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Old 27th January 2009, 02:49 PM   #1
IG81 is offline IG81  Canada
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Default TL design questions

Hello,

I am currently designing my first loudspeaker cabinet and chose to build a TL for Fostex FE126 drivers.

I thought Id first practice with cheap drivers I have at home. They are JVC VGS-1001-018 ,10cm, 15W fullrange salvaged from an early 90ies micro-component system that sounded decent despite having such tiny drivers in a bass reflex enclosure. I think I can make them sing a lot nicer in a TL. I measured their Fs at 110Hz.

Im still hesitating between a straight or once-folded design. If I go straight, Ill place the driver 1/3 down the air column to smooth out the second (3/4) harmonic mode. If I go for the folded design, Ill place the driver 1/5 down the air column to keep it at a decent height, not to have it too close to the fold and to smooth out the third (5/4) harmonic mode. Either way, I will initially stuff the area above the driver to slow down all higher harmonics pressure antinodes and re-adjust as needed.

I know how I want things sized-up, but in case Im missing something big or get an unexpected answer to my questions, I ask you the following:

1. Just how much can you get out of a TL tuning? Assuming typical driver roll-off of 12dB/octave below Fs. Just how low can you tune the TL before its worthless? My guess would be not to go any lower than 1 octave below Fs for quarter-wave resonance mode. Any thoughts and experience on that?

2. What about the cross-section of the TL? I am making mine straight, no-taper and was thinking of a X-section about 3-4 time Sd, which is around 0.0054 square meters for my drivers, with a mouth area equal to Sd and a depth-width ratio of 1.4142. How does a X-section of say, Sd, differ from one of many times Sd, say 5-6 times?

Thanks in advance,

IG81
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Old 27th January 2009, 04:08 PM   #2
GM is online now GM  United States
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Greets!

I recommend educating yourself on current TL, etc. design theory and in particular the Classic TL designer tutorial to help with your specific design: http://www.quarter-wave.com/

If you don't measure T/S specs, then figure the pipe's total volume (Vb) will need to be at least 4x and maybe as much as 10x the mini-monitor's to allow sufficient tuning flexibility/max gain, so its cross sectional area (CSA) is a function of its Vb divided by path-length, not any correlation to the driver's Sd.

GM
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Old 27th January 2009, 04:49 PM   #3
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One thing to add to GM's comments -the 126 isn't ideal for this kind of load, given its very small Xmax. If you haven't bought them already, you might want to look at it's FE127E stablemate which has a little more, or the CSS FR125SR.
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Old 27th January 2009, 05:11 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

FWIW assuming you make some effort to make them look nice
I'd suggest you start off with a driver with known parameters,
at least you will not be guessing and whatever you learn will
be more predictable for the second time round.

Though in maximising possibilities a MLTL (mass loaded TL)
should not be ignored, e.g. the Metronome is very appealing.

/sreten.
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Old 27th January 2009, 05:16 PM   #5
IG81 is offline IG81  Canada
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Hi,

Thanks for the advices.

Is there any version of MJK mathcad worksheets still available somewhere, as he cut off access to it before I stumbled upon it. This tool would greatly help me to learn more.

Thanks,

IG81
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Old 27th January 2009, 05:45 PM   #6
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Not AFAIK.
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Old 27th January 2009, 06:02 PM   #7
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Quote:
Originally posted by sreten
Though in maximising possibilities a MLTL (mass loaded TL)
should not be ignored, e.g. the Metronome is very appealing.
For the FE126 Metronome is likely as close to a TL as you are going to get and have a box that actually works. FE126 really is happier in a horn. FE127 is the driver to use in a TL.

dave
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Old 29th January 2009, 06:35 AM   #8
freddi is offline freddi  United States
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how does Fostex specify "xmax"? - by gap vs coil height? by distortion? a driver could have ~zero overhang and still have some xmax depending upon a few things

do FF225 and 206 have thicker front & rear magnet plates than AN?
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