Line Array Ideas

Thoughts and Ideas behind the design of Line

Since setting up this web site I’ve had requests for information about designing line arrThis page is to explain some of my thoughts on the topic. If you still have any questions further thoughts and issues feel free to contact me. The idea of this page is to give somon what I have been asked. If you don’t agree that is fine by me. Please remember ttheoretical text and that I've tried to keep the maths to a minimum. The concepts are mothan accurate formula.

Before starting please be aware that any images, diagrams and quotations taken from masites belong to the respective companies and I have used them purely for educainstructional and illustration only. For the sake of ease of layout I’ll credit the guilty parti

The basic question is whether it is possible to build a line array. In general terms, yes, ahave sufficient knowledge and ability plus the willingness to put in some time, effort anda line array is a distinct possibility. Whether any particular individual who emails me is caa line array I simply don’t know. Don’t ask me questions about rigging or suspending loudsdon’t think that it is something no-one should attempt, as a safety point if you need to asfully understand the issues involved and are best sticking to floor stacked speakers.

A lot of hype surrounding commercially produced line arrays is simply that. The numbethe manufactures is to make money. To this end they use a lot of smoke and mirrormarketing and pseudo- science to try and convince us that their product as the edge.

Most of the science seems to be directed at coupling the high frequency horns to prodsources” or “isophasic wave fronts”. This is usually accomplished by some patented, prOne reason for this maybe because if the low frequency section comprises of a bunch ofdrive units stacked vertically it is difficult to claim any originality or advantage over the narray with similar drive units stacked exactly the same. Hidden behind the horns and sclaim all manner of great attributes for you device.

Whilst some manufactures claim to use revolutionary new technology others just use mhorn designs stacked up one above the other. That all these systems work as line arrays who prefer each system above another suggests that producing some new patentedwaveguide is not an absolute prerequisite of building a line array.

If you look at line arrays they are almost universally hung vertically. They would work in manner if orientated horizontally or any angle between. Rivers and the ocean shore natural line arrays in a horizontal format. The reason that line arrays are hung vertically a horizontal world with ears on the sides of our head. This makes us more sensitive to vasuch as lobing caused by interference between drive units, in the horizontal plain. loudspeaker stack vertically the lobing is less noticeable than using conventional horizonta

Given the above I think that the best way to design a line array is to forget the line and cohorizontal directivity of each individual cabinet. Using better quality components will obusing a design that maintains an even and smooth (if not constant) directivity is the key to

The previous page, where I outlined my design, explains how I went about trying to aclook at some of the commercial designs you will see a similar goals. The original mode

Página 1 de 24Thoughts on line arrays

12/08/2005http://www.burton-manor.co.uk/Audio/LAthoughts.htm

Vdosc uses two 15” drive units in a cabinet 1.3m wide. If the distance between acoustic cbe 0.75m and the half wavelength rule is applied, the cross over frequency should be 2cross over frequency is 200Hz. Similarly the crossover frequency between the mid andsections occurs where the half wavelength is similar to the dimension of the mid range the smaller two way cabinets some compromise has to be reached and the Nexo geo, whicdriver crosses over at 1.8KHz a frequency whose wavelength corresponds approximately wof the bass drive unit. I suspect that 1.8KHZ was the lowest frequency the manufacturerensure reliability. Interestingly the smaller dV-DOSc loudspeaker uses a lower cross over fthan its bigger brother possibly because it uses a slightly larger cones forthe bass/mid. well the need to pay attention to cross over frequencies and horizontal dispersion.

Given that an array consists of multiple elements it is worth looking at how multiple soufront. The following gif gives the general idea.

The diagram shows a stack oDOSc cabinets. The sm(relative to wavelenradiating area of each frequeband can clearly be seen.



The following diagrams show calculated interference patterns from various sources.consider the sound sources as arrayed vertically so that you are looking from the side, othat you are looking from above.

This diagrshows 4 posources radiating atlow frequenwhere twavelength large compared with tspacing . Twave appeto be comfrom a sinsource



As tfrequency increase twave frostarts flatten. athe darkenedges indicates tthe radiatis becomdirectional.

As tfrequency continues increase tformation lobes and nspots beginappear.



Increasing tfrequency again tnumber lobes aincreases.

More of tsame.



This diagruses the safrequency the previoone but hethe spacbetween tsources been reducwith correspondireduction the numand intensof the slobes.

The white lhere represents perfect lsource. At tfrequency shown the lis jbeginning show directivity.



Even a perfline souproduces lobes.

That worse as tfrequency increases.



This diagrrepresents two loudspeakerplaced clotogether (maybe withorn betwethem?). In tdiagram tfrequency low enoufor them act as a sinsource.

As tfrequency rises tdreaded interferencepatterns stto appear.



And start give a seof deja vu.

This diagrshows tequivalent two loudspeakerangled at 9similar to tarrangemenwith the DOSc. Hethe frequenis low but sshowing siof directivas the wadarken alothe line of tloudspeaker

As tfrequency increases t



radiation continues be smoowith a slidarkening (decrease output) awfrom wwould be tmain axis.

If the opend of tdrive unrepresents tmouth of thigh frequency wave guidecan be sethat tmethod produces linterferencepatterns ththe physicaseparated speakers shown abov

At even higfrequenciesthe directivof tindividual drive unstarts to shwith a lanull spot the main a(45° down).



If you want to mess with interference patterns go here, but I'd finish reading this first anlater

What the above diagrams show is that it is important to pay attention to the layointeraction between drive units. It also shows that even an ideal line array is not perreducing the side lobes of a line array is to use a tapered array. This is where the powerof the array is tapered off as you move out from the centre. The more modern name foIntensity shading is similar to the principle mentioned above where the intensity or poelements is varied. Frequency shading is the same thing only it is frequency dependenvaries the length of the line according to frequency. Angular or divergent shading means less elements at a certain point by either tightly packing the cabinets or angling them furtthe relative intensity. This last technique is employed in the classic J shaped arrays.

Tapering or shading is nothing new and was described back in the 1950s the image below ssource was effectively made shorter at high frequencies by using wedges of fibreglass in funits.



The term isophasic comes up quite often with descriptions of line arrays the following from Acoustical engineering. Back in the 1950s the aim seems to be that of increasing theisophase or phase contour rather than making it flat. What it illustrates is that all loudspproduce isophases.

Another acoustic device that has been rediscovered is the lens. The small image is ...manufacturer describe it.

The SERPIS™ is a D.A.S. designed plane-wave adaptor which provides accurate high frequethe generation of a flat, isophasic wave front. The complex design of the SERPIS™ adaptdestructive interference associated with the high frequency sections of traditional mulThe result is improved over all sonic quality while maximizing the use of input power.



Sound waves on the scale of loudspeaker cabinets don't behave like the rays drawn on thetracing can be useful to illustrate a point. Lenses do work and used to be a common siHopefully the similarity between the plane-wave adaptor and the lens can be seen. The laold PA systems were supposed to have major sonic problems and inferior to the constantthat replaced them. No doubt the original short comings have been overcome.

If all the principles behind modern line arrays appear to originate from the 1950s then thedispel that notion. It is a quote from "Text Book on Sound" first published in 1908 and Rayleigh

The above is better explained by a diagram.

Definition

The locus of all points just reached by a wave disturbanceany instant is called the wave front at the instant in questio

Huyghens' Principle

The wave front at any instant may be derived as the envelof wavelets whose origins are all the points constituting wave front which existed t seconds previously. In an isotromedium at rest these wavelets are spherical and of radiuswhere v is the velocity of propagation of the waves indirections in the given medium.



Using his principle Huyghens (sometimes Huygens) was able to explain both reflection anmain deficit of the principle is that it fails to explain the directionality of the wave. If thein all directions the wave should also converge back to the origin. Subsequently, August1827) elaborated on Huyghens' Principle by stating that the amplitude of the wave atequals the superposition of the amplitudes of all the secondary wavelets at that point (awavelets have the same frequency as the original wave). Fresnel didn't actually resolve th"backward" propagation of waves, but was able to account for diffraction. Fresnel anprominent in the work done by Heil on the V-DOSc. The diagram below hopefully demonsand why slot apertures feature in line arrays.

If we now look at the polar plots for a couple of radial horns, we can see how the aboveThe diagrams are taken from Olson's Acoustical Engineering, and the first shows a 60chosen because the polar plots are given in relation to the radius and with a 60° horn mouth is equal to the radius.

A small opening or narrow slot acts a point source in the dimension thasmall compared to the wavelength.

When the opening is approximately same as 1 wavelength the sopropagates mainly in a forwdirection

Where the opening becomes lacompared to 1 wavelength the sodiffracts around the edge of the slot



Where the radius and hence mouth dimension is small relative to the wavelength therecontrol. As the wavelength becomes comparable with the mouth width the dispersion With an increasing frequency and shortening wavelength the radiation pattern approximhorn walls.

With a 120° horn the width of the mouth is approximately double and the narrowing of dstarts at the half wavelength point.

The graphs are for a curved mouth radial horn, but it does show the effect that the flare size has on the radiation pattern. Also interesting to note is that even with a single horinterference patterns show up on the diagrams.

It is now worth looking at a few more methods deployed by some of the manufacturers in

First of all there is the waveguide used in the grandfather of the modern line arrays. The fshows the use of a sliced cone to produce a constant path length from the drive unit



diagram underneath, criteria No 1 basically states that if you have a load of slot (simildrawing in the diagram above where d>λ) sound sources, they behave as a continuous sougaps in between the slots is less than 20% of the total area.

Within the physical constraints of their size and operating parameters it would be intereone of the DOSC waveguide elements with the more traditional slot tweeters two of below.



Instead of using obstacles to force the sound waves round, some manufacturers use reflection; most notably Nexo. The diagram below is taken from their web site.

Outline use a similar technique with their butterfly array.

Despite its small size, an array of GS830s can produce a remarkable punWhether the parabolic reflector woas described, the cabinet has a greputation.



The pictures look great and the concept seems like a neat idea except that as I mentiondoesn't work like light in the scales that we are dealing with. Ray tracing is useful to give but it is worth remembering that the wavelength of visible light ranges from 400 to 700 nalongest wavelength that is 0.0007mm. If you consider the diameter of a small torch, say 1is 17,142 wavelengths (hope I’ve got all my decimal places correct). To give a similar audio wave whose wavelength is 0.172m, the reflector would need to be 2948 m in dikilometres! To find out how light behaves with openings and obstructions that are comwavelength of light do a search for Young’s slits or Newton’s rings. If you do search, ythere are interference patterns similar to the comb filtering we get when arraying lotracing, as shown in the diagrams above, predicts that the sound level outside the beam isthat doesn’t happen in real life. If the reflector technique works, why has no one mentioto folded, or W, horns?

The JBL Vertec looks suspiciously like the V-DOSc.



The high frequency horn is more conventional but fits three drive units in one cabinet kdriver spacing to a minimum.

While most of the other manufacturers go to great lengths to explain how the wave front produces is perfectly flat, JBL have acknowledged that in some instances a curved wavbetter results and have included spacers between the drive unit and horn to achieve this.



The Apogee cabinet shown below doesn't appear to use any special lens, reflector or otherdevice in its line array loudspeaker.

Martin produce one of the only horn loaded line arrays. As well as being horn loaded it useasymmetrical layout.

I would have expected this to produce some rather skewed polar plots at the cross over



polar plots, however, look quite smooth.

Unfortunately the plots are taken at octave intervals and the cross over frequencies fall shown. Another way to smooth the results (I'm not suggesting that Martin may have measure each pass band separately so that there is no interaction between each section. for those wanting to build their own line array is that from the photographs and drawings to be the normal constant directivity type. The main problem with using the horn basedeven with a minimum stack of four cabinets, each cabinet needs to be large to give ansize at the lower frequencies.

I mentioned the Nexo Geo wavesource above. The Geo-T despite its unusual look is balayout as the dV DOSc, with the high frequency section between two 8" bass units.



The biggest difference is not the shape, but that it uses an additional two rear facing drivthe directivity. The principle behind this technique is quite simple. The drive units are plof a wavelength apart. The front drive unit is then delayed to effectively re align the dthe sound adds together in phase. For a sound wave radiating backwards the effective quarter wavelength physical separation plus an additional quarter wavelength delay. Thesource 1/2 wavelength or 180° out of phase from that of the rear drive unit and whecancel out. The effect is usable over a range of one octave centred on the frequency chos

This diagrashows how usitwo sousources aadjusting tphase of orelative to tother can used to contthe radiatipattern. changing tphase betwethe two the nspots can steered.



Another design that uses additional drive units to modify the directivity is the Meyer

Using four 15" drive units with a large gap between the forward facing ones to accommodloudspeaker appears to defy all the principles that go to make a good line array. Closer ithat all four 15" drive units only operate up to 140Hz. The half wavelength for this frequenis about the width of the cabinet. Above 140Hz just one 15" drive unit radiates up to 500Htakes over. The distance between the centre line of the horn and centre of the 15" dabout the same as one half wavelength at the crossover point. Based on the principle menthe bass/mid cross over frequency of 140 Hz, the directivity control provided by the reawill be effective from 70Hz to 140Hz (the octave below the cross over frequency. Takipoint of about 105 Hz the quarter wavelength is about 80cm. This should set the front tothe cabinet. The actual measurement is 77cm.



The pictures above show the Renkus Heinz line array cabinet. The slot, or Isophasic Planeexits straight on to the front baffle without the additional flare used by other manufactugive a very wide dispersion pattern and allows the 10" drive units to be placed closer togthe close spacing baffles are used to improve dispersion. To quote the literature

The PN102/LA and PNX1102LA's unique diffractor baffle provides mid range diffraceliminates mid range narrowing of the horizontal dispersion to provide consistent widacross the entire frequency range.

I think that about covers everything that I have been asked. Hopefully it will be of some forget to experiment with the interference patterns.



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Line Array Ideas