The Silverman Concert Mapping the soundstage

Sidebar 4: Mapping the soundstage

I included the final track on Concert's CD 2 for those who pride themselves on their systems' soundstaging abilities. With the two B&K microphones still standing in the positions they had been to record the piano, Robert Harley recorded me talking and clapping my hands as I walked first from the far left of the 45'-wide stage to the far right, then from the very back of the 65'-deep auditorium up to and past the microphones and piano (fig.1).

Fig.1 The First United Methodist Church, Albuquerque, New Mexico, plan view. Arrowed line shows paths taken by JA while recording the second CD's final track.

As well as hearing my voice, handclaps, and footfalls illuminating the church acoustic, you should perceive me mapping out a U-shaped soundstage (fig.2). When I start at far stage left, my voice isn't that different in level in the two channels, because the distance between the two microphones is smaller than my distance from the closest one. However, because the sound of my voice reaches the nearest microphone about 7 milliseconds before it does the other one, this time delay locates the sound of my voice in the left speaker position. As I'm a long way from the microphones—about 16'—there's a bloom of reverberation surrounding my voice which moves the image significantly behind the speaker. The image is also quite wide.

Fig.2 Perceived soundstage as captured by two B&K 4006 omnidirectional mikes, mapped out by arrowed line in fig.2.

As I walk toward the microphone, the inter-channel time delay, though decreasing, keeps my voice at stage left. The decreasing amount of reverberation, however, causes the image of my voice both to get smaller and to move closer to the listener until I reach the left-hand microphone, when it should be positioned right at the speaker position. Then, as I walk between the microphones, the rapidly changing time relationship between the two channels causes the image to move very rapidly from the left speaker position to the right. (This pulling of the central images to the sides is why the classic spaced-omni mike setup, as practiced by Mercury, Telarc, and Everest, includes a third, centrally placed mike to stabilize the soundstage center.)

Once I reach the right microphone position, the process repeats in reverse, giving the familiar U-shaped soundstage produced by a pair of spaced omnis. Depending on the microphone spacing and hall acoustic, the "arms" of the U can bend back toward the center, but in this case I arranged things to give a true U shape. If you don't hear precisely what I've just described when you listen to this track, then something in your system or room is impairing the imaging accuracy.

The second half of this track enables you to check out your system's ability to throw accurate image depth. As I run to the back of the church's nave, you should hear my footsteps, roughly placed in the center of the stage, rapidly receding. As I walk back to the piano, you should hear the image of my voice and handclaps moving from somewhere behind the wall of your listening room up the plane of the speakers when I announce that I'm level with the microphone positions. Because of the center-stage instability I mentioned earlier, my small movement of walking around the piano past the keyboard should be heard as a lurch in the image position all the way to the left speaker position, before I then recede a short distance behind the loudspeaker plane. (No stereo microphone technique can distinguish soundsource positions in front of the mike array from those behind—all image depth is therefore perceived as being behind the speakers.)

Again, if you don't hear this on your system, its imaging cannot be regarded as accurate, no matter how much you like what it does.—John Atkinson

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