JBL Technical Note - Vol.1, No.31 电路原理图.pdf

1 Technical Notes Volume 1, Number 31 Progressive TransitionTM (PT) Waveguides Background: The modern constant-directivity horn has evolved slowly since its introduction over 25 years ago. Advances in horn design have been primary evolutionary in nature. Indeed current popular constant directivity horns appear strikingly similar to the first devices, which at their time, defined a revolutionary change in design philosophy. Horn design involves balancing compromise. Key performance parameters that can be controlled by the designer include: frequency response (both on- and off-axis), horizontal and vertical beamwidth, directivity index, electrical impedance, harmonic distortion, and low frequency cut-off. The designer may also manipulate the acoustic wave-front to generate a desirable radiation pattern that smoothly transitions from horizontal to vertical. Horn designers typically optimize one, two, or three performance parameters considered to be of the highest value, and then other areas of performance become an indirect result of the other choices made. Unfortunately each parameter mentioned influ- ences sound quality, arrayability, and accuracy. This in turn impacts the successful application of a horn in a loudspeaker system. Focus on a limited subset of objective parameters may not yield an optimal performance balance for real world appli- cations. Since the human ear doesnt discriminate based on a single area of technical superiority, a balance of each area of performance is required. 2 An Introduction to Progressive Transition Waveguides: PTTM Waveguide Design: To achieve balanced response of all parameters, JBL Professional started with a clean sheet of paper and developed Progressive Transition (PT) Waveguides. Progressive Transition waveguides are unique because a single mathematically-continuous surface defines the waveguide from transducer- throat to waveguide-mouth. Figures 1 through 4 show various PT waveguides. In each case the distinctive feature is the lack of a traditional diffrac- tion slot. Instead the sidewalls transition smoothly from the driver throat through to the square or rectangular mounting flange. Figure 3: PT-H1010HF wave-guide (100 x 100, 12 x 12 inch, rotatable). Figure 1: PT-H64HF waveguide (60 x 40, 12 x 12 inch, rotatable). Earlier designs consider the throat, the diffraction- slot, and the bell of the horn to be separate. This produces a discontinuity at the diffraction slot, where a roughly exponential loading suddenly becomes a rapid final flare (or bell) intended to provide constant beamwidth. While this approach yields uniform beamwidth and DI, the downside is high distortion, rough electrical and acoustical impedance, and often irregular frequency re- sponse. These factors may combine to produce the typical “horn sound”. By applying advanced 3-dimensional surface modeling, it was possible to create a waveguide surface that eliminates the diffraction slot disconti- nuity. This allows the expanding acoustic wave- front to remain perpendicular to, and attached to, the horn side-wall at all times. Figure 2: PT-F64HF waveguide (60 x 40, 6.5 x 12 inch). Figure 4: PT-K95MH waveguide (90 x 50, 18 x 18 inch, rotatable). 3 In a PT waveguide, the wave-front is controlled to generate the correct shape to propagate from the waveguides mouth. Even though geometrical diffraction is eliminated in PT designs, constant beamwidth and constant directivity are achieved. Improved frequency response, and lower distortion result. PT waveguide design principles are patent pending. PT Waveguide Performance Benefits: Smoother frequency response. With JBLs 2451SL compression driver frequency response of 1.0 dB is realized, with minimal equaliza- tion, on many PT waveguides. Electrical Impedance is smoother, and is free of typical “high-Q” peaks that compromise passive crossover design, and indicate difficult throat loading of the compression driver. Advanced constant beamwidth and directivity is achieved. Wide coverage angles are achieved without compromise. PT waveguides may be as wide as 120 x 120, but do not have rough frequency response, severe electrical and impedance anomalies, or poor acoustic loading. Harmonic Distortion is minimized to allow the maximum SPL capability of the compression driver to be used to its full advantage without a harsh “horn sound”. A continuous transition from the transducer exit to the rectangular or square waveguide mouth ensures uniform projection in the intended coverage area. Progressive Transition Waveguide Families: PT waveguides are grouped into two families. The first is “compact”, and second is “optimized cover- age/rotatable”. Compact PT waveguides balance performance in favor of small overall package size. Frequency response is optimal, distortion is superbly low, depth is minimized for use where a shallow enclo- sure is required. Beamwidth and directivity are optimal in the horizontal plane. Vertical beamwidth and directivity are optimized to provide a good match with JBL low frequency and midrange transducers; however, vertical pattern control does not extend as low as optimized coverage PT waveguides. Figure 5 shows a compact PT waveguide. Systems with rotatable PT waveguides optimize pattern control both horizontally and vertically. Pattern control is extended to a lower frequency. The installer can easily configure the loudspeaker for horizontal or vertical use. In systems using an optimized coverage PT waveguide, smooth frequency response, and the uniformity of off-axis coverage, and arrayability are all superior. A rotatable PT waveguide is shown in figure 6. Figure 5: PT-F95HF waveguide (90 x 50, 6.5 x 12 inch). Figure 6: PT-H95HF waveguide (90 x 50, 12 x 12 inch, rotatable). 4 Compact vs. Optimized Coverage PT Waveguides: Each PT design is appropriate for a wide variety of applications: Compact PT waveguides offer these features: Minimized enclosure size. Optimized low distortion for maximum output. Maximum output and superior intelligibility. Stage monitors, distributed systems, and small arrays are excellent applications. Optimized Coverage PT waveguides allow for: Rotatable systems: Horizontal or Vertical orientation. Extremely smooth frequency response at all playback levels. Predictable arrayability in engineered loud- speaker systems. Superior uniform coverage in difficult acoustical environments. Improved intelligibility. PT Mid-High Rotatable Waveguides: Combination mid-high, rotatable waveguides are a part of the PT family. The midrange transducer is a JBL Cone Midrange Compression DriverTM (CMCD). Each CMCD design features a cone midrange transducer integrated with a phasing plug, and an optimal rear enclosure. Figure 7 is a section view of a CMCD transducer. CMCD midrange systems have extended band- width over a full decade. Either from 200Hz to 2 kHz, or from 350 Hz to 3.5 kHz, depending on the specific model. JBL Professional Technical Note, Volume 1, No. 30, describes CMCD midrange components, and evaluates the performance in detail. PT Waveguide Models: Descriptive model numbers have been assigned to each Progressive Transition waveguide. This allows different systems using the same waveguide to be easily identified, and allows different coverage angle waveguides in the same family to be identified. A selection of PT waveguide models are indicated in Table 1: Figure 7: CMCD-81H section-view with PT waveguide. Table 1 PT ModelApplicationCoveragePhysical Description PT-F95HFHigh Frequency90 x 50Compact Rectangular PT-H95HFHigh Frequency90 x 50Optimized Rotatable PT-K95MHMid-High Frequency90 x 50Rotatable Mid-High PT-F64HFHigh Frequency60 x 40Compact Rectangular PT-H64HFHigh Frequency60 x 40Optimized Rotatable PT-K64MHMid-High Frequency60 x 40Rotatable Mid-High PT-H77HFHigh Frequency70 x 70Optimized Rotatable PT-F1010HFHigh Frequency100 x 100Compact Rectangular PT-H1010HFHigh Frequency100 x 100Optimized Rotatable 5 Design and Development: In developing these waveguides, JBL combined advanced acoustical modeling, computer-aided surface modeling, in-house rapid-prototyping, and an automated acoustical measurement system. Waveguide Contours: Proprietary, patent-pending, mathematical models are used to establish the contours that each waveguide surface must pass through. Based on JBLs decades of horn design experience, these models allow the designer to select coverage angles, mouth size, depth, and throat diameter. The vector-splines calculated then define between six and ten boundaries of the PT waveguide surface. Computer Surface Modeling: The waveguide contours are then imported into a high-end surface modeling program typically used by the aerospace and automotive industry. A 3-dimensional model of the waveguide surface is generated. Figure 8 shows the CAD surface model for a design. Prototyping: The unique constantly-varying surface defining a PT waveguide makes it difficult to prototype the design. Most other horn designs have surfaces that change direction simultaneously in only two of the three possible directions. When this is the case, prototypes are easily fabricated by the “model- maker”. PT waveguide surfaces vary in all three directions simultaneously. This makes it impossible to construct a prototype “by-hand”. Instead, to rapidly design and evaluate a potential design, a prototype is machined by JBLs in-house CNC machining center, shown in Figure 9. A CNC milled prototype is dimensionally accurate, and requires only a few hours to fabricate. Figure 10 shows a completed prototype. Figure 8: CAD model of PT-F77HF waveguide. Figure 9: CNC machining-center fabricating a production horn tool. Figure 10: CNC-milled prototype of PT-F77HF waveguide. 6 Measurement: The prototype is then measured acoustically with JBLs Automated Measurement System (AMS). JBLs AMS, shown in Figure 11 can rotate simulta- neously in two axis to measure either full spherical data, or horizontal and vertical data. Five-degree resolution horizontal and vertical polar data is collected in 20 minutes. Five-degree resolution, full spherical measurements are completed in 3 hours. Production: Each PT waveguide in production is a perfect 3- dimensional duplicate of the approved engineering prototype. Tooling is made from the same CAD file used to make the prototype. This assures final performance equals the initial prototype. Figure 12 shows the production waveguide component weve discussed. A Performance Analysis: The balanced technical performance achieved by PT waveguides is explored here. Measurement data is compared with previous JBL horns, and against a competitors solutions. Two rotatable 300 mm x 300 mm (12 x 12 inch) PT designs were selected. Performance evaluated includes: fre- quency response, impedance, beamwidth, directiv- ity index, and harmonic distortion, in each case. Medium Format, 90 x 50, PT Waveguide: This section compares a PT-H95HF waveguide against two equivalent 90 x 50 horns. JBLs previous 2381 horn, and a current horn design from another well established manufacturer are used. The competitors horn is also a rotatable design. The PT waveguide was measured with a JBL 2450SL HF driver. The JBL 2381 used a JBL 2446H driver, and the competitors horn used the popular european-sourced driver that came with the system the horn was removed from. Figure 11: JBL Professional Automated Measure- ment System. Figure 12: Final Production PT-F77HF waveguide. 7 100100010000 80 90 100 110 120 Impedance (ohms) 10 100 2020K dB SPL Frequency (Hz) 1 Watt Sensitivity and Impedance 100100010000 80 90 100 110 120 Impedance (ohms) 10 100 2020K dB SPL Frequency (Hz) 1 Watt Sensitivity and Impedance 100100010000 80 90 100 110 120 Impedance (ohms) 10 100 2020K dB SPL Frequency (Hz) 1 Watt Sensitivity and Impedance Figure 13: Frequency response and impedance at 1w/1m. Figures 13a-c show frequency response of each design. Note the PT-H95HF waveguide has smoother frequency response, and a smoother electrical impedance curve. The improved electri- cal impedance allows more accurate results in systems using a passive crossover. The PT-H95HF also has the highest sensitivity. Figures 14a-c show -6 dB beamwidth for each design. The performance of the PT-H95HF is greatly improved compared to the 2381. Compared to the competitors horn, the PT waveguide has equally good beamwidth. 100100010000 10 100 Vertical Beamwidth Horizontal Beamwidth Frequency (Hz) Beamwidth (-6dB Coverage) Coverage (degrees) 360 100100010000 10 100 Vertical Beamwidth Horizontal Beamwidth Frequency (Hz) Beamwidth (-6dB Coverage) Coverage (degrees) 360 100100010000 10 100 Vertical Beamwidth Horizontal Beamwidth Frequency (Hz) Beamwidth (-6dB Coverage) Coverage (degrees) 360 Figure 14: -6dB beamwidth a) PT-H95HF waveguide. b) JBL 2381 c) Competitors design a) PT-H95HF waveguide. b) JBL 2381 c) Competitors design 8 Figures 15a-c show the directivity index of each design. Again the PT waveguide shows improved power response compared to the two other ex- amples. Figures 16a-c present harmonic distortion data. The response shown was measured with a JBL DSC-260A loudspeaker controller. On-axis re- sponse was equalized flat with a 1 kHz crossover frequency. Input voltage was adjusted to achieve 110 dB SPL at 1m. The on-axis response, and harmonic distortion were then measured. Figure 15: Directivity and Q.Figure 16: Harmonic Distortion at 110dB/1m. 100100010000 0 10 20 Frequency (Hz) Directivity Index (dB) Directivity Factor (Q) 1 10 100 100100010000 0 10 20 Frequency (Hz) Directivity Index (dB) Directivity Factor (Q) 1 10 100 100100010000 0 10 20 Frequency (Hz) Directivity Index (dB) Directivity Factor (Q) 1 10 100 100100010000 80 90 100 110 120 2020K dB SPL Frequency (Hz) Frequency Response and Harmonic Distortion Black: Fundamental Red: 2nd (rasied 20dB) Green: 3rd (rasied 20dB) 2nd 3rd 100100010000 80 90 100 110 120 2020K dB SPL Frequency (Hz) Frequency Response and Harmonic Distortion Black: Fundamental Red: 2nd (rasied 20dB) Green: 3rd (rasied 20dB) 2nd 3rd 100100010000 80 90 100 110 120 2020K dB SPL Frequency (Hz) Frequency Response and Harmonic Distortion Black: Fundamental Red: 2nd (rasied 20dB) Green: 3rd (rasied 20dB) 2nd 3rd a) PT-H95HF waveguide b) JBL 2381 c) Competitors design a) PT-H95HF waveguide b) JBL 2381 c) Competitors design 9 In a high-frequency, horn the primary source of distortion is ideally limited to second harmonic distortion. This is a due to the non-linearities of air at high pressure, and is unavoidable. References 1 and 2 discuss this in detail. JBLs Optimized Aperture horns were designed specifically to reduce this distortion. As a result the 2381, shown in Figure 16b, has the lowest distortion, at the expense of compromises in frequency response, impedance, and beamwidth. The PT-H95HF waveguide produces textbook distortion character- istics, but the distortion is approximately 3 dB higher than the 2381 When the absolute lowest distortion is required,