The lost image
We've all experienced the position of a single instrument wandering in space as if that player got up and moved to different locations for different notes. But, of course, that isn't what's really happening. It's called image wander. Image wander occurs for any number of reasons including crossover design, driver placement, speaker setup, and room conditions. But mostly, image wander is a function of loudspeaker design. The good news is that most of the problems can be reduced through careful setup and room treatment. We hear the image losing its way at the same note or group of notes typically at the crossover point between drivers. Violins, cellos, trumpets, sometimes stand up bass instruments, are prone to image wander, as are others, because they are the instruments whose frequency range cross over from one driver to the other. And while this might make a little sense—the drivers are in different physical locations on the speaker baffle—it doesn't quite explain the distance instruments can move. Sometimes feet. What might help untangle this question are the very different polar patterns of drivers: tweeters vs. midranges or woofers have very different radiation patterns. Thus a violin note mostly from a midrange driver has a very different dispersion pattern than that same instrument's higher notes when reproduced by the tweeter. One way to help mitigate image wander is through speaker placement and room tuning. After all, speaker placement and room conditions are primary tools for achieving imaging in the first place. If your system suffers image wander, try repositioning your speakers and note the differences in placement of specific instruments crossing frequency zones between drivers. Make the changes big to try and get an idea of what's important: on-axis (speakers pointing at your ears), vs. off-axis (speakers pointing straight ahead). Less effective, but no less important, is room treatment. If your room exaggerates or subdues narrow frequency peaks, reducing those will help with image wander too. If you want more info on the matter I've put together a little video you can watch here.
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