JBL Technical Note - Vol.1, No.4 电路原理图.pdf

Technical Notes Vol. 1, No. 4 Constant Directivity Horns L Introduction: In the last three years, constant directivity horns have virtually replaced the older radial and multi-cellular types in large-scale speech and music reinforcement systems. These devices are available from three domestic manufacturers: JBLs family of Bi-Radials, Altecs family of Mantarays, and EVs family of White Horns. While all of these horns succeed in their design aims, there are significant differences between them which contractors and consultants should be aware of, and it is the intent of this Technical Note to present comparative data on these devices in the most objective way. More recently, JBL has introduced a line of Flat Front Bi-Radials, which effectively replace the corresponding older radials in the line. Since a number of users of the older models have been reluctant to give them up, we would like to present performance data on the old and new devices so that the advantages of the new models will be apparent. Additionally, we will present comparative data on the newer Altec and EV smaller horns which compare directly with JBUs new Flat Front Bi-Radials. While most of our discussion will deal with the directional properties of these horns, we will comment as well on certain aspects of smoothness of frequency response and distortion. II. The Large Horns: The horns to be compared in this section are: JBL: Altec: 90 x 40: 60 x 40: 40 x 20: 2360A 2365A 2366A MR94A MR64A MR42A HR9040A HR6040A HR4020A A. Details of Pattern Control: While polar plots present the most complete directional data on horns, they take up considerable space and do not allow for easy comparisons. More usually, we see plots of the horizontal and vertical nominal coverage angles, or the -6dB beamwidth, and directivity index (Dl), vs. frequency. The beamwidth plots present information along the frontal normal on- and off-axis angles of the horn, while the Dl plots tell us something about the pattern control integrated in all directions around the horn. The Dl plot is an especially useful measure in assessing a horns performance in a power-flat system. We now show these plots for the 90 x 40 devices. Figure 1: JBL 2360 Beamwidth the roll-off above 8 kHz is not reflected in any broadening of the horns patterns above that frequency. EVs Dl data for the HR9040A is consistent with their beamwidth data, and it shows that the horn would be a good performer down to only 1 kHz in a power flat system. Moving on to the 60 x 40 devices, we show their beamwidth and Dl plots. Figure 4: JBL 2365 Beamwidth specifically, a Dl of 15 dB at 200 Hz is an obvious error. 2 B. Distortion and Frequency Response: In general, the newer horns exhibit lower distortion than the older radials and multicells because they flare more rapidly. The older designs had fairly low flare rates, and while this may have increased their loading ability at low frequencies, it was at the expense of increased distortion throughout the frequency range. The newer designs generally exhibit smoother frequency response contours than the older models, and this makes system equalization easier. We have noticed, however, that the Altec Mantarays, with their abrupt transitions from one conical flare to another, exhibit irregular frequency responses as compared to designs with smoother transitions. C. Low-frequency Limitations: The factors in horn design which determine pattern control are not necessarily related to those which determine the extent of low-frequency loading. Thus, it is possible for a horn to exhibit good pattern control down to, say, 500 Hz, but not exhibit adequate loading on the driver at that frequency. Obviously, such a horn would be of very limited use in that frequency range. For the six horns we are discussing, the following data is taken from published specifications: Horn Model: Lowest Usable Frequency: Lowest Recommended Crossover: JBL2360A 300 Hz 350 Hz 2365A 300 Hz 350 Hz 2366A 200 Hz 300 Hz Altec MR94A 500 Hz 800 Hz MR64A 500 Hz 800 Hz MR42A 500 Hz 800 Hz EV HR9040A 350 Hz 400 Hz HR6040A 350 Hz 400 Hz HR4020A 250 Hz 300 Hz Both JBL and EV have similar low-frequency loading limitations. The Altec horns do not load down as low, and this is the result of their straight-wall, conical construction. Both the JBL and EV horns utilize exponential flares in their throats, and this enables them to load down to lower frequencies than is the case with the multiple conical flares. We can also arbitrarily set low frequency limits on these horns based on loss of pattern control. Establishing a 1.5 factor(*) on the opening up of the nominal angle as being the useful limit, we have: 3 Horn Model: Pattern Control Limits Horizontal: Vertical: JBL 2360 200 Hz 500 Hz 2365 400 Hz 500 Hz 2366 630 Hz 1 kHz Altec MR94 300 Hz 630 Hz MR64 400 Hz 630 Hz MR42 400 Hz 800 Hz EV HR9040A 200 Hz 1.2 kHz HR6040A 400 Hz 1.2 kHz HR4020A 300 Hz 1.2 kHz *No greater than 30 for 20 nominal No greater than 60 for 40 nominal No greater than 90 for 60 nominal No greater than 135 for 90 nominal One point is clear from an inspection of the data presented thus far, and that is that the JBL 2360 and 2365 horns are the only ones of the group that can be specified for use down to 500 Hz with negligible loss of both horizontal and vertical pattern control and with proper driver loading. Again, we stress that the data we have been examining is that provided by the manufacturers themselves on their own products. ill. The Small Horns: A. A Quick Look at the Old Radials: Most of the radial horns which were the workhorses of the professional sound business for decades were designed back in the forties and fifties. When viewed from above, they appear as sectors of a circle bounded by radii on the sides; when viewed from the side, they appear as a simple exponential flare, as shown in Figure 10 (A) and 10 (B). If the flare development in the throat is carefully derived, then the radial horn will exhibit quite even horizontal coverage. However, horizontal coverage at high frequencies is driver-size dependent. The larger throat drivers (49 mm or 2 inches) will show narrowing at high frequencies as compared with the 25 mm (1 inch) throat drivers. Due to the exponential vertical cross-section, the vertical coverage will always be wide at low frequencies and narrow considerably with rising frequency. The typical beamwidth data of a radial is shown at Figure 10 (C), and the Dl plot is shown at 10 (D). Figure 10 (C) also shows a characteristic of all radials, mid-range narrowing in the horizontal plane. This happens at the wavelength which is approximately equal to the mouth width. Usually this will be in the 500 to 1000 Hz region, and the loss in coverage in that region is a problem. Despite their shortcomings, many designers appreciated the rising Dl of the radials, inasmuch as it did not require much, if any, electrical equalization in order to get flat on-axis response. This advantage of course was bought at the expense of vertical coverage. The larger radials provided good loading down to 500 Hz, and sometimes below, because their exponential flare rates were relatively low. 4 B. The New Designs: With the introduction of the new family of Flat Front Bi-Radials, JBL discontinued the models 2345, 2350 and 2355 radials. In each case, the new horn is smaller than the corresponding older model and exhibits improved coverage at high frequencies. While the new 2370 replaces the 2345 as a systems component, the 2380 and 2385, respectively, replace the 2350 and 2355 horns in the 90 x 40 and 60 x 40 categories. Just as JBL has brought out a line of smaller horns based on new design principles, Altec and EV have brought out their own smaller horns. We will now compare these devices and see how they measure up to the old 2350 and 2355 radials. First, we will look at the 90 x 40 devices. Figure 11: JBL 2345 Beamwidth however, there appears to be nothing consistent in them. The intended uses of these horns is in small reinforcement systems, as auxilliary elements in large arrays, and, especially in the case of the JBL Flat Fronts, for use in music reinforcement systems. C. Low-frequency Limitations: Below are the published lowest usable frequencies and the lowest recommended crossover frequencies for the small horns. 5 Horn Model: Lowest Usable Frequency: Lowest Recommended Crossover: JBL2350 350 Hz 500 Hz 2380 400 Hz 500 Hz 2355 350 Hz 500 Hz 2385 400 Hz 500 Hz Altec MR II 594 500 Hz 500 Hz MR II 564 500 Hz 500 Hz EV HR90 500 Hz 800 Hz HR60 500 Hz 800 Hz Some comments are in order. Altec does not specify different frequency values for the lowest usable frequency and the lowest recommended crossover. Their single value is the one given here in both columns. EV states that their lowest recommended crossover frequencies are beamwidth limitations. Let us now look at the low-frequency beamwidth limitations, assuming as we did before that an arbitrary limit is when the pattern control expands to approximately 1.5 times the nominal. Horn Model: Pattern Control Limits Horizontal: Vertical: JBL 2350 300 Hz 2 kHz 2380 400 Hz 1.6 kHz 2355 400 Hz 2 kHz 2385 500 Hz 1.6 kHz Altec MR II 594 500 Hz 1.6 kHz MR II 564 1.2 kHz 1.6 kHz EV HR90 500 Hz 2.5 kHz HR60 630 Hz 2 kHz This study of constant directivity horns, both large and small, is intended to help the contractor or consultant to sort out much of the confusion surrounding these devices. All of the data we have shown has been taken from manufacturers specification sheets, and we have redrawn graphs so that comparisons can be made easily between similar models. 6 FIGURE 1. JBL 2360 Beamwidth vs Frequency Directivity vs Frequency FIGURE 4. JBL 2365 Beamwidth vs Frequency Directivity vs Frequency FIGURE 3. EV HR9040A FIGURE 6. EV HR6040A Beamwidth vs Frequency Beamwldth vs Frequency Directivity vs Frequency Frequency Hz Frequency Hz Directivity vs Frequency Beamwidth (-6 dB) Beamwidth (-6 dB) Frequency Hz Beamwidth (-6 dB) Beamwidth (-6 dB) Frequency Hz Horizontal Vertical Horizontal Vertical Frequency Hz Frequency Hz Frequency Hz Frequency Hz FIGURE 2. ALTEC MR94 FIGURE 5. ALTEC MR64 Beamwidth vs Frequency Beamwidth vs Frequency Beamwidth (-6 dB) Beamwidth (-6 dB) Frequency Hz Frequency Hz Horizontal Vertical Horizontal Vertical Directivity vs Frequency Directivity vs Frequency Frequency Hz Frequency Hz 7 Beamwidth vs Frequency Frequency Hz Directivity vs Frequency FIGURE 9. EV HR4020A FIGURE 10. A TYPICAL RADIAL HORN Beamwidth vs Frequency Beamwidth vs Frequency Beamwidth (-6 dB) Degrees Beamwidth (-6 dB) Vertical Hoiizontal Vertical Frequency Hz Frequency Hz Directivity vs Frequency Directivity vs Frequency Frequency Hz Frequency Hz FIGURE 7. JBL 2366 FIGURE 8. ALTEC MR42A Horizontal Vertical (-6 dB) Degrees Frequency Hz NOMINAL HORIZONTAL COVERAGE ANGLE HORIZONTAL- VERTICAL NOMINAL VERTICAL COVERAGE ANGLE I Beamwidth (-6 dB) Dl (dB) Frequency (Hz) Frequency (Hz) A. TOP VIEW B. SIDE VIEW 8 Frequency Hz Directivity vs Frequency FIGURE 12. JBL2370 Beamwidth vs Frequency Frequency Hz FIGURE 14. JBL 2380 Beamwidth vs Frequency Directivity vs Frequency FIGURE 15. ALTEC MR II 594 Beamwidth vs Frequency Directivity vs Frequency FIGURE 13. JBL 2350 FIGURE 16. EV HR90 Beamwidth vs Frequency Beamwidth (-6 dB) Vertical Horizontal Beamwidth (-6 dB) Degrees Horizontal Vertical Frequency Hz Frequency Hz Directivity vs Frequency Directivity vs Frequency Frequency Hz Frequency Hz FIGURE 11. JBL2345 Beamwidth vs Frequency Horizontal Vertical Beamwidth ( 6 dB) Degrees - Vertical Horizontal Frequency Hz Frequency Hz Frequency Hz . Horizontal Vertical - Beamwidth ( 6 dB) Degrees Beamwidth (-6 dB) Degrees Horizontal Vertical Frequency Hz Frequency Hz Directivity vs Frequency Frequency Hz 9 Frequency Hz FIGURE 19. ALTEC MR II 564 Beamwidth vs Frequency Frequency Hz Beamwidth vs Frequency Beamwidth vs Frequency Beamwidth (-6 dB) Beamwidth H5 dB) Vertical Horizontal Horizontal Vertical - Frequency Hz Frequency Hz Directivity vs Frequency Directivity vs Frequency Frequency Hz Frequency Hz FIGURE 17. JBL2355 Beamwidth vs Frequency Frequency Hz Directivity vs Frequency Horizontal - Vertical Beamwidth ( 6 dB) Beamwidth (-6 dB) Horizontal Vertical Directivity vs Frequency Frequency Hz FIGURE 18. JBL2385 FIGURE 20. EVHR60 10 NOTES 11 PROFESSIONAL DIVISION JBL Incorporated, 8500 Balboa Boulevard, P.O. Box 2200, Northridge, California 91329 U.S.A. UBL/harman international 2M 64291 6/84 Printed in U.S.A.