According to the “Project K2” literature, “Imaginary Equivalent Tuning” (IET) was a new concept developed by Greg Timbers for the S5500 loudspeaker. By using two LF drivers, each in its own ported enclosure with different volumes and tunings, he claimed to provide a “better balance between the high speed of Bessel tuning and the flat frequency response qualities of Butterworth tuning”. Subsequently, this technique was also used in the M9500 speaker.
So, is “IET” really a “better mousetrap? What do you S5500 and M9500 owners think? Did Greg Timbers ever publish a “white paper” on the idea? Has anyone ever built a custom speaker with “IET”? Did “Project May” consider using “IET? I would love to see some more discussion and technical information on this idea.
It looks like the S5500 and M9500 may be the only JBL speakers that ever used “IET”. (The current JBL S5800 seems to have its two LF drivers in a common enclosure, since the exploded view in the Technical Manual shows only one set of ports at the bottom of the cabinet.) However, I seem to vaguely remember some older JBL speakers that used something called “stagger tuning”. Could “IET” have been used previously under another name? Comments?