Computer Modeling a Useful Tool in Optimizing Sound Quality in Spectator Facilities

Computer modeling is a useful tool in helping architects and building owners ensure sound quality in their new and renovated spaces.

Acoustics Rast0409 Many images are said to be indelible. If you walk into the seating bowl at Boston's Fenway Park and take a close look at the iconic wall in left field, the language of sight allows you to describe it in detailed, multilayered fashion.

The Green Monster is not only green, it's a particular shade of green that can be compared to the grass below it, or to any other natural or manufactured objects. It's not only tall, but its height can be estimated - eyeballed, that is, and guessed at. It's painted and, what's more, the paint has a flat finish. Its surface carries a certain texture - hundreds of slight indentations from decades of line drives - and has distinguishing features, such as a large manual scoreboard and a ladder in fair territory that was once used for retrieving balls from the screen that used to stand up on top. You could go on and on.

Describing the ballpark's sound, however, is not so easy. There's the roar of the crowd, of course, but your ear can't break it down into distinct voices, or a range of frequencies. Your ear can't go deeper into the sound.

But a computer can - and frequently does. Acoustical consulting firms utilizing auralization software can build a virtual space with the exact dimensions of the arena, natatorium, ice rink, lobby or stadium you're constructing or renovating, and help place loudspeakers or acoustical panels, or select finishes within the space, to ensure it sounds as it should.

Consultants say that although clients tend to assume the auralizations they have paid for will exactly reproduce what a space will sound like, sound quality remains elusive - and subjective. Ioana Pieleanu, an acoustics consultant with Cambridge, Mass.-based Acentech Inc., compares hearing with taste, another sense that is highly dependent on the individual's receptors.

"Can you remember a certain flavor?" Pieleanu says. "If you eat something with cinnamon today, and you eat it again in two weeks, you will remember it, but maybe not the precise flavor. There is more place for error, I would say, than with visuals. It's the same with sound. Auralization software allows us to make a relative comparison, which in this kind of loose ballpark of being able to remember things is actually pretty accurate."

Jack Wrightson, a principal with Dallas-based Wrightson, Johnson, Haddon and Williams, which in 1991 was the first consulting firm to perform an auralization on a sports building, says clients' expectations must typically be tempered before they give their auralization a listen.

"An auralization is somewhat like a rendering," Wrightson says. "Very few renderings are so perfect that you're tricked into thinking you're looking at a photograph of a building. The acoustical data we build auralizations from, the math that models it, is imperfect; everything's an approximation. That having been said, it's an excellent tool, the way renderings are, of giving people an idea of what the space will be like. It won't let them know exactly what they're going to get, but it will approximate the nature of the sound quality, and it's especially valuable in letting building owners evaluate different acoustical options."

Auralizations predate computers, but they were difficult and very costly to accomplish. Associated with the highest-end concert halls, these projects involved building scaled, three-dimensional replicas of the hall, putting a sound source into the model (the frequency of the sound in such cases has to be scaled to match the dimensions of the model) and tape-recording the result.

The modern method allows the virtual space to be constructed straight from the architect's CAD drawings or built manually using drawings or measurements of an existing space. Precision is vital, as a mistake in the placement or angle of any wall or obstruction will affect the interplay of the peaks and troughs of sound waves reverberating through the space. Typically, a sound source is selected - in a theater it would be a spot at stage left, center or right - as well as the location of the "receiver" (as a techie refers to a listener). Pieleanu says that in cases in which an existing space is measured and recordings are made, the sound source is typically a popping balloon (in the age of lax security, it was often a round from a starter's pistol), with the resulting "acoustical fingerprint" serving as a basis for comparison with the computer-generated auralization. Such sounds are omnidirectional, with sound waves spreading outward in the way that sound leaves a dodecahedral loudspeaker or loudspeaker cluster; to measure directional sound, an impulse can be recorded and measured as it is played through one loudspeaker aimed at the preselected listener.

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