This is a description of a compression driver for high-powered horn systems fitted with a unique metal diaphragm whose geometrical and physical characteristics combine the use of mechanical parts with a new design and unusual assembly techniques, which has resulted in better performance than can be obtained from the various types manufactured today and the technology they normally use.

It represents a considerable development and one of the first important upgrade during almost seventy years since the very important invention of its forerunner by Wente and Thuras in far-off 1928.
 

Introduction

We're all familiar with the great advantages which horn loudspeakers have over all the others nowadays (even more so than in the past) - particularly those for professional use. Features such as the very high radiation efficiency and directivity control offered by horns thanks to their construction in fact mean they are the only ones able to achieve the high sound coverage levels required nowadays for a wide variety of applications, respecting the dynamics of the sound event, whatever the source.

The enormous difference in efficiency (an order of magnitude) between any type of direct radiation loudspeaker and a horn obliges users to choose the latter in the professional field - for obvious economical reasons if nothing else.

The most frequent application of horns is for the reproduction of mid-high frequencies, for which they're used along with the appropriate active transducer units - compression drivers.

This presentation intends describing this unusual type in particular, whose key components are a remarkable improvement on others currently available on the market.

In order to clearly understand what improvements have been introduced and their importance regarding the upgrading of compression drivers' current performance, it's worthwhile illustrating in principle all the constructive and functional aspects involved,  without indulging in proposals and purely theoretical aspects, on which plenty has already been said and written.
 

Details

All audio pros know that for approximately seventy years (and with no substantial variations), compression drivers have basically been indirect radiation electrodynamic loudspeakers, whose diaphragm or active component is made up of a dome which radiates the sound, firstly via a rephasing and equalization device (the phasing plug), then through the horn loaded on the driver.

The horn, a real acoustic transformer, is effectively a progressively expanding duct which has the aim on one hand of adapting the acoustic impedance which air, the medium in which sound is generally propagated, opposes to the diaphragm, and on the other directing the sound waves emitted in the required direction.

The phasing plug generally consists in a rigid transition element located between the diaphragm and the horn's throat, in which there are a number of apertures, such as round holes and radial or concentric slots forming multiple ducts with a progressively variable cross-section inside, which lead the sound to the throat's surface, compensating the differences in propagation compared to the surface of the diaphragm.

These apertures have a considerably smaller total surface area than the diaphragm (normally 1/10), so when this is operating an actual chamber is created between it and the phasing plug, in which the air is compressed - hence the definition "compression driver".

Fig. N° 1 and 2 illustrates perfectly what you've read so far.

click to enlarge

click to enlarge

This type of set-up, which compels the air vibrated by the diaphragm to be compressed drastically, passing firstly through the phasing plug and then progressively expanding in the horn, gives this particular type of loudspeaker has the theoretical capacity of returning up to half the energy applied to it to the medium of propagation (normally air). In other words, compression drivers have a theoretical efficiency of 50%.

In fact, due to various kinds of loss occurring in this type of system, such as that caused by the well-known "eddy currents”, efficiency normally drops to 30% with the more sophisticated and better built models, and from 20 to 25% in the large majority of those on the market.

Thanks to these values however, compression drivers are therefore by far the most efficient (an order of magnitude higher) of all electrodynamic components currently used for sound reproduction.

But in spite of the great merits listed so far, compression drivers are transducers whose performance is influenced more than any others by component design and to a great extent by manufacturing allowances.

Small changes in just one of the construction parameters cause variations in performance in almost every other parameter taken into consideration.

For example, increasing the cross-section of the coil wire for higher power handling causes considerable loss on the high frequencies.

Slightly widening the magnetic gap to reduce manufacturing waste causes a great drop in sensitivity, as well as a large drop in high frequency response.

Due to the different curvature radius, slightly reducing the depth of the dome to avoid moulding problems results in an inexact match with the phasing plug and a consequent drop in mid-high frequencies.

Small differences in the distance between the phasing plug and diaphragm (i.e. variations in the volume of the compression chamber), due to the presence of glue holding the various components together - from the magnetic circuit to the phasing plug itself - inevitably lead to large differences in response at the top end of the reproduced sound.

Excessive reduction of the distance between the phasing plug and diaphragm to improve high frequency response will inevitably lead to a loss of level at the bottom end and more distortion.

As well as these, other factors due to the types of construction and the relative geometry reduce the advantages which high efficiency gives this particular transducer and limit its use from a practical point of view .

In fact, although compression drivers' high efficiency would enable them to easily deliver 50/100 Watts (according to diaphragm dimensions), and bearing in mind that one single Watt is the level at which a symphonic orchestra's crescendo reaches the conductor on his rostrum, this performance can't be achieved for a variety of reasons.

For example, distortion due to diaphragm break-up and air compression make drivers' sound unbearable at excessively high sound pressure levels.

Diaphragms' limited excursion doesn't enable them to reproduce the low part of the band at this high theoretical sound level without the diaphragm coming into contact with the nearby phasing plug.

Non-linear excursion of the moving parts, caused among other things by the suspension design, and consequent inevitable friction of the voice coil against the walls of the magnetic gap in fact mean it's impossible for the high acoustic levels which the compression driver is capable of to be emitted.

The maximum operating temperature of the moving parts and low capacity for dissipating the heat generated in the diaphragm's voice coil, with a consequent rapid increase in power compression, also drastically limits the achievement of theoretical efficiency figures.

Even if during the years since they were invented by Wente and Thuras, new materials, adhesives and technology have improved compression drivers' performance, in my opinion only a different design of the diaphragm and moving parts, as well paying more attention to solving problems caused by thermal stress can effectively bring compression drivers' performance close to the theoretical figures.

Along with a valuable team of collaborators, I've dedicated time to achieving these results, building a new type of metal diaphragm which is at present made in aluminium, but can also be made in Titanium. The logic behind it's construction is extremely simple and its original structure is compatible with conventional magnetic circuits.

This unusual diaphragm, called UNIMETAL^®, effectively overcomes the key problems afflicting compression drivers from a mechanical and thermal point of view, thus leaving designers greater construction freedom than other ideas based on a search for better acoustic performance in a strict sense. To understand this better, some designs are shown here illustrating in principle the geometric and functional features of compression drivers with metal domes, at present marketed by all the world's major manufactures.

Diagram N° 3-4

click to enlarge

click to enlarge

Current types and systems

Having carefully examined the designs used by the large majority of compression driver manufacturers shown in the above diagrams, it's easy to see how the various types have an indisputable characteristic in common:

all the diaphragms consist in several pieces (at least two in the most sophisticated) made from different material and assembled using glue.

This widespread manufacturing feature is due to the current state of this sector's technology and is the most serious obstacle hindering the achievement of the mechanical (and therefore acoustic) performance theoretically obtainable from a metal diaphragm.

It's easy to understand how the construction of a diaphragm in several pieces requires joints which, no matter how rigid they are, always have the effect of mechanical and thermal breaks, with all the logical consequences:

lower capacity for transmitting current between the voice coil, its support and the dome which has to generate the sound, with such a high loss of transduction as to partially thwart one of the key reasons for choosing  metal to build the diaphragm, i.e. the capacity of the latter to reproduce sound peaks without any damage, thanks to the low internal damping and therefore high sound transmission speed;

poor thermal conduction between assembled parts, due to the use of insulating material (a characteristic feature of all glues), with a consequent tendency to undergo an obvious drop in performance in a short space of time, with alteration of the frequency response from the point of view of quality and quantity, as well as clear reduction of output sound pressure due to the well-known phenomenon of power compression connected with the voice coil's rise in electrical resistance, caused by the heat generated in it during operation;

tendency of the voice coil to burn or break during use, caused by rubbing against the walls of the magnetic gap because of the difference between the dilatation of the connected parts and the coil itself, as the parts are made from materials which are completely different from each other, such as plastics or similar material for the suspensions and/or for the former of the moving coil, normally wound with aluminium ribbon and the metal (aluminium or titanium) used for the dome.

There's no need to go into further details, as it's already clear how compression drivers, whose present-day form is practically unchanged seventy years after its invention, still haven't managed to fully exploit their potential as a high performance transducers.

This situation has at last been definitively solved with the arrival on the market of the UNIMETAL compression driver (patented in Europe and patent applied for in USA, Canada and Japan): after years of experiments and research, applying exclusive sophisticated technology to heat-based moulding of metals (aluminium alloy to be precise), OUTLINE has broken new ground, improving performance from the point of view of quality and quantity of the compression driver as it's been known up until now. More generally speaking, the company plans to upgrade transducers, whether they're indirect or direct radiation units such as dome tweeters, whose sound reproduction would benefit from the use of a metal diaphragm with an innovative design and a geometry totally unaffected by traditional criteria.

Outline compression driver with UNIMETAL metal diaphragm

Diagram N° 5

click to enlarge

Diagram N° 5 shows a simplified schematic design of the compression driver which Outline (using a traditional magnetic circuit with a ferrite ring manufactured by a well-known Italian loudspeaker firm) currently uses as the power-house for the famous UNIMETAL metal diaphragm, which Outline itself manufactures in-house with cutting edge machinery and sophisticated technology.

Although emphasizing the apparently total traditional nature of the Outline compression driver compared to those on the market, a closer look at the design will reveal to experienced eyes a very different functional use of the active part of the system (the diaphragm) as far as both quality and quantity are concerned.

This use, made possible only thanks to the fact that the diaphragm is made from a single piece of metal and assembled with its support in such a way as to exploit to the utmost the advantages which this fundamental characteristic offers, emphasizes how the important shortcomings afflicting all the other products on the market have finally been overcome.

As well as the obvious role of forming the active transduction component necessary for transforming current into sound, the geometry of the diaphragm, using a single piece of  metal lamina (aluminium or titanium) offers other important features, which in my opinion are decisive as far as results are concerned.

The first obvious advantage obtained with this geometry consists in the coil's high heat dissipation capacity and speed, thanks to the fact that it's fitted extremely closely (virtually without use of glue) to the metal walls, in the cleft formed by part of the lamina from which the entire diaphragm is built being folded back over itself.

This results in a very great reduction (if not the actual elimination) of phenomena caused by "power compression" - poorer performance and alteration of frequency response.

This virtual lack of "power compression" is a phenomenon connected and concurrent with the greater capacity of the moving coil to accept electric power, thanks to the very efficient disposal of the heat generated in it, through contact with the metal of the diaphragm; 

but also by means of its close contact with the support which is in aluminium and assumes the role of a radiator; and lastly by means of the contact of the latter with the metal parts of the actual magnetic circuit and the rear cover of the driver (not shown), also manufactured in material suitable for dissipating heat.

The second advantage obtained with this innovative geometry consists in the great  rigidity of the coil/diaphragm connection, virtually forming one piece and resulting in an exceptional capacity for transforming the current into mechanical action of the diaphragm with a surprising and acoustically noticeable ease of transduction, even for the smallest, most unexpected change in voltage which the music signal causes on the voice coil;

the acoustic result is in fact clear, extremely dynamic reproduction of the music signal with unbeatable results on even the most minute sound details and absolute fidelity, even and above all in the event of peaks, whose correct reproduction, in my opinion, is at least as important for achieving top-class sound reproduction as good linear frequency response.

The third advantage is total protection of the voice coil from possible problems caused by the effect of heat or incorrect movement during operation, such as coming unglued from the support, with consequent breakdown or burning due to rubbing against the walls of the magnetic gap, even in the event of irregular lateral movement due to particularly high stress: voice coil breakdown therefore only depends on the voltage applied, which would have to be very high to cause this - at least twice that applied to other drivers on the market.

The physical stability of a structure such as that of the coil seat, even at very high temperatures, will not allow noticeable dilation or alteration of the concentricity of the structure itself, giving the further advantage of using a very small magnetic gap, which among other things ensures greater exploitation of induction with the same magnet, a key factor for better high frequency reproduction.

But the fact that the diaphragm is built entirely from one piece of metal isn't the Outline compression driver's only ground-breaking feature.

The suspension, also made from the same piece of metal lamina as the rest of the diaphragm, has some really original characteristics which clearly highlight its top-class performance.

Its layout, made up of the series of closely placed concentric spirals, allows "screwing and unscrewing” along the medial axis of the diaphragm itself during its piston-type movement, in such a way as to respect the linearity of the axial and lateral movement of the entire transduction system more than any other type of suspension used by others.

This results in a great reduction of distortion due to non-linear movement, as well as giving metal suspensions, even if as ductile as aluminium, an elasticity typical of top quality plastic materials.

Last, but by no means least, the diaphragm support also has a design which, as well as enabling it to function as a heat-sink, allows a very important mechanical component to be included in the system - the rubber damper shown in the design This other original component is also patented and on one hand is intended to prevent the breakage of the diaphragm at the point in which it's rigidly fixed to its support (very frequent in other set-ups due to impulsive power overloading), on the other it has the equally interesting and useful goal of allowing suspension rigidity to be adjusted in relation to the series of moving parts, therefore obtaining different system resonance when required, according to the frequency range which has to be reproduced, without modifying the diaphragm.

This info regarding the unique features of Outline's "UNIMETAL Driver" intends showing beyond all doubt (using clearly understandable terms) the intrinsic superiority of this type of product, not only necessarily for the benefit of trade members, but also for those who have had the opportunity of appreciating the results, by personally listening to the Outline enclosures in which the compression driver in question is already used to great success - the SPECTRA, TRIPLA and DOPPIA models.