The Virtual Orchestra: Acoustical and Audio Engineering Issues

Richard H. Campbell* and Frederick W. Bianchi~

*Electrical and Computer Engineering Dep~rtment and ?Hurnanitics Department. Music Division Worccstcr Polytechnic Institute, 100 Institute Rd., Worcester. MA 01609

Abstract: me interactive multi-channel computer music system known as the Virtuat Gchestra has been used several times in professional opera and theater as an alternative to a live pit orchestra, and was recently-demonstrated at au ASA regional chapter meeting in Boston, and at the National @ra Association meeting in Washington, ~, A companion paper discusses the role of the mnsicidtechno]ogist and the creative issues. This paper discusses technical issues such as the sel~wtionof the tyWs of loudspeakers and their plaeemeat in the pit, the amplifier power requirements and the sound level calibration for each instrumental ~tion based on a mez~forte norm ‘.Wolf’ tones mused by understate extension cavities and other pit deficiencies can be easily modified within the computer program or with external equalization. The computer operator. s abili~ to follo}vin real time the conductors direction in nuanw and tempo guarantees a cohesive performance s}~)chronizedwItb the singers and ~~iththe dramatic action.

DEPLOYMENT OF THE ~R~AL ORCHESTRA

The Virtual Orchestra (VO) comprises a group of loudspeakers. po~veramplifiers. signal processors, Wnthesizers, computers, and other controls which are placed in the pit of an opera house or equivalent location in other auditoria. Although the standard VO deployment is 16 audio channels. many more than 16 loudspeakers are normally used. In a mdtipurpose hall venue. it frequently happens that dl of this equipment must be placed in position and ready to perform in a period of three hours from first access to the hall. This imposes a severe requirement on the type of audio connwtors and cables, and consideration of reliabili~ with rough handing. loudspeaker robustness. and packaging of the electronic equipment for portability. Clearly, carefi a~ance planning is a necessi~. One issue constantly faced \vith VO deployment is the lack of adequate electrical power in a pit. Older hall pits were designed to power a handful of music stand lights, not eight high-potvered stereo amplfiers along wfithmeral hundred watts of computing and processing equipment. Sequence starting the power amplifiers is essential.

~NING THE ~TUAL ORCHESTRA

Once the equipment is deployed and operating properly, the process of tuning the VO to the hall commences. One or more persons with trained ears sits in different locations in the audienee area and calls for adjustments to be made by the VO operator. There is great flexibili~ in deciding what to adjusl to compensate for acoustical deficiencies. We use the overture to Mozart-s The .t{arriage of Figaro to establish balance in the string sounds. wood~vinds and timpani, and the prelude to Bizet’s Carmen to adjust the large orchestral tutti sound which includes fitll brass and percussion. The Musical Instrument Digital Interface (~1) controller allows the pla~-ingof isolated sectionrd instruments for more preeise adjustments of tend Mance. Oboe scales are used to get that section properly eqtiized, Nearly mew kind of pit deficienq ean be co~ed including the addition of small amounts of artificial rwerberation on a per-channel basis. Under-stage cavities which cause pronounced resonances for certain notes are easily “-attenuated. by reducing the g[obd signal Iael WI \oiume) for that partidar note.

671 AUDIO CO~ONENTS ~ THE WRTUAL ORCHESTRA

The selection of VO audio components relates strongly to the OF of musical instrument king played through that particular channel. HeawT brass and percussion demand high audio power and Iargc loudspeakers. Strings demand much less from the audio ~stem but much more from the ~nthesis source. Mw strings and woodwinds are somewhere in between for audio power demand. In practice, for purposes of quick replacement and interchangabili~, all of the audio amplifiers are identical and rated to deliver the required performance of the percussion section. The Ioudspe&ers are individurdly selected to provide the optimm tonal qtiih for the partitiar musical instrument family with which they are used. Timpani, bass drum, contra-bass and tuba require sub-w700fersplaced on the pit floor working with high qtii~ upper range loudspeakers (stressing minimum intermoddation because percussion loudspeakers are used dso for qmbats and snare drum) which are set directly on top of the subwoofers. We find that the orchestral tuba high frequenq Iouds@er shotid face upward for best radiation pattern. We sometimes face the woodwind loudspeakers downward toward the pit floor to simulate their principal radiation axis. By far the most dtiltit sections are the first and second violins. Regardless of which loudspeaker is used, several of them are deployed for each section in a way that generates the time smear associated ~vithmultiple violins bowing together but spaced sigtilcantiy apart in the pit. The “humanizing” algorithm in the ml controller generates minor statistical errors such as fa~u intonation and s~gger~ sta~s. ho~~e~er,tie acoustical time smear is ememely impomnt for violin section realism. We are also now looking at planar distributed-mode loudspeakers for the violin section. There is no attempt at the moment to simualate violin directional characteristics. There are many elements to be considered for loudspeaker selection, not the least of which is survivability in a ve~ rough transport environment. Inevitably, the most desirable loudspeaker may simply not be up to the task and some compromise is needed. Working }vitha professional loudspeaker manufacturer having a reputation for musicdiq and reproduction accuraq is essential.

CAL~RA~ON OF THE WRTUAL ORCHESTRA

The singers on the stage. and the conductor in the pit. must aurally perceive the VO as a well-balanced, accurate orchestral sound source, even at close range. This implies that the VO sound Imel must reproduce as closely as possible the sound level output of each instrumental section for a given playing effort. This is easily done@ cataloging the sound level ofa Vpical pit orchestra P1aYingse~ional chords at mezzo- forte (@ and setting the ~1 veloei~ and volume control to emulate the same sound Imel. The object of this exercise is to assure the conductor that the VO will respnd correctly in sound level since the entire orchestral score has been translated to M~I using velocities and volumes comcnsurate with the loudness marks in the score. The actual sound Imel from the VO is easily changed in real time, under keyboard control , during tic perforrnanm, even on a sectional basis, During rehearsat, these fine adjustments, for example for a weak singer. or for a stage set casting acoustic shadows, can be stored to be used later, automatically, during a performance. An advantage of the VO is that it can reproduce the sound levels of a vev large orchestra. e~’enin a relati~elY small pit. Humperdink’s HanseI and Grete/, which has been publicly yrformed with the VO, is scored for a Wagnerian orchestra. The enormous sound levels produced by 100 musicians in the pit can be a bit disarming in a small space when the VO reproduces it accurately. Since the VO loudspeakers are spread across the entire width of the pit. and the orchestral sections appear in their proper location, the soundscape is rendered with astonishing realism to evev seat in the hall, as well as to the poduim and to the stage. The conductor is able to localize each source easily for p~choacoustic comfort and can call for Sectional loudness modulations as required.

AC~O~EDGMENTS

We would Iikc to thank Eastern Acoustic Works for allowing us to audition all of their products during the loudspeaker selection process, We also thank Htier for their valuable assistance with the power amplifiers.

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