Acoustic enhancement

From Wikipedia, the free encyclopedia

Acoustic enhancement is a subtle type of sound reinforcement system used in some concert halls where classical music such as symphonies and opera is performed. Acoustic enhancement systems help to give a more even sound in the concert hall and prevent "dead spots" in the audience seating area.

[edit] Design and application

Acoustic enhancement systems use "...an array of microphones connected to a computer" which processes the sound and directs it to an array of amplifiers and loudspeakers spread out in the concert hall. The computer processor is designed to augment the natural acoustics in the hall and prevent "dead spots." However, as concertgoers have become aware of the use of these systems, debates have arisen, because "...purists maintain that the natural acoustic sound of [Classical] voices [or] instruments in a given hall should not be altered."^[1]

Kai Harada's article Opera's Dirty Little Secret^[2] states that opera houses have begun using electronic acoustic enhancement systems "...to compensate for flaws in a venue's acoustical architecture." Despite the uproar that has arisen amongst operagoers, Harada points out that none of the major opera houses using acoustic enhancement systems "...use traditional, Broadway-style sound reinforcement, in which most if not all singers are equipped with radio [headset] microphones mixed to a series of unsightly loudspeakers scattered throughout the theatre."

Instead, most opera houses use the sound reinforcement system for acoustic enhancement, and for subtle boosting of offstage voices, onstage dialogue, and sound effects (e.g., church bells in Toscaor thunder in Wagnerian operas). In rare cases, acoustic enhancement systems are used to amplify individual voices, as in the case of child singers or actors, or where it is specified in the stage directions (e.g., in one opera, Wagner specified that a character's voice should be amplified by a large speaking trumpet).

[edit] Types

There exist different types of acoustic enhancement systems: In-line versus Feedback systems with or without electronic reverberators.

Because of the design of In-line systems they are capable of producing very wide acoustic effects. As a matter of fact digital signal processing techniques being now quite powerful, it is possible to precisely control each channel for early reflection or late reverberation. Especially it is possible to produce very long reverberation time. Independently controlling the length and the level of the reverberation looks very attractive. This is the reason why most of the recent systems are "in line" type. In fact, setting up "in line" systems leads to a lot of acoustic limitations, often severe, more especially in relatively already reverberant room, giving artificial coloured sounds or disturbed sound images.

Contrary, Feedback systems seem to be more restricted in the effects they can produce because they influence mainly the late reverberated sound, as if the acoustic absorption of the room was partly cancelled. On another hand the reverberation produced sounds usually very natural.

In-line systems with electronic reverberators: Acoustic enhancement systems include LARES (Lexicon Acoustic Reinforcement and Enhancement System) and SIAP, the System for Improved Acoustic Performance. These systems use microphones, computer processing "with delay, phase, and frequency-response changes," and then send the signal "... to a large number of loudspeakers placed in extremities of the performance venue." The Deutsche Staatsoper in Berlin and the Hummingbird Centre in Toronto use a LARES system. The Ahmanson Theatre in Los Angeles, the Royal National Theatre in London, and the Vivian Beaumont Theatre in New York City use the SIAP system. ^[3]

Feedback systems with electronic reverberators: Another acoustic enhancement system, VRAS (Variable Room Acoustics System) uses "...different algorithms based on microphones placed around the room."

Feedback systems without electronic reverberators: CARMEN developped by CSTB comprises a number of electro acoustic active cells (approximately from 16 to 40), each of them being composed of a microphone, an electronic filtering unit, a power amplifier and a loudspeaker. Placed around the walls and ceiling of the auditorium, the cells form virtual walls depending on the architecture and the acoustic problem to solve. They only communicate between each other by the acoustic way. ^[4]