How many more dB for 2 woofers?

[JohnA]

If I build a dual woofer center channel speaker, how much more output will I get from 2 woofers? I had thought it was 3 dB, but I've read different lately. How does spacing affect output? Spacing affects intereference patterns, but is the spacing measured center to center, or inside edge to indise edge?

[pzannucci]

I also thought it was 3 originally but then again I am hearing 6 too.

The measurements are center to center, similar to an mtm design.

If you are using dual woofers remember that there will be some anomalies in the crossover region depending how far to the right or left you sit from center. As stated many times before, there is only one point in space that there will be time coherence between the tweeter and woofer. This time difference can cause a dip in the response.

Secondly as with an mtm design, there is a cancellation effect from both of the woofers when sitting off center based on the distance between the woofers and the wavelength size of that frequency. Beware of vertical and

horizontal dispersion patterns.

I would rather have 0

o

0 wider horizontally

than 0o0 - wider vertically

[tpg]

If you add another woofer in parallel to the first within one design, sharing the same amp signal, then you get 3dB gain. If, though, you add a second self-powered woofer, then you will get 6dB gain. Note that adding the second woofer in parallel will lower the total impedence. (which is why the output is increased)

[JohnA]

I will have to accept some lobing. It looks like the C-C distance will barely be less than one wavelength at the crossover point.

[Malcolm]

http://www.linkwitzlab.com/faq.htm#Q21

[pzannucci]

Thanks for the correct info.

John, you must be crossing very low, what are you using for the high frequency driver and at what crossover point?

[JohnA]

It will be a Frankenstien Heresy. The crossover point will be 700 Hz from the woofers to the squawker.

[jdire1]

John you have La Scalas? I tried a couple of Mods to my Heresy center but it was never right.I even tried 2 Heresys for the center.I just finished my center channel prototype. I am using 2 K-33 woofers in a 6 cubic foot vented cabinet. The Midrange is a K-400 with a K-55v driver. The tweeter is a K-77. I tried several speakers to match the timbre of K-Horns. There is no match without the K-400. I now have a perfect timbre match for my K-Horns. I bet it would work great with La Scalas.

[j-malotky]

JD

True about the K400. Looks like our paths to finding the right center are about the same. I tried a single and double Heresy center in between my Khorns and they just sounded small for a lack of a better description.

My HT came alive when I went to the K400 for the mid horn.

I am using a pair of K-22 woofers for my center. The bass is weaker (less detailed) than my Khorns (surprise, surprise, NOT) but since the center channel is mostly midrange, It is not too bad listeneing too evan in concert/ music films etc. Its not so much that the bass is quiter, in fact, tweekeing my processor the bass db overall is close (do not remember the exact numbers off the top of my head) but the center having direct radiators has a different bass sound than the horn loaded bass bins.

In thinking about when I played with my HT durring setup, I would say the 6 db boost sounds about right for the second woofer.

JM

[JohnA]

A K-700 is a sawed-off K-400! The difference in crossover frequency is 3/4 octave, or 6 notes. I'd say the big difference in sound is the woofers, plus the autoformer settings in the crossover. The squawker in my Heresies runs at 98 dB and the tweeter at 99 dB, but the woofer runs about 95 dB. My brother just (finally) ordered the parts to reconfigure the crossovers and we'll address the difference in output then. If it works, I'll post the schematic.

[WMcD]

John, I don't have all my technical references here. So I'll have to wing it.

1) The 3 dB figure is correct only when the two sources are fed with noise independent of each other. So nothing is in phase. Therefore, there can be no mutual loading or coupling..

2) The 6 dB figure by Linkwitz is certainly correct in the application. He equates the two drivers to one driver of equal size. Obviously, the assumption is that all parts of the larger driver are moving in phase.

3) Linkwizt is talking about radiation resistance as "seen" by the larger radiator. You are asking about mutual coupling. I think these are the same thing expressed in different ways. But it takes some thinking.

4) These are the same issue as the restricted area in the throat of a horn. That is to say, the motion of the diaphragm could move more air molecules if they couldn't run away to a low pressure area. In the throat, the walls prevent this, or create some back pressure when they hit the small chamber walls.

5) Mutual coupling is really a way of saying that the woofer next door (a) created some pressure, and therefore there is no way for the molecules to run away in that direction from the ( woofer. Naturally, it works the other way too. The other speaker (a) sees the pressure created by ( So, there is some mutuality to the situation.

6) One time I was looking at mutual coupling and pressure in the form of fooling with pennies on a desk top. You can form a circular matix of them with a penny in the middle and six around the edges. They are like seven small speakers with mutual coupling assembled into a bigger woofer which has a diamater which is three times as large as one penny.

6.1) Maybe too wacky an analogy, but it makes some sense when you see that there is a lot of equivalence to mutual coupling and a larger diaphragm "seeing" increase radiation resistance. The penny in the center is surrounded by six. The ones at the edges have three adjoining ones. Essentially, every penny speaker has more more molecules to work against because the neigboors are pushing them away too, again, in phase.

That is my insight for the night.

Gil