JBL L65 Jubal Catalog
Dispersion may be one of the least understood and most neglected of all the specifications listed for a loudspeaker system. Yet, the dispersion specification has important implications for the ability of a system to reproduce open, spacious sound.
It should be understood that the quoted angle of dispersion is not a constant. The low of nature is that the angle of dispersion decreases as the wavelength of the frequency becomes shorter relative to the diameter of the cone or diaphragm that reproduces it.
However, as frequencies get higher, wavelengths get shorter. Taking another point of reference, at 200 Hz, there is a wavelength of 5 feet. Since that is still greater than the 12-inch cone, it disperses well, but not quite as widely.
The listener would be able to hear 40 Hz reproduced by that 12-inch cone from any position in the room. He could hear 200 Hz over a wide area of the room, but to properly hear 2000 Hz he should be nearly on axis, or in a straight line from that loudspeaker.
Another effect to be considered is reflection. The narrowing dispersion causes the direct/reflected sound ratio to change in the favor of direct sound. In the hypothetical room above, if the listener were on axis the 2000 Hz signal would sound very "up-front" while the 40 Hz signal would be perceived as more "distant". The off-axis listener would hear sound that favored the low frequencies since the direct sound of the higher frequency would miss him.
Actually, this problem has s simple solution. Those frequency wavelengths too short to be dispersed widely by the 12-inch woofer can be crossed over by a dividing network to a midrange transducer with a diameter of, perhaps, 5 inches. Then a 2000 Hs signal with a 7-inch wavelength will be reproduced by a 5-inch cone giving superior dispersion.
The dispersion limitation on the 5-inch midrange does not become acute until the frequency has reached around 8000 Hz. So, typically above 6500 to 7000 Hz, frequencies are directed by another crossover network to a tweeter with a diameter of approximately 1,5 inches. Again, wide dispersion. But only up to a point. When wavelengths approach 1.5 inches, the usual tweeter will be unable to disperse them. Radiation starts to focus into a narrow beam.
The Design of Man
JBL Transducer Engineers had to get past the limiting concept that dispersion is a function of the diameter of the diaphragm. For professional use at first, there was designed an extremely efficient, high frequency ring radiator capable of reproducing 6500 to 21,000 Hz.
The 077 has unusually high efficiency as a result of horn-loading, bur the significance of the horn is much greater when its effect on dispersion is considered. In professional use, this assembly has been variously called a slot radiator and a diffraction horn. In fact, the horn is a very unusual development combining a tried and true idea: basically it's an exponential horn, but it incorporates diffraction characteristics.
The dispersion of the 077 high frequency driver is 130 degrees at 15000 Hz, wider than any other single directional tweeter. Even at frequencies reaching 20,000 Hz, the upper end of audibility, the 077 still disperses 110 degrees.
But as important as dispersion is, its only one part of the story. Another part is efficiency. As a horn-loaded, ring radiator, the 077 is effortlessly efficient. It has such reserve acoustical power handling capability that no component amplifier designed for home music reproduction is likely to challenge it. Frequency response of the 077 in the Jubal L65 begins at 6500 Hz and continues smoothly beyond the range of human hearing. With such high efficiency and extended frequency response, the 077 reproduces transients, those bursts of sonic power, with outstanding accuracy and crispness
The Jubal L65 was conceived as a project to integrate a high frequency driver which previously had only been used in professional sound systems with a suitable midrange transducer and low frequency loudspeaker for a compact, floor standing, home entertainment system. The unusually high efficiency of the 077 demanded compatible performance in the midrange and low frequency components.
Midrange was already well handled by a 5-inch transducer affording high efficiency and excellent transient response which are achieved by its very large magnetic assembly, its 7/8-inch edgewound copper voice coil and the traditional excellence of die casting and machining to ]BL's typical close tolerances,
Coincident with the development of the 077 was the evolvement of principles which resulted in a woofer sufficiently innovative to qualify for patent filing. The discovery was a new application of mass to the cone assembly for better performance.
A complete cone would have structure integrity, but a speaker cone is truncated at the apex to accept a voice coil. As a result. Large cones like the L65's woofer could be susceptible to deformation.
The voice coil and cone together should act as a piston which pushes air in response to electrical impulses that stimulate the voice coil.
A complete cone would have structural integrity, but a speaker cone is truncated at the apex to accept a voice coil. As a result, large cones like the L65's woofer could be susceptible to deformation.
Unless the voice coil and entire cone move together, response will not be uniform. This combination needs stiffness to prevent, as much as possible, deformation and breakup during excursion.
To provide the desired degree of mass and stiffness. JBL has traditionally applied a special dampening material, Lansaplas, directly to the woofer cone. This carefully measured and controlled process gives the cone the white appearance which has been emulated by others solely to achieve color, rather than function.
The new discovery adds a die cast ring as mass to the voice coil form instead of adding mass to the cone.
The die cast ring gives greater rigidity to the cone. preventing radial deformation even at the higher frequencies where it is most likely to happen. The voice coil and cone act as a pure piston, so that frequency response is smooth through the transition from low frequency to midrange the typical problem area.
This new center mass application has resulted in a low frequency loudspeaker more accurate than any previous 12-inch speaker. Naturally, it was chosen for the Jubal L65.
Low frequency—A 12-inch, long excursion loudspeaker wich a 3-inch edgewound copper ribbon voice coil, massive magnetic assembly, and target AlnicoV magnet. The cone is straight-sided and ribbed for additional stiffness. A die cast concentric ring on the voice coil provides center mass to prevent unwanted reverberations and avoid distortion of the cone at transitional frequencies. A tuned port in the enclosure is employed to optimize bass efficiency and dynamic range.
Midrange — The 5-inch transducer reproduces midrange program material which encompasses most of the vocal range. A 7/8 inch voice coil and the massive magnetic assembly give outstanding transient response and greater undistorted acoustic output than any other small cone loudspeaker.
High frequency—The 077 operates from 6500 Hz and continues past the upper range of human hearing. A ring radiator that is powered by a 3 1/4-pound Alnico V magnetic assembly, the 077 high frequency driver couples with the air through a unique exponential/diffraction horn. The horn assembly is die cast of solid aluminum with a transparent acrylic internal dispersion element. Internally, the voice coil diaphragm is pneumatically formed of fatigue-resistant aluminum alloy. The 077 offers an unprecedented combination of extended frequency response, high efficiency and wide dispersion pattern..
Dividing network — The tolerances of JBL network components are much more stringent than normal industry practices, since JBL well realizes the complexity of a frequency dividing networks function.
A properly designed network does far more than direct low, midrange and high frequency information to the appropriate reproducer. Vitally important to the sound of a loudspeaker system is precise control of the drivers through the transition frequencies. The network of the Jubal L65 is designed for optimum cross-overs which give a smooth response curve from lowest to highest frequencies reproduced.
Art for Art's Sake
JBL has used art to achieve ait. By employing the arts of design and craftsmanship the art of sound reproduction is heightened.
The most relevant criteria for a loudspeaker system are four efficiency, frequency response, distortion and dispersion. In each of these, Jubal L65 is superior But those are objective measurements when only a subjective judgment is necessary. Listening is all that's needed.
The Jubal L65 is a JBL advance into a cleaner highly sensitive, more open sound with better bass and improved frequency response across the entire audio bandwidth. Articulation, the ability to resolve complex musical passages into individual instruments, is demonstrably better in the L65.
With the Jubal L65 reproduction is literal. Reproduction is rebirth. The art has been refined.