JBL Technical Note - Vol.1, No.28 电路原理图.pdf
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1、1 Technical Notes Volume 1, Number 28 Forward Steered Arrays in Precision Directivity Speaker Systems Introduction Effective high-powered low-frequency systems are a desired feature of many sound reinforcement applications. However the resulting interaction of the many drivers in the resulting syste
2、m creates problems of sound energy directivity that are either unforeseen, difficult to control, or otherwise not desired. For instance, the act of stacking a large number of low-frequency elements together creates the undesirable effect of excessive beaming. Although this narrowing may be desirable
3、 especially when controlled by frequency tapering throughout the band it creates an inherent limitation of the quantity of LF devices that can be effectively added to an array where a wider or more consis- tent polar response is desired without detrimental lobing. Using Bessel arrays or large curved
4、 arrays may also accom- plish the purpose. Bessel arrays may not provide the desired polar coverage, are inefficient, and do not provide a great deal of off-axis attenuation. Curved arrays must be on the order of two wavelengths long or greater to effectively control the polar pattern. Curved arrays
5、 also beam when the length of the array becomes less than two wavelengths and do not provide much useful off-axis (rear) attenuation at low frequencies. Two desirable characteristics of a low- frequency system would be to provide a single “lobe” of energy to the coverage area that is one where there
6、 is no major dips or peaks in the response and to minimize response in areas that are outside of the coverage area. A good example of an application that may require a large number of LF drivers to meet high SPL requirements and requiring a wider vertical polar pattern is a cluster in a sports arena
7、. Generally the system requirements are 105 dB at 120 feet with a vertical polar coverage requirement of 90 degrees. High SPL applications that require a narrower vertical pattern are performing arts venues and touring systems. Both systems benefit from a high degree of rejection from the rear of th
8、e array. 2 Modular, extensible low-frequency arrays have been conceived which allow the creation of multi-driver arrays that create high sound pressure levels, maintain a wider pattern if desired, and maximize the off-axis rejection of energy by steering the sound energy forward. These forward-steer
9、ed arrays have been developed to create a single, main lobe of energy that provides relatively constant SPL levels over a defined coverage angle. The arrays are robust in the fact that they may be configured in a range of sizes, steered, and tapered to create an energy lobe that is most appropriate
10、for its applica- tion. Theory Forward-steered arrays are based on the end-fired array principle that has been briefly described by Olson1. The simplest example to consider is where two drivers are spaced on-axis to the direction of aiming. When the front drivers signal is delayed corresponding to th
11、e sound propagation time between the drivers, there is coherent summing in the direction of the array. If the spacing of the two drivers is chosen to be one-quarter of a wavelength, then at that frequency there will be a null behind the array. This is the result of the forward element being delayed
12、1/4 wavelength added to the physical separation of 1/4 wavelength. The energy directly behind the array is then offset 1/2 wavelength creating a null at that single frequency. With a two- element array, this null changes into useful attenuation for about half and octave or so centered on this freque
13、ncy center. TWO-DRIVER END-FIRED ARRAY 3 When multiple elements are used in an end-fired line-array configuration, the length of the array determines its low- frequency useful limit and the resolution or spacing of the elements determines its useful upper limit that is, where the side lobes are at l
14、east 6 dB lower than the main lobe. At the lower limit, ap- proximately 6 dB of off-axis rejection is provided when the length of the array is approximately one-quarter wavelength. At the upper frequency limit, the side lobes remain 6 dB less than the main lobe when the resolution or spacing of the
15、array elements is less than approxi- mately 0.4 to 0.5 times the wavelength. FIVE-DRIVER END-FIRED ARRAY Consider a five-element end-fire line- array with a spacing of 1 foot. The overall length of the array is then four feet. At 70 Hz, one-quarter wavelength, the array provides approximately 6dB at
16、tenuation, less at lower frequencies. At 450 Hz, where the spacing of the array is 0.4 times the wavelength, the side lobes remain suppressed by at least 6 dB. Intermediate frequencies of 140 Hz and 280 Hz are also shown to help describe the polar characteristics of the array. 4 As described in this
17、 example, in the frequencies between its useful limits, a multiple-element end-fire array pro- duces substantial off-axis rejection. Note that the main lobe has a relatively flat response throughout much of its effective coverage area with a relatively steep polar cut-off. Increasing the number of e
18、lements will provide greater off-axis rejection, however, the main lobe directivity will also increase. The nature of this basic end-fired array is that the width of the main lobe changes as the array goes through its useful frequency range as expressed by the off-axis performance. This may be toler
19、able in applications where the highest total directivity and power from the array is desirable. By adding addi- tional elements above, and/or below to create a three-dimensional array, arrays can be conceived that narrows the pattern of the array, while coherently adding power. By trading off height
20、, width, depth, and resolution of the array, an unlimited number of array character- istics can theoretically be developed. The array may also use frequency shading to create a single lobe of sound energy at a desired power level and polar pattern that is appropriate for the application. 5 APPLICATI
21、ONS The concept of a forward-steered array is to create a three-dimensional array of loudspeakers and delay the drivers back to a point, line, or plane so that the energy from those drivers coherently sums in the direction perpendicular to the reference. The most challenging aspect of this concept i
22、s to create a device that works well acoustically by itself while retaining the ability to be physically positioned with other like devices in a modular system that allows them to be steered effectively through- out their band of operation. JBL has developed three such systems; an 18” subwoofer syst
23、em PD 128, a 15” LF system PD 125, and a 12” mid- bass system - PD 162. These systems were designed to maximize off-axis response throughout their band of operation. The particular applications they were created for demanded wide patterns roughly 90 degrees, both horizontally and vertically. The cur
24、rent configuration of the PD 128 and PD 162 array modules allow for the creation of arrays with wide or narrow vertical patterns depending on the configura- tion of the array. The PD162 Based Low-Frequency Array: 6 The PD162 array development was an effort to pack low-frequency devices together as t
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