Keeping Your Perspective

The basic purpose of a sound reinforcement system is to provide a consistent perspective of a live event to an entire audience. This sounds simple, but the details required to achieve this end can become complex enough that many people involved in the technology will lose sight of this goal. If it were not for the desire to control and enhance the aural perspective the audience perceives, sound reinforcement systems would rarely be used. Everyone would attend 19th Century-style concerts, where your seating location determined the sound quality you could expect. Instead, we have come to expect live concert sound to convey the listener to the front rows, even though the performers may be small objects on a distant stage.

The tonal quality, timing and apparent local of a sound source can be just as important in reinforcing a single talker as it is in the largest touring productions. Much of the work in achieving good results will remain in the hands of the sound system operator. However, design flaws in a sound system may prevent even the most skilled operator from meeting this basic need. A critical aspect of the sound system implementation is the presentation to the entire audience of the same frequency response and relative timing of the sound sources. The timing of the sound arrivals can have a significant impact on the frequency response (and, therefore, the tonal quality of the sound) due to interference effects that can cause small peaks and large canyons in the response curve.

Our auditory system uses the relative timing and tonal qualities of sounds to determine the nature of the source, making judgements on the sound's source and origin. A simple example is a single speaking voice, where adding low-frequency energy can increase the sense of intimacy and proximity. This can be further enhanced by boosting the upper "whisper" frequencies. We sub-consciously assume that distance will reduce these frequency components, therefore, when these qualities are present in the voice, a perspective shift occurs. However, we are now so accustomed to this shift, thanks to dialogue replacement in film and television, that it is rarely a surprise. The performer's style must match this perspective (with a softer more intimate vocal presentation) and when it does not, other assumptions are made by the ear/brain combination. A loud, bravura-style of presentation using the same tonal adjustments will no longer sound close and intimate; instead it will create the impression of a gigantic sound source, producing the "larger-than-life" image of the stadium touring show. This is why the internalized vocal style of many popular singers sound appropriate when they use a vocal mic that is touching their lips, but classically-trained operatic voices sound bizarre if they use the same close-mic technique.

The early reflections of the sound that reach the listener also have a major impact on the perceived perspective. Reflected energy is picked up at the microphone, from sound bouncing off nearby surfaces (the performer' face, instrument, stage floor, etc.), and leakage from the monitors (often the major source of reflected energy from other instruments). The resulting comb-filter effects produced at the mic are indistinguishable from the impact of early reflections occurring near the loudspeakers and near the listener. All of these potential sources of interference combine to create the final quality of sound heard by the listener. Problems in creating an appropriate perspective, especially one of enhanced intimacy in a larger venue, are difficult to overcome at the mixing console when the sound source has been "contaminated" with inappropriate early reflections at the pickup.

To provide the sound system operator with the greatest control over the perspective of each sound source, all of the systemic early reflections must be minimized. This requires careful selection and orientation of the loudspeakers (as noted in my January 1998 Pro Sound News column). Eliminating the destructive interference, which occurs between loudspeaker and listener, allows the sound system to achieve the most "reach" or dramatic change in perspective to listeners throughout the venue.

The problems limiting the ability to control or enhance the aural perspective of a concert or speech primarily exist in the acoustic transmission of the sound as it travels from source to mic, and from loudspeaker to listener. If those elements of the transmission chain (between performer and listener) are optimized, the shaping of the tone; addition of delay and reverberation; and the relative balance of instruments and voice will allow the greatest artistic control of perspective. Without attention to these elements, your control of perspective will be limited in scope.

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