JBL's new SFG.
A geometry lesson worth listening to.
SFG, or Symmetrical Field Geometry, represents a significant design improvement in the science of building loud speakers.
The first low-frequency ferrite magnetic structure good enough to bear the name JBL, SFG not only out-performs conventional ferrite structures, but also the Alnico structures we've used for more than 30 years. In doing so, SFG produces the lowest measured second harmonic distortion levels to date.
Most loudspeaker companies use ferrite magnets. But until SFG, we simply were not satisfied with using ferrite in low-frequency drivers. We had JBL always been able to obtain far better results with Alnico, a com pound of aluminum, nickel and cobalt. But the rising cost and growing scarcity of cobalt caused us to take another look at ferrite. While doing so, JBL engineers identified the unacceptably high distortion levels in the competitive ferrite designs we tested and found they were caused not so much by the nature of the material, but by the de sign of the magnetic structure! A modest technical explanation of SFG follows:
In conventional ferrite designs, the structure creates a non-sym metrical magnetic field above and below the voice coil gap.
This uneven magnetic field causes large amounts of second harmonic distortion at low frequencies. That's where SFG comes in. By reducing the diameter of the pole piece and by making the top plate and the pole piece an integral part of the design, SFG creates a symmetrical magnetic field to substantially reduce second harmonic distortion at low frequencies.
SFG also solves another form of distortion-a flux change resulting from the interaction of the voice coil and conventional magnetic structures. The JBL solution, a unique Flux Stabilizing Ring, is yet another tribute to the JBL ingenuity of JBL engineers. It maintains a constant level of magnetic energy in the voice coil gap, reducing second and third harmonic distortion to less than 0.1%.
Our tests showed we'd reached our goal: A low-frequency ferrite magnetic structure with distortion levels typically far below 0.5%, over the full operating range of the speaker.
But, as always, the real test is in the listening. So, here's your assignment: Go to your nearest Authorized JBL Dealer and listen to the difference SFG makes.
James B. Lansing Sound, Inc., 8500 Balboa Blvd., Northridge, CA 91329
--A unique Symmetrical Magnetic Force above and below the voice coil gap is created by the relationship between the top plate and the reduced-diameter pole piece. This symmetrical force substantially reduces second harmonic distortion at low frequencies.
--Exclusive Flux Stabilizing: Ring counteracts the distortion caused by the inter action of the voice coil and magnetic structure. It maintains a constant level of magnetic energy in the voice coil gap, reducing second and third harmonic distortion to less than 0.1%.
JBL--First with the pros.
(Adapted from Audio magazine, Mar. 1981)
Also see:
JBL Ti Series Loudspeakers (Auricle, Aug. 1985)
JBL L150 speaker system (ad, Aug. 1979)
JBL L166 speaker system (ad, Dec. 1976)
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