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Two PI: A Little Bright OR Light in the Bass? [message #76092] Sat, 30 March 2013 14:04 Go to next message
themilford is currently offline  themilford
Messages: 66
Registered: May 2009
Viscount
Hey,

I've been living with a pair of 2PIs on and off, in and out of my system for the past few years now.

While the tonal balance is a tad on the "middy" or "vintage" side which I like, I think overall they are lacking in bass... or maybe it's just a that they are a tad bright?

I've only ever really compared them to my Audio Note Js and I'm running them with a pair or Bottlehead Paramours... the Js are flatter sounding... not a ton more bass... but they have it where is counts for most rock records... and although some folks think they are a tad on the bright side as well, when they listen at my house... the PIs are surely brighter still. It is particularly egregious when playing music with prominent jangly or distorted guitars and bashy cymbals. Basically around 2-3k...

I've got 14 gauge wire throughout, 0.51mH 14 AWG Perfect Layer Inductors, Solen 10uf caps and a 20ohm wirewound across the tweeter like the older design called for. lightly stuffed with polyfil.

I read one tweak might be to swap the film caps for oil-filled motor run caps... But I really feels it's more a lack of bass.

Any thoughts?
Re: Two PI: A Little Bright OR Light in the Bass? [message #76097 is a reply to message #76092] Sat, 30 March 2013 16:50 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17689
Registered: January 2001
Illuminati (33rd Degree)

That's true, the one π and two π speakers have rather high f3 points. They don't dig really deep, and are intended to be used as surrounds, or if mains, to be supplemented with subs. The two π tower had deep full bass though, so if subs are out of the question, you might consider building tower enclosures.

Re: Two PI: A Little Bright OR Light in the Bass? [message #76105 is a reply to message #76097] Sun, 31 March 2013 10:14 Go to previous messageGo to next message
themilford is currently offline  themilford
Messages: 66
Registered: May 2009
Viscount
Thanks wayne.

I dunno though... I just wonder if I did something wrong. Because I made a pair some years ago (w/ the CTS tweeter so no cap) but don't remember them being so bass shy.

Did I make the port to short or something? Do I need more stuffing? Should I have used R13 instead of Polyfil? Should I have used different coils or caps? is the resistor the wrong value? Do they just need more break in time?

I haven't really put many hours in on them over the years. Just a few sessions here or there...

Re: Two PI: A Little Bright OR Light in the Bass? [message #76106 is a reply to message #76105] Sun, 31 March 2013 11:19 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17689
Registered: January 2001
Illuminati (33rd Degree)

You could always check your cabinet to see if the port is too short, I suppose. The plans call for 1-7/8" length. R13 insulation should line the rear, bottom and side nearest the port. So check everything, if you think you might have sone something wrong. You might even want to have the woofer checked, if you think they used to sound differently.

To my rears, they sound very powerful and not "bass shy" but they do lack the deepest bass. Like most of my designs, you really need to augment them with subs for the bottom octave (20-40Hz).

Re: Two PI: A Little Bright OR Light in the Bass? [message #76109 is a reply to message #76106] Sun, 31 March 2013 12:52 Go to previous messageGo to next message
themilford is currently offline  themilford
Messages: 66
Registered: May 2009
Viscount
DUH, DUH, DUH!

How the heck did I manage to make the port a 1/4" too short!

Hmmm... how much of a difference would that make in the bass?

Also, what's an elegant solution to elongating it without tearing the things apart?

Re: Two PI: A Little Bright OR Light in the Bass? [message #76110 is a reply to message #76109] Sun, 31 March 2013 13:09 Go to previous messageGo to next message
themilford is currently offline  themilford
Messages: 66
Registered: May 2009
Viscount
Also, is there any meaningful difference between the R13 and the polyfil?

I mean the polyfil just sorta gets stuffed in there and the R13 lines the walls... I don't really like working with R13 in the house... but if you feel it's essential I'll grab some this week and try it out.

Also, as a tangent... any thoughts on playing with the crossover a little in terms of fine-tuning? I have parts on hand to try a "solen split" 1st order... I feel like most of the bright energy is focused in this x-over region.

In any case, I'll try the port first... then the stuffing... and report back.
Re: Two PI: A Little Bright OR Light in the Bass? [message #76112 is a reply to message #76110] Sun, 31 March 2013 14:19 Go to previous messageGo to next message
themilford is currently offline  themilford
Messages: 66
Registered: May 2009
Viscount
Ahhh, THERE'S Neil Peart's kick drum!

I just glued & taped little port extensions in there... seems to help a little bit with the very lowest bit of bass.

Wondering if a little more stuffing might help tame the high upper mids a tad and fill out the upper bass a tad more...

I'll report back in a few more...
Re: Two PI: A Little Bright OR Light in the Bass? [message #76114 is a reply to message #76112] Sun, 31 March 2013 15:22 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17689
Registered: January 2001
Illuminati (33rd Degree)

Yeah, having the port too short will make the speaker lack bass in the 60Hz region and overexaggerate midbass and lower midrange in the 80Hz to 120Hz region. Then not having the right damping material will make matters worse, tending to make the speaker be hollow sounding. The insulation attenuates internal standing waves, and in this speaker, those occur in the upper midrange. So I'd say the two accidental modifications probably tended to make the speaker sound kind of "thrummy", having an unnatural over-emphasis of the midrange.

Re: Two PI: A Little Bright OR Light in the Bass? [message #76126 is a reply to message #76114] Mon, 01 April 2013 10:23 Go to previous messageGo to next message
themilford is currently offline  themilford
Messages: 66
Registered: May 2009
Viscount
Ok,

Some pics for yous guys:


index.php?t=getfile&id=1091&private=0

index.php?t=getfile&id=1092&private=0

index.php?t=getfile&id=1093&private=0

index.php?t=getfile&id=1094&private=0

index.php?t=getfile&id=1095&private=0


Cork-lined front baffle, Finland birch plywood, rear panel is particleboard. Finished with about a bazillion coats of tung oil and paste-wax-buffed to a shine.

FWIW: I love the utilitarian, mid-century modern look of birch plywood. I'll take it over fancy-schmantzy veneers any day. Wink

So the tweaking continues...
So, I swapped the .51mH, 14ga coil for a .60mh, 20ga coil I had on hand. It's getting closer to what I remember the older pair of these I made years ago sounded like? Why is that? What is happening here?

Also, added more polyfil just before this swap and that seemed to help a little too... but minimally so...

What is the resistor on the tweeter doing btw? I have a 20ohm there... is that the correct value?
Re: Two PI: A Little Bright OR Light in the Bass? [message #76131 is a reply to message #76126] Mon, 01 April 2013 13:38 Go to previous messageGo to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 17689
Registered: January 2001
Illuminati (33rd Degree)

I love the looks of that cabinet. The cork front is a nice touch, as is the rabbet edge. Good work!

Looks like a nice little Bottlehead system too.

The 20Ω resistor across the tweeer is there just to shunt the Zmax peak around 680Hz. It prevents excessive energy way down low. You can't really tell it's there, it's just a snubber, an impedance control device. Again, it just protects the tweeter from getting excessive energy down low.

Now for the cabinet. I see that you installed the port on the back. This change, paired with the change of stuffing material is probably what has caused the differences. Could be that the polyfill has settled over time, and has become less effective at absorbing midrange standing waves.

I suggest installing fiberglass insulation before evaluating any other changes, like the larger coil. Your 0.6mH coil is not so much bigger that I would expect to to make a huge difference, but still, let's get back to a baseline. You can't really do that since the port is in a different position, but I don't think that will be a problem if you use R11/R13 lining the walls. Might want to cut a hole in the insulation where the port is though, or perhaps line the front instead of the back. But definitely go with fiberglass insulation, and then see where you're at.

I've tried other damping materials over the years and find most to be lacking, some really bad. What I often find in other padding materials is they are less effective at attenuating midrange. Sometimes not only that but they can also modify cavity resonance and detune the box. I'm not sure if the problems with other materials are that the fibers are too heavy and/or rigid to vibrate and absorb energy or if it's more of a bulk/mass thing where the sheets just acts like a solid block. Could be a little bit of both. But whatever the case, I find good old fiberglass insulation works best.

Fiberglass insulation may be a little irritating to your skin when you install it, but once it's there it doesn't enter the air. The sound of the loudspeaker may vibrate the fibers, but they don't break free. And glass is a quite benign, really. It does not cause cancer, and is probably one of the safest fiberous materials to use. It's not like asbestos, but probably gets associated with that in some people's minds because it looks similar. So I think it is probably the best, safest and most effective material to use for this purpose. I find no downside to using the stuff.

Beyond that, I often tell people not to make cabinet mods without testing. The reason is you don't want a port to be in a pressure node of a standing wave. All cabinets have internal standing waves, and that's why we use insulation inside, to attenuate the standing waves.

Large cabinets are particularly sensitive to cabinet mods because internal standing waves are at low enough frequency the insulation is only partially effective. So the positions of the midwoofer and the port are really important. However, the smaller one π and two π cabinets are small enough that the standing waves are in the upper midrange where insulation usually does a very good job. So they tend to be a little less sensitive to cabinet mods, provided the insulation is effective.

That's why I suggest using fiberglass insulation - It could be that you've put the port in a pressure node that causes a midrange peak, and the polyfill you are using is unable to attenuate it. But once you use fiberglass insulation, I think the port position will matter less. Should get you back to where you want to be and sounding good.

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