Did I see a thread, Linkwitz Transform for a reflex?

[mikee55]

Hi, I was browse through the threads the other day. Not sure if it was here or car audio, but I'm sure I saw a thread mentioning an LT type circuit for a Reflex box?

I reckon I might have been dreaming, and all I can remember was, I had to rush out, and closed the page by mistake. My History gets wiped at midnight every 2 days and it was the second day. So I couldn't tack down what I'd read, when I came to find it.

I'm sure someone posted a circuit too, I've searched and its bugging me, that I can't find it.

I can't afford to built every design there is, obviously, but just reading all the clever ideas out there is interesting, from like Graham Holoman's patent to Collaborative Tapped Horn. It all makes for great reading.

Can you help? Thanks

Mike

[Svante]

I don't know if there was a thread, but I suppose one could use the exact same circuit for a bass-reflex box. One would have to be careful, though, in the tuning of the vent.

For example, a solution with the vent tuned to 40 Hz, cannot be "transformed" to a 20 Hz lower cutoff by the circuit alone with any decent maximum output level. The tuning will need to be lowered to achieve this.

[mikee55]

Interesting, so the benefits of reflex loading, could be retuned as well as a sealed box, giving a small reflex cab behaving like a bigger one. This leads to an obvious need, where size of box is important in a car for example. I got a small Hatch, and i use a 10inch driver in a reflex box, 40 litres if I remember, it doesn't leave much room for a pushchair and shopping bags.
I used a reflex, because of the driver's T/S. I tuned for about 27hz, don't know if I achieved that, but it does reach quite low with limited volume.

A Linkwitz Transform type of circuit would be useful, maybe I could get to 27hz with an even smaller box. An added benifit would be weight too.

Cheers, Mike

[GM]

Originally posted by Svante
I don't know if there was a thread, but I suppose one could use the exact same circuit for a bass-reflex box.

Greets!

FWIW, when I brought this subject up many years ago on the old basslist, John Murphy's response was to view the vented box as two cascaded 2nd order systems with each its own LRT wired in series since it didn't change the driver parameters in the way that would be required for it, suggesting using SPICE or similar to work it out.

WRT using a small vented cab, you can't get around the need for a longer vent that could negate a fair portion of the savings plus its harmonics may need damping, further reducing BW output, making you probably wish you'd just went LRT sealed to begin with. That said, it could be worth the effort if the cab is exceptionally large due to its T/S specs, which was why I was interested, but still haven't gotten around to pursuing it.

GM

[forr]

Hi Mikee55
---Interesting, so the benefits of reflex loading, could be retuned as well as a sealed box, giving a small reflex cab behaving like a bigger one.---

Such schemes have been described by Jeff Macauley in Electronics World and by Bob Cordell.

[Svante]

Well, the theory behind the closed box is that it behaves like a second order highpass filter. Such a filter has one "pole pair" which results in a f0 and a Q, and these two determine its behaviour. The Linkwitz transform adds "anti-poles" (zeroes) on top of the poles, which effectively cancels them, and adds a new pole pair at a lower frequency. The net result is that the original poles are replaced by a pair that is freely selectable.

The cost is that the amplifier has to deleiver extra power at the lower frequencies.

The bass-reflex box, on the other hand behaves like a fourth order highpass filter. A fourth order filter can be divided ito two second order filters. The Linkwitz transform can be used to modify one of these sections. In theory, two Linkwitz transforms could be cascaded to get full control of the response of the bass-reflex box, but I suppose that the limitations below the vent tuning will make this unnecessary. I guess that lowering the vent tuning and applying an ordinary Linkwitz transform will do the trick.

[mikee55]

So, can todays Guru's come up with a workaround for the port length, or is this getting into T-Line territory/ big boxes? Is todays PC upto the simulations?

Another minus, the harmonics, could be electronicaly removed but could be complex, way out of my league. But there sure are a lot of clever guy's on this site. Just need to strike up interest, perhaps.

Any takers?, is it worth it?, even her indoors would approve WAF on the reduced size!

What do you think?

[forr]

Hi Svante,

---The cost is that the amplifier has to deleiver extra power at the lower frequencies.---

I think there are some subtleties here.

Suppose a closed box of fc = 50 Hz and Qtc = 1.4 which is LR transformed to an apparent fc of 25 Hz with a Qtc = 0.7.

The initial response at 50 Hz with a Qtc = 1.4 was +3 dB. It is made almost flat with the LR transform. The required power at 50 Hz is then halved .

The initial response at 25 Hz with a Qtc = 1.4 was about -10 dB. It is made -3 dB with the LR transform. This required power for this new response will theorically need about 5 times more power.

However, statistically, the spectral distribution of has 12 dB less energy at 50 Hz than at 25 Hz. I think even peak levels at 25 Hz are very rarely as loud as at those at 50 hz.

This implies that the mean power required from the amplifier can often be curiously less with an LR transform than without.

[mikee55]

I'm trying to keep up. Keep at it, your doing okay!

Mike

[Svante]

Originally posted by forr
Hi Svante,

---The cost is that the amplifier has to deleiver extra power at the lower frequencies.---

I think there are some subtleties here.

Suppose a closed box of fc = 50 Hz and Qtc = 1.4 which is LR transformed to an apparent fc of 25 Hz with a Qtc = 0.7.

The initial response at 50 Hz with a Qtc = 1.4 was +3 dB. It is made almost flat with the LR transform. The required power at 50 Hz is then halved .

The initial response at 25 Hz with a Qtc = 1.4 was about -10 dB. It is made -3 dB with the LR transform. This required power for this new response will theorically need about 5 times more power.

However, statistically, the spectral distribution of has 12 dB less energy at 50 Hz than at 25 Hz. I think even peak levels at 25 Hz are very rarely as loud as at those at 50 hz.

This implies that the mean power required from the amplifier can often be curiously less with an LR transform than without.

Ah, yes for such a high Qtc there will be a peak that will have to be lowered, yes. Also, the since the maximum output level is typically limited by Xmax and not Pmax at low frequencies, it is perfectly ok to have some bass boost.