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Sound Delivery

Jan 22, 2013 2:30 PM, By Bob McCarthy

Sound systems and the rooms they live in

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Figure 1: The summation of two signals that are 1 millisecond apart in time, such as the direct sound and an early reflection, or two speakers. The comb filter peaks and dips are linearly spaced and therefore are perceived as varying percentage bandwidth to our log frequency hearing system. The spacing is octave wide at 1kHz and 1/10 octave at 10kHz. Click here to see a larger image.

I am a sound system designer and tuner, and I work in performance spaces. Every room I have encountered has a unique and extremely complex acoustic response. Each has its own macro dimensions, surface materials, and architectural details—all of which contribute to the unique “sound” of the hall. The combined effect of these factors can be quantified with a stupefying array of mathematical statistics such as reverberation time, early decay time, clarity, bass ratio, strength, and on and on. These numbers alone cannot ensure a room will be successful for particular applications such as opera, classical music, or speech, but they are the best objective indicators to date.

One might assume that as a sound system designer, I would have a great interest in these numbers and that they would be an important part of the design process. For example, you might expect that the sound system in the very lively Carnegie Hall would require a dramatically different approach than a Vegas showroom full of curtains and fiberglass. The approach differs, but to a much lesser extent than you might think.

The Role of the Room

Ask yourself, “Why should it be different?” Where do you aim the speakers in a lively hall? At the people. And in a dead hall? At the people again. In what kind of hall do we intentionally aim sound at anything other than the seats? None that I have ever been involved with. Do we approach this differently for pop music than speech in a house of worship? Do loud shows need to aim away from the walls while quiet ones don’t?

This might seem like a silly line of questioning, but I am bringing this up to make a simple but important point. Sound system engineering is not about the room. It is about the sound system. It is about laying a blanket of direct sound with consistent level and tone onto all of the seats. The room is first and foremost to us, a place to hold the seats. Our approach to the room geometry begins with the arrangement of seats from the perspective of where we are allowed to place speakers. This defines the target, and hopefully, the walls and ceilings are kind enough to lets us get there without their help.

The room comes into play only after we have completed our primary mission: direct sound delivery to the seats. Direct sound delivered to unseated areas is viewed from our perspective as potentially treasonous for its ability to interfere with the unfettered direct sound delivery to other seated areas. I don’t know any sound engineers who look at a particular wall and say, “Nice! That reflection will really improve things.” It is fairer to say that we inspect a hall for the surfaces that are going to start an insurrection against our control over the space—and yes, we are direct sound control freaks. Strong reflections put the room in control and risk degrading areas that would otherwise have undisturbed coverage. As an example, we can adapt an old dog lover’s saying, “The only good balcony front is a dead balcony front.”

Am I advocating for a sterilized world of anechoic chamber music? No, not at all. I love reverberation as much as the next guy, but a little bit goes a long time. In the ideal world, we would have variable control of the reverberation to create a decay character that was appropriate for a given program material or artist, or even a particular song. We have reverberation devices that we can add to the sound mix, but we understand that this is not the same as reverberation in the room. A dead room filled with lots of electronic reverb in the speaker system leaves the performers singing in the rain while the audience stays dry. The most desirable result is a uniform reverberant field surrounding the audience creating a sense of sonic envelopment. That goal is widely shared between acousticians and sound engineers alike.

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