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 Re: Interesting Centre Speaker Discussion
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Joined: May 2007
Posts: 16
frequent flier
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frequent flier
Joined: May 2007
Posts: 16 |
John, I think you have cleared that up for me. Thanks.
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Re: Interesting Centre Speaker Discussion
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Joined: Oct 2005
Posts: 853
aficionado
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aficionado
Joined: Oct 2005
Posts: 853 |
Quote:
The frequently adopted solution of course is to use an MTM configuration which as was said before was designed by Dr. D'Appolito for vertical use with a narrowed vertical dispersion, and instead for aesthetic reasons place it horizontally, contrary to the intent of the design. While this may be acceptable in general, the tests showing off-axis irregularities illustrate that it's far from optimal.
Other solutions using a horizontal shape include the TMMMT configuration of the VP150, which is said to be better in this respect. The best horizontal center I've heard used a W T/M W configuration which in essense is a small vertical speaker handling the mids and treble flanked by two woofers. At least two members here have centers from manufacturers who use that design. In the past I've wondered how an Axiom center using two of the 6 1/2" drivers flanking the regular tweeter above the 4" mid from the QS4 would do in a roughly 9" x 25" enclosure.
It certainly is true that a properly positioned identical speaker on the same plane would be an ideal configuration, however few people can set up the centre that way. Using a similar vertical speaker or monitor as suggested is another alternative but such a setup will also come with its own set of off vertical axis dispersion problems and timbre differences off plane. In a wide HT with rows of seats, such a setup is less than ideal. Placing dual identical monitors above and below the screen has promise but the effectiveness of this situation depends on where one sits, room acoustics and the speakers radiation patterns. I've heard lobing with friends using this configuration plenty of times.
I often suspect that the knock on D'Appolito horizontal centre speakers has more to do with poor design choices than the physics of the design. Choices must be made regarding the size of enclosure, distance between drivers, crossover point and size of drivers. A wrong choice in any of these results in poor off-axis dispersion or lobing. I have an old CW centre which sounds awful and does not follow the most basic principles. The drivers are too large and too close together to a large horn tweeter. And I can only guess how far off the crossover point must be. A badly designed speaker is just that, however it does not follow that all MTM designs suffer from lobing or poor off axis response as has been suggested. Nor was the use of horizontal made for simple aesthetic choices. Rather it was more to deal with the tradeoffs in off-axis radiation patterns.
If you take a look at the VP100 you can see an example of a good D'appolito design. A pair of 5.25" mids crossed at 2.7Khz to a single 1" tweeter aligned geometrically with the tweeter equidistant to the two mids. At the crossover point, one wavelength is 5" (13,503"per sec/2700hz) which is the ideal distance from the centre of the tweeter driver to the mid drivers. One often sees tweeters in MTM speakers placed off centre to stay within this one wavelength criteria. Such a configuration effectively minimizes or eliminates lobing in both vertical and horizontal placement.
Off axis dispersion is very important and again all speakers are not created equal. While a vertically aligned speaker will always have better dispersion off axis vertically, the fall off in response at the sides of the room are less than ideal. Where a properly designed horizontal centre with the requisite drivers will perform better off axis than the vertically oriented speaker, is if it is aimed directly at the listeners across rows of seats or across a single wide sofa . The Axiom and many other horizontal centre speakers follow these important design criteria, while my CW centre does not. The sweeping generalizations in the original Fleishmann article forced him to backtrack and write that rebuttal. One can always find a bad horizontal centre that lobes and has lousy off axis response, but that is not always the case with well designed speakers.
John
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Re: Interesting Centre Speaker Discussion
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Joined: May 2007
Posts: 16
frequent flier
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frequent flier
Joined: May 2007
Posts: 16 |
Excellent post Jakeman. I can see a couple of VP150's in my very near future.
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