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> Sometimes musicians use dispersing devices mounted on the speakers, like "beam blockers". Beam blockers seem to be the most common thing employed, though to my knowledge a much more effective measure is a "Mitchell donut". Basically, a soft foam disc with a soup can diameter hole cut in the center. The disc is sized to completely obstruct the speaker when mounted in front of it on the grill(cloth). With properly chosen foam of the right thickness, frequencies above about 1khz will be attenuated except when traveling through the center hole, meaning they'll behave more like they're coming from a point source and the constructive interference that causes the beam will be much reduced. Equalization may need adjusting since some highs are lost, and because if the player adjusted initially for on-axis sound they are now hearing the off-axis sound. From a stroll through the audio cyclopedia, I think "mitchell donuts" are more properly called "acoustic lenses". A key difference between them and most beam blockers I see is that the blockers assume high frequencies come from the center of the speaker (cause that's where the beam is, and when close micing that's usually where you get the most treble), whereas donuts/lenses assume the entire speaker emits the highs, and that it is their constructive interference that causes both the beam and the trebliness of close micing a speaker on axis. I have not fully verified all the physics yet, but so far my understanding is that this later explanation is correct, at least in the context of guitar frequency ranges. The consequence of that is that beam blockers usually/likely add a comb filtering effect from what I've read, and will still develop some beaminess in the far field despite removing it in the near field. The donuts/lenses should be effective in both the near and far fields. Anecdotally, my experience confirms the donut behavior. A previously painful-on-axis 4x12 was almost completely evened out by the addition of donuts. If anyone has good pointers for honest to goodness physics books on speaker drivers and speaker cabinets, it would be much appreciated. |