers. The reverberation time required for opera is a compromise between orchestra (long) and speech (short).

Electronic architecture is the creation of acoustic fields to alter the acoustic behavior of a space.

The aim of electronic architecture is to provide improved acoustics in existing venues and variable or virtual acoustics in general. These systems work by electronically placing virtual reflecting surfaces in desirable positions, and by electronically removing excess sound absorption.

Electronic architecture has nothing in common with public address systems other than it is achieved through the use of sound equipment, that is microphones, signal processing, amplifiers and loudspeakers. In contrast to conventional sound reproduction systems, sound systems for electronic architecture are generally characterized by having a small number of microphones and a very large number of loudspeakers which “tile” the walls and ceiling of a space.

These systems have some stringent requirements setting them apart from conventional public address design approaches. Electronic architecture is generally used to “amplify” the acoustical space, rather than the performers, in the same way that walls and reflectors would, if the acoustical engineer had the freedom to place them where they are needed. The resulting musical balance is not determined by a sound engineer, but left firmly in the hands of the conductor or musical director.

The development of electronic architecture started in the 1930s with experiments conducted by RCA at the Philadelphia Academy of Music where stairwells adjoining the auditorium were excited by loudspeakers to increase the reverberant energy level in the hall. In 1955 the first

Philips Ambiophonics system was installed in the La Scala Opera House in Milan, Italy.

This was followed by the AR (assisted resonance) system installed into the Royal Festival Hall, London, then came MCR (multichannel reverberation), ERES (early reflected energy system), RODS (reverberation on demand) and ACS (acoustic control system).

Several companies are currently marketing electronic architecture systems using modern DSP techniques, including the European SIAP (system for improved acoustic performance), American

LARES (lexicon acoustic reinforcement & enhancement system) and Japanese Yamaha AFC

 acoustic field control).When it was decided to stage the Paris Théatre du Châtelet’s version of the entire Der Ring des Nibelungen, Wagner’s epic cycle of operas, in the Adelaide Festival Theatre,there was a great deal of concern that the much maligned acoustics of the theatre would not please high ticket price paying patrons (around $1000 each). Acoustical refurbishment of the theatre was considered paramount.

The Festival Theatre auditorium’s acoustics provided good, even coverage for speech, contemporary music concerts and musicals. The reverberation time was long at low frequencies, short at middle frequencies and very short at high frequencies for the auditorium’s size. Low frequency resonance sometimes caused problems for sound engineers.

When the theatre opened in 1973, it was regarded as an acoustically innovative design for its system of retractable acoustic curtains along the rear walls and a false ceiling above. The design of the ceiling was intended to promote reflections back to the auditorium, while still being open enough to allow the development of reverberant energy in the volume above that could filter back down to the auditorium.

After being in operation for some time, the theatre came to be regarded as having less than satisfactory acoustics for symphony orchestras, and to a lesser extent opera, primarily because of its excessively short, middle and high frequency reverberation time.

The walls of the theatre are covered with decorative wooden blocks, which are a hardwood tile varying in size, but typically 200mm square and 50mm deep. The builder, without authorization from the acoustical consultant, incorporated a gap between each tile. The gap was intended to prevent the tiles from dislodging from the wall should they expand as a result of absorbing moisture. The main reason why the auditorium did not perform as expected was a result of the air gaps (varying between 40mm and 65mm in depth) acting as frequency selective (quarter wave) absorbers.