AudioFred Messages: 377 Registered: May 2009 Location: Houston
Illuminati (1st Degree)
Duke wrote on Tue, 17 November 2009 03:40
One of the advantages of the offset bipole configuration is that the baffle step is to a certain extent side-stepped. At the frequency where the front woofer (or fullrange) starts to bafflestep, so does the rear one... so its wraparound helps offset the bafflestep of the front driver.
In our Houston Audio Society meeting Saturday we compared the sound characteristics of several speakers including the TT-2000 and the Jim Griffin bipoles with CSS drivers. The bipoles with their little 4.5" drivers have the more impressive bass of the two plus the open life-like soundstage of a live performance. Jim's design isn't offset, and as I understand it the advantage of offestting the front and back drives is that it preserves all the advantages of a bipole while eliminateing one disadvantage, an anomaly caused by the reinforcement and cancellation of the front and back drivers at various frequencies. Is that correct?
The main reason for the offset is to physically stagger the low frequency sources as much as possible, thereby approximating the delusion of being surrounded by swarms of tiny subs.
The offset also increases the boundary reinforcement the rear woofer gets, giving you a bit more bottom end. In addition this geometry helps fill in the floor-bounce notch, as the rear woofer's output will not be floor-bounce-notching at the same frequeny as the front woofer.
The wrap-around notch (occurring at the frequency were the path length difference between rear woofer and front woofer equals 1/2 wavelength) is only marginally migitated by the offset. Various bipolar designers have used different techniques to deal with the wraparound notch. These include:
1. Lithium and group therapy.
2. Make the box about 1.5 times wider than it is deep (this is what I do).
3. Ignore it (works quite well, because it's not really audible for various reasons).
4. Use a side-firing woofer to cover the lower midrange and bass region (patented by Definitive Technology).
5. Use a rear-facing midwoofer wired in reverse polarity for that portion of the spectrum only (Genesis Model 5).
6. Notch the rear-woofer's output in that region (Mirage M-1, which also used the wide-and-shallow geometry; subsequent Mirage bipolars omitted the notch filter on the rear woofer).
In theory what you don't want is for the wrap-around notch to coincide with the floor-bounce notch. The offset geometry helps keep that from happening. It sounds like a lot of things to juggle and I guess it is, but it's not like a failure to geometrically optimize will ruin the design. Chances are nobody will ever notice.
AudioFred Messages: 377 Registered: May 2009 Location: Houston
Illuminati (1st Degree)
Do you think Obamacare will cover lithium and group therapy for people who have issues with the wraparound notch, or will Glenn Beck's prediction come true and a Washington bureaucrat will dictate some generic antidepressant like listening to line arrays for 30 minutes each day?
Just kidding, but I wanted to be sure you know we do read your responses, and how much we value them. As these Tang Band drivers break in (Or is it my ears that are breaking in?) I find myself listening to them more and more, and the other speakers less and less.
Bob Brines Messages: 186 Registered: May 2009 Location: Hot Springs Village, AR
Master
My TB's went into the cabinets that had the DX3's. I will probably have to build a new pair of boxes for the DX3's because I have no desire to take the TB's back out. The TB's lack the ultimate detail and air of the DX3's, but they sound so right.
AudioFred Messages: 377 Registered: May 2009 Location: Houston
Illuminati (1st Degree)
Santa brought me a dbx DriveRack PA+, and I'm using the graphic equalizer section instead of the passive LR filter. I'm very happy with the results. Now I can drive the TT-2000's to very satisfying room-filling levels even with my Bottlehead Paramour 3.5 watt amps, but I still prefer the convenience of a solid state integrated amp with remote. I've also found the graphic equalizer lets me make small adjustments to the midrange, which smooths out a peak in that area. http://fredt300b.smugmug.com/Hobbies/Speakers/132721_wacsQ#751452403_BdpKM
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