A BR for the Alpair 10.2 - diyAudio
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Old 23rd October 2012, 04:27 PM   #1
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Default A BR for the Alpair 10.2

As you may know, I have designed a rather successful MLTL using the Alpair 10.2 driver. Looking at the specs for the A10.2 it is obvious that it will work well in a BR. So I started modeling.

When I design a speaker that has real bass potential, I like to get a fairly gentle roll-off to avoid boominess in an actual room. (The biggest mistake that DIY’ers make is to design for a max flat alignment and then put the speaker in a small room.) There at least two ways to do this. One is to use an Extended Bass Shelf alignment. This entails using a bigger than normal box and tuning to get the appropriate roll-off. Another way to do this is to use a smaller than normal box and tune it lower than normal. I chose the latter.

OK, off I went with the modeling. I found a 15L box tuned to 35Hz gave me the response that I wanted. The immediate problem is that a small box tuned low requires a large port, and the problem with a large port in a small box is where to put it. I settled on a 2” dia port (a little small, but adequate for all but LOUD listening), but the port is 8” long. Solution: Bottom port the box with the port vertical. That puts the port entrance close to the back of the driver, but in practice this is no real problem.

The next decision was how to vent the port to the room. With a bottom firing port, the air volume under the box will potentially affect the tuning. I tested the prototype of ¾” legs and on 1 ½” legs. No difference, so the final design will be the Dayton 5/8” cone shaped spikes. Adding the thickness of the mounting disks, all is as planned.

So: I give you the B10-A10 prototypes. Real bass with f3=41 and F10=30. Overall size 17”x8 ½”x9 ¾”.

Click the image to open in full size.

Click the image to open in full size.

Bob
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Old 23rd October 2012, 06:22 PM   #2
chrisb is offline chrisb  Canada
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ever try converting round ports to narrow aperture slots? depending on the box dimensions, the internal panels forming the vents can become part of the panel bracing scheme, give a great deal of flexibility on placement within the enclosure and can easily be folded if needed

then there's the cute Fostex BR for FE167 that has a long port incorporated into a pedestal stand - which smaller enclosures tend to need anyway
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Old 23rd October 2012, 06:48 PM   #3
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Quote:
Originally Posted by chrisb View Post
ever try converting round ports to narrow aperture slots? depending on the box dimensions, the internal panels forming the vents can become part of the panel bracing scheme, give a great deal of flexibility on placement within the enclosure and can easily be folded if needed
Interesting thought. I went the route I did out of pure simplicity, but I am no stranger to slot ports. My first MLTL's had slot ports--My A7.3 TL is essentially a slot port and I have a pair of double slot ported BR's wit CHR-70.1's in them. I was going to do something analogous to the funken's. But the driver was discontinued and I lost interest.

One of the problems with the bottom firing port as it stands using a PVC pipe is there is no way to cross brace panels. However, if I build up the pipe, then I can anchor bracing to it. Hmm.

Quote:
then there's the cute Fostex BR for FE167 that has a long port incorporated into a pedestal stand - which smaller enclosures tend to need anyway
It looks sooo Japanese in unfinished BB!! I did model it way back and I wasn't impressed. Too high Qc. But if crossed to a sub at say 100Hz, It should work well.

Bob
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Old 23rd October 2012, 07:22 PM   #4
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I like this box. Simple & to the point.

Out of interest Bob, how did you model it? I ask because, nosey parker that I am, I had a quick waffle around & 15 litres with those vent dimensions suggests a slightly higher Fb around 40Hz, depending on the damping.

I know what you mean re that 167 cabinet; not one of Fostex's better efforts.
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Old 23rd October 2012, 08:44 PM   #5
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Quote:
Originally Posted by Scottmoose View Post
I like this box. Simple & to the point.

Out of interest Bob, how did you model it? I ask because, nosey parker that I am, I had a quick waffle around & 15 litres with those vent dimensions suggests a slightly higher Fb around 40Hz, depending on the damping.
I use Unibox and MJK "Ported Box". OK, I confess. I build to 16L assuming 1L for the port, driver and some bracing. The actual Fb is 38Hz.

Bob
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Old 23rd October 2012, 09:28 PM   #6
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Ha! Rumbled!

That's a relief, thought I was losing the plot for a minute there. Although there are those who would argue I never followed the plot in the first place...
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Old 24th October 2012, 12:32 PM   #7
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Ok, just have to ask. Why can't you just run pieces past the tube or cut a brace w/ a hole for the vent?

Seems like the boxes wouldn't need extensive bracing such that a couple of 3/4" x 1" braces side to side would do the trick.



Quote:
Originally Posted by Bob Brines View Post
. . . One of the problems with the bottom firing port as it stands using a PVC pipe is there is no way to cross brace panels. However, if I build up the pipe, then I can anchor bracing to it. Hmm.
. . . .
Bob
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Old 24th October 2012, 01:15 PM   #8
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Originally Posted by fastbike1 View Post
Ok, just have to ask. Why can't you just run pieces past the tube or cut a brace w/ a hole for the vent?
1. The cross section is 7"x8". The port tube is 2 3/8". With the port tube in the center of the box, any cross bracing that runs past the tube will be so far off center as to be not much good. I could but the tube in a corner of the box, but then the omnidirectional pattern will be screwed up ( if indeed that makes a difference).

2. Window braces have much less affect on panel vibrations than longitudinal braces. Also, a window brace that's 7"x8" with a 2 3/8" tube in is may not allow enough open area to breath properly. Or not!


Quote:
Seems like the boxes wouldn't need extensive bracing such that a couple of 3/4" x 1" braces side to side would do the trick
.

The plan is to run 1/4"x1 1/2" braces top to bottom on three sides. The baffle is 1" thick. I posted an FR plot showing the effacly of such a brace -- Panel Bracing -- but it didn't get many hits. Perhaps a better title would have helped.

Bob
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Old 24th October 2012, 01:30 PM   #9
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I looked at the panel bracing thread. Looked to me like the bracing wasn't doing much until the FR was in the ranges that the driver was pretty much done anyway.

However it also looked like panel resonnace wasn't an issue for much of the driver's range. Perhaps I didn't understand what I was seeing or didn't appreciate the Y axis scale.
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Old 24th October 2012, 09:12 PM   #10
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Quote:
Originally Posted by fastbike1 View Post
I looked at the panel bracing thread. Looked to me like the bracing wasn't doing much until the FR was in the ranges that the driver was pretty much done anyway.

However it also looked like panel resonnace wasn't an issue for much of the driver's range. Perhaps I didn't understand what I was seeing or didn't appreciate the Y axis scale.

I was about to respond to this, but a post by P10 has me thinking. I need to reevaluate the test procedure and add additional tests. For the moment --

Noted.

Bob
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