Time alignment

[jmar]

I've been playing with what is turning into a very nice sounding system. I have a Pioneer B20 on an open baffle with a Vifa D27tg tweeter which match very nicely -- both are rated 91 db spl and the crossover is a simple 1st order 2uf capacitor. (and I ******* love that tweeter).

At first I thought the tweeter was a tiny bit harsh and tried different cap values and some resistors.

Then, after trying a different firing angle and some other experiments, I simply mounted the tweeter on the back side of the baffle.

The woofer hasn't been flush mounted (yet -- I was planning on it).

So, there is a good 3/4 to 1 inch in depth difference between the 2 and honestly, I'm blown away by the difference in what I'm hearing -- much smoother highs and not a touch of "grunge".

The tweeter hole is very large so I don't think it has any dispersion problems.

Is there really something to this accidental "time alignment"? (or whatever you want to call it). I love the difference I'm hearing.

Also using a subwoofer with this -- gotta have it.

Unless I find something better, I'm leaving mounted this way -- tweeter mounted on back, woofer on front and neither flush mounted.

It's only 1/2" ply right now but plan on 3/4" and chamfering the tweeter hole.

[kyrie48]

Sure thing... "time-alignment" is one of the more important elements I've discovered. My first build was not to make as good a speaker as possible, but more an experiment. I placed the small boxes on stands that would give me "ear level" roughly just above midwoofer center... a "time-alignment" of sorts, and the imaging/sound quality is greatly improved.

[jmar]

but isn't it more of a "depth" thing ?

the distance to your ear, not drivers from each other.

[arande2]

I would guess that the woofer has some group delay/phase difference from the tweeter around the crossover (usually more delay) frequency than the tweeter, and placing the tweeter on the back has matched these up better.

Keep in mind this is only my guess.

Maybe it has to do with the absorption of the material the tweeter reflects off of matching up with the tweeter, as well.

Another theory is of how 'large' the reflected sound is compared to what the direct sound would have been.

Many possible reasons..

I really would have to do testing to figure it out... I wonder...


If you can, I would recommend measuring the response of the speaker from where you would listen to it.

[lvw]

I would second measuring the response. If I'm picturing the setup correctly, the tweeter is firing through a hole maybe 3" in diameter? If so, you will be getting a significant cancellation of frequencies in the high treble range, from edge diffraction around the hole. (With a concentric edge the resulting dip can be fairly large, I've seen several db in such a case.) The tweeter response will be quite a bit different as a result.

While I do like the effects of correct time alignment, I think you are hearing differences in the frequency response, mainly.

(Of course, if you like the sound better, that's more important than a flat response...)

[kyrie48]

Originally Posted by jmar

but isn't it more of a "depth" thing ?

the distance to your ear, not drivers from each other.

Yep... you get different arrival times at different frequencies, but you can get a nice balance.
But your point is the same thing that has bugged me for many yrs enjoying audio... all the jargon about design, but always those tweeters blasting in your face. "Time-alignment" was part of most all the systems I've liked through the decades... DQ10's, OHM (model??), Vandersteen's, etc.
Have fun!!

[jmar]

Really intriquing because I'm not really one of those "nuance" guys -- but with this setup, it really made a tremendous difference.

Arande, I think you're right and you nailed it -- the hole is what I made to originally inset the tweeter (about 3.25").

But like I say mounting it on the back really smoothed out the high end. The tweeter isn't shouting at me.

Also, because the Pioneer is full range it's reproducing some of the highs in tandem with the Vifa (about 10k on the crossover -- 2uf) which I'm sure enters into it.

Maybe I shouldn't touch it -- I was thinking about doing a chamfer on the front part of the tweeter hole.

These drivers are a great combination you guys.

I understand the "spirit" of using an inexpensive piezo with the B20 as many have and I tried it, but no thanks -- no comparison to the Vifa.

[chris661]

Phase, Time and Distortion in Loudspeakers

Good read there if you wish to understand it more.