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Ben_Tech 15th January 2013 10:06 PM

HT Front Stage Designs Using Markaudio Alpair 10.2 - Plans, Feedback and Build Log
 
I had a previous thread over in the Markaudio forum where many excellent suggestions were made to improve my design. The initial idea was to build an LCR (identical speakers for the front three channels of a home theater) using the Markaudio Alpair 10.2. However, as I revised the design per the suggestions that I received, the project evolved away from that concept into a more traditional mains and center.

Some of the design goals of this project include:

- A traditional home theater form factor where the mains are taller and the center channel is of the horizontal persuasion.
- No stands required.
- Good bass response in case the speakers are used standalone without a subwoofer at a later time.
- Optimize driver height and baffle angle to aim high frequencies more directly toward listener's ears.
- Excellent dialog intelligibility from the center.
- The best possible voice matching between the mains and center necessitating the cabinets be as close as possible in design to one another.

Many hours of painstaking work later, I have drawn the latest designs in detail as follows:

Version 3, Center:

http://i204.photobucket.com/albums/b...ps4f5e4f36.png

External:
width: 30"
depth top: 7.15"
depth bottom: 8.1"
height: 9.5"

3/4" (19mm) plywood cabinet.

Internal:
width: 28.5"
depth top: 5.65"
depth bottom: 6.6"
height: 8"
depth, driver to back panel: ~ 3.7"

Total volume ~ 23 liters less bracing, ports, driver frame, magnet, cleats and hardware.

Off center horizontal holey brace shown in gray - 2" and 1" random holes. Bracing spars on either side of holey brace (only toward the ends, away from driver). Spars have 2" holes all along their length.

2 x slot ports, 5.6 long x 7" wide x 0.25". Tuning frequency ~ 38-40 Hz.

Version 8, Mains:

http://i204.photobucket.com/albums/b...psc31506f7.png

External:
width: 9.5"
depth top: 6.15"
depth bottom: 10"
height: 39"
distance, center of driver to floor: 29"

3/4" (19mm) plywood cabinet.

Internal (sound chamber only):
width: 8"
depth top: 4.65"
depth bottom: 7.65"
height: 28.5"
depth, driver to back panel: ~ 3"

Total volume around 23 liters in the sound chamber. Void space at the bottom.

Off center holey brace shown in gray - 2" and 1" random holes. Bracing spars on either side of holey brace (only toward the bottom, away from driver, the side panel is widest here anyway). Spars have 2" holes all along their length.

2 x slot ports, 5.6 long x 7" wide x 0.25". Tuning frequency ~ 38-40 Hz.

Please let me know what you all think of these!

rjbond3rd 16th January 2013 01:19 AM

Wow, nice! Just curious, is there a rationale for putting the slot vents very close to the driver?

xrk971 16th January 2013 01:32 AM

Nice designs! With the height and lengths you have you may have room for a MLTL, have you considered that approach? The vertical mains may need more clearance at top slot vent inlet as it is very close to edge and flow may get restricted.

Ben_Tech 16th January 2013 03:28 AM

Quote:

Originally Posted by rjbond3rd (Post 3327844)
Wow, nice! Just curious, is there a rationale for putting the slot vents very close to the driver?

Hi, thanks for your question. The ports are close to the driver so that the port scheme can be identical in the center and the mains, as per the design goals to keep them as close as possible to each other for matched voicing.

Also, it can't hurt to have more of a point source as the 40 Hz range is still quite within the audible range.

Ben_Tech 16th January 2013 03:34 AM

Quote:

Originally Posted by xrk971 (Post 3327856)
Nice designs! With the height and lengths you have you may have room for a MLTL, have you considered that approach? The vertical mains may need more clearance at top slot vent inlet as it is very close to edge and flow may get restricted.

Thanks for your question and comment.

Yes, I did consider the Pensils but rejected due to difficulty designing a truly voice matched (MLTL) horizontal center channel design that made sense. I'm sure the Pencils are nice and certainly much easier to build than my design.

The clearance of the top slot is intentional as the top panel becomes an extension of the port length to match the length of the lower port, it's just an elbow.

Keep the comments coming!

Ben_Tech 10th February 2013 04:54 AM

My local supplier had 5x5 foot pieces of quality 3/4" 13 layer Baltic Birch ply. I bought two for $120 USD with tax and had him rip them in half for transportation in a typical sedan trunk, yielding 4 30x60 inch pieces. That should provide plenty of extra for small mishaps.

The supplier had 4x8 as well but I could not fit everything on one sheet with the cuts required for transportation. So, two 5x5's were less costly than two 4x8's and everything fit fine. I think it looks like a good step up from the Home Depot "Birch" plywood I compared it with. The plys are a lot straighter with fewer voids and none of the 'twisted' areas in the layers (I don't know what else to call it).

Below are the cut sheet layouts for the four panels. I tried to arrange things efficiently, but I'm sure it could be better.

I decided to cut the slot port pieces from 1/2" "oak" Home Depot plywood I have on hand, so those pieces are not shown.

In case anyone is following this later, all of the drawings are somewhat accurate but not perfect, and are on a scale of 20 pixels = 1 inch.

I tried to allow for the Rabbet and Dado joints I plan to use when sizing the panels.

http://i204.photobucket.com/albums/b...ps219e3459.png

http://i204.photobucket.com/albums/b...psd0946f3d.png

http://i204.photobucket.com/albums/b...psb3934169.png

http://i204.photobucket.com/albums/b...ps269337af.png

GM 10th February 2013 01:47 PM

Quote:

Originally Posted by Ben_Tech (Post 3327938)
Also, it can't hurt to have more of a point source as the 40 Hz range is still quite within the audible range.

A 1/4 WL of 40 Hz is ~85", so acoustically, this is how far away they can be and still couple as one.

What matters is that the closer the vent, the less noticeable its comb filtering harmonics will potentially be, which can be well up into the mids where the WLs have been shrinking exponentially in size down to < 12".

This can be minimized though by vent design as you have done and/or 'critically' damping the vent[s], so in scheme of things, not an issue unless maintaining peak vent system efficiency is a design priority, which is rarely the case in a HIFI/HT app.

GM

Octavia 10th February 2013 03:37 PM

What a beautiful design. Since it's home theater what xover do you plan on using for subwoofer support? Am I right to assume that the 10.2's can crank on home theater levels if you relieve them of the low end as is typical with home theater processors? Looking forward to seeing pics and hearing your comments.

Ben_Tech 10th February 2013 06:04 PM

Quote:

Originally Posted by GM (Post 3364020)
A 1/4 WL of 40 Hz is ~85", so acoustically, this is how far away they can be and still couple as one.

What matters is that the closer the vent, the less noticeable its comb filtering harmonics will potentially be, which can be well up into the mids where the WLs have been shrinking exponentially in size down to < 12".

This can be minimized though by vent design as you have done and/or 'critically' damping the vent[s], so in scheme of things, not an issue unless maintaining peak vent system efficiency is a design priority, which is rarely the case in a HIFI/HT app.

GM

Thank you, GM. I did not know all of that when designing these, I was simply following some of the experts in terms of what they sometimes do in these situations. It's good to know the real reason for this style of arrangement. :) All I knew is I needed front vents to place these near a wall (as do most HT setups) to avoid excessive bass reinforcement. I guess I also wanted to avoid elbows and their associated additional woodworking that may have been needed if the vents were at the top/bottom. And, of course, I thought two vents might balance things out a bit better, and also avoid these becoming a TL due to their shape. The vent pressure is low at Mach 0.04.

Ben_Tech 10th February 2013 06:11 PM

Quote:

Originally Posted by Octavia (Post 3364174)
What a beautiful design. Since it's home theater what xover do you plan on using for subwoofer support? Am I right to assume that the 10.2's can crank on home theater levels if you relieve them of the low end as is typical with home theater processors? Looking forward to seeing pics and hearing your comments.

Thank you, Octavia.

I plan to cross over to a sub at 80Hz. This design should be somewhat full range on its own though, which is what I wanted in case they are ever used without a sub. My understanding is that the Alpair 10.2 is the best Alpair for home theater as it can do midbass with the most authority, after Mark's suggested break-in routine has been carefully observed. In my setup, these won't be driven very hard at all, but I'm sure they would have no trouble with 95 dB levels should one choose to listen at that slightly deafening level.


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