JBL Technical Note - Vol.3, No.2A 电路原理图.pdf

Technical Note Volume 3, Number 2A The New JBL LSR6300 Series Studio Monitors 1. Introduction: In earlier days, studio monitor loudspeakers were designed for flat on-axis response, with secondary concern for off-axis response uniformity. With the introduction of JBLs Bi-Radial monitors in the early 1980s, JBL began to address the balance between on- and off-axis response and the importance of this balance in ensuring that the combination of direct and reflected sound reaching the listener was uniform. In the late 1990s JBL introduced the first LSR (Linear Spatial Reference) products, and now the LSR6300 Series has been improved to achieve an even higher degree of performance. The new models use improved structural materials for further damping and rigidity of the enclosures. The LSR6328P now has integral handles, and all LSR6300 products have built-in mounting points for ease and flexibility of installation. The products in the group are all THX pm3 approved. The HF transducer in the full-range systems was previously unshielded, making operation of the systems in proximity to a CRT type video monitor difficult. The new driver is shielded. Most significant is the addition of JBL RMC Room Mode Correction to two of the powered LSR6300 models. RMC provides a means for precise equalization of the dominant room mode, thus providing more accurate LF response of the LSR6328P and LSR6312SP in typical production environments. It is explained in fuller detail in the User Guide that accompanies the RMC system calibration kit. 2. The Linear Spatial Reference (LSR) Concept: Stated simply, LSR provides: 1. A forward listening angle over which the systems direct sound field response does not deviate from a given standard. This ensures that all listeners located within a solid angle of 30 degrees horizontally and 15 degrees vertically will hear substantially the same balance. 2. An early reflected sound field which is substantially smooth, uniform, and free of irregularities over the frequency range. This ensures that the pattern of room reflections in the room will complement the direct sound, not conflict with it. 3. A substantially smooth radiated power response. This complements items 1 and 2. If the room itself is acoustically uniform in its absorption treatment, then the reflected sound field, in actuality the room response, will likewise be uniform. This response is achieved through careful selection of crossover frequencies and slopes, placement of drivers on the baffle, and the use of waveguides, integral to the baffle, to shape the coverage angles of MF and HF drivers. The measurements shown in Figure 1A and B illustrate the degree to which the LSR6332 and LSR6328P attain their design goals. The labeled curves are described below: 1. On-axis response. 2. Averaged response over a listening window which is 30 degrees horizontally and 15 degrees vertically. 3. Averaged simulated early reflections arriving 5 to 10 milliseconds after the direct sound and lasting for approximately 10 to 20 milliseconds. 4. Total radiated sound power. 5. Directivity index of sound power. 71800_JBL.TechNote8/26/04, 7:58 AM1 2 6. Directivity index of early reflections. Notice in particular how closely the on-axis response and the averaged listening-window response track each other. Figure 1. LSR curve families. LSR6332 (A); LSR6328P (B). Figure 2. LSR curves for an earlier three-way model. Figure 3. LSR6332 MF and HF baffle rotation for horizontal or vertical mounting. Figure 4. Cutaway view, LF driver for LSR6332 system. Figure 2 shows the LSR curve family for an earlier three-way monitor that was designed primarily for flat on-axis response. You can clearly see the irregularities in the early reflection pattern and the total radiated power, which translate directly into irregular reflected signal at the listener. The model LSR6332 is intended for both vertical and horizontal mounting at the users discretion. In order to preserve the response shown in Figure 1A, the baffle containing the MF and HF transducers can be rotated as shown in Figure 3. 3. Transducer Development: The LF drivers used in the LSR6332, LSR6328P, and LSR6312SP systems make use of Differential Drive topology. In addition, models LSR6332 and LSR6312SP use neodymium magnets, a technique that results in lighter weight, inherent magnetic shielding, and lower distortion. Figure 4 shows a section view of a Differential Drive transducer with the two drive coils labeled. Located between the drive coils is a braking coil; this is a short-circuited coil that responds only when cone excursions are at maximum. For normal modulation it has no effect; but when high excursions are encountered the braking coil enters the magnetic field and acts to restrain cone motion. This action linearizes cone displacement, resulting in lower distortion at high operating levels, as indicated in Figure 5. The midrange (MF) driver for the LSR6332 system is shown in Figure 6. The driver has a neodymium motor and is thus well shielded magnetically. The voice coil has a diameter of 50 mm (2 in) for high power handling capability. The cone is of woven Kevlar, and the outer 71800_JBL.TechNote8/26/04, 7:58 AM2 3 Figure 5. Action of braking coil in Differential Drive systems. Figure 6. Cross-section view of MF driver in LSR6332 system. Figure 7. Cross-section view of HF driver in LSR6332 and LSR6328P systems. half-roll surround is of butyl rubber. The transducer has sufficient excursion capability and linearity to enable it to be crossed over at 250 Hz, operating comfortably at rated input power. It is free of the midrange distortion that afflicts many similar drivers operating in the 250 Hz range at high levels. The model 053TiS high frequency (HF) driver, redesigned with magnetic shielding, is shown in Figure 7. It has a 25 mm (1 in) smooth titanium dome which has been damped by a thin coating of Aquaplas as well as with a unique low recovery foam pad in the rear cavity adjacent to the diaphragm. Its frequency response extends smoothly beyond 20 kHz. When mounted in both the LSR6332 and LSR6328P systems, the HF driver is loaded by an elliptical oblate spheroid waveguide that controls the drivers dispersion in the crossover range. With the shielding of the HF driver (plus the inherent shielding in the neodymium structures), the new LSR6300 systems can be used in close proximity to cathode ray type video monitors in all applications. 4. Detailed Performance of the LSR6332 System: Overview of System: Three-way passive with 300 mm (12 in) LF driver Sensitivity: 90 dB SPL, 1 W input at 1 m (93 dB SPL, 2.83 Vrms input at 1 m) Impedance: 4 ohms nominal Nominal frequency response: 60 Hz 22 kHz, +1 dB, 1.5 dB Long term maximum power (IEC265-5): 200 W continuous (113 dB SPL) System dimensions (W H L): 635 394 292 mm (25 15.5 11.5 in) System weight: 20.4 kg (45 lb) Integral M6 mounting points In addition to the superlative directional response shown in Section 2 of this Technical Note, the LSR6332 excels as well in areas of distortion and in time domain integrity. We now discuss these: Response Uniformity: Figure 8 shows the contribution of each transducer in the system. Noteworthy here are the steep 24 dB/ octave transitions between sections and the in-phase 6 dB summation at each transition that are characteristic of Linkwitz-Riley filter designs. The high slopes between adjacent elements are crucial in assuring smooth response throughout the target listening windows. All network components in the primary signal paths are of the highest quality, including low loss capacitors and high saturation current, low distortion inductors. The systems response has been so precisely adjusted that JBL felt the only useful option for the user would be a relatively small HF adjustment, as shown in Figure 9. Here, you can see the effect of the 1 dB jumper setting on the terminal block on the rear of the enclosure. While this may look very small, bear in mind that a one-dB adjustment over a range of about three octaves is significant in terms of acoustical power reduction. The user will normally opt for flat response, the 1 dB option being called for when the systems are used in rooms that have minimal absorption at higher frequencies. 71800_JBL.TechNote8/26/04, 7:58 AM3 4 Figure 8. LF, MF, and HF contributions in LSR6332 system. Figure 9. HF 0 dB/-1 dB option in LSR6332. Figure 10. Second and third harmonic distortion in LSR6332, 96 dB and 102 dB SPL at 1 meter. Figure 11. Linear Dynamics Aperture port contour as used in the LSR6300 Series. Wide-Band Distortion: Plots of second and third harmonic distortion components at nominal listening levels of 96 dB and 102 dB SPL, measured at a distance of 1 meter, are shown in Figure 10. The distortion plots in these graphs have been raised 20 dB for ease in reading. At A, the distortion components above 100 Hz are 40 dB relative to the fundamental, while at B the same components are about 35 dB relative to the fundamental. This is remarkable performance for any loudspeaker when you consider that peak program levels in a surround sound environment with the loudspeakers at an average distance of 2 meters from the listeners would be in the range of 100 dB SPL. This would correspond to a per-loudspeaker reference level of about 100 dB SPL at 1 meter. Port Turbulence: At low frequencies in the region of port tuning, many LF systems exhibit what is called port compression, a tendency for turbulence to develop in the port and cause both noise and distortion. Contouring of the port terminations, both inside and outside the enclosure, can reduce this tendency considerably. JBL uses a proprietary Linear Dynamics Aperture port, whose cross-section contour is shown in Figure 11. The effect of this port, as compared to a straight port, is shown in Figure 12. The difference between the LSR6332 port and the normal straight port design can be clearly seen, reaching a difference of 4.4 dB at a power input of 400 watts. Note that for the LSR6332 system there is about 0.5 dB compression at 10 watts input, gradually increasing to 2 dB for 100 watts input. For the straight port the compression at 10 watts is 1.5 dB, increasing to 4.5 dB at 100 watts. Power Compression: Power compression results from heating of the voice coil, which causes an increase in voice coil resistance. The result of this is a loss of overall electromechanical coupling in the driver and a loss of output. Power compression is normally shown by scaling and superimposing curves run at different values of input power. If there were no power compression at all, then the response curves would lie directly one atop the other. Figure 13 shows the power compression in the LSR6332 system at power inputs of 10, 30, and 100 watts. The curves have been normalized so that the flat curve at 0 dB represents the response measured 71800_JBL.TechNote8/26/04, 7:58 AM4 5 Figure 12. Effect of port contours on output of system. Figure 13. Power compression due to voice coil heating in LSR6332 system. Figure 14. Phase and amplitude response of the LSR6332 system. at 1 watt. At 100 watts input, the power compression is less than 1.5 dB. Time Domain Performance: In todays systems engineering, all causes of excess phase shift through the system are identified and reduced to their lowest practicable values. The greatest offenders here have historically been loudspeakers. With reasonable care in baffle layout and well designed dividing network topology, deviations can be kept to a minimum. Amplitude and phase plots for the LSR6332 are shown in Figure 14. The group delay through the system is given below: Delay (seconds) = d/d where is the phase shift in radians and is the angular frequency in radians per second. The portion of the phase plot from about 100 Hz to 5000 Hz has a uniform slope, indicating that the time delay behavior of the system will be constant over that range. The frequency extremes deviate slightly from this constant value, but are consistent with the respective LF and HF rolloffs. 5. Detailed Performance of the LSR6328P System: Overview of System: Two-way powered system with 200 mm (8 in) diameter LF driver Maximum peak SPL (80 Hz 20 kHz): 111 dB SPL at 1 meter Numerous LF and HF shelving frequency controls Nominal frequency response: 50 Hz 20 kHz, +1 dB, 1.5 dB One parametric section for room mode correction (RMC) System dimensions (W H L): 406 330 325 mm (16 13 12.75 in) System weight: 17.7 kg (39 lb) Integral M6 mounting points and handles Response Uniformity: The 200 mm (8 in) LF driver maintains smooth response up to 2 kHz, which allows it to cross over smoothly with the HF driver in the range of 1.7 kHz in a Linkwitz-Riley configuration, as shown in Figure 15. The frequency division has been realized electronically. Figure 15. LF and HF contributions in response of the LSR6328P system. Time Domain Performance: As a 2-way system, the time domain performance of the LSR6328P will show a slight deviation in the uniformity of the phase plot, as compared with the 3- 71800_JBL.TechNote8/26/04, 7:58 AM5 6 Figure 16. Phase and amplitude response of the LSR6328P system. Figure 17. Second and third harmonic distortion in the LSR6328P, 96 dB and 102 dB SPL at 1 meter. Figure 18. LF 24- or 36-dB/octave alignment choice for LSR6328P; HF 1 dB choices. Figure 19. Boundary compensation curves for the LSR6328P. way system. This can be seen in Figure 16, where the phase plot has a slight curve in the range between 200 Hz and 2 kHz. Note however that the total range of phase shift, from low to high frequencies, is the same as that of the LSR6332. Distortion: Distortion curves at 96 dB and 102 dB, both at one meter, are shown in Figure 17. At 96 dB, the distortion at 70 Hz and above is less than 3%, while at 102 dB it is less than 9%. Excellent performance for a 2-way system with a 200 mm (8 in) diameter LF driver. LF and HF Contour Controls: The LSR6328P has a number of user response options that are accessible via DIP switches on the rear panel. Figure 18 shows the effect of an LF alignment switch that converts the normal 24 dB/ octave response to 36 dB/octave. 24/dB/octave is the “natural” alignment of a ported system, but it allows subsonic disturbances to pass through the system. When that position is engaged, the user can observe the excess cone movement caused by LF noises or disturbances that could be objectionable in a recording. Normally, the “VLF Protect” switch is engaged to remove these effects from the monitor system. There are three HF response positions: flat, +1, and 1, for fine-tuning the system to the working environment. Boundary Compensation: The LSR systems are designed to exhibit flat LF response when placed in a free-standing position away from a wall or corner. Using DIP switches, a broad LF cut of 1.5, 3, or 4.5 dB can be introduced into the LSR6328P system, as may be required, when the system is operated in positions