[Aether Audio]

Everyone,

Stepping back and reviewing all of the recent developments regarding the crossovers in our loudspeakers has given me cause to reflect a little.  If one takes it all in, it has become obvious (at least to me) that the “big picture” is there for all to see.  I will explain.

As a preface, I will say that there are other areas that apply specifically to loudspeaker design that do not come into play in the other components of the signal chain.  Specifically, they are matters that pertain to the acoustic domain.  Those are things like dispersion and diffraction, which, if they do have electrical analogs, are very small in comparison to their effects in the acoustical performance of loudspeakers.  As such, the following does not address those issues.

In the world of engineering we have two major subdivisions regarding the transmission or reproduction of information (signals/waveforms).  These divisions identify the dynamic extremes our equipment must be able to handle without injecting errors (distortion).  At one end we have the weakest signal levels, which in engineering terms we refer to as the “small signal condition.”  Logically, the other extreme is referred to as the “large signal condition.”

These terms are most often referred to when discussing amplifier performance, but they actually apply to any device – including loudspeakers.  In the loudspeaker design world though, most designers perceive the most difficult issue to address as that of the large signal condition – and rightly so as it is the most obvious and difficult to contend with.  In fact, this is so true that many designers – including myself in the past – have a tendency in their efforts to overlook the much finer details that encompass the small signal condition.

There is one other good reason for doing so as well…the designer often “believes” that he has little influence over these parameters, if they really even exist.  The issues of the small signal condition are very difficult to measure – if they are measurable at all – and are essentially “built into” the parts we are forced to use.  It appears to me now that once the large signal issue has been sufficiently dealt with, the “last frontier” is that of the small signal world.

As one steps back and analyses the ongoing debate and struggle that the dedicated audiophile often finds himself in regarding cables, connectors and such, we see that the small signal realm is the source of great controversy.  Anybody can hear when a loudspeaker or amplifier is driven into gross distortion.  In fact, the resulting distortion that occurs when any device – say even passive ones such as non air-core inductors – is over driven, is clearly audible and easily measured.  We call such distortion as being the result of “saturation effects” and they must be avoided at all costs.  Why?  Because when the dynamic headroom of a device is extended, all errors and artifacts produced at lower levels are typically reduced as well.  A device that can cleanly reproduce large signals will almost always sound better at any other level below that point.

Therefore, due to the extreme dynamic peaks modern digital sources can now achieve, virtually every designer finds himself forced to focus on this one parameter far more than that of the other extreme.  Rightly so…as it makes little difference what the small signal performance is when the system produces significant distortion at, or even slightly above, average volume levels.  This issue is so dominantly important that once a designer has truly achieved success in overcoming it, he can easily be deceived into thinking that there’s not much left that he can do. 

In regard to amplifiers, the designer is only left with the “noise floor” at the other end of the dynamic scale, and if that is sufficiently addressed his work is essentially “done.”  This assumes of course, that it is a good design such that “static” distortion and the rest are acceptably low as well.  The amplifier designer has one other weapon in his arsenal to correct small signal errors though – negative feedback.  The poor loudspeaker designer is not so fortunate.  We run “open loop.”

In order for us to begin the effort to reduce or eliminate these small signal errors in loudspeakers, we must first address their source.  The most obvious of these is “resonance.”  Resonance occurs most commonly in transducer diaphragms.  We call these “break-up modes” wherein the cone or membrane flexes in a manner that produces “standing waves” that continue to travel back and forth or “ring” long after any signal has passed that has acted as a stimulus to begin the process.  As loudspeaker designers, we have a universe of drivers to select from that exhibit varying degrees of these effects.  Ideally, we select those devices that exhibit these effects to the smallest degree possible while achieving our other large signal objectives.

Having done so, we typically find ourselves with a “completed” design and at a loss to effect much further change.  But there are other sources of distortion and improvement if we choose to look more deeply.  “Hysteresis effects” are always lurking in the background.  Hysteresis is simply described as a sort of “memory” in a device that causes it to behave differently when signal polarities change direction.  A device can “hang onto” a state that it has most recently been in before an instruction (signal) demands that it change vectors (direction of motion or current flow).  Distortion is the result if this occurs. 

Every real-world device manifests some level of hysteresis and loudspeaker drivers are no exception.  Hysteresis effects, if they are severe enough, will generate measurable levels of distortion and thus, drivers are selected that exhibit it at a minimum in a good design.  But…the type of distortion products that hysteresis causes in drivers usually manifests itself under large signal conditions, therefore, lending themselves to easy observation and correction in the driver design.  While the effort to reduce hysteresis in a driver may not be easy in itself, since it can be easily measured, at least we have a place to start.  Inductors in the crossover network can be obvious sources too and behave much the same under similar conditions as that of drivers.  Once all these have been sufficiently dealt with, it appears we are done.

[Aether Audio]

Part 2

Ah…but not so fast.  There are two areas left that escapes obvious scrutiny and which lies as the source of the “great debate.”  Both of these are the result of artifacts being generated under small signal conditions and/or reside at very small signal levels.

Recently, a caller inquiring about our product made the comment that the “crossover was more important than the drivers.”  On the surface this seems ludicrous.  But the caller made the point that “even cheap drivers can be made to sound good.”  Well, if this weren’t true there would be a lot of high-end speaker companies without a product to offer.

But how can this be?  The fact is that if a speaker is intentionally designed “not” to offer wide dynamic performance, then driver demands are greatly relaxed and the cost is significantly reduced.  The greatest advantage really “good” drivers have to offer is their ability to “keep it together” under the extreme stresses encountered during large signal conditions. The bulk of such ability is rooted in their magnet/voicecoil/motor/ suspension systems and is really what drives up the cost.  An inexpensive driver can achieve very good performance and have relatively low levels of hysteresis as long as expectations of large signal performance are limited.

So…it appears our caller was correct.  Then what is it that makes even these cheap drivers sound good?  Apart from generally good compromises being made in the overall system design, it seems such systems can be designed to have very good small signal performance.  If such inexpensive drivers already offer low levels of hysteresis, what else must be done to design systems with them that produce excellent results?

Well, all we have left is the crossover and the wiring used that interface with those drivers.  That means those components must exhibit excellent small signal performance as well.  Specifically, crossover components and (now I believe) wires can exhibit resonances in the form of micro-phonic vibration and “sub-luminal” charge carrier (electron) standing-wave formation.  This is all rather complex and much of this can be found here: http://www.audiocircle.com/index.php?topic=39346.0

Along with the above, capacitors can exhibit the same behavior and are outlined in the above link as well.  All of these effects are the result of non-linear small signal behavior and/or a type of hysteresis that only manifests itself during small signal events.  Therefore, by their very nature these effects do not lend themselves to easy measurement.  Hence, they are at the crux of the entire “tweak-o” debate.

We at SP Technology have now come “full circle” and are beginning to see “the forest for the trees” regarding the entire range of signal reproduction.  It must be realized that human hearing has the ability to discern a dynamic range of 120dB – or even more under certain conditions.  Such a dynamic range has no comparison to any other sense and exceeds the limits of virtually most, if not all modern test equipment.  120dB represents a ratio of 1 trillion to 1 and therefore, makes human hearing one of the most sensitive detector/transducer/processing systems in the world!  Who are we to question it?

Having successfully overcome the large signal limitations of loudspeaker design, SP technology is now moving forward in the evolutionary process of completing the final engineering requirements for an "all inclusive" approach to world-class loudspeaker design and production.  It has been a long journey, but hope springs eternal as long as the quest is not abandoned.

Let the naysayers begin!

-Bob

[Aether Audio]

Marbles,

Thanks!

Although, I seriously doubt we're the "center."  There's a lot of bright folks out there in the loudspeaker design world that I highly respect.  One that's right here in our own AC "backyard" is Danny Richie of GR Research.  That guy tried to tell me some of this stuff a long time ago, but since I couldn't wrap my head around it at the time, I pretty much dismissed it.   

If anything, I think the only thing we've really done is draw attention to these issues through the process of our own discovery - the bulk of which has been lead by the obsessive nature of Karsten and his quest for the perfect speaker.

The most I think that I've been able to do is to tie it all together for myself first, then ultimately other folks, in a way that helps them to understand it all in the form of one big picture.  I'd like to think we've done something "original" here, but actually, it's more likely we're nearer the back of the buss rather than the front... on this one.

-Bob

[Robert C. Schult]

Hi Bob.

...That guy tried to tell me some of this stuff a long time ago, but since I couldn't wrap my head around it at the time, I pretty much dismissed it.   

You probably don't remember but Steve and I met you at Stereo Exchange in South Bend late '80s or very early '90s. You had brought an early version of what I think is now your Timepiece. I tried to tell you the same thing then too. You dismissed it. I've always held a bit of anger about you for that. But now, I feel better ! Glad age and experience continues to make us all wiser.

I think you've met my wife at Staples within the last six or eight months. Maybe we could meet up again. Might be fun....or not

Anyway, congrats to you Bob on SP Tech's success. You've worked long and hard many years and deserve it. Good to be a fellowlaborer with someone who's making a good difference for folks.

Respectfully,
Robert
RSAD

[Robert C. Schult]

Hello Bob,

Steve Rothermel here. That "other" speaker guy in little La Porte, IN.

Congrats on the success you are having...I KNOW how hard it is to get your babies up and walking.

I sometimes get asked at the local stores if I am "that speaker guy, Bob Smith". It's frustrating being confused with you around here!

So now when they ask, I say "YES, charge it to my account and I'll need twice as many this time"!

MUCH SUCCESS, BOB!  aa

Steve Myspeakersareheavierthanyours Rothermel

[JLM]

I've been saying sort of the same thing for years, that the essence of the music is not at the frequency or sound pressure extremes.  Of course this idea isn't new, it was part of the facts-of-life from decades ago.  Since then marketing types have been very effectively convincing us that we need 20 - 20,000 Hz at 120 dB.  Factoring in the male dominance in audio, it's easy to see how the vast majority of the market bought into this "macho" message and a lot of new loudspeakers.

Striving for 20 - 20,000 Hz at 120 dB has led to precious few real advancements in loudspeakers, but has produced tons of different loudspeakers that try to maintain those standards while also being all other things to all other people.  Of course no one has invented the perfect loudspeaker, so the result has been trading off one parameter for another as designers and consumers chase their tails.

And as you say Bob, that is exactly what we've been designing to, while neglecting the heart of the music (IMO something closer to 80 - 5,000 Hz at 70 dB).  You also mentioned "the digital effect" as pushing the need for dynamic range.  Another factor that is usually overlooked in the continual increase in background noise in western society.  (Background noise in hospitals for instance has been increasing 5 dB every 10 years.)  For instance at home the dishwasher is now a noisy machine, the water heater has a power vent, the furnace is equipped with an air conditioner, the neighbors are closer, we have more power groundkeeping equipment, and when's the last time someone read or used a computer without the TV or music on?

As we're being conditioned to higher background noise levels the result being needing to turn up the volume (and suffer possible hearing loss).  One of my answers to this noise issue is use of a dedicated room that is insulated from the rest of the house.  Frankly I have a hard time understanding folks that spend $10,000 on a system that goes into a poorly designed room.  When I was faced with that problem in college the answer was headphones.

[Russell Dawkins]

The measurement protocols for both amplifiers and speakers also reflect this bias towards the large signal realms of performance.

For a couple of examples, Stereophile, a respected source of measurements, usually displays amplifier distortion on a graph with a range of 0.1 W to 1 kW. With a speaker of a sensitivity of 90 dB/W, you are looking at amplifier performance over the theoretical spl range of 80 dB to 120 dB. Much of what is talked about in terms of amp performance is addressing the micro dynamic range of, presumably, - 40 to -60 dB, this is being completely missed in the measurements. For the average speaker, the power requirements for reproducing quiet signals like background instruments and room ambiance are around 0.01 mW, not .1 W.

Often I see a rising trend towards the low end of the scale - the question is what happens beyond the scale, at output levels down around .01 mW, which is 10,000 times lower level? That is where the amplifier performance would give some clue as to the way the micro dynamics are handled.

In a related way, speaker frequency response curves are given for output levels around 85 - 95 dB. We hear of speakers that seemingly "come alive" only at slightly higher levels. Perhaps this could be actually seen if response plots were also taken at 50 dB levels. With modern microphones and electronics and reasonable test acoustics this is still out of the noise floor.

[JLM]

Good points Russell.

Kinda like buying a 150 mph sports car to drive on 70 mph roads, eh?

[Aether Audio]

Robert,

Sorry so late getting back to you.  I’ve been up to my eyeballs in work.

You probably don't remember but Steve and I met you at Stereo Exchange in South Bend late '80s or very early '90s. You had brought an early version of what I think is now your Timepiece.

Yes, I remember.  I remember going to your house too and listening to some speakers you had built.  Although, I’m getting a little forgetful in my old age and that was a long time ago, so I don’t remember much detail.

I tried to tell you the same thing then too. You dismissed it.

Well, I wouldn’t say I “dismissed” it so much as it was probably more like (I really don’t remember one way or the other) I couldn’t wrap my head around it.  You must remember.  I come from a more orthodox engineering background.  “If you can’t measure it, it doesn’t exist.”

I've always held a bit of anger about you for that.

Gee, that’s a shame. I had no idea, as I don’t remember any indications of such at the time. As far as I remember we had a good time together.

Glad age and experience continues to make us all wiser.

Age has a tendency to do that with the more fortunate of us.  Although, I am glad I stayed the course I was on at the time. 

True to its promise, science has been a faithful guide.  It has empowered us to develop systems that have essentially eliminated the most common sources of non-linearity.  Dynamic compression and the various distortion artifacts resulting from large signal conditions have been all but totally eliminated.  Acoustic domain issues such as diffraction and dispersion have greatly benefited as well.  I guess you could throw bass performance and extension in there too.  Not that any of those things really matter all that much.

Having achieved a substantial level of success regarding the above issues has resulted in a platform that enables us to clearly discern the subtle issues that reside in the small signal domain.  It has been our experience that if a system is optimized for superior large signal conditions, linearity and resolution is improved across the entire dynamic spectrum.  We are now able to clearly discern the subtlest changes in components and other peripheral sub-components with virtually no obfuscation caused by the more common forms of distortion.

In light of the above, I suspect an endless process of changing “pieces and parts” would have been the story otherwise, had we not followed the path of science.  I believe that unless you optimize every possible and obvious (measurable) source of non-linearity first, you will be forever treading water in an ocean of confusion regarding the true source of a potential problem. 

To sum up, I say there’s no sense worrying about which pimple is ugliest on a potential girlfriend’s butt, if that butt is 3 feet wide!

Anyway, congrats to you Bob on SP Tech's success.

Thanks!  – And to you too.  We have different design objectives and that translates into different product definitions.  My hat’s off to you as well.  We’ve had more than our share of trouble working with wood... I don’t even want to think about granite!!!  And I thought I was a glutton for punishment.

-Bob