Time coherence - how important and what speakers?

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

Bigtree,
Vandersteen 2's and 3's have an unusual arrangement where the rear driver is both active AND passively driven. It is an active woofer below ~100HZ and is also a passive radiator reacting to the cubic air volume driven by the front woofer- which is ingenious. The 5's woofer is in an enclosure with an amplifier and a lot of EQ to make up for being mounted so far away from the upper woofer, and to compensate for its small enclosure.

Vandersteen has reduced the baffle size greatly, but that does not mean that these speakers are free of baffle reflections. There are still large amounts of reflected sound:
from the tweeter impinging on the mid (a little),
from the mid onto the front woofer and onto the entire cabinet,
from the front woofer reflecting off the entire cabinet.

Why? The felt applied to the face does not absorb much below 800Hz. And below that frequency is the range where the mid and front woofer also both want to be fully omni, thus reflecting off the entire cabinet and each other, and the felt does not prevent that. If you would like to see some of the math behind this, read my latter postings at this site in Europe, called The Vinyl Engine:

http://www.nakedresource.com//yabb/cgi-bin/yabb/YaBB.pl?board=general;action=display;num=1038342561;start=

I respect what Mr. Vandersteen has accomplished- his engineering is far better in many obvious ways, and in many subtle ways, than virtually all other speakers, which is why he has so many satisfied customers who can play just about any kind of music with them. But he has not banished reflections, just reduced them. Those still cause abberations in the total sound output. This is one reason the crossover circuit is complex- to make the final measurements read OK in spite of the reflections.

Best,
Roy

Roy,
I have not investigated T-lines thoroughly enough to have all the answers, but I will say that much of the received wisdom is downright wrong. Examples: (1) The commonly bandied-about equation describing speed of sound changes based on stuffing density is patently wrong on its face, and almost no one seems to notice. (2) There are all these theories about how the stuffing works, from the air causing movement in the stuffing to adiabatic/isothermal changes to who knows what else. From what I have seen, these are 90+% BS. Viscous damping due to air movement past the fibers is almost all you need to understand stuffing.

This is why the impedance curves come out so flat. In an undamped line, you have a whole series of sharp impedance peaks at n/4 for all odd n. (Note that these are pipe resonances just like in an organ, and that this is very different from a ported box, which has only two peaks which are compliance/mass resonances.) The stuffing removes these peaks entirely at even midbass frequencies and damps them extremely effectively at lower frequencies (including at the lowest 1/4 lambda resonance). On the other hand, ported design is specifically intended to function without damping, for all practical purposes.

In addition, although this isn't discussed much, T-line woofers have their fundamental resonance frequency dramatically reduced when placed in the line (as opposed to a sealed box, which always drives it upward). This is most likely due to the effect that at the lowest frequencies, the entire mass of the air in the line becomes coupled almost 1:1 to the cone. This is a very substantial increase in effective mass. An argument could be made, however, that due to compressibility, the initial attack at higher frequencies is much faster than if an equivalent real mass were added. Contrast this to a sealed box, where the only way to drive the resonance down is to add real mass, which hurts the transient response at higher frequencies. (And in addition, an argument could be made that decay at all frequencies occurs much faster as well, due to the high level of damping the stuffing provides.) This increase in effective mass at low frequencies is very nearly "something for nothing", and is probably why T-lines seem to have both "speed" and "weight".

I cannot disagree about the delay of the back wave, but I question whether it is an audible effect at the very lowest frequencies (because, again, a properly stuffed line will absorb everything from the lower midbass on up). The question becomes whether an 8-ft delay is audible at 35 Hz. I can't say because I don't honestly know. It could well be.

I am not trying to disparage the quality of a low-Q sealed box in any way, as I too think it is often the best real-world solution, but I think that there is a lot more going on in a "T-line" than is commonly appreciated, and worse, a lot of plain misinformation floating around.

Cheers,
Karl

Roy, I certainly agree that all reflections are not removed from the Vandersteen's minimal baffle designs. However, it is much better to attempt a solution that helps minimize these reflections than the way a lot of designers have basically ignored them using a large area baffle. When you look at the front of a Vandersteen, you see very, very little cabinet structure around the drivers. I think Vandersteen has attempted to address a lot of issues with sound engineering in a very reasonably priced product.
I was actually attempting to respond to Karls statement that the Vandersteen's were ported designs which they are not with the exception of the 1 series which is stated to be a transmision line of sorts.
I did know that the woofer arrangement was unique in its implementation. However, again, I was responding to Karls since the driver(s) are active, not passive, although, as you stated, the front woofer will move the back woofer, etc since they share the same sealed chamber.
I have certainly enjoyed reading your posts. They are very informative. Its nice to cut through the hype and get to the point.

Roy...in your own designs...how do you approach low frequency reproduction while retaining time/phase integrity?This seems to be the biggest challenge(there are many)...somehow containing the rear energy wave...and then knowing what to do with it...which I assume even in a sealed design...there is a high degree of stored energy...

ALso...although many frown on ported designs(myself included)...not all designs are created equal...and yes the number of poorly designed port speakers(often due to cost restraints) far outweigh the few that seem to "get it right"....lets face it...ports are found on many costly, highly regarded speakers(especially in hi-end monitors)....which brings me to this:very good speakers with ports exist...which leads me to believe it is the execution of the design rather than the design itself that is paramount...and that time/phase relationships are one of many concerns a designer must face ...in short...there is more than one way to skin a cat...