Part 1. Design Ideas and Theory
Page 2 Design criteria    

 

Accordingly the design criterion for the WD25A speaker kit was to revisit an efficient 10 inch driver that had good midrange qualities and yet produced high quality bass performance in a not-too-large enclosure. The key here is cone and surround quality. Researching the archives brings up all sorts of horrors which resulted in many large bass units being consigned to three way loudspeakers. For example the stiff and heavy cones and large motor systems which typify traditional reflex designs result in barely controllable and undesirable cone breakup modes in cone diameters over 6 inches.


We were looking, therefore, for smoothly performing drive unit designs which suited closed box parameters when we chanced across a design classic of the early ‘70s, the Dynaco A-25. This unusual design by SEAS for Dynaco in the US featured a soft coned ten incher in a cabinet that wasn’t much bigger than the drive unit itself. The key to its bass performance was ‘aperiodic loading’ – a formula for driver and cabinet design which has rarely had commercial success.
As very few of us are au fait with aperiodic loading it is worth taking a closer look at this type of cabinet design. At first glance the system looks like a damped reflex port and, in fact, some designs have approached the loading in such a fashion with disastrous results. Actually the behaviour of a well designed aperiodic loading speaker is closer to that of a well damped closed box of larger internal volume. And that’s the key to it, to design the drive unit and cabinet parameters such that the combination behaves like an optimally damped closed box system.


However there is an advantage to the aperiodic enclosure which is not, at first, apparent. A well damped closed box tends to sound exactly that – well damped. In other words the high degree of internal damping, usually arranged by a complete stuffing of the enclosure volume with acoustic absorbent, seems to suck the life out of the midrange dynamics of the driver. In the aperiodic enclosure the damping is performed by the resistive vent which, if correctly designed, offers a leakage path for pressure build up inside the enclosure at the large cone excursions encountered at low frequencies. The interior of the enclosure can therefore be fitted with a minimum amount of acoustic damping, just enough to cut down internal reflections, and therefore has less effect on midrange transient performance. In fact the ‘tuning’ of an aperiodic system is managed in such a way that the transient performance throughout the bass and midrange is optimal.
As for the drive unit itself who better to turn to than SEAS themselves? Our modern version of this design classic uses a soft paper cone and butyl rubber surround that produces beautifully controlled breakup for excellent midrange qualities combined with a motor system that ideally suits a 23 litre aperiodic enclosure. Good dynamic power handling is aided by a 39mm voice coil with a vented magnet, whilst the whole is mounted in a strong, cast chassis. Truly a modern classic driver.


Part of the key to the Dynaco A-25’s success was its simple crossover technique. The bass/midrange unit was so well controlled that it could be used without crossover, whilst the treble unit possessed a low enough fundamental resonance to be used with a single capacitor and variable L-pad resistor level control. We asked SEAS for a modern version of the original treble unit and two hf units were suggested, the 29TFF/W and the T29CF001. The latter is closer in theory to the ideal of low resonance and wide bandwidth, but its response with a single capacitor crossover gives a shallow roll-off which is not best suited to the required midrange crossover.


The answer is a simple crossover for the basic version that uses the 29TFF/W and a more advanced crossover for the luxury version using the T29CF001. The latter unit is part of the SEAS Excel range and utilises a Neodymium magnet system surrounding an underhung voice coil to provide a motor system of extremely low distortion.  This is coupled to a SONOMEX fabric dome and housed in a precision machined aluminium front plate with a zinc injection moulded rear chamber. As you can imagine this is a luxury treble unit for this project for those who want the ultimate performance.


Audiophiles used to tinkering with different components can also try their own combinations of crossover components. We will detail these options in the next article along with how we developed the crossover and what affects the resulting subjective performance. The final article will give you all the construction notes and drawings necessary to build up the kit.
For those who can’t wait, or just want to get on with it now, the full construction details are given with the drive units and crossover kit available from World Designs. There is also the option of a cabinet kit as well for those who don’t feel up to the woodwork!

 Graph2
Graph1


  Aperiodic - what does it mean?

‘Aperiodic’ actually means ‘without periodic repetition’ and, in the case of a loudspeaker, defines a system which has optimally damped resonance control. In the case of a closed box or reflex ‘vented’ box loudspeaker the primary resonance of the speaker driver in the box is high ‘Q’, in other words as the frequencies fed into the speaker get close to the primary resonant frequency they easily excite resonant behaviour.
In a poorly damped system what you hear from this resonance is a ‘boom’, in fact many of the early reflex loudspeakers were called ‘boom boxes’. It is generally left to the amplifier to control this resonance, though some help through damping is applied by stuffing the cabinet with absorbent, by damping the speaker using the amplifier’s negative output impedance or ‘damping factor’. This demands considerable instantaneous current flow from the amplifier to apply a ‘braking’ effect to the drive unit as it starts to oscillate – all well and good if you have a high power amplifier but not ideal for low power or valve amplifier users.
The Aperiodic enclosure applies resistive control of airflow in and out of the cabinet to effectively help damp out the primary driver resonance. You can see the difference in Graph 1. This is a plot of driver impedance and is the load the amplifier ‘sees’. You can see the primary resonance at 70Hz in the closed box ‘blue line’ as a sharp, high ‘Q’ peak. The high impedance at this point shows that the drive unit needs very little input in order to produce a large output, i.e. the driver is resonating in its box.
Compare this to the aperiodic plot (red line).This is the same driver in our optimally damped aperiodic enclosure. The high Q peak at 70Hz has been dramatically reduced. The flattening of the impedance curve also means it is an easier load for the amplifier.
Now look at the difference in response curves (Graph 2).  The blue line is the response in the closed box. Due to resonance it exhibits a relatively sharp knee in the response through the 80-100Hz region before it falls away. Compare it to the red line of the aperiodic system. The ‘knee’ is reduced and the bass response is given a little more subjective extension at very low frequencies, albeit with the loss of about one dB at 70Hz. A reflex ‘vented’ system would give greater LF extension but, again, you would be back to the highly resonant behaviour. The only drawback to the aperiodic system is that, at subsonic frequencies, cone movement is higher than that experienced in a closed box system. This won’t be of concern when using CD or other subsonically ‘clean’ sources, but turntable owners should be careful their arm/cartridge/suspension system is not prone to resonance itself and avoids exaggerating record warps (this warning applies equally when using reflex speakers).
 

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