For those who have not been following the dicussion thread related to this review, there has been some discussion of whether or not the SongTower cabinet is a "true" tansmission line design. I thought I'd post my reply to this discussion here for those who do not care to wade through the audioholics thread. Here it is...
First of all, I would like to thank Gene for making this review possible and Tom for a thorough, well thought out and well written review.
I would also like to publicly thank Dennis Murphy for another spectacular crossover design and Paul Kittinger for his great work on the cabinet design. And, of course, thanks to Martin King for developing the software that made the cabinet design possible. Without the efforts of these individuals, the SongTowers would not exist.
While I could certainly quibble about this point or that, I only have one comment I'd like to share on the review and subsequent discussion. I am somewhat amused by the discussion of TL or not TL. I have had this discussion with a couple of speaker designers in the past and it is purely a matter of semantics.
If you adopt a very tight definition of what constitutes a TL cabinet, the SongTower cabinet may not qualify. But to say that it is simply a standard bass reflex cabinet is certainly not an accurate statement either. If you compare the bass quality and extension of the CA15 in a properly designed bass reflex cabinet with the same driver in the SongTower cabinet, it is quite obvious that there is something very special taking place with the ST cabinet.
Technically, the ST cabinet is a "mass-loaded quarter wave tube." Just as with a more traditional TL cabinet, this design takes advantage of quarter wavelength resonance.
As Gene accurately stated, a classic TL design would require a line length of roughly 7 feet to achieve a tuning of 40 Hz. But as Martin so eloquently pointed out, you can achieve the exact same tuning by using a shorter line length coupled with a mass loading port.
In this case, the line length is tuned to a quarter wavelength resonance of just under 80Hz. The mass loading port then lowers the system tuning to 40Hz.
If you simply change the dimensions of the cabinet by making it shorter and deeper for example, while maintaining the current internal volume, you could certainly end up with a classic bass reflex cabinet.
In this case, the cabinet's physical appearance does closely resemble a standard bass reflex cabinet. But from a performance perspective, it more closely resembles a classic TL cabinet.
For me, there is a simple question that resolves the issue. Is the performance based on helmholtz resonance (bass reflex) or quarter-wavelength resonance (TL)?
Since this design takes advantage of quarter-wavelength resonance just as the most tightly-defined TL design, but accomplishes it in a different manner, I (and many others) feel it is more accurate to define it as a TL variant. But if it makes you happy, simply call it a pseudo bass reflex cabinet on steroids. No matter what you call it, I think we can all agree on one thing - it works.
Thanks again to audioholics for making this discussion possible.