The Metronome

[Scottmoose]

Glad you guys are liking them.

[Steve Cresswell]

Originally posted by ecir38


Looking foward to these, it will probably be my next project. How far off are you?

I've got the drivers and the timber. All I need now is for the school holidays to start in three weeks and I'm good to go.

Steve

[bchristmas]

Hey All,

Has anyone used the DCA 5.5 driver in the Metronome enclosure? I am considering this driver but it is not listed in the Metronome variations table. Hey Scott or Dave could you run the calc's if you have the driver specs?

Thanks,

Bernard

[planet10]

Originally posted by bchristmas
DCA 5.5 driver in the Metronome enclosure... Dave could you run the calc's if you have the driver specs?

I have a set of the drivers. Haven't done anything with them yet. I need to run them in a bit before measuring.

dave

[schro20]

I am putting together a FE207e Metronome according to the tables at http://www.frugal-horn.com/metronome-table.html. There it states that the tube should be 3x3 in. 3in inner diameter I presume. What about the length? Is 3in the inner length (this would account for the part going through the wood) or the exposed length on the inside? (This would be tube length minus wood thickness.)

Peter

[Steve Cresswell]

Hi Peter

The inner length of the pipe is 3 inches.

Steve

[schro20]

Dear Colleagues,

I have started to measure my Met build (FE207e; using build dimensions as found on http://www.frugal-horn.com/metronome-table.html). I have glued the front and sides and put the back on with clamps so I can learn something about the stuffing. For starters I have put 1in acoustic polyurethane foam with an NRC rating of 0.75 (http://www.mcmaster.com/nav/enter.asp?partnum=5692T13) on the sides and back just by the driver (20cm in height at the height of the driver). I also put some 1in egg crate polyurethane foam at the top and bottom (it only has an NRC of 0.25; probably doesn't do anything). I have then endeavored to measure the response with ARTA. I elevated the speaker off the floor (on top of a marble topped nightstand kind of piece of furniture) and have 1.5m of free space all around the driver (as verified by looking at the first echo in the impulse response). I am using the pink noise measurement with 4 times averaging. I am attaching an image of the raw (1/9 octave averaged) measurement as well as the gated measurement (stopping just short of the first reflection) with 1m distance between driver and microphone (head on). The gated response calculation is trustworthy (according to ARTA) from 200Hz onward.

I'd like to ask your help in understanding what I can learn from this. For example, there are severe dips at 78 and 106 Hz. I've taken a number of measurements with different mic distances (e.g., 50cm and 100cm and such, and these dips are fairly consistent though they move a little as I change measurement conditions (78/84Hz resp. 106/124 Hz. Next big dip is at 200 Hz which corresponds to the beginning of the confidence interval (I presume this implies that it is actually due to the wall reflections (floor resp. ceiling). (Why?) What's up with the peak at 2500 Hz? It seems to be at the end of a general upward slope (see the green curve of the gated response) with a rise of 3dB/octave. The green curve also has these dips at 980 and 1450Hz. Are they significant?

I realize these are very elementary questions so I ask your indulgence as I am trying to learn this stuff.

peter

[Scottmoose]

Room acoustics for the LF stuff. Anything over about 250 - 300Hz is the driver's own response as it's above the cabinet's operating BW, so you'd have to ask someone with experience in driver design.

[schro20]

Originally posted by Scottmoose
Room acoustics for the LF stuff. Anything over about 250 - 300Hz is the driver's own response as it's above the cabinet's operating BW

Just to be sure I understand. With the room reflections I have only things above 200Hz really mean anything in these measurements (else they are contaminated by the reflection). But if anything above 250-300 is due to the driver and not the cabinet I don't really have much left that is under my control... Am I reading this right?

What I am fishing for here (in a way) are things having to do with the lining of the cabinets and how that should show up. Also the baffle refraction and how it shows up. Are you saying that the measurements I made really don't help me with those questions?

If I think about corrections I think about the initial 3dB/octave slope and making that flat. But beyond I wonder about any of the peaks and dips. Is that all the FE207e driver? That doesn't match at all what the Fostex measurements show. Is all the ripple due to room effects? (even at high frequencies), or is some of it due to (effectively) missing lining? Where can I see the cabinet action? Is it visible in these measurements? Or am I looking in the wrong place?

Lots of question...

Peter

[Scottmoose]

Basically correct. The cabinet's operating BW is very narrow, below about 250Hz or so. Above that, it's not doing a whole lot, other than providing a solid mounting point for the driver. Below about 200 - 300Hz, the room dominates the response & aside from digital Eq, there's little you can do about it. That's why in-room response plots are really only useful to the person who owns the speaker & the room.

You can see a degree of step-loss coming in from about 480Hz, give or take, due to the cabinets being a ways out from the wall -moving them back will lower that somewhat. The rest is simply the driver's own response. You can actually see the peak you mention at ~2.5KHz in the driver plot Fostex provide. It's just that they happen to smooth their driver plots rather heavily, like all manufacturers do (1/3 - 1/2 octave is usual: 1/9 octave is nowhere near sufficient to sell drivers.)

As for trying to measure damping, you'll want to be measing cabinet (i.e vent) output & look for unwanted harmonic resonances. If there are any, it's not sufficiently damped.

TBH, the more important thing is whether you enjoy the sound. If you do, it doesn't matter one jot what a graph says.