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MADE IN FRANCE

FLAX SANDWICHNEW DIAPHRAGM

Patented

n1350

116

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THE SEARCH FOR THE HOLY GRAILOF DIAPHRAGM TECHNOLOGIES

More than 30 years of FocaltraditionThe drive unit is at the very heartof Focals work. For more than

30 years we have been exploringthe possibilities offered byrevolutionary new materialsin order to develop the idealloudspeaker diaphragm. Thelist is long: Polyglass, "K2"sandwich, "W" sandwich, Kevlar,titanium, Tioxid, beryllium,aluminium-magnesium.

The diaphragm is the criticalinterface for the transducer; itis what sets the air moleculesin motion, and the fidelity of

the transfer depends on theintrinsic qualities of the materialused. Ideally, a diaphragmshould satisfy three key butcontradictory criteria:

it must be light so that it will

accelerate quickly.

it must be rigid so that itfunctions like a piston,

it must be well damped so thatit does not colour the sound.

Across our whole Home, Carand Pro ranges we currentlyuse two different diaphragmtechnologies for mid-woofers:Polyglass for the less costlyranges, "W" and Kevlarsandwich, with their outstandingqualities, for our top of the rangeproducts. (The "K2" sandwich isderived from and very similar

to the "W" sandwich). Becausethe W sandwich has to bemanufactured by hand, its rangeof application remains limitedfor reasons of cost.

Glass sandwich for rigidityIn the course of the last fewyears, many innovations

have appeared as a resultof engineers revisiting

natural materials andcombining them withmodern composites,thus creating new typesof hybrid material.There are very fewsolutions using a singlehomogeneous materialthat satisfy the three key

criteria set out above.Solutions using a mixture

of materials offer a muchwider range of possibilities.

Sandwich construction usingthin skins of fiberglass, with

which we are totally familiarthanks to the "W" sandwich,

remains the supreme methodof controlling rigidity.

What material for the core?

The core of a sandwich materialprovides the mechanicaldamping, which is responsiblefor the sonic neutrality of thediaphragm. We wanted to makeuse of non-woven fibers whichcould be formed into a sandwichcone by a stamping process inorder to automate the formingprocess. This would makesandwich construction, whichhas hitherto been reserved fortop of the range applications,

available to a wider range ofproducts. With the "W" sandwichthe only way to form the core inPlexiglas foam and to apply thefiberglass skin is by hand.

Flax very quickly became aserious candidate. France isthe main cultivator in Europe offiber flax, principally in Flanders,Picardy, Normandy and the Pas-de-Calais region. French flax isconsidered the best in the world.

Flax is one of the oldest textilefiber, which has been cultivatedsince the 4th Century BCand which was used by theEgyptians as a protectivecloth for mummies. A bundle(called technical fiber inspinning) is composed of tento forty fibers. Each fiber is asingle, very elongated cell, 6 to10 centimetres in length andwith a diameter of 7 to 40 m,composed of 70 to 80 per centcellulose.

DIAPHRAGM TECHNOLOGIES

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The unique qualities of flaxComposite materials haveproperties suitable forapplications where bothrigidity and low mass arerequired. However, their

disadvantage is that they lackdamping, which leads to verypoor control of vibrations.Often, as in the case of sportsequipment, it is necessary toapply damping materials toabsorb the intrinsic vibration ofthe material. It is clear that thisis a particularly critical issuefor loudspeaker diaphragms,one which requires us toensure that there is sufficientinternal damping within thestructure of the material.

Recent developments in thisfield combining compositesand natural fibers have broughtunexpected opportunities.Flax is twice as light as

fiberglass, because the fiber

is hollow.It also has very lowelasticity which makes it ideal

to increase the flexural rigidityof our sandwich. Moreover, itcan be obtained in non-wovenbundles of considerablediameter and it can thereforeproduce the thickness requiredfor a sandwich. Syntheticfibers or foams originatingfrom petrochemical resourcesare subject to inflation linkedto the sharp rise in crude oil

prices. Organic or ecologicaltextiles, like flax, are thusbecoming more and more price-competitive. Flax combines thethree key factors required ofthe core of a sandwich materialfor loudspeakers: it has lowdensity, a high tensile (Youngs)modulus of elasticity of up to60GPa, and excellent internaldamping.

PS 165FX (high end powerfull new Car Audio 2 way component).

Flax offers unique qualities: light because it is hollow and very low elasticity. It constitutes a choice suitable for the manufacture of the core of a sandwich structure.

lignin (wood)

hollow centre (void)

fibre bundle

fibrous cell

epidermis

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France is the largest producer of flax in Europe. French flax fiber isconsidered one of the worlds best. Cropped in Flanders, Picardy andNormandy it enjoys very favorable climatic conditions.

Performance comparisonof various solutionsMany prototypes have beendeveloped by Focal in thecourse of the last two yearsusing diaphragms made fromsingle materials (includingmetals and thermoplastics),composites (with thermosetting,thermoplastic and acrylicmatrices) and even in coatedpaper. We very quickly restrictedour choice to composite/sandwich structures: single-skin structures with a pre-impregnated glass fiberface and a rear woven ornon-woven linen face, andsandwich structures withinternal and external skins inpre-impregnated glass fiberon a core of braided or non-braided flax, with straight orsemi-exponential cone profiles.A measurement report wasdeveloped internally to make

our research less empirical.At each stage test sampleswere produced to assess themechanical performance ofthe new structures. To providereference data, test samples ofPolyglass and "W" sandwich indifferent thicknesses, as well asaluminium and glass fiber, werealso produced so that we couldvalidate our experiments withreference to known materials.Five criteria were analysed,

with results that are shown inthe graphs which follow. Thedesignations for the sandwichstructures are:

"W" sandwich/Focal patent:introduced in 1995, this is asandwich with glass fiber onboth sides of a core of acrylic/Plexiglass foam.

"F" sandwich/Focal registeredin 2013 (N 1350116): this is a

sandwich with glass fiber onboth sides of a core of flax.It will be used in Focalproducts from 2013.

Low mass a guarantee ofhigh dynamic performance[Graph 1]For a 165mm mid-woofer, the mass of thediaphragm required to conservegood sensitivity is of the orderof 10 grams. This correspondsto an ideal surface mass of the

order of 300g/m2. The diagramopposite shows that a 2 mmthick "W" sandwich due toits mass is more suitable forapplication in pure woofer.Aluminium needs to be usedin a very fine thicknesses of0.1mm, which is completelyimpractical. It appears in the listonly to validate the coherenceof our tests.

Flax 0.4mm

Glass 0.04mm

Glass 0.04mm

Flax sandwichDetails for midrange driver

Graph 1: Surface Mass in g/m2

600.00

450.00

300.00

150.00

0

"W"san

dwich2

mm

"W"san

dwich1

.5mm

"F"san

dwich0

.4mm

Polygla

ss0.5m

m

Alumin

um0.1

mm

Fiberg

lass0.1

2mm

304

570

304 327 305

270

174

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Car Audio Flax component kits: A concentrateof 100% Focal technology for the new high endof Performance Expert range.

High tensile modulus aguarantee of extendedfrequency response[Graph 2]Youngs modulus

represents the mechanical loadwhich would cause elongationof 100 per cent of the initiallength of a material, if it werepossible to apply it. In reality,the material is permanentlydeformed or breaks well beforethis value is attained. If theYoungs modulus of a materialis high it is said to be rigid. Itis easy to understand that thehigher the Youngs modulusof the core of a sandwich

material, the more resistant tobending it will be and the bettera loudspeaker diaphragm madefrom the material will functionas rigid piston. Flax has a highYoungs modulus of 60GPa, ofthe same order of magnitudeas the glass or Kevlar fibers oraluminum.

High internal damping aguarantee of neutrality[Graph 3]Internal damping ischaracterised by a coefficientknown as the dissipationfactor. At the diaphragms"breakup" frequency, the

higher the dissipation factor,the less severe the resonanceas surplus energy is dissipatedwithin the material. This is aguarantee of low coloration,of true tonality. Aluminiumhas very poor dampingqualities, which disqualifies itirretrievably. Polyglass, with itscellulose pulp base, is excellentin this respect (the reputation ofpaper diaphragms is alreadywell known to loudspeakerdesigners); "F" sandwichperforms very well too.

"W"san

dwich2

mm

"W"san

dwich1

.5mm

"F"san

dwich0

.4mm

Polygla

ss0.5m

m

Alumin

um0.1

mm

Fiberg

lass0.1

2mm

Graph 2: Youngs modulus in GPa(logarithmic scale)

100.00

10.00

1.00

304

3.492.64

11.57

1.83

69.75

6.89

"W"san

dwich2

mm

"W"san

dwich1

.5mm

"F"san

dwich0

.4mm

Polygla

ss0.5m

m

Alumin

um0.1

mm

Fiberg

lass0.1

2mm

Graph 3: Dissipation factor tan delta

0.05

0.04

0.03

0.01

0

3040.017

0.02

0.026

0.049

0.005

0.018

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High speed of sound in thediaphragm a guaranteeof definition[Graph 4] An importantcriterion, the speed of soundpropagation in a diaphragm

is directly proportional to itsbreakup frequency. The speedof sound of the flax sandwichis on a par with that of theW sandwich, with a valuedouble that of Polyglass. Highvelocity of sound is also a

guarantee of high definition inthe midrange. Polyglass, whichwas introduced nearly 30 yearsago, now falls far short of thehybrid materials developed byFocal in the interim.

High flexural stiffness aguarantee of well controlledbass[Graph 5] This parameterdefines how stiff the diaphragmis in bending. High bendingstiffness is an essentialcriterion for a diaphragmworking at bass frequencies,where significant differencesin the air pressure on eitherface can otherwise causedeformation. High flexural

rigidity brings a materialclose to the theoretical idealof a diaphragm functioning asa rigid piston over its entirepassband.

The advantage of sandwichconstruction is obvious heresince flexural rigidity increaseswith diaphragm thickness. Asthe lower graph shows, theW sandwich is without rivalin this regard. Despite its relativethinness, the flax sandwich isnearly three times stiffer thanthe thicker Polyglass alternative.

SummaryTo design a high performancediaphragm you have tooptimise a complex set ofcriteria, which are for themost part contradictory. Thecomposite solution usingflax in a sandwich structurewith fiberglass skins is a very

harmonious way of combininglow mass with high internaldamping, high velocity of soundand high flexural rigidity.[Graph 6]

"W"san

dwich2

mm

"W"san

dwich1

.5mm

"F"san

dwich0

.4mm

Polygl

ass0.5

mm

Alumin

um0.1

mm

Fiberg

lass0.1

2mm

Graph 4: Speed of sound in m/s

6.000

4.500

3.000

1.500

0

304

3793 3706 3629

1774

5199

2165

"W"san

dwich2

mm

"W"san

dwich1

.5mm

"F"san

dwich0

.4mm

Polygla

ss0.5m

m

Alumin

um0.1

mm

Fiberg

lass0.1

2mm

Graph 5: Flexural rigidity index(logarithmic scale)

1000

100

10

1

0

304

344.7

90.1

5.31.9

0.6

0.1

6.000

5.000

4.000

3.000

2.000

1.000

0

0.000 0.010 0.020 0.030 0.040 0.050 0.060

Aluminum

Fiberglass

Polyglass

Ideal

"W" sandwich

"F" sandwich

Graph 6 : Internal damping (tan delta)

Speedofsound(m/s)

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New 2013 - Aria 948Black High GlossNew Home classic Flax technology 3 way loudspeaker.

Based on these analyses,the d iaphragms weremanufactured to evaluate infull scale their performancein a blind listening trial. Wedecided to conduct the testsusing a Chorus 806V and a

Chorus 806W. Focal has along tradition and recognisedexpertise in designing two-waybookshelf speakers, whichrequire a mid-woofer whosediaphragm must satisfy allthe contradictory demandsalready identified: high rigidityfor good bass combined withhigh internal damping to avoidcoloration in the midrange.All the prototypes not usingsandwich construction wererejected because their bass

performance was inferior tothat of Polyglass.Flexural rigidity can also beimproved by adjusting thegeometry of the cone. Thesemi-exponential profiles givevery good results, for example,

with Polyglass because wereinforce the stiffness. Withthe sandwich structures, whichare intrinsically stiffer, we havefavoured straight cones as theydeliver tighter bass.

The "F" sandwich based onflax has a promising future:its qualities in the midrangeare in total accord with ourbrand values, since Focalsound is characterisedin particular by the

richness of reproductionin the midrange register.Moreover, the industrialprocess developed tomanufacture this materialbring it within reach of productranges such as Aria 900 for

the home or PerformanceSeries for the car, very price-competitive ranges whichcould not aspire to the "W"sandwich for reasons of cost.[Graph 7]

Graph 7: Relative cost / Polyglass base

0 1 2 3 4 5

304

"W" Sandwich

"F" Sandwich

Polyglass

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