Phase Coherence or Time Alignment: Which More Imp?

Yes, I surely do like my C/3-Ls, and they've served me well for 15 years or so. I'm in the process of getting them upgraded to C/6 status.

Suits me:

You don't need a stepped baffle to get time-alignment. A sloped front baffle actually works better.

Bud did recommend that most of his bookshelf speakers be listened to from a certain axis.

This brings me to my next point, and it involves an earlier comment I saw about the new Monitor 7 and Studio 7 not being time-aligned.

The acoustic center of a typical tweeter is closer to the plane of the front mounting plate than in a woofer. Since you're sitting out in front of the speakers, we can say that you're closer to the tweeter (to the tweeter's acoustic center). That's a bad thing if we're looking for time-alignment.

So we need to move the tweeter farther away (or the woofer closer). Stepping the baffle can move the tweeter farther away, as can sloping the baffle.

With something like the Monitor 7s, you're meant to listen on-axis with the woofers. The tweeters are off-axis, so they're farther away from your ears. In this case, time-alignment is accomplished without sloping or stepping the baffle.

All we're really looking for is to align the _distance_ from each driver to your ear to be the same.

So Bud had designs that had sloped front baffles, and he also had designs where you were meant to listen from a certain point where the distance from the drivers to your ear was equal.

I used to listen to my Betas on stands that tilted them back. An possibly equivalent method would have been to place them on taller stands. In this case Bud recommended tilting. I can only assume that he looked at the positioning of the drivers and their respective acoustic centers and determined that placing them tilted back was correct.

He recommended that the A/3s that my brother owned be placed on taller stands, with the woofer at ear level.

As you can see, it's difficult to judge whether a speaker is time-aligned or not, unless you also know where your ears are meant to be in relation to the drivers.

BTW, I don't feel stepped-baffles are a good idea because you can get big problems with reflections and diffraction. Fried suggests another problem:

"We have developed as our first possibility three drive systems, in time proper enclosures, joined by a network that makes them “phase coherent”. These we must get to work in a “phase aligned” fashion, i.e., all providing the proper data to the listener at the proper time. The only way known to the author is to place the drivers on a sloping baffle, so that the propagating center of each driver is in vertical alignment with each of the others. If we place them on a flat board, we will be phase coherent but not phase aligned; if we place the bass units out in front, with a step back to the mid, and another step to the tweeter, we will get proper time at just one seat on axis, every other seat in the room being very out of phase; and we will have introduced severe problems in the vertical plane. The best we can do is to slope the baffle, either by designing it in; or by recommending that the speaker be used on a tilt back stand."

Note that he re-thought this later and arrived at time-alignment as done in the Monitor 7s as an alternate. (I first saw that same arrangement on the Studio IVs.)

"I have noticed the positive recommendations you are receiving here and other places on your speakers. With the apparent unfortunate situation with Meadowlark, it looks like you, Richard V. and Thiel must carry the load. "

There are others. Karl Schuemann is working on phase-correct designs, and then there are a couple of people or groups of people carrying on Bud Fried's work.

In addition, I wouldn't count Pat McGinty out just yet. :-)

The "X number of listeners thing it sounds swell" argument doesn't impress me because it also applies to Bose speakers...

"Professional" listeners? Like recording studio engineers or musicians?

I'm not sure I'd rate most studio engineers as a good recommendation for speakers. They often are listening for different things than the end user of the recording is.

I might respect a musician's opinion more, but I have no way of knowing whether they have actually compared "more correct" speakers to "steep slope" speakers, or whether they just find that Brand X "steep slope" speaker is better than a bunch of other incorrect speakers.

My opinion is that a speaker with steep slope crossovers doesn't stand much of a chance of being able to reassemble the original waveform at my listening position.

Whether that's important, and whether "time and phase coherent" speakers can reassemble the wave form correctly or not is where there's a lot of argument.

At that point I have to fall back on my own listening experience, and say that I feel that phase-coherent designs work better for me. They provide a better window into the recording.

That should have been "X number of listeners _think_ it sounds swell."

Trelja,

I'm very surprised to hear that Bud didn't know about Zeta in re: series crossovers. He certainly had known about 1st-order series and quasi-second order series crossovers since at least as early as the Betas.

Suits_me> Is there a vintage site with a picture of the C/3L?

There is a picture at: http://groups.yahoo.com/group/IM_Fried/?yguid=180415640

They appear to be of early-middle vintage, judging by the drivers. My C/3-Ls resembled these at one point, although I used a single piece of foam to cover the tweeters without a cutout for the dome. (The foam they sell for air-conditioner filters works quite well.)

I regret that I never got subwoofers to match my C/3-Ls. I am planning to buy a pair of Monitor 5s, which likely don't image as well, but have better bass and maybe even better midrange due to less reflections back through the cone.

The Monitor 7s evidently may have bass that's a bit better than that, but they use a tweeter that is not as good.

Perhaps a much deeper C style cabinet would help avoid reflections, but you're still dealing with pretty tight spacing. The larger cabinets of the floor-standers give you much more room for the line and its folds, but you get a larger front baffle along with it. Topping a rectangular cabinet with a truncated pyramid would help, but then your cabinet costs go way up.

I own a pair of C/3-Ls, and have seen crossover schematics for other models, and they use a simple 1st-order series network. Period. There's no way to get a higher-order electrical network out of what's there.

The published articles I've seen from Bud all talk about 1st-order series crossovers as being superior.

Perhaps he was talking to you about the acoustic response? Or perhaps the "archived" info you're referring to dates back to before modern drivers allowed symmetric 1st-order crossovers?

"Phase coherence" and "time alignment" are related concepts.

I think what you're thinking of as time alignment is aligning the acoustic centers of the drivers with either a stepped or slanted baffle. This "phase aligns" them, so to speak. (Differences in phase are differences in time. Don't forget that a typical plot of a sine wave has two axis - amplitude and _time_.)

It's possible to have alignment without a slanted or stepped baffle. You have to look at what the listening axis is, and see if the extra distance between your ear and other drivers spaced farther away will make up the alignment.

Let's take a 2-way (TM, not MT) as an example. The woofer's acoustic center is 5 feet away from your ear. The tweeter's acoustic center will be farther forward. Step the baffle to put the tweeter farther back and you get alignment. Or tilt the front baffle back and you also get alignment of the acoustic centers.

Now lets assume that the woofer is at your ear level. So it's 5 feet away. The tweeter is 10" above the woofer on the front baffle. You now have a triangle with 3 sides, 60, 10, and 60.8. Just by the increased distance to your ear, you've done the equivalent of moving the tweeter back.

I'm sure that the designers of the Studio 7 took this and other factors into account, and that they're a minimum-phase design.