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From Hi-Fi World - February 2009 issue

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Designing

Loudspeakers

 

 

sfb3ad

 

 

Open Baffle loudspeaker – drive unit choice.

By Peter J Comeau


Encouraged by listener’s responses to my lecture demo of the Wharfedale SFB/3 at the autumn 2008 Heathrow Show I decided, rightly or wrongly, to get as close to the performance of the Gilbert Briggs bass and midrange section as I could.


Not surprisingly there are few bass units currently available that have the necessary specifications for a 12 inch or 15 inch moving coil unit that works in an open baffle. Bear with me while I go over the ideal requirements briefly.


The SFB/3 had a fundamental resonance in the 30 – 35 Hz region and a powerful magnet to keep sensitivity high. In addition the voice coil was engineered to give a long throw so that bass lift could be applied at the amplifier without ‘bottoming’ the bass unit. (Bottoming is where the voice coil movement exceeds its maximum travel (Xmax) and hits the suspension limits or batters itself to death against the back of the magnet)!


Of course the SFB/3 was around when the majority of amplifiers had tone controls fitted and Gilbert Briggs was particularly thinking of the QUAD 22 which enabled the user to apply a judicious amount of bass lift without muddying the midrange too much.


exoticf8

SEAS Exotic F8 full range drive unit with Alnico V magnet, papyrus fibre cone and ‘whizzer’ treble cone.


Nowadays, with tone controls being considered something of an anathema to good sound, we don’t have the ready opportunity to leave it to the user to apply a modicum of bass boost. Of course there are ways round this, for example we could supply a passive, or active, equalisation circuit to put in the tape loop circuit of the user’s amplifier, much like Bose did with the 901. However I can’t see this going down too well with the purist users of our open baffle speaker (and I can’t say I much like this level of complexity either)! So, for the WD Open Baffle, I settled on a bass performance which was naturally balanced with the midrange without any amplifier boosting of low frequencies.


This means that we have to add one more factor in the drive unit specification, namely a relatively high Qts.  By experiment I’d already discovered that a Qts in the region of 0.5 – 0.6 would deliver the required bass performance in normal living rooms. The difficulty is in delivering this from a bass unit designed along the lines of the one in the SFB/3.


Put simply the standard way of achieving a high Qts is to either:

1. Reduce the electrical damping.

2. Increase the suspension compliance.

3. Increase the moving mass.


Of course you can use a combination of all three of these factors to achieve the desired end but let’s look at the problems endemic in each:

1. Reducing electrical damping requires a reduction in magnetic force, with a subsequent loss of sensitivity.

2. Increasing suspension compliance runs the risk of the bass unit suspension becoming unstable either allowing the voice coil to ‘rub’ in the magnet gap and/or being more prone to ‘bottoming’.

3. Increasing cone mass reduces sensitivity and adds to the inertia of the moving system, again being more prone to ‘bottoming.’


I hope that you can see, from this, that it is no simple task to balance these factors into the drive unit makeup in order to achieve our objective. Yes, it’s fairly simple to make a robust drive unit with an Fs of 30Hz, a sensitivity of 96dB and a Qts of 0.23. It’s also pretty easy to make one with a Qts of 0.8 and still keep the sensitivity at 96dB but with the Fs at 85Hz. The difficulty comes in hitting the target of a low Fs and high Qts.


Scouring the drive unit market yielded a few examples which come close to the criteria but you have to remember, also, that the majority of modern drive unit designs are either aimed at the reflex box hi-fi loudspeaker or the pro ‘guitar’ loudspeaker. Most of these designs just won’t work satisfactorily in an open baffle where there is no constraining force from the air mass and springiness in a box.

DESIGNING TO A RECIPE

Accordingly there was little to do other than turn to a reliable speaker manufacturer prepared to make to a recipe.  It’s best, in these cases, to find a manufacturer that already makes something close to what is required in order that only the necessary adaptations need to be made to an existing driver.


Such drivers no longer exist in the hi-fi drive unit manufacturer’s repertoire, so the Pro unit guys are where we start looking. Don’t forget that, unlike hi-fi drive units, the Pro market requires robust, reliable and unbustable drivers, so finding a unit which will withstand the rigours of open baffle use is easier.


Now I’d love to tell you that the ingredients for the recipe turned out to be as tasty as I expected, but I can’t. At time of writing I’m still to receive a driver that both myself and the manufacturer are happy with. At the moment it’s a toss-up between a single 15 inch covering the required range or twin 12 inch units. I await developments!


In the meantime I can, at least, describe my choice of midrange unit. Again, keeping to the Briggs open baffle recipe, a 200mm (8 inch) unit is the ideal. This size of unit can encompass the necessary low crossover frequency to the bass units as well as keep the sensitivity high.

It would also be nice if this unit covered the majority of the frequency range to give the musical reproduction from our open baffle design the seamless, effortlessly flowing quality that always seems to come from a well designed single drive unit speaker.


Having heard what an ancient Lowther full range unit could do in an open baffle, supported with two Goodmans Axiom bass units, at our WADFEST last year I’ve been eager to arrive at a design with a similar performance.


A few months ago the engineers at SEAS finally delivered a drive unit that had both the pedigree and performance that I was looking for. The Exotic range, from SEAS, encapsulates the combined experience of drive unit engineers ranging back to the ‘50s. Produced in small, handbuilt, quantities the Exotics are totally unlike anything you’ve seen in a modern commercial loudspeaker.


The first design in this series is the Exotic F8, a full range unit with a lightweight papyrus based cone, special foam rubber surround and Alnico V magnet – all the hallmarks of a traditional design. Equally traditional is the use of a ‘whizzer’ cone to extend high frequencies.


A ‘whizzer’ cone is a small cone attached directly to the voice coil just inside the major cone itself. It thus behaves as though it was a treble cone, adding to the output of the midrange section of the cone and taking over completely as the frequency increases beyond the range of the midrange cone.


Because the outer edge of the cone is unsupported and unterminated the term ‘whizzer’ originally described the typical resonant ‘whizz’ that the cheaper paper add-ons provided as basic additions to speakers designated for use in desktop radios and the like.


But that shouldn’t rule out their use for hi-fi because a correctly designed cone can perform equally as well as the best cone treble units. Of course it doesn’t have the radiation characteristics of a dome tweeter, but then nor does it have the cavitation and rear reflection problems of a dome either. Instead it presents a treble output closer in character to that of its supporting cone, which helps the drive unit achieve that seamless performance that you expect from a full range unit.


At first sight the frequency response looks unpromising with its general rise from the midrange towards the treble. But remember, you are looking only at the on-axis response. Because of the narrowing dispersion of treble power as the frequency increases, and the corresponding increase in dispersion of midrange power as frequencies get lower, the overall power response in room, and on a wide baffle, will flatten out.


exoticf8specs

SEAS Exotic F8 frequency response and impedance. The heavy line shows the

on-axis response while the dotted lines below indicate the performance off-axis.

Overall average sensitivity is 93dB when placed on a baffle.


Average sensitivity lies at 93dB for the 8 Ohm unit and 96dB for the 4 Ohm version, which gives us a choice to suit the bass driver sensitivity once the final units arrive. I’m expecting the 8 Ohm unit to match best as most of the bass unit’s sensitivity will be used to bolster the very low frequencies with a deliberately engineered 3dB depression towards the crossover.


Qts of this unit is 0.44 which is ideal for this application and the maximum coil travel of 14mm means that we should be able to pump the long term power handling of 35W into this unit without problems, given a suitable crossover of course. However we won’t know the exact outcome until the bass units arrive!


So, apologies that this particular project seems to be extending ever onwards but I never thought it would be easy to engineer something which meets modern performance requirements from a classic design. I am, however, convinced that this open baffle design is worth waiting for.



 

 

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