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In Reply to: RE: On the subject of directivity posted by arend-jan on July 05, 2009 at 05:05:54
Thank you for this post. I did not want to post with this great of detail regarding directivity and directivity idex. It can be a complex subject for some to understand.
What is clear is Roger Sanders and Peter Walker/Peter Baxandall had completely different ideas about the importance of directivity. While I have not heard the Sanders speaker and the designer gives little if any details about his speaker, it is clear by the geometry of the electrostatic element, at some frequency, there will be a rapid and dramatic change in the directitivity index of the speaker. This change in directivity will be neither smooth or gradual in nature. PJW/PB would claim this sort of rapid change will produce an unnatural overall balance and unpredictable off-axis response(in-room).
PJW commented about this type of approach, i.e. rapid change in directivity 'Of course, out in the open that sounds fine, directivity doesn't come into it. And if the loudspeaker is raised up off the floor fairly close to the listener so that he hears mostly direct sound anyway, it makes practically no difference what you do with directivity. But when you move the speaker to different rooms and try them the reproduction varies from room to room more than you would like.'
'This is because the after-sound, if you like to call it that, has got the characteristic of the power output firmly attached to it, and so it's all bassy. That won't do. Neither is it any good to make the power response level and have the directivity jump up 6dB around 200-500 cycles - that would sound most unnatural.'
'So we tried about a dozen directivities and found that the best thing to do is to change the directivity gradually, never by more than a decibal in an octave, preferably less than that. The trouble with most loudspeakers is that where you change from the woofer to squawker or squawker to tweeter the diameter of the speaker changes sharply and so does the directivity.'
Nothing new or exciting about PJW comments, just science. Few loudspeaker designers pay any attention to directivity and power response. The focus is on flat on-axis frequency response. But without flat power response any given speakers performance will change dramatically from room to room. This is not the case with ESL63. It sounds remarkable predictable/similiar in various room.
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