4ft Met Build Thread

[Cressy Snr]

OK,
so here is the official thread for the build of the 4ft, Metronome, quadratic-tapered, quarter-wave resonator speaker I'm on with at the moment.
The idea is to keep everything as simple as possible, with a mechanically applied roll-off to the bass/mid unit.
HF is handled with a second order 12dB/octave filter on the tweeter.

Main driver is the 150W, Faital 6PR110, pro driver with cast aluminium frame and doped, paper-pulp cone.
Tweeter is the SB Acoustics SB29RDNC-C000-4 ring driver.

As is usually the case with a pro driver of this size, the main driver Fs is 100Hz, which is a bit high, but the enclosure/port combination is configured to reinforce output below this and allow wall placement to boost the low bass.
Driver sensitivity is 96dB/W.
FR looks nicely flat across the telephone band; just a slight rise but its even all the way.

This 6" driver Metronome is a further development of the one I designed for our Ant.

The quadratic taper' quarter wave resonator speaker with down-firing air mass loading port, is such a piece of form-follows-function design purity that it deserves to be developed to the max potential. Real room operation, close to walls is what it's all about, with the Metronome.

Maximum WAF is the number one priority, absolute fidelity is secondary, I mean what's that anyway? Nobody can agree.
Clarity, neutrality and a fine midrange will do for starters, we can work from there as without those three characteristics
present and correct, a speaker may as well be binned.
So the requirements are clear and the design compromises have been set out.
We know what we want and that is half the battle won.

[Cressy Snr]

Bass/mid drivers:

[pre65]

How are you going to join the panels together ?

Routed joints ?

[Cressy Snr]

The panels are simple butt joints held with biscuits and glue. The top, where the panels all meet when assembled, is profiled flat with respect to a level surface, then a top cap with appropriately angled sides is glued on and made flush all around. The base is profiled with an angled blade table saw, then inserted from underneath against a set of battens around the interior, and glued.
The bottoms of the legs will be profiled flat with respect to a level surface.
An off-centre brace to the rear cabinet inner face from the rear of the bass/mid driver magnet also needs to be included.

The long, sides front and rear panels are cut with a hand held circular saw against a true metal straight edge, clamped to the work.
If any are to be made in quantity, then jigs would need to be built to ensure repeatability and consistency between samples.
Of course everything needs to be machine sanded flat to get a perfect substrate for the veneer once the cabs are completed.

[Cressy Snr]

Tweeters:

[Cressy Snr]

The bass/mid drivers have been tested in our Ant's Metronomes, using the first order crossover we developed between us, for the Faital 6FE100 to SB ring tweeter.
The proposed crossover for my own build is second order, 12dB/octave, which will give a 180 degree phase shift to the tweeter, through its operating range, so should make it a little easier to get a good phase coherent stereo image from the newies by reversing the tweeter connection with respect to the bass/mid unit as commonly done in speakers with 2nd order filters.

Note here I'm speaking only about what I know and I'm perfectly well aware that there is more to it than that, when it comes to time aligning the drivers for wideband phase linearity. I'm in the 70s when it comes to speaker design and quite content to remain there. Tone is king for this builder. but there's nothing wrong with striving for maximum imaging performance as well. I won't sweat over it though. A bit of third octave pink noise analysis will be adequate to check out the in-room response.

So in our Ant's cabinets, the all-important midrange is correct, with no shoutiness evident on 'hot' recorded vocals, of either the male or female variety.
Through informed and careful choice of driver we have this vital part of the spectrum right, so the rest of the frequency range can be tweaked up around it. This is how I have always designed my speakers and it works for me.

[pre65]

I've always used PVA glue for wood, but do modern adhesives like no-nails or gorilla glue have any advantage on BBP (baltic birch ply) ?

[Cressy Snr]

Wouldn't know Phil. Like yourself, I've never tried anything other than PVA.