Sound Image Costs and Benefits, Part 2
Apr 4, 2013 1:38 PM, By Bob McCarthy
Practical sound system design for the best preservation of imaging.
In “Sound Image: The costs and benefits”, we established how sound image perception can be characterized in three main categories: horizontal location, vertical location, and perceived range (distance). Our goals are to make audience members perceive the sound as plausibly emanating from the source on stage without localizing to the speakers. Our success is measured by how well we minimize angular distortion in both planes, and if we can decrease the perceived range between the stage source and listener without detection. We will now put into practice the image control techniques previously described.
As discussed in part one, a standard theatrical or concert venue requires a minimum of four sound sources to create a reasonably stable image location for a good number of seats: left-low, center-high, right-low, and most importantly, the stage source: center-low. The more level we get out of the stage source, the more we can expand our area of quality image. The closer we can get our left, center, and right speakers to the stage source, the more we can expect to keep the image tracked to the performer. Additionally, we found that our listeners can detect image errors much better in the horizontal plane than the vertical plane. Therefore, when it comes to tradeoffs, we can get away with moving speakers upward much better than moving outward.
Venue Size and Shape
Are there certain venue shapes that are more conducive to low image distortion than others? Certainly. First of all, size matters. Smaller is better for two reasons: We will have a higher proportion of original source level and time is on our side. In any size venue, the original stage source loses 6dB with each doubling of distance. It is a certainty that the source level will be less in the rear than in the front rows. As venue size increases, they add rows to the rear, but they don’t take away rows in the front. Therefore, the level difference between the early seats and rear seats rises for every row that is added in the back. We can use directional loudspeakers and proper placement to make the sound system achieve equal level from front to back, but as we move back the listeners will certainly hear more speakers and less original source. On the positive side, however, is the fact that as we move back in the hall, the angular offset between the source and speakers will decrease in both planes. Therefore, we can get away with a higher mix of amplified sound.
Now onto the venue shape. Skinny makes things easier. Wide makes it more challenging. The classic shoebox or gently widening fan shape can be very favorable shapes, while wide fans, thrust stages, and in-the-round create the ultimate challenge.
Let’s consider the favorable venue shapes first. The primary reason that skinny is better is that we can minimize the horizontal image distortion. Since the horizontal plane errors are so much more detectable it helps to have a room shape that puts physical limits to the game. Nonetheless, a skinny venue alone can’t do it. We need skinny speaker placement as well. In short, if our left and right sources are placed out at the extreme corners of the hall, we will never fool the people located in the outer seats of the early rows. These seats face a double challenge. They have the highest angular offset and are likely to have very unfavorable level ratios. The angle offset is easy to visualize, but for the level part, we have to remember that our stage source has a directional pattern too. The human voice can be roughly modeled as a 60-degree speaker and in the high-frequency range can be quite directional. Therefore, the near outer seats are likely to have an inward, late, off-axis signal from the stage source competing with an outward, early, on-axis response of a nearby loudspeaker. Ouch!
The more inward we can place the speakers the better off we are in terms of image control. How far inward we can place the speakers will be governed by the stage width. Therefore, the extent to which the stage width is narrower than the seating is movement in our favor. The challenging near-outer seats have a fighting chance of imaging toward the stage if the speakers are placed inward to them, thereby minimizing the horizontal image offset. As the stage width goes down proportional to the room width, and our left and right positions move inward, we can tolerate wider halls since the horizontal geometry lines up similarly for both the stage source and speakers. This has it limits, however, since stage sources are not necessarily omnidirectional. This leaves the outer seats with a late, weak, off-axis signal from the stage that is unlikely to pull its weight in the imaging equation.
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