Time coherence - how important and what speakers?

Just because it can do a square wave does not guarantee it will sound excellent on music. A nice square-wave response does not reveal many other problems that affect what we hear. However, it is a GIANT step in the right direction.

Roy

Can a ported or transmission line design be truly time/phase coherent? For that matter can a dipole or bipole with their trailing backwave be time/phase coherent? I suppose the same question can be asked of omni-directional designs as well. Is a point source design mandatory? Are sealed boxes required? How important is amplitutde linear(ty?) to sucessfull speaker accuracy, especially with regard to the vacillating ability of power delivery by amplifiers and the speakers dependency upon them?

A ported or transmission line speaker can never be time coherent, nor dipole, bipole or omni, OR SEALED. See my 1/16/03 post, about why any moving system has a natural time delay down at its resonance. The ported or "transmission line" (which is a port variation and NOT a transmission line) designs have the SAME phase shift as a sealed box for the sound leaving the front of the cone, and the port opening's output has additional time delay AND polarity inversion.

For living room use and mix monitoring, I believe point-source design techniques are the best way to achieve fidelity- primarily to avoid hearing time-delayed output from more distant drivers, or from more distant panel regions.

Amplitude linearity is most important when the speakers are of minimum-phase design, as then one can hear small deviations from amplitude linearity. But if the speaker has lots of phase shift, that skews the harmonic structure of the music, which puts those harmonics out of phase and thus alters the perceived timbre of the instrument or voice. Which makes it harder to "accept" what your test microphone is saying is "flat" amplitude response. The warped phase response keeps amplitude deviations from being noticed as much. In those designs, phase and amplitude are not independant parameters. When you remove phase nonlinearities, then amplitude response IS an independant parameter.

Amplifiers have problems- if a speaker has phase shift, it will change the sound of those problems, usually making them worse. It is distorting distortion. The amplifier has its lowest distortion working into a flat impedance curve from the speaker, but many speaker designers try for that by adding extra parts in the crossover- which act to flatten the impedance curve, but reduce clarity. And some of those impedance-flattening techniques do more harm than good- especially the ones for the woofer resonance- what a mistake!

Good questions, Mr. Unsound.

Best,
Roy

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.