Time coherence - how important and what speakers?

Soix -

Thanks for pointing out my oversight! I stand corrected.

Theil actually has at least two concentric drivers, one of which is a mid/tweet module (requiring a separate woofer), and one of which is a midwoof/tweet module, requiring no separate woofer.

The radiation pattern would still vary considerably with respect to frequency, but output from the concentric units would be free of the off-axis anomalies that otherwise occur with non-coincident drivers using first-order crossovers.

A couple comments about concentrics: It has always seemed to me that the presence of a moving cone surrounding the tweeter would actually frequency-modulate the tweeter's boundary reflection signal, at least in the range where the tweeter is not beaming, leading to some rather strange colorations. That, plus the inevitable horn-loading colorations imparted by the midrange cone onto the tweeter's signal, makes me not-so-impressed with this concept.

Audiokinesis- thanks for your kind comments about Jeff and me! Jeff and I have yet to wrestle behind the Alexis Park- we'd buy each other beers instead.

I find many of the smaller speaker designers (smaller than corporate entities such as Vandersteen, Theil, Martin-Logan) share tips and ideas. We'll give out supplier information, talk about how to join a particular wood, a brand of power tool not to buy, or a better way to pack for shipping. In tight spots for parts, some of us have been known to help out another. This does not happen in amplifier design or in digital or turntable work (it would not be kind to speculate why).

Consider that
-Jeff and I know that truly professional loudspeaker design requires a lot of hard physical work done in isolation, in many different areas from materials science to field theory.
-We rarely sit down with someone who has actually experienced and understands the difficulties of bringing any speaker design to full production.
-We read research papers of all sorts, looking for an insight on a particular measurement technique, or to find if a certain type of cabinet loading has pitfalls the researcher missed, so we don't have to stop and make that measurement or cabinet ourselves.
-We learn that materials and drivers suppliers don't know enough about their products to help us.

We share tips because advice is worth A LOT from someone whose judgment and experience we respect. Besides, any tip about who makes a good acoustic felt is never going to upset the competitive balance anyway. After all, we're taking on the big guns of "speaker design", with their not-too-insightful designs that border on outright laziness. We are the Panoz up against K-cars. ~twas ever so... The big firms attract customers who drop big bucks, only to be bored with the music; sound so un-inspiring that their friends hear there's no way they'd EVER spend that kind of dough.

There are no schools for speaker design, and no peer review as in other science or engineering fields. It doesn't help that magazine writers aren't technically competent like they were 25 years ago. So, just because you are the head of B&W, Bose or the Candadian Research Council, doesn't mean you know what you're doing. Nor will anyone find out... remember, few people can call us out on what we really know about sound.

Besides, when any design doesn't "sound quite right" or perform well on aggressive music, it must be the cheap amplifier or the dreaded "poor recording" and of course the room at the stereo show.

It couldn't be that the designer hasn't ever looked at time-domain math, or at cone breakup (putting in notch filters!!!), or looked for shear vibrations in the standard folded-up cabinetry that their cabinet shop assured was "the best way to make the cabinet". Or hasn't even looked up the absorption coefficients of the internal stuffings to see that his crossover is "correcting" for a 1/4-wave internal cabinet resonance that shouldn't have been there in the first place. Nor has he done the 6th-grade wavelength math that shows how non-parallel cabinet sides don't do anything to supress internal echoes. He only has to claim they do, `cause that seems to make sense- and he knows so much more than the listener or reviewer!!

There are poor speakers because there are poor designers. If these were cars, many of them would have five wheels, and three would steer!

So, the more customers that move over to Jeff's and my speakers (and to Soundlab), the more they'll enjoy the music- which keeps them customers of ours, and their friends in the market too.

There will be time to discuss some more of the ins and outs of speaker design- my thanks to all who have posted, because they at least listen, and think about what they are hearing.

However, anyone interested in understanding the art of speaker design needs to get a good grasp of the fundamentals of soundwave propagation. Step one is to know wavelengths vs. piston diameters, necessary to understanding the reasons for directionality, reflection, and "radiation resistance".

As an example, the designer who espouses a tweeter should be placed in the center of the mid's cone, ignores 6th-grade math that clearly shows that, in the crossover region, any tweeter will always try to be fully omni-directional (the wavelengths there are 3-7 times the dome's diameter!). The truth of that omni-directional "mathematical assumption" is always verified on sine wave tests, on noise tests, on impulses, tone bursts, on TEF, MLSSA, indeed on EVERY test.

Thus, being PROVEN an omni source, any tweeter's low end will ALWAYS "splash" off the mid's cone. Which means the designer will always screw around with the crossover, sucking out the bottom end of the tweeter until it "sounds OK on Holly Cole". It should also sound OK on Janis Joplin, and Billy Holiday, Dinah Washington, James Brown, Screamin' Jay Hawkins, Willie Nelson, Garth Brooks, the Klezmatics, Britten and Barber and Stravinsky, and the three tenors, and Tool, Primus, Metallica, ... but somehow it never does.

That designer needs to justify putting the tweeter in the center is a great idea, so he will manipulate the crossover to make 2 or 3 types of measurements look good, make the speaker sound fine on non-aggressive music, and ignore the other tests and other music.

It is also easy to say "we are not sensitive to phase", and to claim that after all, "the recorded sound is mixed up in phase before it ever gets to the speaker". There are several holes in that logic that I see. Can someone in the forum point them out? And it's not because the word "phase" is used instead of "time".

No reviewer will ever call a designer on his technical claims, from politics, from a lack of education. The designer of the concentric-tweeter model will then proudly display the complicated crossover that makes the design function on those few tests and recordings that "prove" how good it is. And the giant "wall of sound" that emerges is unlike anything else- so it must be better, as it's from a wide-selling, respected designer! And different is often confused for better- until we play enough variety of music (another thing that isn't done during reviews) to hear that speaker's signature.

I prefer time-coherent designs, simply because they are more revealing of the musical intent. On non-minimum-phase speakers, you listen to the separate parts of the sound, as the speaker picks it apart in time. The "separate tweeter" phenomena is one example, another is the image sticking to the face of the speaker (lack of depth), and another is the sensation of height- a tweeter out-of-phase artifact. But the real test comes on instruments with harmonic structures that span the crossover region. Without time-coherence, they sound flat, lifeless, definitely not "real".

To avoid listening to the sound of the sound (can I say that?) instead of the instruments, listen to ALL sorts of recorded music, at all loudnesses, on all kinds of stereos- ESPECIALLY music you don't much care for nor ever will. Try to experience many different types of live performances, from unamplified acoustic music in a living room, to a marching band. That's how you become an experienced audiophile- by knowing sound.

Thanks again to Audiogon for providing a place for such a forum. Jeff and I will be out back behind Audiokinesis' store taking apart a Soundlab if you need us.

Roy Johnson
Green Mountain Audio

PS: something to think about for what's "real sound" from a speaker- here, the working definition is "the perfect speaker gives us the clearest, single pinpoint image from each mic." ~cause the mic itself cannot pick up more than one dimension of the soundfield- and that dimension is distance. Which is time-arrival differences. Which is another indicator of how time-domain information is indispensible in enjoying music, and in reproducing the clearest pinpoint image from each mic.

We are stuck with pinpoints as "perfection". If you don't hear specific pinpoints (images that have no height, no width, only depth) in ALL the tonal ranges of the music, then you are hearing time-domain distortions in those ranges where the pinpoint is smeared out (up to "life size"!). Even in the nearfield of the time-coherent Soundlabs and Wisdom Audio ribbons, all we hear are pinpoints, especially when we remember to close our eyes to allow our ears to better function as location sensors. And you'll find that even surround sound is only 2-D, with only depth and angular location as the dimensions.

Karls- you got it right.

If you wanted to know the actual % modulation distortion, you'd have to know the stroke and frequency of the mid's vibrations that are affecting the tweeter's sound. Which are random, as far as the tweeter is concerned. Which means the modulations are unpredictable on music- so all we can say is that they should probably make the sound hazier or dirtier.

Jeff's crossover is probably the only one that could make a co-ax design work well. Even then, the tweeter dome would require a modest horn around it- a waveguide to keep the tweeter's sound from bouncing off the mid's cone. Of course, the mid's sound will bounce off that waveguide's exterior...

The "horn-loading" coloration is a common term- what we're hearing are the quasi-transverse reflections from the sidewalls of the horns, and the reflection from the mouth of the horn back to the throat.

At the mouth of the horn, the sound pressure goes from travelling in a high acoustic impedance to a low acoustic impedance. Thus, from the un-equal impedances, a reflection/standing wave takes place inside the horn. Then there is the matter of a horn's possible throat-compression ratio that boosts efficiency and distortion (love them PA horns, don't you!). You can't compress/rarify the air more than 1% or you get harmonic distortion from the air itself.

Roy

Well, Roy you share a quality with Jeff that I truly admire. You speak your mind. You also seem to share the same fault. You condemn those who take a different path. I do think that you seem to be a little overly critical of the "corporate entities". Jim Thiel and Richard Vandersteen both started their corporate entities out of their garages. Those who belittle success doesn't hold much promise for themselves. The designer who lacks 6th grade math skills (and we all know who you are referring to) seems to have fooled his university professors into giving him an engineering degree. This same designer seems to be aiming at the same target you are with his co-axial drivers (the mirror image of a microphone and it's pin point radiation pattern). You find fault with his approach. That's fine, your entitled to your opinion. It's also interesting that his success has allowed him to experiment with designing new drivers, a luxury many competitors don't enjoy. Attacking his education or intelligence doesn't seem like such a good marketing concept. I either don't remember learning this elementary math or never learned it in the first place. I'm sure I'm not alone. You may have just insulted a large percentage of potential customers. Don't worry I'm not that thin skinned. I can't help but feel that one can be opinonated and back up one's position with mathmatical evidence and still be diplomatic. See, I really do want you guys to succeed. I applaud both of you for sharing the evidence that brought about your design philosophies. I look forward to more from both of you and hope you lead others to be so frank.