Published in July 2005

Acoustic ‘Enhancement’ Systems…
By David E. Marsh

…as electronic architecture?

     The phrase “electronic acoustic enhancement” describes application of loudspeakers, microphones and electronic signal processing to improve or enhance the perceived acoustics of a space used for music performance. This might be a dedicated performance hall or a conference hall sometimes used for musical performances. Another common application is to provide a means of varying the room acoustic characteristics of a venue to accommodate different types of events. These systems have even been used to raise the level of crowd noise in sports facilities to give the home team a greater advantage!
      Most people think of room acoustics in terms of reverberance, so one goal of acoustic enhancement is to lengthen the reverberation time. Ideally, the system also should be able to increase the loudness of a performance, add musical warmth (bass ratio), improve clarity and control the “aurally perceived” size of the room (intimacy). Some systems are more effective than others at simulating early sound reflections produced by architectural features found in well-designed concert halls. In this sense, the concept is referred to occasionally as “electronic architecture.”
      Is it really possible for electronic systems to produce authentic sounding reverberance, to compensate for missing lateral reflections or to create an enveloping sound field where the architecture fails to do so?
      The quest to simulate room acoustic effects is older than the spring reverb in my first guitar amp (that was 40 years ago!). Of course, today’s high-quality digital reverb units sound a lot better than my old springs. However, you can’t just “add reverb” in the house audio system if the goal is to realistically simulate reverberation. You must have sound coming at you from all directions. So, you say, “What about adding reverb and then amplifying the signal through a gazillion loudspeakers located all over the room?” To understand why that won’t work, let’s consider how real reverberation is created.
      Sound emanates from an acoustic source and “bounces” around the space. Each surface encountered has an effect on the traveling sound wave, depending on the sound-absorbing properties of the surface material as well as its size and shape. Some frequencies are reflected efficiently while others may be diffracted, scattered or absorbed. Some sound waves travel only a short distance (e.g., one or two bounces). Others travel long distances, causing them to arrive much later, weaker and tonally altered.
      The “gazillion loudspeaker approach” suggested earlier would provide sound from all directions, but it fails in several important ways to simulate quality reverberation:
• Unlike the long, drawn-out sequence of sound arrivals in a naturally reverberant space, the amplified sound from all loudspeakers would arrive over a relatively short period (depending on the size of the room).
• Real sound reflections are absolutely unique in tonal quality in contrast to loudspeakers all reproducing a signal with the same frequency response.
• Likewise, each reflection varies in level, depending on the amount of sound absorption encountered in its path.
• And finally, the initial signal is generated by a single pre-mixed source instead of being “picked up” at multiple locations in the room and distributed from each of them in unique ways.
      Driving each loudspeaker with a separate power amplifier channel and applying multiple signal-processing paths can address the first three issues mentioned. This provides a way to simulate sound reflections or reflection sequences from each loudspeaker with unique temporal, frequency and level characteristics.
      Now, we have to address the issue of picking up the sound in a natural way. Simple: Install a gazillion microphones all over the room and feed their outputs into a signal-processing matrix. This offers flexible routing to a gazillion loudspeaker channels! Now we’re onto something. But with all these microphones and loudspeakers working together in the same room, how do we keep the system from becoming unstable (ringing or feeding back)?
      Designing these systems involves careful layout that ultimately determines the device quantities and relative distances between microphones and loudspeakers. As it turns out, it doesn’t really take a gazillion to get the job done. The trick is in having enough of each located in enough of the right places to meet the goals stated at the beginning of this column, and then to achieve maximum acoustic gain before feedback. System manufacturers have employed a variety of proprietary methods to improve acoustic gain; some work better than others. In practice, far fewer microphones are required than loudspeakers to attain a significant increase in reverberation time, but more microphone locations gives better control over early sound-energy levels. This affects not only the clarity and intimacy characteristics, but also the enhanced envelopment provided by early lateral reflections.
      Skillful application of this technology produces remarkable results. But…interested parties should remember that it is not really a sound system design. Clients expect results in terms of acoustics. Therefore, someone experienced in performance-hall acoustics should be involved, at least in the layout and tuning of these systems.

David E. Marsh, a member of Sound & Communications’ Technical Council, is vice president of Pelton Marsh Kinsella. He is a fellow of the Acoustical Society of America and on the board of directors of the National Council of Acoustical Consultants. Send comments to him at dmarsh@testa.com.

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