TY

Here you go. One atop the other, on a pole, one with the BMS/horn assembly installed.

There's still quite a bit of woodwork to do, mainly at the back where you can't see.

OK, the woodwork is basically done, the drivers fit and I can dry-assemble the whole cabinet. I'll do a round of tuning and then start gluing things.

One problem is that once things are glued, there is less that I can adjust (some things, like A and AX inertience will always be adjustable though). So I can make big changes based on inaccurate measurements (because the box is not glued) then only limited changes based on more accurate information. For sure, the final TY will not match the model, but hopefully I can get the model to match the measurements and thereby figure out the best tweaks from where I am. More to follow...

I'm learning that prototyping is a real deal. I hope you'll get the result predicted. So before "closing the cab" do you make some measurement to get the adjustment? or just by ear? Really interesting at all. smiley20

I'm taking driver impedance measurements which are revealing because you get the clearest indication of the resonant frequencies and their respective Q values. I'm also measuring the SPL in the internal chambers using what I call "portals" which are really just holes in the cabinet through which I poke the microphone part of an SPL meter. Unused portals are plugged. Finally, I'm taking SPL at the mouth and at 1 metre, and by ear. The 1 metre SPL measurement is actually the least accurate of all the measurements I can do, because I don't have an anechoic chamber. At the moment I'm just using them as rough confirmation of the expected sensitivity and response.

I'm not trying to measure group delay because most actual group delay measurements are completely wrong, for reasons I'll explain in a future post.

So far I've got one round of results, but have not analysed them fully yet. Suffice to say, the TY concept works, but I don't yet know how well I'll be able to make it work. It's nice to hear convincingly loud tones at 0.6mW input power though.

Overall results are mostly as expected: all the resonances are somewhat damped, as expected in a dry-assembled box with seals that are probably not very good. "A" tuning is off by a tiny amount, basically in the error margin, so I'll adjust when I have better measurements (lla can be tweaked on a finished box).

The C system fundamental is high and its 3/4-wave mode is out at 900-1000ish, as expected since I haven't put in the L-gates yet. AX is spot on (hurrah!)

B is very high. It's supposed to be 4 litres per driver and the results I'm getting suggest 2.5ish. This is partly down to the fact that I haven't put in any wadding. Since B is sealed, I'm going to put in loads. Don't have enough so ordered from Terralec.

It turns out that the drivers needed breaking in. Specs say fs=90Hz. Measured 107Hz which means compliance was 30% too low. Fed in a 10 volt 35Hz sinewave for 3 hours, and Fs down to 87Hz, which is within tolerance. Moral of the tale: break in your drivers!

Hopefully this will give me the right effective vb. If not I can get a little more by routing the internal walls (don't really want to do this). When the wadding arrives I'll do another rough measurement and start making L gates.

it looks like its starting to take shape very nicely......... im looking forward to hearing them, maybee at the next fsp meet.......... smiley20 this kind of stuff is well above were my learning stage of designing is, but im still enjoying watching and learning from your posts.... smiley32

It would be cool to bring them to a meet! Anyway, no more TY work till the weekend. I want to watch the alternative election coverage tonight with beer+pizza.

lol.....

Made L-gates. I am now at one with my hole saw.

Long time no TY update.

So I've done several (maybe 5 or 6) rounds of tweak and measure, based on dry assembly (just screwing the cabinet together and trying to seal the gaps with tape, foam etc).

A big problem that emerged as I got better at measuring and understanding the results was parasitic quarter-wave shunts. Essentially, parts of the interior shape of the resonator chambers resemble a duct going off to the side of the cabinet (ie in shunt) which is stopped at some depth. The stopped pipe resonance is the quarter-wave resonance, and for example a 10cm effective length resonates at 850Hz.

Such shunts have an acoustic impedance which drops to close to zero near resonance. This can tend to drain or "short-circuit" most of the pressure in that region of the system, adding a notch filter to the response. The notches can be big enough to dominate the frequency response which seems surprising considering the shunts themselves are physically quite small, basically corners and nooks and crannys, like round the outside of the comp or into the narrowing around the horn throat.

I've seen a very pronounced one at about 900 in the AX chamber which, though out of band, was still dragging down the upper-end response. Pushed it up to around 1.3K with extra baffles backfilled with foam in the corners and around the comp, extending the comp's incursion to a strip from left to right. Seems high enough to be out of the way now.

I then found one in the C chamber, which was actually notching at about 750, not as deep but still unacceptable. C is bigger than AX and filling the shunts consumes more air volume, compromising TY's unique efficient-use-of-cabinet-volume proposition. I have demonstrated I can add about 1-2 litres of expanding foam, around comp and horn, and drive the notch up to 850 where its effect is more tolerable. Maybe I can get 900 or higher which would be better.

With these shunt notches hanging around threateningly just outside the passband, like stroppy hoodies with roll-ups, I will have to move the A and AX port tuning to compensate. It looks like I can do this by adding the shunts to the Akabak model and respinning.

I've decided now is the time to glue up the boxes. Because now the major headaches and unknowns are sorted, the rest is just fine tuning. Gluing the box will shift the tunings by itself anyway, and I can adjust A and AX ports on a finished box (designed that way from the start, that's where experience comes in

In listening tests, TY is already doing nice things with snare drums hits! But I don't want to overdo listening tests until the measurements are all giving the green light. Or at least greenish. Lime green will do.