Feedback: New LCR Design for Alpair 10.2

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This design is specifically meant for home theater. I was thinking of a floor stander that could be rotated 90 degrees and used as a center channel without any changes to the cabinet in order to maintain identical voicing.

Here is a quick drawing:

alpair-102-lcr_zps7a51ff1d.png


Specs:

0.75" ply
External dims:
Height: 39"
width: 9.5"
depth: 6.5"
(1500 cu in / 24.6 liters)

ports: 2
port dims: 0.25" H x 8" W x 4.59" L
port tuning: 40 Hz
port mach: 0.03
Sim F3: 38 Hz
Sim F10: ~ 30 Hz

The driver would only be 19.5" off of the floor, on center.

What do you guys think?
 
Yes, I thought it might be too low to the floor... no, I won't be using stands. I've seen some small commercial floor standers with drivers not much higher than that, but still... it would cause main listening position to be off axis vertically (the driver will always be below ear height, but this would be a bit more so than typical). I could maybe raise the driver and ports nearer to the top for the LR. But then, it might sound a bit different than the center. :) Or, maybe not noticeably so.

Group delay, Dave? What am I looking at there? Sorry for my ignorance as to what group delay is.... I thought it was something to do with multi-driver systems? Or maybe you mean the sound emitting from the slot ports and the driver? Why is the F3 only 50 Hz whereas the sim shows it to be lower?

As to it being big, it is smaller than a Pensil.

Thank you in advance for the education.

Ben
 
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So not REALLY trying to push you away from doing a matching LCR but if they're going to be on the floor I'd imagine that the imaging is going to be way off for music. Why not make your left and right with the drivers toward the top, at or near ear level then just adjust the port to be a single at the bottom, adjust its dimensions since you'll have just one port, and be done? The box dims could be the same so all your major cuts would be virtually identical making construction similar. I don't think you'd audibly hear a difference between them aligned this way with the center as you have designed vs them all being identical. Just my 2 cents.

Scott

PS I did SuperPensils with a custom Mar-Ken12 shallow that Dave designed and I beta'd. With some simple adjustment of the stuffing I was able to get an almost acoustically identical LCR. With the 80hz crossover the difference in bass is non-existent to my ears. It's a fantastic front stage. Just look at my SuperPensil build to see pictures and such.
 
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frugal-phile™
Joined 2001
Paid Member
Group delay, Dave? What am I looking at there? ... Why is the F3 only 50 Hz whereas the sim shows it to be lower?

The blue curve that goes off the top of the graph... too high a group delay is one of the things that gives BR a bad name.

The sim does not account for the "R" added by the high aspect ratio slot port.

As to it being big, it is smaller than a Pensil.

Pensil is a TL not a BR

dave
 
frugal-phile™
Joined 2001
Paid Member
So not REALLY trying to push you away from doing a matching LCR but if they're going to be on the floor I'd imagine that the imaging is going to be way off for music. Why not make your left and right with the drivers toward the top, at or near ear level then just adjust the port to be a single at the bottom

With the vent at the bottom he will end up with an ML-TL. He has done most of what can be done with the current design to fight that.

dave
 
Ok, here is a revision to shift the drivers higher on the LR. The driver centerline will be around 29" off the floor, which is quite standard. I adjusted the ports a little bit to allow room at the top (the part after the elbow is shorter.)

LR:

alpair-lcr-lr_zps9ae79608.png


Center:

alpair-lcr-c_zpsf3c7a6c8.png


The elbow in the ports is to prevent mids from escaping quite as much. Good idea?

Let me know what you fellows think of the revision. :)
 
Boxed in volume created by slotted port fixtures confine driver; reflections off of them and back panel increase amount of higher frequency energy bouncing back and re-radiating through driver. This is much bigger factor than mid radiation through slots. In pictured drawing the slotted port fixtures reduce distance that energy travels through damping material before bouncing back at/through driver.

Slotted ports break interior into three ported chambers, each alone and in combination with resonances.

Elbow in slotted port not much more effective in escape of mid frequencies than straight port; turbulent plus reflective properties at corner are complex and greatly influence behavior of port. At frequencies much above tuning of port/enclosure little energy enters port.

Critically lined and stuffed enclosure is route to controlling mid and high frequency energy without overly damping low end.

Regards,

Andrew
 
^^^

Maybe moving the ports on the center to the outside then moving the port to the bottom on the LR and making it one port would be better in this regard and help the internal reflection issues. That definitely is a big consideration with these thin cones. Good point for sure!

Scott
 
You are probably right, Andrew. However, the way I justified that in my mind is that the Alpair 10.2 has been put in smaller cabinets than the distance between the slotted port components in these.

However, I'm thinking of a version 3 design already to address this. :)

As far as putting a single port at the bottom, that would turn it into a transmission line, which I am trying to avoid as it would be difficult to make a matching center channel design.
 
Here is version 3 of the mains:

alpair-lcr-lr-v3_zps8f8109cc.png


The specs are the same as in the OP, except the driver has been moved up on the baffle and the slot ports have been minimized internally to reduce reflections. The slots needed to be moved closer toward the driver as well to provide enough space at the top without folding the port to keep things simple.

Comments? :)
 
Bigger than i'd do. Look at the group delay (the dip above the liw frequency max is a give away). The high ratio vents will knock that back thou. F3 is gong to be in the neighborhood of 50 Hz, F10 about the same.

dave

Dave, sorry to bring this up again, but what would you do to the version 3 design just above to improve the group delay issue? Make it shorter with a void at the bottom as a 'stand'?
 
I use maximum 20 litres for A10.2 (the large miniOnken for them are 16 litre). Do whatever you need to make them tall enuff -- try tilting them back 5 degrees as well

dave



even a little tilt helps a lot with enclosures that place the driver lower than we want or think they need to be - worked like a charm with several DIY designs that I can think of - Tysen, FH3, Vampyr, and commercial models as well - Terry Cain's Abby, various Thiels etc, etc
 
Yes, I have built speakers with complex angles before - I'm a lazy woodworker, but not unskilled. :) I could make the front baffle sloped. However, in the version 3 design the driver is at the same height as other popular designs, like the Pensil. Are you saying it should still be sloped even though the driver is now at a very standard height? I mean, sure, it's still below ear height, so it makes sense, just wanted to double check.
 
Of course, the goal of this project was to make three speakers as identical to each other as possible. That's usually what "LCR" implies in speaker vernacular. :) So, I think I'm inclined to keep the mains' front baffles at 90 degrees as the center would be at a different height anyway, and a slope on that might just make things more complicated. Sound reasoning? :)

Happy Holidays, by the way!
 
Of course, the goal of this project was to make three speakers as identical to each other as possible. That's usually what "LCR" implies in speaker vernacular. :) So, I think I'm inclined to keep the mains' front baffles at 90 degrees as the center would be at a different height anyway, and a slope on that might just make things more complicated. Sound reasoning? :)

Happy Holidays, by the way!

No one questioned your woodworking skills, and even though I might build a dozen or more pairs a year, the laziest way possible is always appealing.

Unless you're blessed with a projection system, it's hard to avoid the placement / orientation constraints usually required to accommodate placing a center channel enclosure under or over the flat screen - and as the screens get bigger, this problem will only be exacerbated.



In my own system, the only possible configuration places the TV immediately in front of a large window, with the screen a bit lower than ideal, but it is what it is. With Alpair7s as LCR, ("mains" are Pensils) there's an approximately 5" difference in driver heights in the front row, but you can still understand what's happening, and the "discontinuity" is not as bad as might be expected. If I was to ever get off my lazy a$$, I'd probably build a new centre based on the existing design, but with a slanted front baffle.

There, I was looking for another project for the upcoming holiday weekend :rolleyes:
 
Thanks for the reply. Sorry, I was not thinking anyone was questioning me, sorry if it sounded that way. Hard to get the 'tone' across on the Internet! I do have an 85" front projection screen but it's permanently mounted to the wall. No such luck to have room for an AT screen with serious speakers behind it. Perhaps some day.

Below is version four of the mains. I shortened up the internal volume to 20 liters not counting the driver or port volume, so it would be a tad less in practice. There is now a void at the bottom. I also adjusted the ports to maintain the 40 Hz tuning and take advantage of the top panel as sort of an elbow extension of the port (why not... it was so close anyway it would have had some effect on the tuning).

Here it is:

An externally hosted image should be here but it was not working when we last tested it.


Specs again, as so much has changed since the OP:

0.75" ply
External dims:
Height: 39"
width: 9.5"
depth: 6.5"
Internal dims:
Height: 30.5"
width: 8"
depth: 5"
(1221 cubic inches / 20 liters)

ports: 2
port dims: 0.25" H x 7" W x 5.53" L
port tuning: 40 Hz
port mach: 0.04
Sim F3: ~ 40.5 Hz
Sim F10: ~ 31 Hz

Let me know what you think. :)
 
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This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.