Tannoy Monitor Gold 12" build.

[awkwardbydesign]

 A couple of sweeps on REW.  Not too bad, but an Hf peak (which I can't hear anyway) and the room dominates lower down.  A start.  1/12th octave smoothing, and slightly different HF setting.

[wenig watt]

The dips at 200 and 4-600 are quite severe. can you attribute them to the room proportion? would moving the speakers help? is the low bass enough for you? Do you have a measurement of the bass port?
Best regards

Arndt

[awkwardbydesign]

The 200-ish dip disappears at 160mm mic distance, and the 600mm dip remains. Those traces are at 800mm from the foam grilles. No test of the ports (slots) yet, but I will remove the drivers and test them in free air, DATS V3 and REW.  They are now back near the wall, so a re-test will be needed anyway, as that is where they will live.  The fronts are around 900mm from the wall; they could go a bit closer.
And they look closer together than they really are.

[wenig watt]

Ahh, sorry Richard,
I only have a hand-held measuring device that is actually intended for room acoustics and for soundchecks at concerts.
That's why I usually measure at the listening position and think like that. I was simply surprised at the low bass yield, which is no wonder with near field measurements.
I suspect that the Tannoy are quite clearly focused in the mid-high range. So it will take many differently placed measurements to get a meaningful picture. Do your power amplifiers have a high damping? If not, an impedance circuit might be necessary or helpful.
I wish you a lot of fun with the tuning.

Best regards

Arndt

[awkwardbydesign]

Valve amp with low damping factor.  Which is partly why I run them from the 4 ohm tap, which doubles the damping and halves the hum and noise.  Plenty of power (Icon Audio MB90 Mk2s), so I can afford to lose some.
Where I sit close to the back wall gives a big boost around 30Hz, which doesn't help, but I am stuck with that.
Fortunately my wife likes the look of them, but then she has had 50 years to get used to the stuff I build!

[awkwardbydesign]

I've just been looking at a floor (and ceiling) bounce calculator.  Nulls at 80, 174, 241, 401, 521, 562, and some higher up.  The trace shows dips between maybe 150 and 250, and 400 and 600.  Not far off the calculated figures.  Food for thought.

[wenig watt]

Hello Richard,
please stop me if I'm being too offensive.
You have three and a half options for influence in the bass.
The position of the speakers, the position and frequency of the reflex port and (expensive in the bass because of the large components) an impedance adjustment. Of course, you can still use bass traps like helmholtz resonators in the room, but that is only a last resort, as it destroys energy.
If the room has a 30 Hz resonance, I would set the resonator tuning higher, e.g. to 50 Hz, or damp the resonator heavily or tune it extremely low. Is there an impedance measurement of the chassis in the cabinet? this may provide an additional explanation for the low frequency response.
If your power amplifiers have a very low damping in pentode mode, you can try the triode circuit or work with an impedance linearisation in the crossover. In German, I know of a small tool for calculating this.

Best regards

Arndt

[awkwardbydesign]

All advice is welcome, Arndt.
I have now added 8 litres of ballast into the bottom of the cabinets to further reduce the volume, and some more polyester damping to the top and bottom of the cabinets.  The bass is now dryer, but maybe a little too much, and the slight squawk is now ameliorated, but again, it may have dulled the sound a little.  I will measure the FR with REW before changing again.  The impedance hasn't changed much, and the bass resonance is still lower than I was aiming for, so I have to work out how to modify that.  I think I can lengthen the slots from the inside without removing the panel, but shortening them would require unglueing a block or two.  Somewhat harder.

[wenig watt]

Hmm,
I would only install damping behind the driver to reduce reflections from the diaphragm and cancellations. The bass will only become less precise with more damping material inside. If you shorten the reflex slots, the amplification comes at higher frequency and increases. If you lengthen them or add a little damping material as a flow filter, the gain will be lower and the tuning frequency lower. What does the measurement look like with completly closed slots?

Best regards

Arndt

[awkwardbydesign]

We have had visitors, and it's early days, so not many measurements taken yet.  Plus a kitten is due to arrive this weekend, so cat proofing the stereo comes first!  Foam speaker grilles and hot valves will need protecting.

[awkwardbydesign]

I have covered the bottom half of the fronts with a removable board, to stop the kitten (also known as f**king cat) clawing them.  I forgot to remove them the other night, and for late night listening the reduction in bass worked rather well.  Once I get the slot tuning sorted, I will explore idea of a slot plugging board.
The rest of the system is blocked off with clear polycarbonate or wire mesh.  So far, so good.

[awkwardbydesign]

I've been playing with them again.  My pair of MG12s have an Fs of 18.5Hz and 19.5Hz, rather than the quoted 28Hz.  I have run them sealed, from around 130 litres up to 150 litres, and preferred them at 150.  Now I am playing with the slot loading at 150 litres, considering the low Fs.  Modelling them in WinISD, using the parameters from the DATsV3, I got 4 slots tuned to around 22Hz for a flat response down to resonance.  Of course they don't measure like that!  Initially I had the slots at 222mm deep, but the two impedance peaks differed by around 20 ohms.  Shortening them dropped the differences to around 7 ohms, and then to about 4 ohms, with the bottom of the trough around 20Hz.  Left and right are different lengths due to driver variations, but oddly while the left peaks equalised as expected, the right one increased again as the slots were shortened the second time! 
But still, they now sound punchier and more coherent than before.  Bass has better internal definition, and filled out the overall sound.  I will have to investigate the anomaly.
Of course, this only applies in my room, at my seat.

 Left, 177mm.


 Right, 168mm.

[awkwardbydesign]

Testing the right speaker again (and again!) I discovered why results have varied.  The 10mm acoustically transparent (!) foam grille is damping the slots.  Without it, the resonance peaks are as expected, but lowered (unsymetrically) with it on.  Having it over the driver has little effect, it's just over the slots.  Must test the left speaker too.

[Tomcat1969]

Wow! That's quite a difference. How have you gotten on?

[awkwardbydesign]

It's been a while, but now that the amp is settled (Accuphase E-800) I have started on the bass loading.  The slots haven't really worked as I hoped, so now I will try a more "normal" reflex pipe.  As my MG12s have such a low resonance frequency, and I don't tend to listen very loud, I am trying a single 100mm diameter tube.  Twin tubes would need to be too long to fit, and smaller diameter ones would still have a high air velocity.  
So I will be starting with a full volume cabinet, around 150L, which I can reduce easily with bags of wood pellets, and a chosen Fs of around 23Hz.  I am hoping for a gradual roll-off, and no peak, to minimise the effect of the ~30Hz bump at my seating position.  The simple modelling in WinIsd, which is all I can manage, suggests this might work.  But the port is made from 110mm  OD plastic soil pipe with an aluminium sleeve which I can extend to adjust the length, even as it is playing!  Minimum length of 220mm to a max of around 380mm.  Any more and it will hit the back of the cabinet!
The new panel is a straight swap for the existing one, so I can go back if I want to.
I will measure the response with REW and the DAT3, plus obviously listening.  At the moment there is a bit of a peak around 1kHZ too, which might change; if not, a small increase in the bass inductance looks likely drop that.  But first, the bass loading, and it will be fun to hear what happens.
I have rounded over the front panel to make a smooth transition from the pipe to the room, and tied a layer of car roofing felt around the pipe.  The front is covered with the same brown leather as the other front panels, and the screws will line up. Fingers crossed!