What frequency should I tune my micro-TL to? - diyAudio
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Old 5th February 2006, 01:42 AM   #1
owdi is offline owdi  United States
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Default What frequency should I tune my micro-TL to?

I have searched, read, and read again... and I can't seem to find a straight answer. What frequency should I tune my micro-TL to?

The driver in question is a HiVi B3S. My two drivers are not the same, one has an fs that is 5hz higher than the other (105hz and 110hz). The Qts for both is high, somewhere around 1 for each driver. Vas is around .6 L.

Since it is a high Qts driver I know I should tune around half an octave below fs, but is that free air fs, or fs in a sealed box the size of the TL? There is a 15hz difference between the two.

Dan
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Old 5th February 2006, 03:35 AM   #2
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Greets!

Depends on what sort of performance you're expecting from it and what you mean by micro-TL. For instance, if we use T/S, then it's a ~0.52 ft^3/44.3 Hz Fb cab. When a 1/4 WL long and stuffed it will have a ~IB response, but the amount required will also suck the life out of it.

GM
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Old 5th February 2006, 06:43 PM   #3
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Quote:
Originally posted by GM
Greets!

Depends on what sort of performance you're expecting from it and what you mean by micro-TL. For instance, if we use T/S, then it's a ~0.52 ft^3/44.3 Hz Fb cab. When a 1/4 WL long and stuffed it will have a ~IB response, but the amount required will also suck the life out of it.

GM
I call it a micro-TL because I'm tuning very high compared to all the designs I've seen out there. Since the fs of the HiVi B3S is 105hz it goes off the alignment tables MK provides. Those tables were still incredibly useful, because I could extrapolate the values of Dr and Dz by seeing how they change between 20hz and 70hz.

I'm not looking for stellar performance, just the most bass extension possible while keeping impedance flat.


So this is how my reasoning goes.

1. The free air fs of the driver is 105 hz.

2. In a .16 cu ft. sealed enclosure the fs rises to 115hz.

3. Since it is a high Qts driver, I should tune to half an octave below fs. 115hz/sqrt(2) = 81hz.

4. Some guides recommend tuning to the free air fs of the driver, which is 105hz.

Points 1 and 2 I'm certain about, but I don't know if 3 or 4 is correct.

Dan
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Old 7th February 2006, 05:07 PM   #4
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Greets!

Flattest impedance and flattest bass extension is mutually exclusive since the former occurs at ~Fs/Qts and the latter based on its ~Fs, Vas, Qts.

Historically, tuning to a 1/4 WL of Fs was preferred since the 1/2 WL pipe mode will reinforce the driver at the point where its excursion is the greatest. This assumes little stuffing, so the system is quite lively sounding, with the amount of gain adjusted by pipe volume (Vp).

WRT to tuning a 1/2 octave below Fs, this is just the lower practical limit before the alignment 'falls off' the impedance peak. Typically you want to tune a TL up to ~1.6*Fs so that either the impedance peak at Fs can fill in the bottom end to some extent if driven with a tube amp or compensate for the lowering of Fp due to the effects of heavy damping.

Anyway, if they were mine, I'd cobble together a 0.707*Fs pipe prototype out of scraps or cheap boards and see how it sounds with various stuffing densities before considering going with a flat impedance alignment:

L = 42"
SO/SL = ~21.3"^2 (recommend 3.625" x 5.875")
zdriver = 13.375"

GM
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Old 8th February 2006, 03:20 AM   #5
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Thanks for the reply GM. I figured out the size of the TL enclosures and am seriously considering sticking with the current open baffles. I didn't realize they would be so big.

Dan
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