thoughts on my center channel idea

[Zymrgy]

hello all, I am new to this forum and to speaker building in general. Right now, the weak spot in my system is my center channel. I have previously build a sub that worked out great, but having a external crossover simplified that project immensly. Now I would like to replace my 12 year old Polk center channel. It is a small speaker, having 2 4" mids and a tweeter in a MTM arrangement. It does not sound that great, having a sound that can be described as "dead". Sitting off center of the speaker highlights this problem.

So, after doing some research and asking a few questions here, I have decided that I would like to build my own Center Channel, using a Seas P17RE/XTVF coax with a Seas P17RE/TV in a W-Coax-Port type arrangement. This will work very well with my future plans, right now I have a pair of AR-M6's as my main speakers, and if this turns out as good as I hope it will, I would like to use the same drivers in a coax-W-Port arrangement as my main speakers (maybe using the non-shielded version tho)

Now is the time for me to ask several questions.

1- it has been suggested that I do this in a 2.5 way system. What is a 2.5 way, and how does it differ from a conventional 2 or 3 way system?

2- If 2.5 way is the best route to follow, will I need to build a seperate sub-enclosure for the coax speaker? If yes, how do I determine the correct box size for the sub-enclosure?

3- What is the best method of choosing the crossover frequency for doing a 2.5 way system? And can anyone suggest a crossover for me?

4- Someone here earlier pointed out a link to me for Tony Gee's USB. Why is the tweeter wired out of phase to the mid/woofer?

By no means are my ideas set in concrete. I am learning alot day by day, and constantly learning through all of your input. Thanks for all the help!

[michael]

a 2.5 way is basically a two way with 2 1 woofer producing mid and bass and the other woofer making only bass, this means that the drop in low end response due to baffle step is compensated for by the higher efficiency. the XO frequency for the ".5 woofer" is usually set at about 200-300Hz, im not an expert at making XO though so ask some more people for a more detailed explanation of what i am aiming you at.

[tacomaboy]

Pardon my newbiness, and I dont intend to hijack the thread, but could someone explain baffle step to me?

[rabbitz]

Originally posted by tacomaboy
Pardon my newbiness, and I dont intend to hijack the thread, but could someone explain baffle step to me?

Have a look in the Wiki on the home page or use this link
http://www.diyaudio.com/wiki/index.p...e-step+Physics

[Timn8ter]

The tricky thing about center channel design is dealing with horizontal dispersion problems and lobing. If you have a single point source (one driver) there is no issue. A two-way system is less likely to suffer from these problems than an MTM arrangement which often will need a mid-range added. I like the idea of using a coax and helper tweeter. Reversing the tweeter leads is to compensate for the 180 degree phase shift in the second order XO.
I like this article concerning baffle diffraction step.

[CeramicMan]

Originally posted by tacomaboy
Pardon my newbiness, and I dont intend to hijack the thread, but could someone explain baffle step to me?

"Baffle step" basically boils down to the fact that speakers tend to be more omnidirectional at lower frequencies than at higher frequencies. Saying that the difference in loudness between low and high frequencies is up to 6dB is just an approximation, and doesn't take into account many other important aspects of the sound that is created.

It has to be remembered that in most listening situations it's not just the direct sound that reaches our ears, but reflected sounds as well. This makes the total power output more important than the on-axis frequency response and compensating the baffle-step effects. And on top of that many people go hard out with room damping which again has a large effect at high frequencies but very little effect at low frequencies anyway.

Originally posted by Timn8ter
The tricky thing about center channel design is dealing with horizontal dispersion problems and lobing. If you have a single point source (one driver) there is no issue. A two-way system is less likely to suffer from these problems than an MTM arrangement which often will need a mid-range added. I like the idea of using a coax and helper tweeter. Reversing the tweeter leads is to compensate for the 180 degree phase shift in the second order XO.
I like this article concerning baffle diffraction step.

What I would do is a centre speaker with a tweeter that's just above or below a midrange/midwoofer, but not offset horizontally. The enclosure could still be low and wide if need be. Reversing the polarity of the tweeter doesn't really compensate for anything. The "180 degrees" is really a variety of different angles which are always different depending on things like enclosure size, speaker, crossover type, and of course frequency. It's really a time-delay of the woofer, and even that is an over-simplification.

Generally there's always some lobing near the crossover frequency of the woofer and tweeter, while most co-axial drivers don't even measure-up well enough for anyone to bother about that in the end. Reversing the polarity of one of the drivers will mean that the lobing will have different additive and subtractive effects, at the various frequencies and at different physical angles. With a simple low-order crossover generally the best you can do is try it both ways and see which way it sounds best.

CM