Computer-Aided Design of Loudspeaker Crossover Networks

[Thomas savage]

After chatting to a speaker designer a while back who was a believer of active methods but of late feels the gap has been closed by the use of CAD programs to design x over networks I though I would ask the opinions of our members..

 

[Purité Audio]

When Ben Lilly of ATC visited we chatted about crossovers, ATC stil use traditional electronic crossovers but they do use a MiniDSP unit to prototype , he said it saves times and money and allows them switch quickly between options.
Keith.

 

[Thomas savage]

A segment from the vivid audio web site regarding this subject ( and some sexy graphs )...,

Thanks to the intrinsically smooth response of the drivers in their respective enclosures, the business of crossover design is made relatively straightforward but when combined with the use of computer aided analysis it has become possible to create complete sub-system responses which are astonishingly faithful to the ideal function; levels of accuracy which were once thought only possible with active systems.

 

[Thomas savage]

Being a peasant I have no access to this but it's on topic and I know some of you are members .. Or say you are

http://www.aes.org/e-lib/browse.cfm?elib=3779

 

[Purité Audio]

Computer simulation programmes have been around for ages, SPICE ? but the advantage of creating different crossovers digitally is that you can actually hear them and A/B between options.
Keith

 

[Thomas savage]

Computer simulation programmes have been around for ages, SPICE ? but the advantage of creating different crossovers digitally is that you can actually hear them and A/B between options.
Keith

Details on SPICE can be found here
http://bwrcs.eecs.berkeley.edu/Classes/IcBook/SPICE/UserGuide/overview_fr.html

Yes but I am referring to the advantages of active vs passive speakers and how that's mitigated to a extent by computer aided network optimisation.

I never claimed it was anything new...The date on that paper is 1980 however I am not sure speaker designers have been using software to optimise networks in the way highlighted above since 1980.

Either way these techniques seem to mitigate if not eliminate the traditional notions of advantage for the active type of loudspeaker implementation.. Some discussion on that would be appreciated

It's that true? Is there more to it? ( it's a topic for discussion)

 

[Purité Audio]

Active loudspeakers are technically the way to go, everyone accepts that, digitally implemented crossovers such as Grimm and KII offer further advantages over traditional 'electronic' crossovers.
I will leave you to come up with reasons why active speakers are not more widely promoted by magazines and amplifier manufacturers.
Keith.

 

[Thomas savage]

Yep and organic carrots are better than non organic carrots... Everyone accepts this
Beer is better out of bottles than cans... But is it? Everyone says so. ( technically it's not)( if you insist on bottles make sure they are brown! Not clear or God forbid green!)

 

[Purité Audio]

Think about what a traditional passive crossover consists of , coils,inductors, capacitors etc etc the signal has to go through all of them, electronic crossover is better and a digital implementation allows you to use steeper slopes , use dSP to correct any driver anomalies , correct phase ,delay etc etc.
Active means one amplifier dedicated to one driver, how can that not be advantageous?
Of course there is no oppurtunity to 'upgrade' amps, cables etc.
Keith

 

[Cosmik]

The question is: if passive crossovers had never been invented, and active systems had always been around, would anyone now be pushing for passive crossovers?

I am sure that CAD can design them to achieve arbitrarily complex responses - I have in the past written a program around SPICE to home in on a desired response, myself - but there must always be the issues of damping and other interactions with the drivers that change with temperature and power. And also, without DSP-based active systems you cannot sensibly achieve linear phase and time alignment (- although I know that everyone says this is utterly pointless).

As with the debate over time alignment and linear phase, I think that starting from ancient technology as the default position - and spending more on it than the undeniably more accurate active system - is perverse.

 

[Purité Audio]

Wise words indeed!
It is interesting that in 'pro' audio active loudspeakers are accepted without a second thought , the advantages are acknowledged and accepted , yet the traditional manufacturers, magazines have managed to persuade punters that 'traditional' is somehow best.
HiFi is truly a unique and bizarre pastime.
Keith.

 

[Thomas savage]

Wise words indeed!
It is interesting that in 'pro' audio active loudspeakers are accepted without a second thought , the advantages are acknowledged and accepted , yet the traditional manufacturers, magazines have managed to persuade punters that 'traditional' is somehow best.
HiFi is truly a unique and bizarre pastime.
Keith.

Yea, here's a quote from the PCM web site on the issue..,

While it's hard to better a well-engineered set of active speakers, passive monitors, when designed with care and respect like PMC's range, will always outperform inferior active designs. Aimed at customers who prefer to define their own speaker/amplifier combination, our passive speakers are just as uncompromising when it comes to quality.

 

[Purité Audio]

ATC used to supply PMC with their mid driver then PCM commissioned Volt to make a copy.
I am certain from a technical standpoint ATC would prefer to only make active versions, but they are a business
and have to offer what the market demands.
Keith

 

[Thomas savage]

I would prefer my speakers were active but only for convenience really, if there was a huge audible advantage between the two approaches we would know about it as there would be demonstrations at every hifi show to highlight this.

Personally I would love to compare a active G3 to my passive version in my own home.

With the issues of room construction ( dimensions and build material) and speaker placement in many cases this all becomes a rather moot point.

Unless you believe all amps sound the same there is merit in being able to chose your own.

 

[Purité Audio]

ATC make active and passive versions of the same loudspeaker so a comparison is perfectly possible.
Solid state amps IME ( given the usual caveat of being properly designed ) do sound the same given that they are being used to drive loudspeakers ,which they are physically capable of driving, I'd not clipping.
All the more reason for a dedicated amp per driver.
Keith

 

[Cosmik]

...if there was a huge audible advantage between the two approaches we would know about it as there would be demonstrations at every hifi show to highlight this.

I have pondered on doing this, to demonstrate at hi-fi shows, but there are issues of course. The main one is that going active allows you to build a different speaker in the first place. Merely converting an existing passive speaker to 'active' would not demonstrate the true capabilities of the active approach.

 

[Thomas savage]

I have pondered on doing this, to demonstrate at hi-fi shows, but there are issues of course. The main one is that going active allows you to build a different speaker in the first place. Merely converting an existing passive speaker to 'active' would not demonstrate the true capabilities of the active approach.

Umm, most actives I see are just a box with drivers in so I am curious to know what these 'different 'speakers you speak of are other than the relatively new beolab 90 I can't think of many.

 

[Purité Audio]

Cosmik would be far more knowledgeable than me, but if you are using DSP, for crossover implementation, correcting phase, delay and any driver aberrations, I presume you would not be so constrained by driver choice/size /enclosure ,baffle arrangement etc and of course the Kiis and Beolabs take the use of DSP so much further.
Keith.

 

[Cosmik]

Umm, most actives I see are just a box with drivers in so I am curious to know what these 'different 'speakers you speak of are other than the relatively new beolab 90 I can't think of many.

It is things like:

• not having to worry about efficiency
• being able to go three- or four-way instead of two, without extra complexity and side effects
• being free to use sealed enclosures and drivers suitable for bass EQ-ing
• not worrying about sloping the baffle but being able to dial in the correct delays anyway
• having a wider choice of compatible drivers

Someone who knows a lot more about passive crossovers than I do (which is very little):

In my opinion, passive crossovers are useful for small 2-way systems, or where you aren't looking for the ultimate in performance. They are also useful with a biamped system, where the passive network only handles the transition from midrange to tweeter. In this area, the design isn't overly difficult to get right and the power demands are comparatively low.

Should you attempt a 3-way passive design, you will almost certainly need to include a Zobel network for the bass driver, as well as resonance correction for the midrange. When you add this complexity it becomes quite obvious that the passive approach will be large, complex and expensive. The losses introduced will be such that sensitivity will be significantly lower than you might like, the damping factor for the woofer will be severely limited by the series inductor, and the system will still be a compromise.

http://sound.westhost.com/lr-passive.htm

 

[h.g.]

Umm, most actives I see are just a box with drivers in so I am curious to know what these 'different 'speakers you speak of are other than the relatively new beolab 90 I can't think of many.

Here is a reasonably priced pro box speaker with an active crossover that has been around for many years as an evolving configuration. Compare the directivity plot shown in the measurements section on the RHS with the narrow beam directivity plot for the Beolab 90. Notice any similarities? An older discontinued design (again click measurements on the RHS) used to use linear phase filters but this was dropped. What might have prompted that?

CAD programs are equally useful for passive and active crossover design. Software like Keith's SPICE to simulate a passive crossover network has been widely and freely available since the 1970s and would have been used in the larger more technically able speaker companies since around that time. That might not be the case for smaller companies where the chap with the task of coming up with the crossover may also be responsible for a wide range of other tasks and may not have a suitable background to work with the computer software and hardware.