Wayne Parham Messages: 18722 Registered: January 2001
Illuminati (33rd Degree)
You're right that the lower the frequency, the more "wiggle room" you have. As long as the sound sources are acoustically close, e.g. within 1/4λ, then you can expect coherent summing. So that means lower frequencies can be further apart. Bass frequencies have longer wavelengths.
But remember that when the distances between the listener and two sound sources reach 1/2λ difference, summing is destructive and nulls form. And also realize that drivers are stacked vertically, so when the listener moves vertically, the distances between each driver and the listener change. So the critical 1/2λ distances are reached at different places along a vertical arc.
This is what forms the vertical nulls. You can see them in a polar chart. A well-designed speaker has nulls spaced uniformly above and below the speaker at a fairly wide angle, high above and far below the forward centerline.