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JBL's 21st Century Speaker Technology From the time that the first successful sound systems were installed in cinemas in America, JBL Loudspeakers have been at the forefront of quality sound reproduction. As this article shows, they still are. . . I t is now almost 80 years since James B. Lansing formed a partnership with Ken Decker in Los Angeles to manufacture loudspeakers for those new-fangled radios that hobbyists and affluent Americans aspired to own. Jim’s timing was perfect. He just happened to be in the right place at the right time to apply his innovative ideas to solve what turned out to be the motion picture industry’s most pressing problems. In 1927, Warner Bros introduced talking pictures with “The Jazz Singer”, starring Al Jolson. “Talkies” were an immediate success. Although the Depression subsequently cast a long shadow over Hollywood (and the rest of the world), there was no stopping the demand for talking pictures or the cinemas in which they could be shown. But there were problems associated with the “talkies”, many of which initially proved to be quite intractable. The most complex of those problems revolved around the absence of loudspeakers with sufficient power output and quality to suit large cinemas which frequently exceeded 1000 seats. The Western Electric Company was one of the first firms to tackle this problem and they assigned a large team of engineers to the task. But 4  Silicon Chip Western Electric didn’t have the field to themselves and there were numerous private researchers working on the same problems. It was during this period that one of the most successful researchers was Jim Lansing. As a result, his speakers had a marked edge over virtually every other loudspeaker in the market place at that time. Jim Lansing’s most significant achievement was the development of a milling procedure for producing flat voice coil wires. The flat wire could be wound into a much stronger, more durable ribbon voice coil that had James B (Jim) Lansing died in 1949 but his legacy lives on through products bearing his initials. By LOUIS CHALLIS the added advantage of significantly higher power-handling and a more effective use of the space available in the voice coil air-gap. Without that development, there was no simple way to resolve the fundamental design problems associated with producing an effective compression driver. At the same time, Jim Lansing developed an effective phasing plug. The combination of flat wire, more efficient voice coils and the phasing plugs ensured that his compression drivers were the best that money could buy. These two fundamental developments are as important today as they were 70 years ago. Throughout the rest of the 1930s, Jim Lansing focused his efforts on the design of high-powered loudspeakers, better power amplifiers and the associated crossover networks and DC power supplies. His products were snapped up by the most prestigious cinemas and were sought after in the major motion picture sound studios. During World War II, Jim worked on submarine detection systems. It was only following the end of the war that, with much soul-searching, he decided to leave Altec Theatre Services in which he had been a director, to form his own company, James B Lansing Sound Incorporated. The adoption of the Lansing in the company name upset his former partners at Altec-Lan- sing, so he changed the name to JBL. In the few short years before his death in 1949, Jim developed his famous D130 15" loudspeaker with a 4" flat wire voice coil that revolutionised theatre sound and set the standard against which other low frequency drivers would be assessed for the next two decades. JBL continued after Jim Lansing’s death and about 30 years ago, it was acquired by Harman International. In the ensuing period, JBL has continued to produce more powerful loudspeakers without sacrificing that equally important parameter of ‘fidelity’. Twin voice coils Let’s briefly discuss the topic of power-handling capacity. When a loudspeaker is called on to deliver high outputs, the large cone excursions typically cause the voice coil (or portions of it) to move out of linear flux region of the air gap in which it operates. As the excursions become greater, the thermal dissipation simultaneously increases and with it the risk of voice coil burn out. Even if the voice coil doesn’t burn out, there is still the nasty problem of gross non-linear distortion as the voice coil’s travel moves into the non-linear region at the fringes of the magnetic path gap. To solve this problem, most loudspeaker manufacturers have tended to concentrate on building bigger and/or more efficient magnets, larger voice coils or even larger speaker Two amplifiers - one for the woofer and one for the tweeter Built-in active crossovers Low mass titanium diaphragm Low distortion BiRadial Horn Thermal protection shuts down ampifier on overload Die-cast heat-absorbing baffle Active cooling from alumium fins Computer-designed EON woofer cones Polypropylene enclosure Flattened voice coil wires give 22% more wire in the magnetic gap Patented “differential drive” (two voice coils). Neodymium magnets - ten times lighter than conventional magnets Toroidal power transfer for minimum weight. Cutaway diagram of a typical JBL “EON” system showing significant levels of innovative technology diaphragms. Each of those approaches imposes significant functional, weight-related and thermal disadvantages. JBL came to the conclusion that there had to be a better way, without incurring gross harmonic distortion and premature failure of the driver. An early (1954) dual coil drive from Wolff, taken from the patent documents at the time. CM is the magnet, A1 & A2 are two air gaps, C1 & C2 the two voice coils, P1 & P2 two steel plates and PP2 is the pole piece. In the 1950s, Wolff and Kritter both described a loudspeaker transducer which had two voice coils wound in opposite directions and spaced apart on the same voice coil former. Their patents described magnetic circuits using Alnico permanent magnets and adopted two magnetic air Another early patented dual coil drive, this time from Kritter, 1959. Identified components are 1, the magnet; 2 is the steel pole piece and 3 & 4 steel plates. 7 & 8 are the two voice coils, 14 the speaker cone and 15 the speaker housing. DECEMBER 1999  5 Cross-section of a loudspeaker with “conventional” outside magnet construction – except that this one has two voice coils. Note also the square cross-section coil wire. gaps at outer ends of a magnet, with one coil placed in each air gap. As it turned out, neither of these researchers ever constructed a commercial example of their ideas or proved that they were capable of producing a working solution. Other researchers who experimented with Wolff and Kritter’s concepts discovered to their chagrin that there were almost intractable problems to be resolved in the construction of a voice coil that could handle twice the input power and safely dissipate the associated heat. JBL subsequently decided that the concept had merit and a decade ago developed its Eon loudspeaker. This used a dual voice coil and a neodymium magnet nested in an aluminium heatsink. The Eon gave an extremely effective differential drive offering some remarkable features. The main attributes of the Eon loudspeaker are its ability to provide twice the power output compared with that provided by a conventional single voice coil design. At the same time, it achieves a weight reduction of between 75 and 80%. The researchers at JBL were initially concerned that under high drive conditions when maximum excursion occurs, either one coil or the other would leave the air gap, with possibly adverse effects on the other coil. In the presence of any asymmetry in the loudspeaker, a voice coil could be expected to literally jump out of the gap in the direction of the asymmetry. Even if everything was perfectly 6  Silicon Chip Again with two coils, the lower weight and better magnetic efficiency having the magnet on the inside of the coils provides a far lighter and more effective solution. balanced, there was still a risk for the voice coil to jump out of the gap. Once the voice coil is out of the gap, the amplifier loses control, power dissipation goes up and the distortion is really bad. In a conventional loudspeaker driver, this is addressed by providing progressive or non-linear stiffness in the suspension elements and spider. The problem with that is the risk of premature fatigue failure and a relatively short driver life. The Eon design adopted a smarter solution. Claimed to be the strongest and lightest 14-inch driver ever, this is JBL’s 600W DCD Driver 2254J “professional” speaker. Dynamic braking JBL have cunningly placed a third shorted coil midway between the two drive coils. This becomes what we might term a ‘dynamic brake’. As the voice coil excursions become greater and greater, the shorted coil moves into the active magnetic field of either the front or rear coil. When this occurs, the damping generated by the shorted coil effectively limits the motion and the speaker experiences a form of ‘soft clipping’. The net effect is a highly effective protection system. JBL discovered that there were some other unusual magnetic features displayed by its dual-coil Eon design. When they examined the distribution of flux density along the voice coil air gap, they discovered that it was decidedly irregular and asymmetrical. They were even more disturbed to discover that the flux density on the inside of the voice coil was substantially different to that on the outside of the voice coil, ie, on the opposite side. But while the individual gap fluxes were not symmetrical, the overall distribution turned out to be almost perfectly symmetrical and the flux distribution on the inside and the outside of the voice coil performed in almost the same way as a single voice coil design behaves. The most significant advantage of the dual voice coil design is that it inherently has no asymmetric bias at all. As a consequence, the second harmonic distortion is then primarily associated with the non-linearity of the suspension rather than being at- tributable to the magnetic field or flux modulation. Because the dual design has double the coil surface area and a slightly higher magnetic flux density, the force factor is more than doubled when compared with a conventional single-coil design. JBL experimented with two other versions of the dual-coil design - one with the magnet on the outside of the coil (typically using a ceramic or neodymium magnet) and the second version with the neodymium or Alnico magnet on the inside of the coil. As they discovered, the lower weight and better magnetic efficiency of the inner option provides a far lighter and more effective solution than the outer magnet option. Neodymium offers further benefits with the main advantage being that it has lower flux modulation, leading to harmonic modulation distortion 10dB lower than that with a ceramic magnet. Double-blind listening tests The first dynamic loudspeakers were developed a little over 80 years ago. While they might have had some similarity in appearance to the current generation, there is simply no comparison in sound quality. In the beginning, researchers relied solely on their ears to assess the improvements in audible fidelity. Since then, many objective test methods were developed to measure improvements in performance. Nevertheless, a The EON 10: one of JBL’s innovative and high performing “EON” speaker systems intended for professional and commercial applications. well trained and perceptive set of ears still constitutes the most critical test of any loudspeaker’s fidelity. JBL’s current product philosophy has been developed by Harman International’s new Vice-President of Engineering, Floyd E. Toole. His experience had convinced him that any successful organization in this field simply had to have appropriate state-of-the-art research and testing facilities. Accordingly, JBL built a new Acoustical Research Centre with more than 1000 cubic metres of speaker development laboratory, equipped with three anechoic and hemi-choic test facilities. JBL is now able to perform complex automated speaker tests that comple- By adding a shorted coil between the two driver coils a dynamic brake is formed. Normally the shorted coil is between the two voice coil gaps so it has little, if any, effect. On large cone excursions, the shorted coil moves more and more into one or other of the voice coil gaps to produce a braking effect. The effect is shown in the graph at right. ment its outstanding subjective test facility - they have the finest and best equipped listening room that I’ve yet seen. It is not generally appreciated just how critical are the loudspeakers’ positions in a listening room when you wish to compare the performance of two or more sets of speakers. If those assessments are to be valid, then each pair of speakers has to occupy exactly the same positions. Now that is no easy task, particularly if you want to conduct a “double-blind” test, with neither the subject nor the tester being aware of which pair of speakers is being listened to at any one time. Let’s face it, when you go to a hifi The green curve shows the displacement of a dual coil standard 10-inch woofer while the blue curve shows the same woofer with the shorted centre coil for braking. DECEMBER 1999  7 retailer’s listening room, such demanding requirements cannot be met. But JBL has done it. They have constructed an automated platform system to move multiple pairs of speakers back and forth behind a curtain for subjective blind testing. I was intrigued to observe how each pair of loudspeakers can be moved almost silently into position in less than two seconds. As a consequence, listeners are able to make comparisons without adverse interaction problems generated by the listening room walls, floor and ceiling. The critical spatial parameters remain constant and the subjective test assessments are both accurate and, more importantly, credible! Few consumers understand just how important a loudspeaker’s position becomes in determining the quality of the sound you hear in a typical living room. JBL’s research team has devoted an immense amount of effort to accurately quantify the critical parameters that need to be determined. Their ultimate aim was to develop a procedure through which they could optimise a monitor loudspeaker’s effective sound output in both the horizontal and vertical planes. Now there are relatively few situations in either a domestic living room or even in a commercial sound dubbing suite or studio where there is a single significant or ‘sweet’ listening position. In the home, variable seating positions are inevitable. And in most typical commercial dubbing suites, two to four people frequently sit in on a crucial mix-down. A frequently forgotten element is the reflected energy from walls and ceiling. Those components interact with the primary (or direct sound) energy to produce the integrated sound field that the listener hears. Around twenty years ago, most loudspeakers were designed solely on the basis of achieving a flat frequency response on axis. JBL’s research and development team decided that loudspeaker should be designed to provide an appropriately integrated sound field. They describe it as ‘the listening window’, encompassing an arc of 30° in the horizontal plane and 15° in the vertical plane. Their research led them to the conclusion that the loudspeaker that sounds most natural provides uniform energy within those limits, even though their measurements confirmed that there may well be small differences in frequency response and perceived sound uniformity at the outer extremes of the field. As a result of that work, all current JBL consumer and professional monitor speaker systems are designed to satisfy this performance goal, and frankly, as I have observed, they do so remarkably well. I was fortunate to be able to listen to some of JBL’s developmental multi-channel speakers in one of its new test suites in Los Angeles. Prior to my visit to JBL I had no view on the desirability of installing a multi-channel sound system in my home. After Floyd Toole demonstrated the attributes of one of JBL’s new multi-channel systems, I realised just how effective and impressive such a SC system can be. Acknowledgement: Thanks to Jands Electronics (distributors of JBL Professional Equipment) for their assistance in the preparation of this feature. MORE FROM SAVE YOUR EFI CAR! UP TO 60% ON EFI TECH SPECIAL Electronic Components Own an EFI car? Want to get the best from it? You’ll find all you need to know in this publication A valuable collection of the best EFI features from ZOOM magazine, with all the tricks of the trade – and tricks the trade doesn’t know! Plus loads of do-it-yourself information to save you real $$$$ as well . . . HERE ARE JUST SOME OF THE CONTENTS . . . www.futurlec.com large range fast delivery 8  Silicon Chip  Making Your EFI Car Go Harder  Building A Mixture Meter  D-I-Y Head Jobs  Fault Finding EFI Systems  $70 Boost Control For 23% More Grunt  All About Engine Management  Modifying Engine Management Systems  Water/Air Intercooling  How To Use A Multimeter  Wiring An Engine Transplant  And Much More including some Awesome Engines! 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