|17th January 2013, 07:03 PM||#1|
Join Date: Sep 2008
Line Array Tweeter line length?
I'm in the design phase of an Array of 50" tall mounted on 24" bass bins and have been getting conflicting info on the tweeter line length. For obvious cost saving reasons, i'd like to shorten the line length to 24". These will be for midfield listening, about 14ft away. Drawbacks?
|17th January 2013, 07:50 PM||#2|
Line Array length?
I am not an expert on line arrays using conventional dome tweeters and bass / mid mid drivers all but the attached document is a good start.
I have been experimenting with line array's of full BMR range drivers and got good results with as lines as short as 6 drivers.
The drivers are 4.5 inch drivers ( Sd = 60) and they come in a 112mm square chassis. So my six driver array is 780mm - approx 31 inch's long.
I must say things get way better at 12 drivers and 16 drivers per side, really jaw dropping in fact...No subs, no crossover, no " fractured mirror" to listen through, just pure flowing music...I love em!
I have attached some pics of a very cool 8 driver version ( 1,600mm total length) that sound very sweet, the 200mm centre to centre spacing is a bit more than the 112mm centre to centre of the straight array's, so a bit less performance compared to the 1,850mm tall 16 driver array.
Regarding your array design, watch out for floor and celing bounce, both will show up on measurements and be easy to hear.
If you can, use a very thick loose weave floor rug(s) and ceiling treatment sound absorber. Layers of fibre glass / underlay / close cell foam / and cotton or silk covers work a treat and can be made to look "almost acceptable" in most wives opinions...!
Generally I think you must have the array at least 70% of the floor to ceiling height.
Hope this helps and good luck!
|17th January 2013, 07:54 PM||#3|
Doc wont attach, so here it is!
November 23, 2011, by John Murray
" At last count… well, I stopped counting. Suffice to say that dozens of companies now offer line array loudspeaker systems that are more than simple column designs.
Rather than discussing dozens of different product types, I thought we might approach the subject by defining the technological terms of line arrays. This way, we get a better grasp of the issues involved with line array systems and will be able to discern both the similarities of - and unique differences between - these now ubiquitous products.
A Little History
Line arrays have been around for over a half of a century as column loudspeakers, and other than those made by Rudy Bozak in the U.S., most were voice-range only.
Their application was generally for highly reverberant spaces, where a narrow vertical dispersion avoided exciting the reverberant field, provided a higher Q (narrower dispersion pattern) and, as a result, improved intelligibility of the spoken word.
Never losing popularity in Europe as they did in America, it’s no wonder that L-Acoustics V-DOSC line array loudspeakers from France were the first (in the mid-1990s) to show the concert sound world that more level and smoother frequency response can come from fewer drivers in a line array.
After everyone realized that for a given listening area, the drivers have no destructive interference in the horizontal plane and combine mostly inphase in the vertical plane, the race was on.
Basically, a line of sources will create a wavefront of sound pressure that is loosely cylindrical in nature at a particular range of wavelengths (frequencies).
It’s idealized shape is actually more like a section of a cake, and the wavefront surface area, as it expands only in the horizontal plane, doubles in area for every doubling of distance. This equates to a 3 dB SPL loss of level for every doubling of distance.
An idealized point source, imperfectly represented by a loudspeaker or nonlinear cluster of loudspeakers, radiates in a spherical waveform rather than cylindrical. The wavefront expands to four times the area with each doubling of distance, which equates to a 6 dB SPL loss for every doubling of distance.
This is commonly known as the inverse-square law, and it applies to all point-source radiant energy.
Hence the big advantage for a line array is that for a given number of transducers, the resulting long throw level can be much greater than for a non-line array, or point-source, loudspeaker system.
This is the term applied to the dispersion pattern, or response balloon of a line array. It simply means that when you stack a bunch of loudspeakers, the vertical dispersion angle decreases because the individual drivers are outof- phase with each other at positions off-axis in the vertical plane.
The taller the stack is, the narrower the vertical dispersion will be and the higher the sensitivity will be on-axis. In the horizontal plane, an array of like drivers will have the same polar pattern as a single driver.
Some believe that the horizontal pattern is wider than for a single driver, but they are mistaken, likely fooled by the fact that the level is louder off to the side due to the higher sensitivity of multiple drivers. However, the actual polar pattern remains the same as for a single driver.
In addition to the narrowing vertical coverage angles, the array length also determines what wavelengths will be affected by this narrowing of dispersion. The longer the array, the lower in frequency (longer in wavelength) the pattern control will occur.
There is a limit to the 3 dB per doubling loss, and it’s at this point where the array is far enough away to appear to be more of a point source and its level begins to attenuate according to the inverse-square law at 6 dB per doubling of distance. The transition between these two regions is known as the critical distance for the line array.
The region closer than critical distance, and the region beyond it, is termed as the Fresnel and Fraunhofer regions, respectively, so named by Christian Heil of L-Acoustics. Unless you’re a true math dweeb, near-field region and far-field region roll off the tongue a bit easier.
The critical distance for a given line array length varies inversely with wavelength (frequency). Shorter wavelengths (higher frequencies) have much farther critical distances than longer wavelengths (lower frequencies).
In theory, this means that at greater distances, a line array will maintain more high-frequency content than low. However, air attenuation of the highs will counteract this characteristic.
Articulated is the ten dollar term for curved. This describes the popular J-Array shape that most manufacturers currently offer.
On the other hand, the Duran Audio Intellivox system is a line array that covers from extreme near-field to far-field seating with a straight-line dead-hang approach. (Talking about articulated arrays with your clients is what gets your day rate increased and your job title changed from “sound tech” to “audio engineer.”)
This is also a term for curved arrays of a particular type. Spiral arrays describe a curve that is increasing in the rotational angle from one end to the other, just as the common J-Array does from top to bottom.
Arithmetic Spiral Arrays
Mark Ureda, when working as a consultant to JBL, mathematically determined that spiral arrays that increase their angle of curvature in even increments perform better.
For example, at the top of a line array, the splay between cabinets is 0 degrees. Going down the array, the element boxes are successively splayed at 1 degree, 2 degrees, 3 degrees, etc. Or it could go in increments of 2 degrees (i.e.: 2 degrees, 4 degrees, 6 degrees, etc.).
These are arithmetically increasing spiral arrays.
Lobes describe all the acoustical energy that emanates from a loudspeaker or group of loudspeakers. The specified coverage angle of a horn is its main lobe.
Spurious lobes are those that emanate out in a non-useful direction from the source.
Much has been made about lobe steering. Visions come to mind of front of house guys moving loudspeaker coverage around with a joystick.
Lobe steering is generally done by incrementally delaying drivers in a line array. This can only be done when the sources, (the drivers), are about 1/2 wavelength apart for a given frequency, and only in the direction of the line array’s axis. For typical live sound HF drivers with a 9-inch diameter, this means that they cannot be positioned close enough together to steer anything above 750 Hz.
However, using adaptive apertures to mimic a long line of smaller sources enables some steering at shorter wavelengths.
Side lobes are artifacts of line arrays, and actually, they emanate from the ends (not the sides) of the array - at the top and bottom, as a typical line array is viewed in use. They’re caused by the individual elements being in-phase at a particular angle and wavelength at some off-axis position from the array’s main lobe.
It’s possible to eliminate side lobes, but there are limits and consequences to side-lobe elimination in line arrays.
Gradient Side Lobes
This is a synonymous term for side lobes. Gradient describes how these lobes occur at particular angles or grades with respect to the line array’s orientation.
Professional progress terminology tip: use gradient side lobes rather than side lobes in your technospeak. Sounds more complicated, making you seem even smarter than you already are…
Another of the fundamental parameters of line arrays is the spacing between individual elements.
The accepted limit is that for good line array behavior, the sources should be no more than 1/2 wavelength apart for a given frequency. This means that loudspeakers reproducing longer wavelengths can be spaced farther apart without any deterioration in performance.
But since 1/2 wavelength at 15 kHz is just under one-half of an inch, HF devices can never be close enough. One manufacturer maintains that because of this, line arrays do not really work at very high frequencies.
However, I disagree, because even at very short wavelengths, the 3 dB loss per doubling of distance still holds true, and this is what defines the line array effect (in my humble opinion). What does result from driver spacing of more than 1/2 wavelength is more pronounced gradient side lobing.
Logarithmic Driver Spacing
Duran’s Intellivox Series line array loudspeakers, for example, employ the logarithmic driver spacing technique. This provides denser driver spacing at short wavelengths and economizes on the number of drivers needed for longer wavelengths by spacing them in larger and larger logarithmic increments.
Isophasic aperture is one of my favorite high-tech terms. It describes the phase characteristic of the slot that loads the horn bell of some line array box HF sections.
The perfect line array driver, particularly for very short wavelengths, is a ribbon driver like those used by SLS Loudspeakers. Compression drivers are more rugged and capable of higher output levels than a ribbon driver, but they do not have a linear phase signal at the mouth of a horn.
Ideally, the signal at both the top and bottom of the driver’s horn mouth would arrive in-phase with the signal at the center of the horn mouth to mimic the ribbon driver’s characteristic.
Since the center of the horn is closer to the driver’s diaphragm than the top and bottom, the more central paths to the horn from the driver must delay the signal to arrive in phase with the longer paths to the top and bottom of the horn. There are two ways to accomplish this.
The first is to make the path length progressively longer towards the center of the horn via a phase-plug type of device. This technique was employed in the old JBL “slot tweeter” super-tweeter and was adapted by Christian Heil in the V-DOSC system for wavelengths at 1,000 Hz and up. Other line array manufacturers have employed similar devices.
The other method is to use variable density foam, which slows the speed of sound through the more dense foam medium towards the center of the horn. Electro-Voice and McCauley use this technique to provide an isophasic horn section in their line array offerings.
An interesting technique for an isophasic device is the patented mid-high frequency aperture by Adamson. It employs the longer path length method, and utilizes directional vanes to prevent excess vertical dispersion as well.
This approach is used for both the high- and mid-frequency sections of their line array systems. The mid-frequency energy exits via two vertical slots on either side of the high-frequency exit slot. The paths of the mid-frequencies curve around the HF chamber housing. All slots are isophasic.
With the slots of the MF section on each side of the HF slot, diffractional problems of each slot on the other could be very problematic.
However, Brock Adamson came up with a unique solution: overlapping the crossover points between the mids and highs. This provides in-phase pressure fronts from the other slots to prevent diffractional interference in the frequency range where it would be a problem.
The term “tapering” is also commonly called “shading.” They are essentially interchangeable. One of the first tricks used to take advantage of the line array effect was frequency tapering.
My earliest exposure to this technique was the Electro-Voice LR4B column loudspeaker. For low/mids, it used 6-inch by 9-inch cone drivers that had low-pass filters at successively lower frequencies for loudspeakers placed farther out to the ends of the column. (Read more about the LR4B here.)
This resulted in a longer column at longer wavelengths and a shorter column at shorter wavelengths, producing a similar dispersion pattern and critical distance for all frequencies, which in turn provides a more balanced frequency response at all listening distances.
Another tapering/shading technique is amplitude shading. This is used in many current line array products to accomplish front fill coverage where the bottom hook of a “J-array” covers the extreme near-field listeners.
This technique is simply lowering the volume of the loudspeakers covering the nearfield seating with respect to the long-throw loudspeakers higher in the array.
Some line array systems offer more than one choice for vertical dispersion of the individual box elements in the array. This is done as a solution to cover the near-field and extreme nearfield seating in most venues.
Several years ago, EAW went one step further by offering two different models, matching the vertical dispersion and output level so that the drivers produce equal mouth SPL throughout the array. They avoid any amplitude shading for the drivers covering the closer listeners by increasing the coverage angle of those box elements.
Why is it important to avoid amplitude shading?
According to David Gunness, who was EAW director of research and development at that time (and is now a founder of Fulcrum Acoustic), whenever two wave fronts with different pressures are combined, there will be a discontinuity at the juncture of the two.
This discontinuity will be audible as though it were a separate, non-coherent source (delayed loudspeaker). The result is transient smear and uneven frequency response.
Divergence shading provides a wave front whose curvature varies, but whose pressure magnitude does not. Therefore there is no introduced time smear to the signal.
Horizontally Symmetric Arrays
The majority of available line array systems are horizontally symmetric. Ideally, each band pass is a 1/2 wavelength wide strip that runs the entire length of the array. The advantage is that it avoids horizontal lobbing at the crossover-frequency band. It also requires symmetric pairs of inner mid and outer LF drivers flanking the HF sophistic ribbon.
The drawback to this approach is that for the mid-drivers to be within 1/2 wavelength of each other, they must be incorporated into the bell of the HF horn. The normal 90-degree angle causes reflections between the MF drivers and the discontinuous horn walls cause HF problems as well.
Horizontally Asymmetric Arrays
This approach avoids the mid-frequencies in the horn bell problem and contends with the horizontal lobbing at crossover problem inherent in asymmetric designs. Choose your poison.
Cardioid & Hypercardioid LF Sections
Line arrays have great directional control in the vertical axis. Subwoofer systems, by nature of the very long wavelengths involved, do not have any directional control unless arrayed.
Even then, because of the omni-directional nature of each element in the array, there is no front-to-back directionality. This causes muddiness on stage and low-frequency feedback problems.
Enter cardioid and hypercardioid low-frequency sections.
Cardioid and hypercardioid loudspeaker systems are similar to microphones, just in reverse. In the case of loudspeakers, two transducers, separated by an exact distance within the enclosure, with delay on the rear driver, create the directional radiation pattern.
The cardioid type has maximum level cancellation straight back at 180 degrees behind it, and the hypercardioid has maximum level cancellation at about 120 degrees off-axis. As examples, Meyer employs cardioid low-frequency sections, while NEXO employs hypercardioid.
FIR-Based Vs IIR-Based DSP FIiltering
IIR (Infinite Impulse Response) filters in a DSP processor act just like analog crossover and equalization filters. Their amplitude and phase characteristics are in a fixed relationship. So much boost or cut produces an exact corresponding change to the phase response.
FIR (Finite Impulse Response) filters are able to manipulate phase independently of amplitude and correct for distance-related cancellations between drivers if each driver is under individual DSP control. Some systems employ separate DSP processing and amplification for each driver in the array.
These types of systems are one of the next big steps forward in loudspeaker technology.
So the next time you want to impress someone at the local bar, tell ‘em: “We’re gonna hang a logarithmic spaced, articulated spiral array in a horizontally asymmetric configuration employing frequency tapering and divergence shading, which will include isophasic high-frequency and mid-frequency apertures, hyper-cardioid low-frequency transducer sections, is controlled by finite-impulse response filtering digital signal processing, and works well with a psychoacoustic infector.”
You might just get lucky… "
John Murray is a 30-plus-year pro audio industry veteran, working leading companies such as for Electro-Voice, Midas, Peavey MediaMatrix and TOA. John has presented AES papers, chaired several SynAudCon workshops, and is a member of the TEF Advisory Committee and ICIA adjunct faculty.
|17th January 2013, 08:49 PM||#4|
Join Date: Nov 2009
Location: The Mountain, Framingham
I used to design the line array models for Mcintosh and wrote and AES paper on the subject. In general arrays should either be very long or very short. If they are long (approaching floor to ceiling) then you will experience very little variation at any listening height such as seated or standing positions. If they are very short then their directivity broadens and they become less critical for postion.
Mid length arrays will be more difficult and fairly directional. If you can aim them well with regard to height and always stick to a certain listening position, then you may be perfectly happy with them, but the length you are talking about won't likely be good for both sitting and standing.
One model I worked on was the Mac XRT24 with 16 domes (about 90mm center to center). Its directivity was greatly improved with a weighting sequence (also know as level tapering). I think the outer domes were rolled off about 10dB relative to the center ones. This not only broadens the response but it makes the response variation with height much more uniform.
You might look at the long thread about CBT designs. The CBT design has level and frequency tapering and gives good off axis performance.
|17th January 2013, 09:05 PM||#5|
Join Date: Nov 2003
Location: Brighton UK
The very short answer is the line array needs to be at least as long
as the lowest wavelength it reproduces, so 24" will be fine generally.
The centre point should be average ear height.
(Living with 24" ribbons x/o at 750Hz to a 8" bass taught me a lot.)
There is nothing more practical than a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
|17th January 2013, 09:06 PM||#6|
Combe Filtering in rooms Vs Anechoic chambers
Fab that you saw this thread, your line array knowledge is as good as it gets...
I have a question about combe filtering please!
In theory my BMR Line Array 112mm centre to centre driver spacing should result in nasty, audible peaks and troughs / lobing starting at 3KHz (ish).
Yet at anything over 2 meters listening position ( I listen at 4 meters) they sound amazing, seamless and even soundscape all over the room.
The 4.5 inch BMR does have broadest polar respose I have seen, but I also believe that room reflections play a huge part in " filling in the gaps" and or leveling out the peaks...
What do you think?
Thanks in advance and and all the best
|17th January 2013, 11:31 PM||#7|
Join Date: Sep 2008
Thanx for the info and advice. While waiting for replies, I scoured the net looking for opinions and line array theory....vast body of knowledge with many conflicting viewpoints but the theory seems to lead towards Jim Griffins white papers....which I've read extensively and therefore shouldn't have posed the question at all really.
I came across the thread here using the Apex Jr dome tweeters which would make a full line much more viable financially but the builders experience with the tweeters wasn't exactly positive.
A line of ribbons or planers is simply out of the question......too costly even for a DIY project for me.
My thought was also to use frequency tapering, adding an inductor for the top and bottom midwoofers in a 2.5 topology....like a wwwmmmmmmwww. In such a case, I suspect a line of tweeters the length of the M drivers would act like a conventional array and yet the .5 woofers would reduce the combing and add to baffle step. All of the dynamics of an array with the advantages of combating floor and ceiling bounce of a WMTMW. Thoughts?
|18th January 2013, 02:52 AM||#8|
Join Date: Nov 2010
I've been listening to my Revision 3 line arrays for the past 6 months
Their use is "garage speakers" or something that will work with a cement floor without sounding too harsh.
This is what they are made out of
12 Sony 5" woofers (blue ones with neo magnets and grills)
20 Aurasound 3" full range speakers 16 ohm versions
48 Apex Jr. 10mm dome tweeters
The 5" woofers are wired at 6 ohms, the 3" full ranges at 13 ohms and the Apex tweeters at 6 ohms. The woofers have an acoustic crossover point (LR 12dB/Oct) at 375Hz and the full ranges cross to the Apex Jr. tweeters (Bessel 12dB/Oct) at 6KHz.
Those Aurasound 3" full ranges are TV speakers so have a strong rising response which smoothed out nicely when arrayed 20 tall. They are driven by an old Pioneer silver faced receiver with loudness control on (+3dB at 200 Hz and +6dB below 100Hz) A mono sub amp pushes a pair of PA 15" woofers tuned to 32Hz to add the bottom end.
What I have noticed is the mids/treble does not drop off like traditional speakers with distance, at a city block away (200 meters or 660 feet) The vocals come through clearly and treble is heard. I'd say that has to do with the array effect on the treble since they "see" the "infinite line" since their frequency is so short. The near field effect last longer the higher the frequency goes so that counters the air sucking up the shorter wavelengths.
My use is a garage speaker and they are used outside for BBQs and such, I do like the treble/mid boost that they provide over longer distances. For in room listening, I do boost the tweeter lines +6dB at 10KHz due to personal preference (age) and C to C distance etc. The 15" PA woofers are crossed
at around 135Hz to bring up the bottom end.
From what I read and attempted to grasp was the treble lines don't need to be as long as the midrange lines. You could get away with having them long enough to cover the sitting down and standing up distances. There is a formula for calculating the tweeter line to have it mesh with the near field/far field switch to be even with the midrange.
My lines are 6 feet or 1.8 meters tall. The boxes themselves are 6 feet 3.5 inches or 1.9 meters tall so they would fit in a pickup truck with the tail gate down. The bass bins they now stand on are 19.5 inches (49.5 CM) so the entire mess stands 7 feet 11 inches (241 CM) so it will fit in a "normal room" ceiling height (barely)
They do have a unique and very "large" soundstage to them. It works perfectly standing on the bass bins in a cement floored garage. The ceiling height is 10 feet (3.3 meters) in the garage and I don't get any floor/ceiling bounce when listening 3 to 5 meters away. Since it is a line array, I don't get blasted if I walk to close to them and my friends that have had a few too many won't be deaf when they put their ears on them
The best thing I like is I will never build another 3-way column vertical line array for the rest of my life! Getting 160 speaker drivers to work together to create a good stereo image sucks time faster than a black hole eating stars.
Very educational project, I'm not sure if I learned more about audio or woodworking--either way it was a valuable gain in knowledge.
A side benefit besides drunk people counting the tweeters, is they do look really cool I would strongly recommend them for use in garages, outdoors and in really bad acoustic areas where their directional nature and pattern is a good solution.
They don't have the best sound quality I've ever heard--but they do sound great in a cement floored garage with metal and stuff stacked up. No remorse on the drivers for the price I paid--the crossovers cost more than the speaker drivers. I used poly caps to withstand the temp swings and to last 30 years. Figure by the time the caps dry out...I'll be either deaf, dead or crazy...
|18th January 2013, 11:22 AM||#9|
Join Date: Sep 2008
Sounds like a nice project and from what I gather, they turned out well. Congrats!
There seems to be a consensus on the use of midwoofers with a rising response which when arrayed smooth out?......is this due to cancellations of higher frequencies where the 1/2 wavelength begins to exceed the individual drivers center to center spacing? The midwoofer I planned on using was the Vifa TC9 which is incredibly smooth and flat. Makes me wonder if its a poor choice.
When looking at commercial arrays, most systems use a horizontal M-T-M arrangement for each module. Is there an advantage to the double row of woofers and a central line of tweeters?
|18th January 2013, 02:07 PM||#10|
Join Date: Oct 2003
Location: North Georgia
A central line of tweeters would result in horizontal polar symmetry, possibly benefiting from Joe D's 3rd order MTM XO. A taller line of (more) tweeters would help support a lower XO so you have a neat balancing act.
Maybe build some quick & dirty test mules before cutting good material. If you don't want to do test mules, consider going with a full line of tweeters and use cap shunts on the outer tweeters to balance vertical directivity with the .5 mids.
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