October 14-17, 2015, Shanghai New International Expo Center, China's most professional large-scale outdoor array demonstrations - line array will return to the high profile over the same period!
In recent years, the line array system has been widely used in large-scale sound reinforcement sites with its unique advantages. Properly designed and installed line array speakers provide flat frequency response, high quality tone reproduction, and strong, controllable coverage characteristics.
In essence, the line array is the same output signal sent from different drivers and meets the "in-phase" requirement throughout the coverage area. To achieve such technical parameters is by no means a simple matter. It is important to first understand the basic principles of line arrays.
basic concept
Everyone knows that sound is a periodic wave that spreads in the air. In other words, sound travels in the air, and the air itself does not move. Therefore, in the discussion of sound output, all opinions that sound is "air movement" are wrong. This is a very important feature.
Another basic concept to understand is the "breakpoint frequency." Above this frequency, its directivity can be controlled by controlling the degree of the radiator (in this case, the degree of the linear array).
Breakpoint frequency is inversely proportional to speaker length and radiation angle. The formula for the breakpoint frequency (below) is a basic concept that applies to all speakers. For linear arrays, audio professionals can use this to estimate the size of the line array and the directional controllable start frequency.
BF = 24,000/Φ*Is
Where: Φ represents the speaker coverage angle corresponding to -6dB
Is represents the length of the line array, in meters
Speakers and Line Arrays
For a better understanding of the formula, imagine taking a section of the line array as a single speaker model. The coverage limit for each line array horn depends on the frequency. A single horn is not directional at low frequencies; the frequency directivity depends on the size of the radiating element. The horns are combined with adjustable vertical angles of view, and the range of the box can directly determine the effect of the line array.
A typical (well-designed) speaker can ensure 20 degrees of vertical coverage at 6 kHz and only half coverage at 12 kHz. This only changes with frequency, also known as "monotonic contraction" of vertical coverage. So if we know the length of the line array, we can easily calculate the -6dB coverage angle on the vertical plane based on the frequency in the breakpoint formula. In contrast, knowing the -6dB coverage angle and the corresponding frequency, we can calculate the coverage angle at other frequencies.
Φ-6=24000/fl
Among them: Φ-6=-6dB corresponding coverage angle (unit: degree)
F = frequency (unit: Hertz)
l= line array segment length (unit: meters)
Line array type
There are two combinations of line arrays: linear and curved. The linear type represents the "pure" line array characteristics, but in actual use, it must be bent to meet the high frequency coverage requirements. The following are several commonly used line types:
Straight line arrangement (Figure 1): In this arrangement, the vertical coverage is monotonically contracted - the longer the line array, the smaller the coverage angle. Therefore, the linear arrangement has a very narrow high-frequency vertical radiation width, and the projected distance is proportional to the frequency and the length of the array. This arrangement is the "best listening position" and the most consistent array of sounds in the far field. However, because the high-frequency coverage in the distance is too narrow and difficult to control, it is rarely used in practice.
Bow-shaped (constant radius curve) arrangement (figure 2): Curving the arrangement with a constant radius results in consistent directionality, but it impairs the coherence of the sound. "Coherence" should be understood as a "coherent" state. Coherence, by definition, refers to sound waves that have similar three conditions of pointing, size, and phase.
In many cases, the bow curves used are all compromised. For example: an array of eight speakers, with an angle of 1 degree between each speaker, then its total vertical coverage is about 7 degrees. Therefore, for the directivity of all spectra above the breakpoint frequency, this arrangement will have a constant directional characteristic.
At the same time, a tiny opening angle also means good coherence. If the opening angle between the two horns is increased to 5 degrees, the vertical coverage angle increases to 35 degrees. At this time, the directivity remains unchanged, but the coherence decreases. The reason is simple: the greater the degree of bending, the less the horns heard by distant listeners, and the delay in the output of each horn in a particular listening axis, so the coherence is reduced. Therefore, any curve that is too extreme is not desirable. It is best to get the desired coverage angle by moving the speaker position and using a more gentle curve.
J-line (figure 3): Users of line arrays first discovered that the linear type did not provide enough high-frequency sound near the stage, so they let the horn below the array point downwards. This creates a "mutation" in the array, which in fact makes the array not well combined, resulting in the separation of the array. Directivity has also been affected. Some users try to adapt to various areas by applying separate signal processing to "correct" this situation, but cannot correct the interruption caused by the transition between two different arrangement parts.
Involute (figure 4): Involute is a modification of the J-shape. Change the angle of the horn to remove the hard break in the J-line. Involutes provide a constant directionality and a uniform coverage angle, and the total coverage angle corresponds to the sum of the individual opening angles. In the vertical direction, the involute transition is smooth from the point pointing from the bottom of the array to the top.
Coverage and coherence
The most important factors affecting the choice of line array location and configuration are coverage and coherence. In Figure 5, an involute array suspended from the top is shown, and the distance to the nearest and farthest listeners is approximately equal. The sound pressure level and frequency response of the front and rear speakers in the loudspeaker coverage are almost the same. Because the vertical coverage angle is wide, the ratio of direct sound and reverb sound is the same for most listeners.
Figure 5
Turning now to Figure 6, the position of this array is much lower and the angle of view of each horn is much smaller. The sound pressure levels in the front and rear zones are more balanced, the frequency response is more flat, and the ratio of direct sound and reverb sound is also improved. This situation makes the sound energy control of the listening venue itself more convenient, and there is less room for reflection.
But from a sound point of view, this position is unnatural. Because the "positioning" of this sound is not the actual location of the sound source, it may cause people's attention to leave the ongoing performance on the stage. A good solution to this is to add speakers in front of the stage so that the sound can be heard coming from the top of the audience closest to the stage back to the stage.
Figure 6
Near field VS. Far field
When working outdoors, this method also reduces the impact of air. The fewer curves in the shape of the array, the smaller the sound is affected by wind and temperature. what is this? Because the sound is radiated over a wide range, sound waves will refract and "bend" at the junction of different temperatures and wind speeds during propagation. Therefore, when the vertical radiation angle is increased, the transmission range of the sound in the air is increased, and it is more susceptible to the influence of temperature.
In the near field and far field, the role of the line array is completely different. The near field is an interference field. When the two line arrays are close, the frequency components of the loss become more and more. The increase in these losses will even affect the same horizontal axis.
Figure 7 shows the model built by picking eight horns on an array and four different points in the near field. At the farthest "D" point, the response is relatively flat. However, near the array, the high frequencies begin to attenuate because the test points are affected by the edge sources. At the nearest "A" point, only the high frequencies of the two speakers can cover this test point, while the low frequencies are covered by all the speakers. Therefore, in the frequency below 100 Hz, the far field and the near field are basically the same, but when the frequency increases, the far field frequency response will increase rapidly.
Figure 7: Point A in Figure A and Point D in Right
Therefore all of the far-field effective areas of this array are the collection of points at which the listener can hear all frequencies output by the entire array. From this, one can also get a very simple point: the closer you are to the line array, the less high frequencies you hear, which leads to an unbalanced frequency response. This is why you can't sit in the near field while mixing indoors - the sound from the array heard here is definitely not correct.
Figure 8
The level attenuation and distance can be compared by the frequency factor of the line array. Figure 8 shows this at three frequencies: 100Hz, 1kHz and 10kHz. (Note that this diagram has been standardized to show the difference in level at 100 meters.) This diagram clearly shows the change in level as the frequency of the sound changes.
Figure 9: The number of horns that make up the line array, 8 on the upper line, 4 on the midline, and 2 on the lower line
From another point of view, Figure 9 depicts different data at 8 kHz for linear arrays of different lengths. If you standardize a grid to 1 meter, you can easily see the huge difference in high frequency output in the far field. The far field distance is directly proportional to the frequency and the length of the array. The following is a linear array of this formula, but it can also be used to calculate the distance from the array with radians to the far field position:
DF=fl2/700
Among them: DF represents the distance to the far field (unit: meters)
f indicates the frequency (unit: Hz)
l indicates the length of the line array speaker (unit: meters)
Another factor to consider is the overlap between horns. In Figure 10, note the overlap between this array of arrays. There is very little overlap between the horns at the bottom, and at certain frequencies, only individual horns sound.
Figure 10
The sharp-edged array may have these problems at the high-frequency coverage of each horn's edge, but these problems do not occur together because the curves themselves are protected. At the same time, linear and high-curvature arrays can have quite a few different points in the balance of frequency response.
The general solution in this case is to separate the bottom speakers for processing, but care must be taken. There is a gradual change between the horn and the horn, but the trend is to use the same process to make all the horns reach the same working condition. Air is also a factor. The absorption of high frequencies by the air will reduce some of the differences in the high-frequency balance between far-field horns and near-field horns.
Obviously, there are many factors that can directly make the coverage uniform and coherent, and the balance of the spectrum—balanced in the far field and near field—so all links must be carefully considered.
Attenuation VS. Distance
Figure 11 compares the frequency response at 4 kHz for linear and bowed arrays. The linear array is 5 meters long and the arcuate array has a bow of 45 degrees and a radius of 8 meters. The total length is also 5 meters. We see that linear arrays attenuate by 6dB per doubling of the distance in the far field, and columnar filtering effects in areas where the distance has been attenuated by 3dB per doubling level.
Figure 11
On the other hand, bow-shaped arrays rarely produce a comb-like filter effect, and its turning is also softer.
Referring to Fig. 12, three different arrays (composed of two speakers, four speakers, and eight speakers, respectively) are compared here, and the horns in the array remain uniform (7.5 degrees). Therefore, the three arrays have angles of 15 degrees, 30 degrees, and 60 degrees, respectively. Note the attenuation corresponding to the change in distance in the three arrays. At the measuring point of 40 meters, the transition between the 3 dB attenuation per distance and the 6 dB attenuation per distance of the arcuate array is very consistent.
Figure 12
Line array correction
In the most basic understanding, “correcting” a line array speaker refers to the use of physical or electronic methods to form or adjust the effective coverage of an array to meet specific listening needs. There are several ways to correct the line array:
1, dispersion correction is used to adjust the angle between the speakers
2. Amplification correction is used to adjust the level of the horn
3, effective delay correction can change the virtual shape of the line array
All these methods are limited when used. Multivariate compensation (that is, using different compensation for each horn) is also a form of amplification correction, but it is a frequency-based method.
Decentralized correction is probably the most reliable method of correction. In general, speakers that directly point to the listener do not attempt to electronically control the output, resulting in the best subjective hearing experience. In addition to small effects such as temperature gradients and relative humidity, these factors may affect high frequencies. Have a big impact.
The delay correction can well adjust the frequency range limited by the vertical coverage of a single speaker. Imagine that the maximum vertical coverage angle of the horn in the array you want to delay correct is 40 degrees. In addition, you also want to reduce the output of this speaker by 30 degrees. In other words, you want to make the effective output provided by the line array 10 degrees below its coverage limit by delaying the input signal. In fact, the delay correction cannot exceed the device's inherent effective area.
Amplification correction is basically used to process line arrays that are already decentralized. In other words, the shape of the line array has been adjusted and needs to be further and better adjusted.
For a linear array, the polar response of the high frequency part can be improved (or widened) by weakening the horn for the furthest distance, but at the same time, this reduces the total radiant energy of the entire array. Therefore, the resulting polar plot depends on the performance of the high-frequency output.
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