Line Array Design

7/27/2019 Line Array Design

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houghts on line arrays

Thoughts and Ideas behind the design of Line arrays.

ce setting up this web site Ive had requests for information about designing line array loudspeakers. This page is

plain some of my thoughts on the topic. If you still have any questions or want to discus further thoughts and issues

l free to contact me. The idea of this page is to give some of my thoughts on what I have been asked. If you dont

ee that is fine by me. Please remember that this is not a theoretical text and that I've tried to keep the maths to

nimum. The concepts are more rule of thumb than accurate formula.

fore starting please be aware that any images, diagrams and quotations taken from manufacturers web sites belon

respective companies and I have used them purely for educational (I hope), instructional and illustration only. Fo

e of ease of layout Ill credit the guilty parties at the end.

e basic question is whether it is possible to build a line array. In general terms, yes, assuming that you have suffic

owledge and ability plus the willingness to put in some time, effort and money, building a line array is a distinct

ssibility. Whether any particular individual who emails me is capable of building a line array I simply dont know. D

me questions about rigging or suspending loudspeakers. Whilst I dont think that it is something no-one should

empt, as a safety point if you need to ask then you dont fully understand the issues involved and are best stickin

or stacked speakers.

ot of hype surrounding commercially produced line arrays is simply that. The number one priority for the manufac

o make money. To this end they use a lot of smoke and mirrors in the form of marketing and pseudo- science to tr

d convince us that their product as the edge.

st of the science seems to be directed at coupling the high frequency horns to produce virtual line sources or

ophasic wave fronts. This is usually accomplished by some patented, proprietary device. One reason for this may

cause if the low frequency section comprises of a bunch of 8, 10 .. 15 drive units stacked vertically it is diffi

claim any originality or advantage over the next persons line array with similar drive units stacked exactly the sam

den behind the horns and slots it is easy to claim all manner of great attributes for you device.

ilst some manufactures claim to use revolutionary new technology others just use more conventional horn designscked up one above the other. That all these systems work as line arrays and all have fans who prefer each system

ove another suggests that producing some new patented high frequency waveguide is not an absolute prerequisite

lding a line array.

ou look at line arrays they are almost universally hung vertically. They would work in exactly the same manner if

entated horizontally or any angle between. Rivers and the ocean shore are examples of natural line arrays in a

rizontal format. The reason that line arrays are hung vertically is that we live in a horizontal world with ears on th

es of our head. This makes us more sensitive to variations in sound, such as lobing caused by interference betwee

ve units, in the horizontal plain. By configuring a loudspeaker stack vertically the lobing is less noticeable than usi

nventional horizontal arrays.

en the above I think that the best way to design a line array is to forget the line and concentrate on the horizontaectivity of each individual cabinet. Using better quality components will obviously help, but using a design that

intains an even and smooth (if not constant) directivity is the key to a good design.

e previous page, where I outlined my design, explains how I went about trying to achieve this. If you look at some

commercial designs you will see a similar goals. The original modern line array the Vdosc uses two 15 drive unit

binet 1.3m wide. If the distance between acoustic centres is taken to be 0.75m and the half wavelength rule is ap

cross over frequency should be 226Hz. The actual cross over frequency is 200Hz. Similarly the crossover frequenc

tween the mid and high frequency sections occurs where the half wavelength is similar to the dimension of the mid

ge drive units. With the smaller two way cabinets some compromise has to be reached and the Nexo geo, which u

8 bass driver crosses over at 1.8KHz a frequency whose wavelength corresponds approximately with the diamete

le:///E|/clients/hi tech/data/web site infmn/Technical papers/sound/Thoughts on Line Arrays.htm (1 of 23)8/22/2006 5:40:50 AM


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bass drive unit. I suspect that 1.8KHZ was the lowest frequency the manufacturers felt was safe to ensure reliabi

erestingly the smaller dV-DOSc loudspeaker uses a lower cross over frequency (800Hz) than its bigger brother poss

cause it uses a slightly larger cones forthe bass/mid. It does illustrate well the need to pay attention to cross over

quencies and horizontal dispersion.

The diagram shows a stack of V DOSc

cabinets. The small (relative to

wavelength) radiating area of each

frequency band can clearly be seen.

en that an array consists of multiple elements it is worth looking at how multiple sources form a wave front. The

lowing gif gives the general idea.



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e following diagrams show calculated interference patterns from various sources. You can either consider the sou

rces as arrayed vertically so that you are looking from the side, or horizontally so that you are looking from above

This diagram

shows 4 point

sources

radiating at a

low frequency

where the

wavelength is

large compared

with the

spacing . The

wave appearsto be coming

from a single

source



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As the

frequency

increase thewave front

starts to

flatten. and the

darkening

edges indicates

that the

radiation is

becoming

directional.

As the

frequency

continues to

increase the

formation of

lobes and null

spots begin to

appear.



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Increasing the

frequency again

the number of

lobes also

increases.

More of the

same.



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This diagram

uses the same

frequency as

the previous

one but herethe spacing

between the

sources has

been reduced

with a

corresponding

reduction in the

number and

intensity of the

side lobes.

The white line

here represents

a perfect line

source. At the

frequency

shown the line

is just

beginning to

show directivity.



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Even a perfect

line source

produces lobes.

That get worse

as the

frequency

increases.



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This diagram

represents two

loudspeakersplaced close

together

(maybe with a

horn between

them?). In this

diagram the

frequency is

low enough for

them to act as

a single source.

As the

frequency rises

the dreaded

interference

patterns start

to appear.



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And start to

give a sense of

deja vu.

This diagram

shows theequivalent of

two

loudspeakers

angled at 90

similar to the

arrangement

with the V

DOSc. Here the

frequency is

low but still

showing signs

of directivity asthe waves

darken along

the line of the

loudspeakers.



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As the

frequency

increases the

radiation

continues to be

smooth with a

slight darkening

(decrease in

output) away

from whatwould be the

main axis.

If the open end

of the drive

units represents

the mouth of

the high

frequency wave

guide, it can be

seen that this

methodproduces less

interference

patterns than

the physically

separated

speakers as

shown above.

At even higher

frequencies the

directivity of

the individual

drive units

starts to show

with a large

null spot on themain axis (45

down).



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ou want to mess with interference patterns go here, but I'd finish reading this first and I'll remind you later

at the above diagrams show is that it is important to pay attention to the layout, coupling and interaction betwee

ve units. It also shows that even an ideal line array is not perfect One way of reducing the side lobes of a line array

use a tapered array. This is where the power to each element of the array is tapered off as you move out from the

ntre. The more modern name for this is shading. Intensity shading is similar to the principle mentioned above whe

intensity or power to individual elements is varied. Frequency shading is the same thing only it is frequencypendent and effectively varies the length of the line according to frequency. Angular or divergent shading means

nting more or less elements at a certain point by either tightly packing the cabinets or angling them further apart

y the relative intensity. This last technique is employed in the classic J shaped arrays.

pering or shading is nothing new and was described back in the 1950s the image below shows how a line-source wa

ectively made shorter at high frequencies by using wedges of fibreglass in front of the drive units.

e term isophasic comes up quite often with descriptions of line arrays the following diagram is taken from Acoustic

gineering. Back in the 1950s the aim seems to be that of increasing the curvature of the isophase or phase contour

her than making it flat. What it illustrates is that all loudspeaker drive units produce isophases.

other acoustic device that has been rediscovered is the lens. The small image is .... well I'll let the manufacturerscribe it.

e SERPIS is a D.A.S. designed plane-wave adaptor which provides accurate high frequency summing and the

neration of a flat, isophasic wave front. The complex design of the SERPIS adaptor eliminates the destructive

erference associated with the high frequency sections of traditional multi-box clusters. The result is improved

sonic quality while maximizing the use of input power.

http://www.burton-manor.co.uk/Audio/Test%20Applet/Ripple.htmhttp://www.burton-manor.co.uk/Audio/Test%20Applet/Ripple.htm


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und waves on the scale of loudspeaker cabinets don't behave like the rays drawn on the diagram, but ray tracing c

eful to illustrate a point. Lenses do work and used to be a common site on PAs of old. Hopefully the similarity betw plane-wave adaptor and the lens can be seen. The large lenses on the old PA systems were supposed to have maj

ic problems and inferior to the constant directivity horns that replaced them. No doubt the original short comings

ve been overcome.

ll the principles behind modern line arrays appear to originate from the 1950s then the following should dispel tha

tion. It is a quote from "Text Book on Sound" first published in 1908 and written by Baron Rayleigh

Definition

The locus of all points just reached by a wave disturbance at any

instant is called the wave front at the instant in question

Huyghens' Principle

The wave front at any instant may be derived as the envelope ofwavelets whose origins are all the points constituting the wave front

which existed t seconds previously. In an isotropic medium at rest these

wavelets are spherical and of radius v t, where vis the velocity ofpropagation of the waves in all directions in the given medium.

The above is better explained by a diagram.



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ng his principle Huyghens (sometimes Huygens) was able to explain both reflection and refraction. The main defic

principle is that it fails to explain the directionality of the wave. If the wavelets expand in all directions the wav

uld also converge back to the origin. Subsequently, Augustin Fresnel (1788-1827) elaborated on Huyghens' Principl

ting that the amplitude of the wave at any given point equals the superposition of the amplitudes of all the secon

velets at that point (assuming that the wavelets have the same frequency as the original wave). Fresnel didn't act

olve the question about "backward" propagation of waves, but was able to account for diffraction. Fresnel and

fraction are prominent in the work done by Heil on the V-DOSc. The diagram below hopefully demonstrates diffrac

d why slot apertures feature in line arrays.

A small opening or narrow slot acts like a

point source in the dimension that is small

compared to the wavelength.

When the opening is approximately the same

as 1 wavelength the sound propagates mainly

in a forward direction

Where the opening becomes large compared

to 1 wavelength the sound diffracts around

the edge of the slot.

we now look at the polar plots for a couple of radial horns, we can see how the above comes into play. The diagram

taken from Olson's Acoustical Engineering, and the first shows a 60 horn. This was chosen because the polar plo

en in relation to the radius and with a 60 horn the width of the mouth is equal to the radius.



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ere the radius and hence mouth dimension is small relative to the wavelength there is no directional control. As t

velength becomes comparable with the mouth width the dispersion pattern narrows. With an increasing frequency

rtening wavelength the radiation pattern approximates that of the horn walls.

th a 120 horn the width of the mouth is approximately double and the narrowing of dispersion pattern starts at th

f wavelength point.

e graphs are for a curved mouth radial horn, but it does show the effect that the flare angle and mouth size has oniation pattern. Also interesting to note is that even with a single horn and drive unit, interference patterns show u

diagrams.

s now worth looking at a few more methods deployed by some of the manufacturers in their line arrays.

st of all there is the waveguide used in the grandfather of the modern line arrays. The following diagram shows th

a sliced cone to produce a constant path length from the drive unit to the slot. The diagram underneath, criteria

ically states that if you have a load of slot (similar to the bottom drawing in the diagram above where d>) sound



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rces, they behave as a continuous source as long as the gaps in between the slots is less than 20% of the total area

thin the physical constraints of their size and operating parameters it would be interesting to compare one of the

veguide elements with the more traditional slot tweeters two of which are shown below.

tead of using obstacles to force the sound waves round, some manufacturers use the principle of reflection; most

ably Nexo. The diagram below is taken from their web site.



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Despite its small size, an array of Geo S830s

can produce a remarkable punch. Whether

the parabolic reflector works as described,the cabinet has a good reputation.

tline use a similar technique with their butterfly array.



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e pictures look great and the concept seems like a neat idea except that as I mentioned before sound doesn't work

ht in the scales that we are dealing with. Ray tracing is useful to give a general outline but it is worth rememberi

t the wavelength of visible light ranges from 400 to 700 nanometres. At the longest wavelength that is 0.0007mm

u consider the diameter of a small torch, say 12mm across -that is 17,142 wavelengths (hope Ive got all my decim

ces correct). To give a similar ratio for a 2KHz audio wave whose wavelength is 0.172m, the reflector would need

2948 m in diameter; nearly 3 kilometres! To find out how light behaves with openings and obstructions that are

mparable with the wavelength of light do a search for Youngs slits or Newtons rings. If you do search, you will fin

t there are interference patterns similar to the comb filtering we get when arraying loudspeakers. Ray tracing, as

wn in the diagrams above, predicts that the sound level outside the beam is zero, something that doesnt happen

l life. If the reflector technique works, why has no one mentioned it in relation to folded, or W, horns?

e JBL Vertec looks suspiciously like the V-DOSc.



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e high frequency horn is more conventional but fits three drive units in one cabinet keeping the inter driver spacin

minimum.

ile most of the other manufacturers go to great lengths to explain how the wave front that their device produces

rfectly flat, JBL have acknowledged that in some instances a curved wave front produces better results and have

luded spacers between the drive unit and horn to achieve this.



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e Apogee cabinet shown below doesn't appear to use any special lens, reflector or other wave sculpting device in i

e array loudspeaker.

rtin produce one of the only horn loaded line arrays. As well as being horn loaded it uses an asymmetrical layout.

ould have expected this to produce some rather skewed polar plots at the cross over frequencies. The polar plots,

wever, look quite smooth.



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fortunately the plots are taken at octave intervals and the cross over frequencies fall in between those shown. An

y to smooth the results (I'm not suggesting that Martin may have done this) is to measure each pass band separate

t there is no interaction between each section. More importantly for those wanting to build their own line array is

m the photographs and drawings the horns appear to be the normal constant directivity type. The main problem w

ng the horn based approach is that even with a minimum stack of four cabinets, each cabinet needs to be large to

effective mouth size at the lower frequencies.

entioned the Nexo Geo wavesource above. The Geo-T despite its unusual look is basically the same layout as the d

Sc, with the high frequency section between two 8" bass units.



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e biggest difference is not the shape, but that it uses an additional two rear facing drive units to control the

ectivity. The principle behind this technique is quite simple. The drive units are placed one quarter of a waveleng

art. The front drive unit is then delayed to effectively re align the drive units so that the sound adds together in p

a sound wave radiating backwards the effective separation is the quarter wavelength physical separation plus an

ditional quarter wavelength delay. The result is a sound source 1/2 wavelength or 180 out of phase from that of t

r drive unit and when combined they cancel out. The effect is usable over a range of one octave centred on the

quency chosen.

This diagram

shows how using

two sound sources

and adjusting the

phase of one

relative to the



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other can be used

to control the

radiation pattern.

By changing the

phase between

the two the null

spots can be

steered.

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

ng four 15" drive units with a large gap between the forward facing ones to accommodate the horn this loudspeakpears to defy all the principles that go to make a good line array. Closer inspection reveals that all four 15" drive u

y operate up to 140Hz. The half wavelength for this frequency is 1.2m which is about the width of the cabinet. Ab

0Hz just one 15" drive unit radiates up to 500Hz where the horn takes over. The distance between the centre line

horn and centre of the 15" drive unit is again about the same as one half wavelength at the crossover point. Based

principle mentioned above and the bass/mid cross over frequency of 140 Hz, the directivity control provided by t

r facing speakers will be effective from 70Hz to 140Hz (the octave below the cross over frequency. Taking the a

fway point of about 105 Hz the quarter wavelength is about 80cm. This should set the front to back distance of the

binet. The actual measurement is 77cm.



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e pictures above show the Renkus Heinz line array cabinet. The slot, or Isophasic Plane Wave Generator exits straito the front baffle without the additional flare used by other manufacturers. This should give a very wide dispersi

tern and allows the 10" drive units to be placed closer together. Even with the close spacing baffles are used to

prove dispersion. To quote the literature

e PN102/LA and PNX1102LA's unique diffractor baffle provides mid range diffraction loading . It eliminates mid ra

rrowing of the horizontal dispersion to provide consistent wide angle coverage across the entire frequency range.

ink that about covers everything that I have been asked. Hopefully it will be of some use. Finally don't forget to

periment with the interference patterns.

Diagrams are from

Acoustical Engineering Harry Olson

L'Acoustics

DAS

JBL

Outline

Martin Audio

Nexo

Renkus Heinz Meyer

Apogee

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Line Array Design

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