JBL Technical Note - Vol.3, No.1 电路原理图.pdf

Technical Note Volume 3, Number 1 The Evolution of JBLs Large Format Monitor Loudspeaker Introduction: Modern recording technology is presently driven by continuing improvements in digital signal processing and the demands of sound with picture. The coming of the DVD has changed the very nature of the studio environment, both physically and economically, resulting in more (but smaller) workspaces. JBLs response to these ongoing improvements is the DMS-1 digitally controlled monitoring system, a two-way design that makes use of new compression driver technology and high-performance cone transducers, resulting in distortion figures that are more typical of electronics than loudspeakers. During the early eighties JBL pioneered the use of uniform coverage high frequency horns in studio monitors, producing the highly regarded Bi-Radial systems. These systems were tailor made for the control rooms of that day, and the model 4425 and 4430 Bi-Radial monitors remain staples in the JBL line. They coexist with the newer designs, and this Technical Note will trace the engineering evolution from Bi-Radial monitors to the DMS family. The Bi-Radials Revisited: Background: The JBL Bi-Radial monitors made use of state-of-the-art transducers of the early eighties, bringing JBLs SFG (Symmetrical Field Geometry) to new monitor design for the first time. What truly set the JBL Bi-Radials apart from earlier monitor designs was the use of a 90 by 90 uniform coverage HF horn that was crossed over with the LF transducer at the precise frequency at which the LF transducers radiation angle matched that of the horn. The result of this was a system with horizontal and vertical patterns that began wide at low frequencies, progressively narrowing to 90 degrees, and remaining at that value to beyond 10 kHz. This can be seen in the beamwidth and DI plots for the 4425 and the 4430, shown respectively in Figures 1 and 2. 2 In many control rooms of the eighties, the ratio of direct to reflected sound at the prime listening position was about unity, and absorption in the room was fairly uniform. A listener seated at the console in effect heard as much direct sound as reflected sound. In the Bi- Radial design, the direct field and the reflected field can be uniformly maintained above the transition to the horn, as can be seen from the nearly constant DI above 1 kHz. In practical terms, what this means is that, when the direct sound at the prime listening point has been equalized for some target response contour (house curve), the accompanying ensemble of reflections will also reflect that same contour over the frequency range of the horn. Typical monitor systems of the period had rather uneven DIs and the advantage of the Bi-Radial monitors over those systems shown through the progression of curves illustrated in Figure 3. Maintaining the Desired Contour: Electrical or Acoustical Equalization? It is well known that HF compression drivers exhibit a 6 dB/octave falloff in response above their mass breakpoint. (See Technical Note volume 1, number 8) In most drivers, this occurs in the range of 3000 to 3500 Hz. If flat power response is desired above that frequency, then it will be necessary to boost the drive signal to the compression driver above the mass breakpoint. Figure 3.Smooth versus irregular power response in the control room. Figure 2.Beamwidth (6 dB) and DI for JBL 4430 monitor. Figure 1.Beamwidth (6 dB) and DI for JBL 4425 monitor. Alternatively, if the horns directional response is allowed to narrow with rising frequency, the effect of the rising DI will have the same effect on-axis as electrically boosting the signal to the horn. This was the philosophy dominant at the time JBL used acoustical lenses in monitor design (e. g., the model series 4320, 4330, and 4340). The actual signal to the HF driver was maintained electrically flat, and the acoustical falloff above the mass breakpoint was effectively compensated for by the sharp rise in the axial DI of the horn/lens assembly. The effect was excellent for on-axis listening in a fairly dry acoustical setting, but in a normal control room, where reflections were significant, the sound texture was not uniform, even though the on-axis response was flat. Time Domain Response: Since the Bi-Radial monitors are 2-way designs, it is relatively easy to engineer them as minimum phase bandpass sections that will exhibit smooth time domain response along a specific vertical design axis. In the JBL 4430, these design details are shown in Figure 4A. Note that the preferred “launching angle” is about 10 degrees upward from a point midway between the frame of the LF transducer and the lower lip of the horn. Figure 4B shows the Blauert Blauert among them were: 1. An increase in the ratio of small to large workspaces, with consequent greater acoustical absorption in those smaller spaces. 2. The requirement for advanced film and video postprocessing, with surround sound capability. This called for separate left, center, and right loudspeaker models for frontal presentation. 3. The requirement for lower distortion, to match the quality expectations generated by improved digital recording and processing. 4. The requirement for flat amplitude response (with minimum phase characteristics) over a passband from 40 Hz to 20 kHz. JBL responded to these challenges through: 1. Use of the Coherent WaveTMlarge format phasing plug for extended HF response. 2. Use of a new family of rapid flare Optimized ApertureTMdrivers and horns for lower HF distortion. 3. Use of neodymium-iron-boron magnets for high flux densities and for excellent magnetic shielding. 4. Use of advanced techniques for heat removal to reduce LF power compression. 5. Use of digital control electronics for basic frequency division, precise power response equalization, time domain alignment, and control room/environmental equalization. System Philosophy: JBLs aim in designing the DMS-1 was to provide a monitor system whose intrinsic linearity could easily be compared with its associated recording electronic systems. This is especially true at normal operating levels, where the distortion of the system is well below 1%. The DMS-1 uses a vertically symmetrical in-line array in separate left and right models, as shown in Figure 9. The two LF transducers are mounted above and below the HF horn. For listening at fairly close quarters, this over-under placement of LF units produces LF localization at the HF horn itself, very much the way a coaxial design does. By necessity, the center channel baffle layout places the LF transducers horizontally below the HF horn. The relatively small size of the horn provides excellent 90 horizontal dispersion from 800 Hz to 12.5 kHz, narrowing to 60 at 20 kHz. The small vertical dimension of the horns mouth also allows the LF transducers to be placed fairly close together in order to minimized vertical pattern lobing. 6 However, the small vertical mouth dimension of the horn results in fairly wide vertical dispersion in the 1 to 2 kHz region, gradually narrowing to the 40-60 range at higher frequencies. Figure 10 shows the 6 dB beamwidth and DI data for the system. Note that the DI is maintained at 10 dB, 1.5, from 500 Hz to 20 kHz. While the DMS- 1 fulfills the same general requirement for equally smooth power and axial response as the Bi-Radials, the on-axis DI of the DMS-1 rise slightly from 1 kHz to 8 kHz, lessening to a slight degree the amount of electrical boost at higher frequencies needed for flat on-axis response, as compared with the 4430 Bi-Radial monitor. Frequency Division and Power Response Equalization: Frequency division and response equalization for flat on-axis response of the DMS-1 are shown in Figure 11. The curves shown here are programmed into the DSC-280 digital controller and are used to drive the power amplifier sections that are used for each loudspeaker. Each LF transducer is powered with up to 600 watts and the HF sections up to 200 watts. Figure 12 shows overall on-axis amplitude and phase response of the DMS-1. Some users may prefer to use the JBL SMC 24 analog controller, a stereo unit which provides a Linkwitz-Riley 24- dB/octave crossover function at 1 kHz, basic HF power response equalization, and both HF and LF limiting. Limiting thresholds for both LF and HF can be set on the rear panel. When the SMC 24 controller is used, additional system equalization, may be necessary to fine tune the systems to the desired room reponse curve. Figure 9.Front views of left, center, and right models of JBL DMS-1 monitor system. Figure 10.Horizontal and vertical beamwidth (6 dB) and directivity index for DMS-1. Figure 11.Low and high frequency drive voltages for flat on-axis frequency response. 7 Time Domain Response: With the frequency response shown in Figure 12, it is no surprise that the square wave response of the DMS-1 is exemplary, as shown in Figure 13. Distortion: Figure 14 shows the second and third harmonic distortion (both raised 20 dB for ease in reading) for the DMS-1 driven to an output of 110 dB Lp measured at a distance of one meter. Note that second harmonic distortion at 10 kHz is no higher than 3%, a remarkably low value at these extremely high output levels. For the entire system, the distortion remains in the range of 1% or lower from 30 Hz to about 2500 Hz. Conclusions: The DMS-1 represents the best performance available today from any monitoring system in its class. Users invariably comment on the extremely smooth amplitude response of the system and its complete lack of listening fatigue over extended periods of use. Other users comment on the accuracy of imaging, both in stereo and in left- center-right applications. No compression driver system made by anyone can produce equivalent output levels with as low distortion. Figure 14.DMS-1 2nd and 3rd harmonic distortion (+20 dB) for output level of 110 dB Lp at one meter. Figure 12.On-axis amplitude and phase response of DMS-1. Amplitude shown by solid curve (5 dB per division); phase response shown by dotted curve (degrees shown on left scale). Figure 13.Square wave response of JBL DMS- 1 at 365 Hz. 8 JBL Professional 8500 Balboa Boulevard, P.O. Box 2200 Northridge, California 91329 U.S.A. A Harman International Company TN VOL 3 NO 1 CRP 5M 3/97 Additional Reading: 1. J. Eargle and W. Gelow, “Performance of Horn Systems, Low- Frequency Cut-Off, Pattern Control, and Distortion Trade-Offs,” presented at the Audio Engineering Society Convention, Los Angeles, November 1996, preprint number 4330. 2. JBL Technical Note, Volume 1, Number 8, “Characteristics of High- Frequency Compression Drivers.” 3. JBL Technical Note, Volume 1, Number 21, “JBLs New Optimized Aperture(tm) Horns and Low Distortion Drivers.” 4. JBL Technical Note, Volume 1, Number 22, “JBLs New Super Vented Gap(tm) Maximum Output Low Frequency Transducers.” References: D. Smith, D. Keele, and J. Eargle, “Improvements in Monitor Loudspeakers,” J. Audio Engineering Society, Volume 31, Number 6 (June 1983).