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A Question About Time Alignment

I was reading a review of the Wilson Alex V on Stereophile recently. (Published just in time. I’m thinking about picking up a pair. Maybe a couple for the bedroom, too.) And it raised a long-standing question of mine, one that I hope the wiser minds on this site can answer. 
 

Wilson’s big selling point is aligning the different frequencies so they all reach your ear simultaneously. As I understand it, that’s why they have minute adjustments among the various drivers. The woofers put out bass notes that move slowly thanks to their long sound waves while the tweeters are playing faster moving, high frequency notes with short waves. Wilson lets you make adjustments so that they all arrive at the ear at once. 
 

It seems to me, however, that live music isn’t time aligned. Suppose I’m playing the piano and you’re sitting across the room. When I stretch out my left hand to hit the low notes, those notes travel along the same long, slow wavelengths as the notes from Wilson’s woofers. Similarly, the treble notes I play with my right hand move quickly through the short wavelengths. The notes from the piano are naturally out of alignment. If Wilson’s goal is to achieve a lifelike sound, aligning the frequencies doesn’t seem like the way to do it. 
 

Wilson has been selling lots of zillion dollar speakers for lots of years and people continue to gobble ‘em up. Something must be wrong with my line of reasoning. Would someone please point out where I’ve gone wrong? Nicely?

paul6001

Showing 6 responses by erik_squires

erik_squires
16,477 posts
 

Steve,

 

@hifidream You should read up on Head Related Transfer Function research.  It's a lot more than timing.

Having said that, I've heard speakers with great, near perfect impulse responses and they did not by themselves move me.

erik_squires
16,477 posts
 

Ahem, if you are looking at 2-ways, the Fritz models IMHO are highly competitive with the Wilson Tune tots. :)

erik_squires
16,477 posts
 

Let me clarify a little.

Each driver has a different distance to it’s acoustic center. Tweeters are shallow, woofers are deep, and we must take this into account in designing the crossover.

Here’s the problem. From the front the tweeter is say 2" closer to your ear than the woofer but at 90 degrees to the side they are the same distance. This means that the phase relationships you take into account in the crossover only hold perfectly true in front of the speaker.

If on the other hand we step the drivers back so that the acoustic centers (usually near the magnets) are exactly aligned, the phase relationship directly in front and at the sides is the same.

Great! this simplifies crossover design. However, if you look at an impulse response in either case, they will still not be perfect. That is more involved. :-)

Another approach here is also to use higher order filters, which D'Appolito recommends for MTM designs.  Steeper filters (also like Joseph Audio's older infinite slope speakers) minimize the phase matching problem off axis in all directions.

erik_squires
16,477 posts
 

So yes, moving the drivers does change the time/phase alignment, BUT!!

It does not alone ensure a perfect impulse response. :D

However, if you align driver's acoustic centers, it means that the crossover works better off axis.

erik_squires
16,477 posts
 

PS - Looking at the step response (Figure 5) this is not a time-aligned speaker.  If it were there would be a single rising impulse followed by a long decay.  The two peaks are indicators that these speakers are phase aligned. 

 

erik_squires
16,477 posts
 

High and low frequency signals travel exactly the same speed.  Proponents of fully time aligned systems like Vandersteen and Thiel, are concerned with say, 1 note of a horn looking like 1 note from a horn when it reaches your ears.  Many benefits are supposed to come from this, like increased imaging, clarity, etc.

Time alignment however is the minority approach when it comes to speaker design.  Hard to implement in passive crossovers and may over-stress tweeters. 

From a personal perspective, I have never heard a time aligned speaker which sounded so good I'd forego all others forever for it.

We should also be careful about not confusing time-aligned with phase-aligned.  Phase alignment is similar, but only concerned with the correct and smooth sums from multiple drivers through the individual crossover regions. 

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