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Pi Studio One Tower [message #50492] Tue, 09 January 2007 16:40 Go to next message
FredT is currently offline  FredT
Messages: 704
Registered: May 2009
Illuminati (1st Degree)
Some time ago I decided to experiment with the Studio One Pi kit in different volume floorstander enclosures, and since then I get the occasional email inquiry about the details of the enclosures. I never did any drawings, but I've posted a picture, and that picture along with this description should be adequate for anybody to build a pair of their own.

But first I want to explain why I built a tower version of the Studio One Pi. The One Pi looks puny compared to all the other Pi kits, especially the three and larger models, but it's not a small speaker by contemporary standards. The 8" Eminenec Alpha 8 woofer is larger and capable of moving more air than most mainstream speakers in the $1.5K and under range with their typical low efficiency 6.5" or smaller woofers. When you consider its high sensitivity along with its bass and dynamic capability, it's a great speaker for most small tube amps and also for all mass market receivers, which may have high power ratings but not much drive current capability. The Sherwood two channel receiver I bought on sale for $80 at Circuit City will drive the one pi to very high levels. Add to this it's incredibly low price ($150/pr), and it's a logical speaker choice for any budget system.

Ditto for the Two Pi tower, but its much larger enclosure might not fit too well into a small room, especially if there's a wife involved in the decision:) So, I thought, why not build a smaller footprint and not quite so tall tower whose volume will take advantage of all the bass capability of the Alpha 8, and the One Pi tower concept was born.

I've built a few variations on the Pi One towers for friends and have finally decided which version offers the best sound. Interior volumes have varied from about 1.2 c ft to 2 cu ft, and I have finally decided a 2 cu ft tuned to about 50hz sounds best to me.
I make mine 38" high, which places the tweeter about ear level. To achieve this volume I make the front and back panels 10" wide and the side panels 11-3/4" wide. I like the look of a narrow baffle, and the 10" dimension is about the least you want with an 8+ inch driver. The external dimensions are 10"W, 13-1/4"D, 48"H.

I use three brace inside. The top one abuts the front panel between the woofer and tweeter, and is 8.5"W by about 4" deep. The middle brace, placed a couple of inches below the woofer, is an 8-1/2W X 11-3/4"D window frame brace. The lowest brace, also a window frame, is placed halfway between the middle brace and the bottom of the enclosure. The port is mounted halfway between these braces, and the speaker terminals are placed between the bottom brace and the bottom of the enclosure.

The best tuning is about 49 to 50hz. In a 2 cu ft enclosure this requires a 3" port cut to 1.5" length or a 4" port cut to 4" length. The Parts Express #260-411 4" port is a good choice. It's marked in half inch increments and you can cut it with a hacksaw. The port can be placed on the front or the back of the enclosure. I usually place mine on the front only because many of my friends listen to low power SET amps in small rooms with the speaker close to the back wall. I prefer back placement if I know the speaker's back will be at least one foot from the back wall - the lowest bass will be augmented a bit more with this placement.

It's a good idea to have some kind of base for the speaker. You can make one out of a sheet of mdf that's two inches wider and two inches deeper than the enclosure. I prefer the easy way - I use four Parts Express #260-770 rubber feet. Also, since you're already ordering the ports and rubber feet you might as well order a #081-435 bag of black #6 driver mounting screws. These will work on the woofers, tweeters, speaker terminals, and the feet.

For the enclosure material you can use mdf or plywood. Have the store cut a 4X8 sheet to two 4X4 for handling ease. When you get it home, cut each to 38X48. Then cut each to yield two 10X38 boards and two 11.75X38 boards. The remaining material can be used to make the 8.5X11.75 tops and bottoms. You will need some additonal material to make the braces. I usually have enough scraps on hand, but if you don't you can use a 4X4 piece of 1/2" plywood.

I use R-13 fiberglass home insulation for the interior damping material. It's about 4" thick, which is too thick for this application, so I "peel" each piece into two identical 2" thick pieces. I line the back and one side of the interior with one sheet between each of the braces and the top and bottom, and I also place a full thickness piece at the bottom.




Towers and speakers on stands [message #50494 is a reply to message #50492] Tue, 09 January 2007 20:34 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18048
Registered: January 2001
Illuminati (33rd Degree)

I really like your speaker, Fred.

I thought about adding it to the line, as we discussed. But I think it's just as well that we just let people know your version is around - that's kind of how the Professional Series three π is too. This all started out as hobby stuff anyway, and it isn't too hard to make the speaker with existing kit parts or whatever.

Another hybrid I wanted to toss around is a dual woofer version of your speaker. Might do this with the tower two π as well, but in that case, the larger enclosure volume is really useful for additional damping. The characteristics of the Alpha 8 make it suitable for very small boxes, so you don't really need the enclosure volume. Adding another woofer is no trouble at all in the space you have.

What is gained when adding a second woofer is elimitation of floor bounce amplitude notch in the midbass, where distance to the floor is 1/4λ. This gives a smoother response curve, overall.

When you measure a tower speaker or a bookshelf speaker on a stand, what you see is a dip in response at the frequency where woofer distance from the ground is 1/4λ and a peak where distance is 1/2λ. There is sometimes a slight rise where distance is 1/8λ and a slight dip where distance is 3/4λ, rising slightly again at 1λ. Above this, it tends to level off. Lay the speaker down on its side, woofer closer to the ground, and response is smooth. But move the woofer up away from the ground more than a couple of feet, and floor bounce starts making these notches and peaks.

By adding a second woofer, you can fill in the dips and average the response. This is one of the characteristics I like about your arrays, they tend to average out the response curve and remove the problem of floor bounce. By using a second woofer, this can be accomplished. Moving the woofer lower will do it too, but that hurts midrange, if the driver is acting as a midwoofer. The Alpha drivers are inexpensive enough that adding a second to eliminate floor bounce is a good option.


Re: Towers and speakers on stands [message #50495 is a reply to message #50494] Tue, 09 January 2007 20:54 Go to previous messageGo to next message
Matts is currently offline  Matts
Messages: 359
Registered: May 2009
Grand Master
if you added a second woofer, would that also increase the overall efficiency a couple of dBs?

Re: also... [message #50496 is a reply to message #50495] Tue, 09 January 2007 21:03 Go to previous messageGo to next message
Matts is currently offline  Matts
Messages: 359
Registered: May 2009
Grand Master
would it lower the bass response a touch necessitating a slightly lower tuning of the box?

and...thanks for your post, Fred T.! very cool design & thanks for taking the time to post.

Using side wall bounce to counter floor bounce [message #50497 is a reply to message #50496] Tue, 09 January 2007 21:25 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18048
Registered: January 2001
Illuminati (33rd Degree)

Yes, with two woofers, sensitivity would increase. There are a lot of ways you could go, using a 2.5 way, with the bottom woofer crossed over at 200Hz or so, or maybe go with an MTM.

You can also use the side walls as acoustic mirrors, effectively making them act as additional virtual woofers to do the same thing as actual physical woofers would. For example, place the speakers so the side wall is around half the distance as the woofer is to the floor. That will tend to fill in the notches from floor bounce with peaks from side bounce, and vice versa.


Two Woofer One Pi Tower [message #50502 is a reply to message #50494] Wed, 10 January 2007 04:25 Go to previous messageGo to next message
FredT is currently offline  FredT
Messages: 704
Registered: May 2009
Illuminati (1st Degree)
Sounds like something I might try. Making it a 2.5 way would provide some baffle step compensation, enhancing the bass without an unwanted increase in midrange sensitivity. The simplest crossover would be an Erse iron core inductor in series with the lower woofer. I probably would also increase the volume a bit by making the enclosure deeper and taller.

A Question About the Alpha 8 [message #50503 is a reply to message #50494] Wed, 10 January 2007 06:55 Go to previous messageGo to next message
FredT is currently offline  FredT
Messages: 704
Registered: May 2009
Illuminati (1st Degree)
I'm wondering how near the TS parameters for this driver are to the advertised specs, which appear to have been changed within the past couple of years. Here's a comparison of the specs on the Parts Express 2005 catalog versus the 2006 catalog (I understand the change was actually made before 2005 but wasn't reflected in that catalog):

Parameter - Old/New
LE - .65mH/.44mH
Fs - 76/73
SPL - 97/94
Vas - 0.5/0.6
Qms - 4.5/4.6
Qes - .6/.68
Qts - .53/.59

I'm not suggesting that WinISD always yields the optimum solution, but when I enter these different parameters, for the old parameters the software suggests a ported 1.23 cu ft box tuned to 57.5hz. For the new parameters I get a 2.1 cu ft box tuned to 49.5 hz. The F3 for the old alignment is 51hz; for the new one its 44hz. Quite a difference.

Re: Pi Studio One Tower [message #50504 is a reply to message #50492] Wed, 10 January 2007 09:53 Go to previous messageGo to next message
LAL is currently offline  LAL
Messages: 40
Registered: May 2009
Baron
Fred,
Perhaps the next step is to add a side firing subwoofer and a plate amp on the back to power the subwoofer.

Larry

Re: A Question About the Alpha 8 [message #50505 is a reply to message #50503] Wed, 10 January 2007 10:12 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18048
Registered: January 2001
Illuminati (33rd Degree)

You're right. Eminence made a change late in 2005. They do this periodically and keep the same part number, without a version number to identify the difference. It's kind of frustrating, to tell the truth. But the good news is they are pretty good about keeping variations compatible with one another. The specs are different, but when put in a box designed for an earlier version, the response doesn't change all that much. I can see how Eminence would want some manuvering room to source their materials, so maybe slight variations from time to time are unavoidable.


Re: Two Woofer One Pi Tower [message #50506 is a reply to message #50502] Wed, 10 January 2007 10:16 Go to previous messageGo to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 18048
Registered: January 2001
Illuminati (33rd Degree)

I like the speaker as it is, that's for sure. But with the low cost of the Alpha series drivers and the small size of the cabinet needed for the Alpha 8, a dual-woofer model could be easily done. Two to four cubic feet is plenty for a pair of Alpha 8's.


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