Tannoy Monitor Gold 12" build.

[wenig watt]

If you have a 30 Hz resonance in the room, a closed cabinet might be better. have you tried that? Reflex constructions tend to feedback...

But it already looks good! You could simulate a smooth transition between reflex and closed with some cotton wool in the tube. the more cotton wool, the closer the whole thing comes to the closed cabinet. A little cotton in the tube acts like a tube extension.

Best regards

Arndt

[Reffc]

Vented enclosures will have higher peak extension before a more rapid 24dB/Octave roll off.  There is no feedback mechanism.  If the peak extension matches or is close to room resonance mode peak then obviously you'll have a higher peak bass mode at that point.  Going sealed won't remove it but may well reduce it to acceptable levels so trying the ports bunged is worth it.  Also, moving the cabs off the back wall should reduce perceived bass and may not excite corner modes as much although peak room power response will be about the same.

[wenig watt]

Each surface adjacent to the loudspeaker generates 3 dB more sound pressure in the bass...
Since the set-up is given here and the 30 Hz resonance is known, the reflex tuning at 23 Hz may cause trouble by increasing the room resonance. That's what I meant by feedback. Either you try to lower the tuning even more. Alternatively, tuning down to 50 Hz might be even better.
Richard will hear it then...

Best regards

Arndt

[Reffc]

Each surface adjacent to the loudspeaker generates 3 dB more sound pressure in the bass...
Since the set-up is given here and the 30 Hz resonance is known, the reflex tuning at 23 Hz may cause trouble by increasing the room resonance. That's what I meant by feedback. Either you try to lower the tuning even more. Alternatively, tuning down to 50 Hz might be even better.
Richard will hear it then...

Best regards

Arndt

Placing close to rear wall actually generates +6dB, not 3dB as once within 1.2m of the rear wall to front of cabinet, the wall acts effectively as the baffle so sound remains mostly within 2 Pi space.  Baffle step loss usually requires speakers for free space to be compensated in crossover design to allow for the baffle step effect but those closer to the wall don't need it.  The thing to remember is that there is no net gain as such to the room power response (conservation of energy), only to bass within the 2Pi radiated space, however standing waves can built up especially when the cabs are equidistant from rear and front walls  causing the peaking that Richard is experiencing.

Worth noting Richard that your 18.5 Hz Fs is due to aging and therefore increased compliance of the suspension.  I'm guessing these are RS models?  I've yet to test any 12MG with pleated surround that does anything but increase the Fs point due to hardening of the pleat doping compound.  I wouldn't tune with the currently measured Fs as the T&S parameters for the driver (whilst these weren't known at the time of manufacture as it wasn't a "thing" then) are optimised for a more linear response at factory original values.  You might find as Arndt suggests that tuning higher up with a slightly higher forward going peak works better anyway with these in vented tuning configuration.  The old adage that twin initial tuning peaks should be equal is a bit of an old wives tale (arrived at before T&S parameters more accurately determined driver behaviour) since it depends entirely on a design's T&S parameters. Tannoys generally do better with the foregoing peak some 5 Ohms up on the second peak so you tune your driver slightly above box resonance.  50Hz imho is too high for that volume.  It will result in over excursion and possible damage to the drive unit.  I would suggest tuning between 35 and 40 Hz and see how that sounds.  Make it far enough above room mode peak remembering that port output continues to 1/3 octave below tuning point and check that driver excursion remains within permissible limits.  You'll find that upper bass is more impactful.  Putting resistive stuffing (and it really needs only surprisingly little) such as acoustic cloth over the back of the port will drop the foregoing peak significantly and modulate bass impedance.  It has a similar effect to increasing cabinet stuffing but it reduces port efficiency.  In your case this might be worth looking at but for now I'd suggest your tuning point is a little on the low side.  Raising it and then playing with resistive loading might be safer for driver health, give better upper bass and solve peaking issues or at least compensate enough to make it acceptable.

[awkwardbydesign]

Thanks for the helpful replies, just what I was hoping for.
I have tried sealing the box, by blocking the slots, but although that does reduce the lower bass, it also reduces the upper bass, giving a thin sounding presentation.  The idea with sticking with reflex of some type is to get less of a downward slope above resonance.  I may have that idea wrong, but I can vary the port length on the fly to experiment.
They are indeed the rubber surround version, Paul, so your points about how to tune them are useful.  And the other day I moved them forward about 40mm, and it appeared to improve the response.  Unfortunately, any more and it would make it impossible for my wife to reach the on/off switch for the system.  Of course, if I moved our sofa forward a foot or so that would remove the bass hump, but again, that is definitely out of the question.  Still, since selling the bike* I need something to fiddle with, and as I am no longer making amps, it's back to speakers.  And I can change the front panel back again if I wish.

* From this -
   
 to this -

 

[awkwardbydesign]

The new panels are in, after a couple of rookie errors.  I forgot the old ones had 3mm routed out at the edges to allow for the rubber seals, and didn't quite get the overall height correct, so had to plane 1mm off the edges.  And the grille frame cross strut half covers the port!  Getting old and sloppy, but all fixable. 
They sound slightly better, I think, but that is with the ports set at 310mm long , more or less at random, so plenty of measuring and tweaking to do.  Only played a few tracks so far, so I will wait a few days before changing anything. But indoor work is preferable to working in a cold garage!

[Colin_EJJ]

If sealing the ports reduces the low bass but then gives a thin upper bass, it is likely that the enclosure is too large for a sealed alignment, resulting in a Qts of 0.5 or lower. This will give a higher rolloff and a much less warm presentation.

It sound ridiculous but you could reduce the volume by placing something solid inside the cabinet (bricks, suitably wrapped) and reduce the volume by increments. That would allow you to see if sealing it really is able to arrive at the bass response you require. It might be easier than modifying the cabinet.

[awkwardbydesign]

A long delay, but I am back at them.  The system has changed quite a bit too, nearly all Accuphase now, including a DG-68 DSP equaliser.  The cabinet now has a single 100mm diameter port with a sliding tube, so I can tune it.  I found that at its shortest length the impedance peaks are equal with a 22Hz resonance!  Too low to be really useful for bass reinforcement, but well below the ~30Hz room node, so using the DG-68 I can get a flat response in the room.  Next will be sealing the cabinet and equalising that.  It's very simple to use (at least at a basic level), just set the mic at your listening position and press start, it uses warble tones and does everything automatically.  I can now sit close to the wall with no bass peak.  Later I will try aperiodic loading, but the cabs are probably too big for that to be useful.  Still, with the DATS-V3 I can quickly see what it's doing. And there are 30 memory slots on the DG-68 so plenty of room for multiple settings.
Now I can hear more clearly what is going on, it will be worth trying to fine tune the compression drivers and crossovers.  It may be that I can bypass the crossover switches, as I won't need different settings.  We'll see.

[awkwardbydesign]

Another bit of (very slow) progress.  I have made them into closed boxes, Fs.31Hz, and used the DG-68 to lift the lower frequencies a bit, which has given good body to the sound.  I will use REW to measure the response at my seat and tweak from there.  The crossover is set to level, but when I get some more connectors (I am not sure if they are GX 12 or GX 16), I want to remake them without the switches, and with better components.
I am now my wife's full time carer, so it may take a while!

[wenig watt]

Accuphase DG-68 DSP equaliser... I had to understand that first... wow, you can do magic with it! at least you don't need any other tools.

With best regards

Arndt

[awkwardbydesign]

It can equalise almost anything, and save up to 30 settings, I can write the curve on the fly and listen to the changes. It won't make a bad speaker (or amp) into a good one.  But it will improve a room's acoustics, or at least ameliorate it's deficiencies.  I could make settings for late night (less bass), different types of music, etc.  But first I need to get familiar with the basics.
It was stupidly expensive, but hey, I am no longer spending money on motorbikes.