Re: High Frequency Compression Driver Evaluation
Michael,
The “revolutionary” JBL D2 utilizes two voice coils, each with its own polymer annular diaphragm, the same arrangement as the BMS 4592 driver has used for many years.
I have found no published distortion specifications for the JBL D2, but the BMS 4592 published distortion specs, though decent, are not as good as the EVDH1A. That, and the driver's cost, is why I did not request one from Jack Arnott for comparison.
I would not expect the JBL D2 to be much different than the BMS 4592, the designs are quite similar, it is even rumored that BMS makes these drivers for JBL.
The triple D2 VTX cabinet uses a long, almost non-expanding wave-guide coupler. Although the D2 may have “significantly lower levels of nonlinear distortion” (than what, a K-Mart megaphone?) the long non-expanding air column insures plenty of nonlinear distortion to go around.
John Meyers was quite proud about their line array driver to horn coupler length being the shortest of any such coupler, reducing nonlinear distortion.
Tom Danley’s Paraline driver to horn coupler path length is shorter than Meyers’ line array horn coupler, and also has a faster expansion rate.
I use his design in my small format line array.
In side by side comparisons of my line array using EV DH1AMT to a JBL 4889 line array, many hearing it commented that the Paraline HF was more “clear”.
My system was using only 10) DH1AMT, compared to 48) JBL 2435H, 3 inch diameter beryllium diaphragms used in the 16 box 4889 system.
The EV drivers were also crossed over much lower than the JBL, which should increase distortion. The 90 degree conical waveguides in each system are similar, the primary difference being the driver to wave-guide coupler.
John Meyers quoted from Beranek's book “Acoustics” , which describes the nonlinear distortion mechanism:
"If the horn were simply a long cylindrical pipe, the distortion would increase the farther the wave progressed..."
Doubling the distance doubles the second harmonic distortion in a pipe.
The nonlinear distortion increases linearly with distance traveled in the tube, or the length of the horn.
"in the case of an exponential horn, the amplitude of the fundamental decreases as the wave travels away from the throat, so that the second-harmonic distortion does not increase linearly with distance."
John Meyer’s) interpretation of Beranek basically says that in a long, narrow driver to wave-guide adapter like JBL (and many other line arrays) uses, while SPL is falling at roughly the inverse distance law, the nonlinear (air) distortion increases with distance.
My ears agree with Beranek and John Meyers.
It would be very interesting to see actual distortion measurements of the HF sections employed in various line arrays compared at actual usage levels and see if my ears are “telling the truth”.
Art Welter
