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Alpha Transmission line - can the driver be offset?
Alpha Transmission line - can the driver be offset?
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Old 23rd June 2006, 02:14 AM   #1
adamt is offline adamt  Australia
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Default Alpha Transmission line - can the driver be offset?

I am building my first alpha TL and note the article in AudioXpress indicates the driver must be located close to the closed end of the line.

Is it possible to offset the driver as in conventional tl designs? I am considering locating the driver about 0.3 to 0.5 down the line toward the open end.

I have also folded the line which I note from other posts is Ok.

Thanks for any help.

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Old 23rd June 2006, 02:23 PM   #2
GM is offline GM  United States
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Location: Chamblee, Ga.

Sure it can, but unless you 'do the math', experimentally try different positions to find what point works best in your room/app, or at least sim it in MJK's s/w, you may be disappointed with the results. For sure, mounting the driver at the 50% point of a constant taper (straight) TL is a bad plan IMO since the theoretically optimum point in a golden ratio cab is only at ~46.2%, so the higher the aspect ratio, the closer to the closed end it theoretically needs to be.

FWIW, his more recent Omega 1/4 WL reflex design is based on a (L*0.21) driver offset, so maybe either this design or at least this offset will work better for you. I tried this offset in a couple of Alpha TL sims I saved and it yielded an OK response with a mild midbass 'bump' to give it a bit more 'slam' factor.

As always though, YMMV.

Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.
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Old 23rd June 2006, 09:09 PM   #3
nunayafb is offline nunayafb  United States
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I wouldnt advise changing it, if the design has all of the midband ripples worked out then what you have is a tl that produces better bass than an offset tl would according to mjk's articles anyway.
The other side of it is if it's over stuffed to tame ripple then the bass will suffer do to that and offsetting and reducing stuffing will enhance bass and still control ripple.
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Old 26th June 2006, 11:36 AM   #4
adamt is offline adamt  Australia
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Default test enclosure built

Thanks GM and nunayafb

I rebuilt my existing ported enclosure to alpha tl spec.

The alpha is the best sounding enclosure I have had these drivers in (Dynaudio 17W75XL with a ts of 0.462). As described in the AudioXpress article, it seems to have the fast transient response of a tl, with the bass response of a closed box without the loss of efficiency and detail.

My previous ported enclosure had the best modelling outcomes from a frequency aspect, but suffered from a bloom in the bass in my heavily carpeted room, making the drivers sound heavy, slow and overblown in the bass (it took over).

I am very happy with this new sound, even though the f3 is higher than the fs. As described in the article, by tuning the cabinet at 1.4 to 1.6 times the fs seems to give a better low end response (or cleaner at least).

So thinking outside the square is sometimes rewarded.

I would recommend the alpha tl to anyone with a driver with a ts > 0.4 who is struggling to get the bass and transient reponse right.

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