JBL Technical Note - Vol.1, No.30 电路原理图.pdf
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1、1 Technical Notes Volume 1, Number 30 Cone Midrange Compression Drivers (CMCD) Introduction: Throughout the history of professional audio it has been realized there are distinct performance advan- tages to compression and horn-loading a midrange transducer. The advantages include increased sensitivi
2、ty and bandwidth, desirable pattern control, and arrayability - all provided by horn-loading. Other advantages include reduced harmonic and inter- modulation distortion, and increased maximum SPL. Unfortunately, these theoretical benefits have been difficult to simultaneously achieve in practice, du
3、e to the lack of transducers that are suited to this appli- cation, but also due to compromises in the design of the horn and/or phasing plug. Because of these limitations, many previous solu- tions have suffered from poor frequency response, restricted high frequency bandwidth, and non-ideal area e
4、xpansions that introduce other anomalies. These limitations, and others, contribute to audible irregularities often referred to as a “horn midrange sound”. This technical note will discuss how these limitations were overcome in the family of JBL Cone Midrange Compression Drivers (CMCDs). Pertinent p
5、erfor- mance figures are presented in comparison and in contrast to other past and present solutions. Finally, typical CMCD performance characteristics are provided. Historical and Technical Background: The design of horn-loaded midrange systems can broadly be categorized into two philosophies: The
6、first is based on high-frequency compression driver designs that are optimized to operate over a lower frequency range. The second approach uses a cone midrange loaded directly by the horn, or coupled to the horn with a phase plug. Solutions derived from a high frequency compres- sion driver may hav
7、e numerous problems, includ- ing high harmonic distortion (due to high compres- sion ratios, and limited excursion capability), and limited low frequency extension (due to improper Theil-Small parameters for this application). These designs may break under high output, or inadvert- ent abuse. They r
8、equire a very large horn in order to operate optimally. The second solution a cone midrange driver coupled to a horn can correct most of the problems of a compression horn-loaded midrange system. However, other limitations are often introduced. One of these limitations in the past has been limited h
9、igh-frequency extension. Even an ideal cone transducer has a mass break point of ap- proximately 600 Hz to 900 Hz which limits high frequency extension. While many manufacturers attempt to correct this with various phase plug designs, the limitation still exists the moving 2 mass is too high, and th
10、e motor force provided by the magnet structure limits sensitivity. As well, many phase plug and displacement plug designs do not provide the uniform compression of the wavefront required to produce extended and smooth frequency response. After evaluating the state-of-the-art, JBL engineers developed
11、 a new family of midrange devices that eliminate performance limitations, and maximize sound quality. The resulting midrange components are referred to as a Cone Midrange Compression Driver (CMCD). These proprietary JBL implemen- tations are patent-pending. CMCD models are based around industry ac-
12、cepted, and proven, high-output JBL midrange transducers such as the 2250, used in VERTEC VT4889 line-array systems, and the 165H, used in Cinema ScreenArray systems. These transducers meet and achieve the requirements for optimal performance in a CMCD configuration. Key Performance Features: CMCD m
13、odels include the CMCD-81J, CMCD- 81H, and the CMCD-61H. Each CMCD assembly incorporates a JBL cone midrange driver, a 3 slot annular-ring phasing plug, and a thermally conduc- tive rear chamber. An optimal 4-inch exit-diameter and a correct exponential expansion provide excellent coupling to the en
14、tire family of Progres- sive TransitionTM midrange waveg uides. Section- views of the CMCD-81H/J, and CMCD61H, coupled to a PT waveguide, are shown in Figures 1 and 2. CMCD-81 models have a recommend bandwidth of 250 Hz to 2.0 kHz, and power handling of 350 watts. Sensitivity is 107 dB SPL/1w/1m on
15、a typical 90 x50 waveguide. Frequency response and electrical impedance are shown in Figure 3. Due to pistonic response within the pass-band, re- sponse deviations of less than 0.5 dB result with simple equalization. Maximum continuous SPL exceeds 133 dB SPL at 1 meter on appropriate waveguides. The
16、 CMCD-61H recommended bandwidth is 400 Hz to 3.5 kHz. Power handling is 125 watts. Sensitivity at 1 watt/1 meter is 107 dB SPL on a typical 90 x50 waveguide. Frequency response and electrical impedance are shown in Figure 4. Due to pistonic behavior throughout the pass-band, response variations of 0
17、.5 dB are realized with simple EQ. Maximum SPL exceeds 128 dB at 1 meter. The CMCD-81 is ideally suited for applications that call for maximum output, or where a lower cross- over to the low-frequency section is required. The CMCD-61 is suited to applications where extreme SPL is not required. In in
18、stallations where cost is critical but pattern control, low distortion, and excellent midrange clarity are still required, systems incorporating a CMCD-61H are an ideal solution. Figure 1:Section View of CMCD-81H and PT Midrange Waveguide. Figure 2:Section View of CMCD-61H and PT Midrange Waveguide.
19、 3 100100010000 70 80 90 100 110 Impedance (ohms) 10 100 2020K dB SPL Frequency (Hz) 1 Watt Sensitivity and Impedance 100100010000 70 80 90 100 110 Impedance (ohms) 10 100 2020K dB SPL Frequency (Hz) 1 Watt Sensitivity and Impedance Figure 4: Frequency Response and Impedance of CMCD-61H with 90 x 50
20、 PT Waveguide. Figure 3: Frequency Response and Impedance of CMCD-81H with 90 x 50 PT Waveguide. 4 Technical Details: CMCD assemblies use 8-inch and 6.5-inch cone midrange drivers designed from the ground-up to operate as true compression loaded midrange devices. The phasing plugs, and optimal rear
21、chambers, provide maximum sensitivity and wide bandwidth. Design features for improved performance include: Small diameter midrange cone drivers, thermally conductive rear enclosures, and annular ring phase plugs with 4-inch exit diameters. The benefit of each feature is elaborated on in the followi
22、ng sections: True Annular Ring Phasing Plug: CMCD designs feature a three-slot annular-ring phasing plug to provide optimal loading of the cone diaphragm to frequencies as high as 3.5 kHz in the case of the CMCD-61H. CMCD assemblies employ phasing-plugs die-cast in a high-density polyester-fiberglas
23、s thermoset- composite. The phase plugs are rigidly-bounded three-piece annular designs. This construction allows for close dimensional tolerances in produc- tion, and eliminates variability. CMCD phase plugs are typically spaced 0.075 inches from the cone. Many solutions have incorporated either on
24、e-piece displacement plugs, or molded polystyrene foam phase plugs. In either case the results are not optimal due to acoustical losses in the phase plug, and excessive spacing from the speaker cone. Sensitivity and high frequency extension are compromised. The low-loss composite structure of CMCD p
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