JBL Technical Note - Vol.1, No.29 电路原理图.pdf

1 Technical Notes Volume 1, Number 29 JBL Precision Directivity PD700 Series Co-Axial Mid/High Speaker Systems Background: In very large fixed installations, such as sports arenas, large houses of worship, or theatres, the requirements for wide bandwidth, dynamic sound reproduction have steadily increased over the past two decades. Traditional solutions focused on arrays of “bins and horns”, but such solutions are often no longer in keeping with the increased aesthetic requirements of the venues themselves. Furthermore, wide- bandwidth, high-fidelity-sound-reproduction at higher SPLs is difficult to achieve with traditional solutions. In the more recent past, a number of manufactur- ers have introduced packaged systems to address some of these requirements, but with only varying degrees of success. One of the limitations of many of these packaged systems is the inability of a single enclosure, aimed in the correct direction, to provide the required SPL in the audience area, due to the long distances between the loudspeakers and the audience. In a sports arena, a common specifica- tion for SPL is 105 dB continuous in the seating area, approximately 40 M (120 feet) from the loudspeaker. There are a number of solutions which attempt to achieve these requirements but most are “work- arounds” based on existing technology, instead of true solutions. One possibility, often employed, is to “overlap” the coverage of the loudspeakers. The downside of overlapping coverage patterns is the resulting destructive interference, which hampers the quality of reproduction, and only produces an increase in average SPL of approximately 3 dB over that of a single enclosure. In comparison, perfectly coherent summation between two sources through their entire coverage pattern would increase average SPL by 6 dB. Another way to increase the SPL generated by an enclosure is to decrease the coverage angle of the enclosure in order to increase the directivity or “Q”. But to maintain pattern control as the coverage angle is decreased, the size of the horn must increase in direct proportion. For example, a 40 horn that is 1 m (40 in.) tall, would need to be 2 m (80 in.) tall to maintain its nominal coverage angle at 625 Hz, if the targeted coverage were de- creased to 20. Clearly the size of such a horn would be impractical for most applications. 2 As a compromise, if the coverage of a horn is made narrower but the size is kept the same, at lower frequencies the pattern control of the nar- rower coverage horn remains identical to the wider coverage angle horn, as is shown in figure 1. The example 40” horn that is nominally 20 coverage only provides full pattern control above 1250 Hz. In some solutions, two or more high frequency horns are installed in the same enclosure, and the resulting destructive interference is accepted as a compromise, or simply isnt discussed. But once again only a 3 dB increase in average SPL results, instead of the 6 dB increase that is possible if the devices summed in a coherent fashion everywhere in the coverage pattern. Introduction to the PD700 Co-axial Series: After reviewing the existing physical limitations it was clear that a loudspeaker enclosure that is capable of maximum SPL 6 dB greater than current designs would require two high-sensitivity midrange-cone transducers, and two large-format compression drivers. The wavefronts of the devices need to combine in a fully coherent manner everywhere within the coverage of each enclosure to meet the goal of a 6dB increase in maximum SPL. Additionally, a co-axial mid-high horn arrangement with a square face was chosen so the side walls of the midrange horns could be brought close to- gether for improved midrange arrayability, and also to allow the enclosures to be rotated in an array. To allow the face of an array to be a virtually gapless spherical section, the enclosures were made trapezoidal in both planes. The systems developed to meet these require- ments are the JBL Precision Directivity PD700 series. Models available include the PD743 and PD764 (40 x 30 and 60 x 40 coverage). PD700 models are designed to crossover to the low frequency system at 225 Hz. For example, a low- frequency system consisting of PD162 Forward Steered Array modules is an ideal solution for larger venues. Models in the PD700 Series of mid-high enclosures offer the following features: Square faced enclosures which are trapezoidal in both planes allowing systems to be rotated. Co-axial mounting of the high frequency horn flush with the face of the midrange horn. Dual large-format 75 mm (3 in) diaphragm Neodymium 2430H compression drivers. Unique 53 mm (2.1 in) aperture dual driver high frequency throat section for interference free summation. Constant directivity horns for predictable arrayability. Dual 2250J 200 mm (8 in) diameter Neodymium Differential Drive (NDDTM) midrange drivers, for 700W total power handling and maximized midrange clarity. Midrange acoustic damper to eliminate midrange throat reflections. Optimized recommended processor settings to provide matched crossover polar response. 100100010000 10 100 20 horn, 80 wide 20 horn, 40 wide 40 horn, 40 wide Frequency (Hz) Predicted Beamwidth (-6dB Coverage) Coverage (Deg.) Figure 1: Predicated beamwidth of three horn designs, comparing mouth size and design coverage angle. 3 Coaxial Mid-High Systems An Over- view on Performance: Although co-axial systems can provide distinct performance advantages, they are often plagued by design difficulties and flaws that negate the performance gains. Well begin by examining some of the typical performance benefits of a co-axial design. Then well examine the performance limitations which may occur in a co-axial design. Finally well describe how the PD700 series Mid- High systems address the limitations, while realiz- ing performance benefits available. Co-axial Benefits: An important benefit of a co-axial mid-high system is that, through the crossover region, the system behaves symmetrically about the horizontal and vertical planes. As a result, off-axis interference between the mid and high frequency sections can be virtually eliminated by carefully controlling the geometry and location of the components, and by applying optimized signal processing to the pass bands. Such processing is described in a later section of this paper. The second benefit is the compact size of a co- axial design. The frontal area of the system can be greatly reduced in a co-axial configuration. As an example, the PD743 is 990 x 990 mm (39 x 39 in.) in height and width. If the high frequency horn were above, or to the side, of the midrange horn then the resulting system, with equivalent pattern control, would be 1350 x 990 mm (53 x 39 in.). In other terms: the frontal area of the enclosure would be 36% larger. The smaller frontal area can be a substantial benefit in meeting architectural goals of compact array size. An additional benefit of a co-axial design appears when the enclosures are used in an array. The co- axial enclosure allows the midrange horns to be placed sidewall-to-sidewall, both horizontally and vertically, allowing the midrange wavefront to radiate from each enclosure with virtually no gaps. As a result, the coupling of the midrange horns is far more predictable, the system is easier to tune, and the final coverage is more uniform in the audience areas. Finally, if the face of the co-axial enclosure is square, and the enclosure is trapezoidal in both planes, then the enclosure will ideally have no preferential axis, and will be equally effective in both orientations. For example a 40 x30 system would function equally well as 30 x40 system. To the consultant this allows much more freedom in the design of a cluster. Addressing Co-axial Design Challenges: Co-axial systems have not been universally accepted, even though there are clear benefits. A major reason for this lies in the difficulties in designing a co-axial system, which are often not adequately addressed and result in design prob- lems that negate the potential gains. Well now discuss these problems, and how they were addressed in the PD700 series. In a co-axial mid-high horn loaded system the compression driver may be mounted behind the midrange driver, and then fire through the pole piece of the driver. The alternative arrangement is to place the high frequency driver in front of the midrange cone, or phase plug exit. An example of each configuration is shown in Figure 2. Figure 2: Two typical designs of co-axial mid-high systems. H.F. Horn H.F. Horn Mid. Horn Mid. Horn H.F.Driver H.F.Driver Midrange Driver Midrange Driver 4 The problems that result from having the high frequency driver fire through the midrange driver include high 2nd harmonic distortion caused by the low expansion rate in the narrow long throat section. (See Reference 1 for a discussion of compression driver throat distortion). Often, to avoid excessively masking the mouth of the midrange horn, this configuration requires that the high frequency horn terminate before the mouth of the midrange horn, which can result in HF energy diffracting and reflecting into the midrange horn. For these reasons this configuration was elimi- nated as a possibility for the PD700 series. The second configuration places the high fre- quency driver in front of the midrange driver, or in front of the midrange horn throat. This configura- tion allows more flexibility in the design of the high frequency horn and potentially offers better performance. Eliminating Midrange Reflections and Resonances: Two design challenges result when the compres- sion driver and horn are located in front of the midrange transducer, in the mouth of the midrange horn. The first problem is reflections off the rear of the compression drivers, back down the throat of the midrange horn, and the aberration in frequency response, and polar response that may result. The second problem is the high-frequency horn acts as obstruction to the proper expansion of the midrange horn and may generate resonances. To address the first problem, the PD 700 series incorporates two features. First, the high frequency transducers are placed very close to the midrange drivers, this eliminates reflections at the lower frequencies. To eliminate reflections at higher frequencies, an “acoustic throat damper” was designed. The damper is specified to be moder- ately acoustically absorptive above 700 Hz, but not to be absorptive at lower frequencies. The throat dampers are constructed with an inside and outside shell of flame-retardant-treated and acoustically transparent woven fabric. The benefit is much smoother polar response, and a visibly and audibly cleaner impulse response, as shown in Figure 3. The acceptable compromise is a net reduction in output of 1 dB from 1 to 2 kHz. To address the problem of the high horn interfering with the expansion of the mid horn, unique high frequency horns were designed that have both an interior surface, and a molded outer surface as well. JBLs extensive experience in composite construction was applied. The outside surface is molded to provide the correct area expansion of the midrange horn for proper acoustic loading. The space between the inside and outside shell is filled with urethane foam which provides structural rigidity and acoustic damping. Typical co-axial designs in the past have placed a “thin-wall” high- frequency horn in the mouth of the midrange horn, which results in frequency response and pattern control aberrations. Figure 3: Midrange impulse response with and without acoustic throat damper. Solid curve shows response with damper, lighter curve shows re- sponse without damper. 5 Understanding Co-axial “Shadowing”: Another design issue in a coaxial system is “shad- owing”. If the percentage of the area of the midrange horn blocked by the high frequency horn is too large, then shadowing may occur. The effect causes the midrange horn to behave as two distinct “cells”, or signal sources. When this occurs, the midrange off-axis response has nulls within the nominal coverage angle. To solve this problem the size of the high frequency horn must be minimized, but must remain large enough to maintain pattern control at the crossover. A delicate design balance must be achieved. Figure 4a is shown to visualize the problem of the midrange horn mouth being divided into separate acoustical radiating areas. We see three distinct areas indicated. These are defined by the top and bottom edges of the high frequency horn. Two large areas labeled 1 are formed above and below the high frequency horn, and two smaller areas 2 and 3 are shown on either side of the high frequency horn. Figure 4a: Midrange horn shadowing the radiating area of the midrange horn is visually divided into three zones 1, 2, and 3 for further analysis. Figure 4b shows as the listening or measurement location is moved to the left, as indicated by the arrow, the vector that sound must travel through indicated by the X , shifts to the sidewall of the horn. At this angle of observation, acoustic energy originating from areas 1 and 2 is in the same vertical plane, but energy arriving from area 3 is offset in time. Figure 4b is a top view that shows this more clearly. The difference in time-of-flight for the delayed energy is apparent. If the area “shad- owed” is too large, this difference in arrival time causes narrowing of the beamwidth, and visible lobing in the polar. Similarly the same effect may occur in the vertical plane. Empirically, the height and width of the high frequency horn should be roughly no more than 0.3 to 0.4 of the height and width of the midrange horn, which keeps the area masked (area 3 on figure 4a) to between 13% to 19% of the total radiating area of the midrange horn. H.F. M.F.1 1 23 39” 39” H.F. M.F.1 1 2 3 123 P ath 1, 2 P ath 3 Front View: Top View: Figure 4b: Shown in the front view The acoustic source moves left as the observer rotates the speaker. Top view The result- ing difference in path lengths. 6 Assuming the intensity of the sound field is uniform across the face of the radiating area of the mid- range horn, and assuming the worst case situation where the energy radiated from the “shadowed” zone is shifted 180 out-of-phase compared to the primary arrival of energy at some frequencies, the following results are calculated. For 13% masked area, a worst case, 2 dB maximum variation in response may occur. For 19% masked area this variation may be as much as 4 dB. If the high frequency horn is not square, then the percentage masked is different in each plane In the case of the PD743 the ratio of high/mid mouth height is 0.33 vertically, and 0.28 horizontally. Based on this analysis response variations should be no more than 2 dB. Polar plots and beamwidth plots show the resulting freedom from any masking problems in the PD700 series. Geometry of the Dual Midrange High pass filter slope; High pas