Time coherence - how important and what speakers?

Roy..according to your "definition"...Meadowlark,Thiel,and Dunlavy would all be incapabable of true time/phase accuracy... and since Vandersteen incorporates a T-line in their designs...I guess they would also be eliminated...

Vandersteens are definitely NOT T-lines, marketing hype aside. They are ported, end of story, and passive radiators are just a variation of porting. True T-lines are VERY big and VERY difficult to build. And I will take exception to Roy's (and Martin Colloms') lumping of T-lines in with ported designs, as they are not the same. The crucial difference is that a properly filled T-line has the best and most uniformly damped impedance curve you will see in any speaker anywhere, better than most sealed boxes and the exact opposite of all ported boxes. Impedance peaks are due to resonance, plain and simple, and the sharper and higher the peaks, the worse the resonance Q and the higher the stored energy. This is directly related to bass transient performance and that is why properly damped T-lines have a legendary reputation in the deep bass (and why ported boxes suck). I say this as a huge fan of sealed-box loading, often the best "real-world" compromise. But beware, a claim by the manufacturer of "T-line" loading is not sufficient to achieve this level of performance. It takes a lot more work than writing ad copy.

Karls, I think the issue with transmission lines in this thread may have more to do with time.

There is NO port on the Vandersteen 2,3 and 5 series speakers. They are not claimed to be trasmission lines in literature or anywhere else. They have an 8" woofer and a 10" driver (operates below 35hz) in a SEALED enclosure. The 10" driver is active, not passive. The Vandersteens are also as close to time aligned speakers as there is out there. They are also phase correct. Also, they used a baffleless design that does away with reflections from the front of the cabinet and the drivers are staggered for alignment. Vandersteen has covered all the bases in his designs. All drivers operate in the same acoustic phase(something a lot of speakers don't)
I don't know where you got your information from but you need to recheck it. End of story!

Karls, Phasecorrect, no speaker is without time delay in its lowest three octaves, because a moving mass on a spring has 90 degrees of wave-period time delay at its primary resonant frequency.

Also, any port's output is time delayed, from the interior time-of-travel, and from the exterior extra time-of-travel from that opening. Port outputs are also polarity inverted.

The specific reasons a properly-engineered t-line (which are few) has low distortion bass:
--The port opening is reproducing the lowest bass and so the cone is not moving very much at that resonance point- this is part of the definition of a "ported" speaker.
--The upper impedance peak always seen in a ported speaker is mostly absent- a peak due to the port's air mass bouncing off the compliance of the air in the enclosure. This, Karls, is what you are referring to. Why is it not there? From the proper application of the wool stuffing and the shape of the small enclosure right behind the woofer.
--The t-line cabinet CAN be shaped so that its rear wall generates less echo directly behind the cone- but not via the usual tapered short horn leading into the line. A smooth taper only efficiently loads the returning third harmonic (of the t-line's fundamental resonance) back into the rear of the cone, causing a serious dip in the cone's output at 3X the fundamental t-line resonance.

Also, resonance is not always accompanied by an impedance peak- there's always resonance at an impedance minima, which even a t-line has. That's the frequency where the cone is driving the port or t-line most efficiently. So a t-line is a resonator, and no more well-damped at THAT resonant frequency than a ported speaker. The ported speaker has trouble at the next resonance- its upper impedance peak, as noted above.

A t-line also often uses a very low resonant-frequency woofer. In combination with the actual cubic volume contained in that t-line, this leads to a really high impedance peak at a very low frequency, usually well below 20Hz, which can be hard on an amplifier's power supply when excited.

T-lines are less efficient, ONLY because the woofer chosen has a longer voice coil, for more stroke to reach down to that impeance minima. A longer VC means greater moving mass. It is not because they are "more well-damped behind the cone", "which sucks energy from the cone". Utter nonsense, if the t-line is properly designed, as shown 35 years ago in the AES papers, available from Old Colony Sound Labs.

Wool is used in a t-line A) to make it an acoustically longer line (saves floor space), and B) to suppress upper-bass resonances. Wool is transparent to the lowest bass- it offers very little attenuation, which means that the low bass is no better damped. This is in the AES papers as well.

The best way to think of a t-line is as a very small enclosure with a very long port, needed to tune that enclosure to resonate at a low frequency.

A ported speaker is a medium-size cabinet with a modest port length, but without much acoustic stuffing, which would close off the volume of air needed to drive the port. So, with less stuffing, the ported enclosure is "noisier". A t-line enclosure is usually much quieter in the upper bass than a ported speaker's enclosure, and often much quieter than poorly-designed sealed boxes.

From a properly-done t-line (like the old IMF's), you hear extended, low distortion bass. Which arrives so much later than the upper bass, it sounds like it came from another part of the house. And because it took a while to get up to full amplitude, it takes the same amount of time to stop. Which means this resonance puts its signature on different recordings. Which is why sealed-box woofers offer better sound- still putting their own signature out there to hear, just less of one.

A transmission line, by definition means "transmitting energy without reflection". Except that "t-lines" in speakers reflect energy back to the cone, taking several cycles to build to full resonance at the impedance minima. So a t-line speaker is not a transmission line, as the energy came back to the cone.

The only true transmission lines for speakers would be A) an infinite horn (energy goes one way w/o reflection), and B) a muffler (energy goes away and cannot return). A t-line is neither.

Best,
Roy