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DIY loudspeakers series - Part IV

Choosing drivers

how to achieve the sound you desire

trusting our instincts

[what to put here]
[Italian version]

Product: moving coil cone loudspeakers
Approximate cost: pennies to fortunes € ¥ $ in any currency
Tortured soul: Mark Wheeler - TNT UK
Tested: April 1973 to date

What do spec-sheets and hyperbole mean?

In parts I to III (Part I & Part II & Part III) the Tao of the box emerged; its walls and filling combining to make the best of whatever drive-unit it hosts. How do we choose the right drive unit to suit our priorities? Every manufacturer would have us believe that only a fool would buy anything from their rivals. All seem to propose different solutions to the apparently simple task of turning voltages into symphonies.

While juggling mechanical Q, electrical Q, box size, box alignment according to the gospels of Thiele & Small it might be all too easy to overlook any hint about what they actually sound like. While poring over catalogues, quickly entering the T/S parameters into our bass-alignment software and scouring the Internet for the best deal on boutique crossover capacitors, spare a thought for what we actually want from a loudspeaker.

WE MUST NOT assume that we want to build a 2-way system with a 160mm bass-mid driver with a polypropylene cone and a 25mm fabric-dome tweeter because we heard a nice speaker at a hifi show and it too used a 160mm polypropylene coned woofer and a fabric-dome tweeter. We are very likely to be disappointed with our efforts and would be better off buying the ready made product; it will sound exactly as we would expect and it will cost far less money, let alone time than designing and building our own. Frankly, until we are very experienced speaker builders, with sheds full of power tools that have already paid for themselves, anything below 1000€ per pair would be cheaper to buy ready-made than to build an identical copy, even before we factor labour costs.

Readers who wish to build speakers to show-off their cabinet making skills and match the figured burr walnut veneer with the coffee table they made last winter, should buy a kit of drivers and crossover and follow the cabinet dimensions in the instructions but with superior materials. I guarantee that they will look and sound better than their MDF equivalents if when woodworking skills are good enough, and here do save a lot of money over the ready made solid wood equivalents. Half-way between this option and designing from scratch is the popular route of buying a kit of drivers and crossover, then tweaking and refining the design to suit personal priorities. Turning a clumsy deep stand-mount design into an elegant solid wood floor-stander with thicker hardwood walls is an obvious audible and visible upgrade, and using software to exploit larger cabinet volume to lower the bass Q for better PRaT and room interface is an obvious starting point.

The only reasons to design and build our own speakers are:

This final point is typical of the control freakery that plagues us audiophools as we delve ever further into the arcane depths of our hobby. Furthermore as we get higher into the high-end it does become less expensive to create a pair of loudspeakers that cost less money than the equivalent ready made product, but only because we have not costed our time, nor have we costed the years of research & development with our previous projects that taught us the lessons incorporated into our latest masterpiece and actually cost us similar resources as the development budget of a modest audio manufacturer, which indeed is what we have become. Very small. Very costly.

So we want to achieve a particular combinations of attributes that we have not heard together in the same off-the-shelf pair of loudspeakers anywhere in the world. Now we must make a list of all the attributes desired in the next project. Now we must make a second list of all the parameters that are less important to us. Now we join both these lists in order of preference and keep this in mind whenever making any single decision in the design process. The list should never make assumptions about driver size, because it may be that there are big 300mm drivers designed for small sealed boxes with high Q alignments whose frequency response and bass transient performance may be similar to a smaller 150mm driver, reflex loaded in the same size box. Other factors such as power handling, sensitivity and dispersion will determine which of the two is more suitable. The list should include cabinet size and shape somewhere if domestic harmony is of any importance.

Now estimate what we think every component is likely to cost in trying to achieve this goal (before we open any catalogues or price lists) including any special tools we might need such as new router bits, new blades and clamps. Now look through catalogues and web sites and correct any figures that were too low, but ignore any overestimates. Now add all these figures together to begin to estimate the total cost. Now double that figure because, like any building project, this is realistic quantity surveying practice. The figure that we now have before us will help to understand what it is really likely to cost financially, assuming we don't hit any unforeseen snags; which we will if this is our first scratch-build.

Are you sure our first loudspeaker project will cost that much?, demand plebs, stage left
Oh, Sorry, I didn't realise that this is your first project, replieth ye humble scribe, you'd better double that second figure again if this is your first project, plus allow some extra for tools you didn't expect to need.

When estimating construction projects experienced estimators (not new graduates) know to break down every little detail of the project and accurately cost every single detail using the latest price lists. Then add all these rigorously researched figures together. Then they double that figure for the true final cost. You have been warned.

So let us say we require a state-of-the-art moving coil loudspeaker with shattering dynamics, reasonable dispersion (for even in-room response), reasonably rhythmic and tuneful bass, fast treble to fill a big room with orchestral climaxes. Less important to us for this project are size, cost, bass extension or perfect soprano voices.

Sales blurb alone won't guarantee we'll choose the right driver, but nor will Thiele Small parameters and IEC baffle frequency response curves. We should be familiar with these and how to interpret them before designing our cabinets (buy and own Martin Colloms' book), but all they will do is help us design for loudness (sensitivity and power handling) and bass (-3dB and -6dB frequencies, Q, slope or filter order).

To understand the other priorities of the drive unit's designer, and therefore to establish whether they match our own, we have to learn how to decode sales blurb. The copywriter's hyperbole soon collapses under scrutiny and deconstruction.

The AKKURA Linear Eight

Akkura's Linear Eight has been designed with low distortion rear-spider suspension and ultrasoft cone roll surround developed using stroboscopic laser technology for improved linearity. The motor assembly has been optimised for flat frequency balance and assembly adhesives chosen for their vibration absorption properties. The ultra-long throw large 75mm diameter voice coil with FLUFFOFLUID (TM)cooling can handle phenomenal power inputs without heating causing power compression. The whole chassis has been cast from flexible plastic to minimise vibration transfer to the cabinet.

This driver is the perfect driver because it adds nothing of its own to the music.

If low distortion and low colouration are the sole criteria pursued by this designer, the recipe could equally lead to low excitement. It may handle large power inputs without compression but the large voice coil diameter and length imply low sensitivity, and excessive inductance used to tame the usual rising response. Not what we're looking for from this design.

The 200mm SILKKO SMOOTHIE

The Silkko Smoothie range has been devised to meet the needs of builders who want a super-hysteris (TM) cone for minimum colouration and natural timbre. The cone is doped with added damping DENSOGLOP (TM) to ensure freedom from resonances or unpleasant breakup artefacts. A short voice coil gap and 25mm diameter pole-piece ensure that the heavy cone is kept well damped and controlled by low QES. Bass is thus extended without boom.

This driver is the perfect driver because it adds no colouration to the music.

Possibly more MOR than more...more...MORE...MORE!!!? This is unlikely to appeal to the dynamics demanding designer. The language of the brochure emphasises what this driver does not do, rather than what it does. Not what we're looking for today.

The 8" DA KKAPPO

The 8" version of the Da Kkappo combines a low mass pulp cone with a large magnet for maximum sensitivity. The small diameter 25mm voice coil extends the bandwidth higher, and the parasitic whizzer cone aids high frequency dispersion too. The rising overdamped response has been designed to match a rear exponential flare horn with mouth up to 6 feet diameter for bass extension. For filter-less full-range single driver excellence look no further.

This driver is the perfect driver because it adds drags every last dB from every tube watt.

One for the SET300B brigade, but not for this project, so how about....

The TANKK 200 Turbo

The Tankk 200 Turbo, with BIG MUTHA (TM) magnet generates shattering dynamics from the smallest signals. The Carbon-fibre & Kevlar (TM) cone is designed to be ultralight to respond quickly to the slightest nuances. The cone area is the largest possible within a 200mm chassis for maximum air-shifting properties and the cone surround & coil suspension have been designed to be as free moving as possible. The BIG MUTHA magnetic field is concentrated into a small gap with a short throw to maximise flux density for spectacular microdynamics and resolution, enhanced by the edge-wound 40mm voice coil.

This driver is the perfect driver because it explodes with music.

Even before we read the electrical and mechanical small signal parameters this driver looks like a contender for the shortlist. It may transpire that it is unsuitable for our proposed cabinet dimensions, but the designer is lauding the performance criteria mention in our brief. It may be that the TANKK 200 Turbo is horribly coloured or has a very unbalanced frequency response and falls at the next fence, but at this point in the selection procedure it makes it through to the next round.

If we had been asked to design nearfield monitors to put on the ends of a long analogue mixing desk this driver would be unsuitable. Monitors need to be accurate and uncoloured; at no point did the sales blurb mention these as priorities. Nearfield monitors also need to have very even dispersion for accurate balance at the ears of 2 or 3 people standing side-by-side but close to the speakers; large thin undamped cones are unlikely to achieve this, whereas the 200mm SILKKO SMOOTHIE looks like it has been designed for this very purpose.

For a photographer nearly 3000 words and one lousy picture is disgraceful, so here is a picture.

[bastard child of AKKURA and TANKK on steroids]

Above is one interpretation of a 300mm driver and below is another. They are very different in their priorities and execution, as is most clearly evidenced by the rear view. Both no doubt loved by their designers and both enjoyed by me for their very different interpretation of the task to make music effectively.

[Da Kkappo on steriods]

In parts 5 and 6 we'll take a sideways look at decoding those Thiele Small parameters for other reasons than bass alignments.

Conclusion

All sales blurb is written to pursuade readers that only one product, ours, will be worth buying. However, the priorities chosen to reinforce those arguments become our clue to whether this is the right product for our project.

Decoding advertising is easier for us when shopping for DIY audio because we are all educated customers by the time we choose to lift the saw and the soldering iron. Every drive unit designer is as motivated as every customer to make the best possible drive unit for the price, ACCORDING TO THEIR PRIORITIES. No one sets out to design an incompetent product, but their differing goals may make theirs an unsuitable component for this project.

If the things that excite a designer are the same ones as excite the customer, there is every chance that it will be a match made in heaven. If the things that motivate the designer contradict those demanded by the customer, when the first music emerges from the project newly completed after hundreds of hours of painstaking design, accurate cutting, careful glueing, immaculate silver soldering and pinpoint positioning, will not stir the soul and calm the savage breast but will invite a match of a different kind: the one that reads "Remember to close cover before striking".

Music enjoyed during this review

Plenty live, but sadly still none on the hifi at home due to building work.

© Copyright 2008 Mark Wheeler - www.tnt-audio.com

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