[ghamel]

Ok everyone, thanks for all of the recommendations. I've had a read through lots of great suggestions and after careful consideration i think that I'm going to attempt something completely audacious, time and $$ permitting..

Hugh's Aksonics are getting rave reviews here and I like the components, but I'm still looking for a more fullrange speaker, like down into the 30 Hz range. I've run into some interesting designs using the ScanSpeak 18W/8545 woofer which can get down into this range in the right cabinet, and I've been reading up on the state of the art in transmission line design based on G.L. Augspurger's work, and what i'm considering now is..

(drum roll..)

Hugh's Aksonics with ScanSpeak 18W/8545 in a tapered TL cabinet..

Hugh (or anyone) can you see any obvious problems with this driver when used with your Aksonics Xover? The high end of the frequency response and efficiency of the two drivers are about the same, but the driver itself supports a more extended low end.  
 
Thoughts? Comments?

Am i truly insane?

[Jens]

Hi Ghamel,

For deeper bass the Scan-Speak 18W/8546 (kevlar) is actually better than the 8545.

I've used both these units extensively and can only say that I think the 8546 is the better unit. Not only does it have a much better midrange reproduction than 8545, it also has better articulation in the bass region and is in fact able to go deeper.

Where I live the units cost more or less the same.

However, you might encounter some problems if you try to use either of the Scan-Speak units instead of the Vifa unit intended. This is because the x-over is finely tuned to the Vifa, and since the magnet systems of the Scan-Speak and the Vifa units are not the same, their electrical behavour will not be the same either.

Why not go with the Vifa and simply put it into a TL kabinet to extend bass?

Cheers,

[AKSA]

Hi Ghamel,

This is a vexed question.  Will a different driver work well with the AKSonics crossover?  Heck, I dunno......  You'd have to try it, it's all a gamble.     The crossover will certainly work, no question, but it won't be optimized for the 8545 (or 8546) driver, and careful tweaking could well be required.

Speaker design comes back to crossover design.  Generally speaking, that's where the magic is.  If you change the drivers, the myriad of compromises incorporated into the crossover all shift, and there lies the problem.

I spoke to Ron, and this was his comment.

He is well aware of the Scanspeak driver, and emphasizes that this driver is substantially different to the M18WO-08-09 Vifa.  He has designed with them for his clients, and says that they are priced high very carefully to cynically attract the DIYer whose labor input building the speaker is then absorbed by the higher price driver.  He believes the Scanspeak drivers are only marginally better than the quality Vifa and Peerless offerings, and dismisses them out of hand as TOO EXPENSIVE!!!  Resonance on the Scanspeak 8545 is nominally 26Hz, compared to 35Hz;  VAS is 72 litres compared to 28.5 litres;  Mechanical Q (a vital statistic which largely contributes to the slam and placement of low notes) is 2.5 compared to 6.7, and cost is $US131 compared to $US64.

In closing, and my thanks to Jens for his input on this bass driver, I'd say this, all checked and passed with Ron   :

1.  Three ways cover the range better, but mandate use of a band pass, which is a low pass and a high pass in series, and which CAN have a dislocating effect on the phase (and hence the image) of the music signal.
2.  Two ways are a bit stretched, so you tend towards a smaller woofer, say up to about 170mm, which goes up to around 2,500Hz, the usual crossover point for a two way.  But a smaller driver compromises low bass - you just can't have it all.....
3.  Even a 5" driver will comfortably go down to 100Hz, so you really need only a sub to cover the range 30-100Hz, and this is eminently doable with quite steep low pass filters because psychoacoustically it appears the ear is not to peturbed with phase dislocations in this range (it certainly doesn't disturb imaging, and in any case sub bass is non-directional).
4.  The logical outcome of the foregoing is to employ a sub woofer for augmentation, run the two way full range with its simple high/low pass crossover, and use steep electronic and mechanical crossovers to augment the bass with a sub-woofer.  In any event this will give you much more bass power than moving to the Scanspeak driver which has an almost identical diaphragm size anyway.

Generally, #4 has been my choice.  However, with the AKSonics the deep bass is actually very good, and I never bothered with my isobaric 2 x 12" sub any more.  I find I don't crave low bass these days, any more than I want to sit behind a 6 litre V8 auto engine.  (Must be getting old....... )  

So, on balance, it does seem to me that a sub is really all you need, but YMMV, and likely does, and that's fine.  But be assured that whatever decision you take, it will always be swings and bloody roundabouts!!!  You just can't get away from the engineering!  

Cheers,

Hugh

[stvnharr]

Hello,
May I add some additional ideas to this speaker dialogue.
The Scanspeak drivers are indeed more expensive, and certainly would require some changing of the crossover for quality sound.  The 8545 and 8546 drivers do offer very good bass, really all the low bass one really can desire.  The midrange detail is okay, the 8546 being better than the 8545, but neither one can approach the midrange detail of the Seas Excel drivers, that are priced about the same.
Though the SS drivers are twice the price of the Vifa, that cost is more than offset against the price of a subwoofer.  Crossover design costs may get things a little closer.  But even a diy subwoofer is going to cost a bit.

Vifa is the midprice line from DST, and are known as exceedingly good value performers, and are used in a great many commercial speaker designs for this reason.   However, the Scanspeak drivers are the best performers and are not just priced more to gouge people.   They are used in a lot of very exspensive commercial speakers.

A bit of caution needs to be exercised if one decides to change the driver in the Aksonics, as one is really making a new speaker.  Some speaker design and crossover knowhow would be needed to be sure.
Much easier to do as suggested and modify the cabinet, although some knowhow is needed for making a good T/L.

[StevieM]

Hi all

can I muddy the waters even further ? I've got a pair of the mk1 Aksonics and I'm still delighted with them. If the mk2 is even better then it's got to be worth investigating.
On reflection the following issues are important in speaker choice too
1/ Efficiency - my Aksonics are more efficient than the Kefs they replaced and this is audible in tems of dynamics and 'ease', you can hear that the amp sounds less strained.
2/ The room. The Aksonics are subtly tunable and in the long term this is worth having.
3/ Bass. I listen to alot of live concerts and whilst I don't want to replicate the volume I do miss the depth to the sound. To recreate this I have bought a small subwoofer (http://www.iplacoustics.co.uk/ipl_sw2_active_subwoofer.htm) to supplement the Aksonics.
I reckon the combination of Aksonics and subwoofer gives so much choice in tuning options to suit individual taste that you should consider this route.
Hope this helps
Steve

[Tinker]

I know that speaker design is something of a black art, and there are many who would disagree with my opinion. But I offer it anyway.

It seems very hard to make a high-quality 2-way that has good bass, good imaging and a decent response curve. The AKSonics (MkI and II) certainly do very well in all these regards, but perhaps need a bit of bass assisstance, having traded this off against exceptional imaging, flatness, and distortion in a system so easy to build.
Incidentally, I pursued the TL approach about 4 years ago before building the Linkwitz "Phoenix". I had some sucess with folded and labyrynth designs, but I digress.  The TL gives great bass, and good midrange, but is very difficult to get a good alignment that doesn't have a weak spot in the low mids which is often further acentuated by baffle step diffraction. Even with a TL IMHO a good FULL-RANGE system needs a sub.

The argument for why a 3-way is mandated is as follows. We make as our goal getting the best possible image, low distortion, fairly flat response (unless you subscribe to one of the various theories about non-flat responses for reproduction) :

1. Smallish mid-basses need a lot of excursion to produce decent sound levels at low frequencies. Apart from the mechanical problems associated  with this, there is also doppler and other non-linear distortion products at high excursions. Small speakers experience loss of control as the voice coil is driven further out of the magnet. A small speaker must not play too low. All speakers have an absolute lower frequency limit set by resonance, which is inversely related to the speaker size.

2. Narrow cabinets (requiring small diameter speakers) have been repeatedly shown to produce better imaging (subjective and measured  properties) than wider cabinets. This argues for a narrow midrange cabinet and by point 1 above dictates a smallish driver. This means less bass or a separate (sub)woofer.

3. The highest upper crossover point is largely dictated by driver size. At higher frequencies speakers "beam", that is the angle over which they disperse sound decreases. At some frequency this angle becomes so small the speaker is useless. This frequency is inversely related to the speaker diameter. For 5.25-6" speakers this point can be as high as 2800Hz. Larger speakers put the point much lower than this, and very large speakers are going to put the XO point so low the tweeter will go up in smoke trying to keep up. There are also very good arguments for not putting the tweeter crossover point too low, forcing us to choose a moderate size mid driver or small mid-bass.

4. There are also issues of baffle step diffraction and transient response in the bass region. It is noteworthy that powerful low bass with smallish speaker is really only possible using ported enclosures. Such enclosures that are optimised for the lowest possible bass have degraded transient responses and (wait for it) a phase shift at frequencies belwo 100Hz, which is induced by the combined mechanical and electrical properties of the speaker and box. In general, the smaller the speaker the worse this is. Ported speakers can produce very good transient response, but must sacrifice bass extension to do this, although in general such speaker will have better better extension than a simlar closed box design.

In short: Small speakers can't go too low. Big speakers can't go too high.

The conclusion one reaches is that you must compromise between volume, imaging, dispersion and transient response in a 2-way system. Using conventional drivers you can only get a good full frequency response (say 25Hz - 18kHz) with a 3-way system. A moderate compromise can be reached using a 2-way system with 2 identical small mid-bass drivers and one tweeter. The two mid-basses produce 6dB more oomph than a single one of the same size and can extend the bass, usually require equlisation, which is another evil altogether.

Finally I can concur with Hugh's assertion (it has appeared in a number of JAES articles in recent years) that phase distortion below about 100Hz is (almost) inaudible. If the crossover is at 100Hz then the wavelength at this point will be 3.4 metres, so placing the "sub" anywhere within about 1m of the cabinet should give great results. The band-pass problem of a 3-way can be partly circumvented using the natural roll-off the the mid cabinet.

Substituting one driver for the other in the AKSonic and expecting the same crossover frequency will depend largely on how similar each driver's effective resistances are (I haven't checked the specs of either driver). The effectiveness of the impedence equalisation will depend on how similar the resonant frequency and inductance of the two drivers are. If any of these parameters are radicaly different, then some parts tweaking will almost certainly be needed.

Apologies for using all this space. Hope this helps a little.

AKSub Hugh? OK. DAC first.

T.

[JohnR]

Hi Ben, hope all is well. I can't resist commenting on the following:

3. The highest upper crossover point is largely dictated by driver size. At higher frequencies speakers "beam", that is the angle over which they disperse sound decreases. At some frequency this angle becomes so small the speaker is useless.

Not true. This assertion assumes that the driver is "pistonic," but drivers that are designed for "full" or "extended" frequency response are not pistonic. The dispersion does decrease with frequency but it is far from useless. Various people will argue about the decreased dispersion at HF of "full range" drivers, off-axis response, etc etc, but I have heard plenty of disastrous two-ways (*) in my time (and in some cases, paid for them ) and will take a decent full range over a poor two-way any day.

(*) I exclude the Aksonic from this category, of course. I did hear them at AksaFast and enjoyed them a lot.

PS hope I haven't veered this thread too far off topic

[stvnharr]

Gotta love the muddy water!!!

An alternative to the 2 way speaker, which compromises either the low bass with a small woofer, or the upper mids, with a large woofer, there is the 2.5 way with a second woofer (the .5 one) to help out the bass.   This is easier to design than a 3 way, but gets much of the sound of the 3 way. The main woofer is released from the baffle step duties, which the .5 woofer assumes, as well as reinforcing the bass.   Still, 7" drivers are used and don't get into the lowest bass areas, but it is a nice compromise.

Of course cost is an issue as there is the cost of another pair of drivers.

Ultimately, one has to decide on one's compromises.

[Tinker]

Hi Ben, hope all is well. I can't resist commenting on the following:

Not true. This assertion assumes that the driver is "pistonic," but drivers that are designed for "full" or "extended" frequency response are not pistonic.

Touche! You are completely right. I should have qualified this statement more carefully. However, it is probably true that a lot of easily available drivers are cones that are pretty much meant to be used in the piston range. Plus these kinds of drivers have well-known mathematical models which makes it easy for the DIYer to be confident of a good result, and I think this is an issue for the amatuer builder who doesn't want to do hours of testing and rebuilding.

Full-range drivers are interesting. I recall a vocal PA my uncle made when I was very young, which was an array of over 100 small full-range drivers. Amazing power and clarity. There's a lot to be said for them. There are a growing number of experimental surround systems and wavefront synthesis technologies that use arrays of full-range drivers, but I digress.


I put forth another contentious opinion, that for the DIYer, a "simple" 2-way will deliver the most satisfaction for the time and money, and it is relatively easy to add a sub later. Just an opinion, that more experienced builders are free to ignore.

Anyway, if the TL becomes a go I'd be interested to hear how it works out.

T.

[StevieM]

Sorry Hugh, this is straying form AKSA territory somewhat........but on the theme of interesting speakers look at this website. This guy lives a couple of miles up the road from me and I keep meaning to get in touch with him
http://www.soton.ac.uk/~apm3/diyaudio/Speakers.html
hope you all find it of interest
Steve