EXPERIMENT - Small driver in a big pipe/horn - does it need a sub? - diyAudio
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Old 6th August 2008, 03:18 AM   #1
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Default EXPERIMENT - Small driver in a big pipe/horn - does it need a sub?

PERSONAL EXPERIMENT OBJECTIVES
1. Only one pair of enclosures will be made.
2. Small single fullrange drivers will be used.
3. Starting with the smallest of 4 chosen drivers, the limits of the drivers will be tested in terms of displacement limited spl and overall sound quality in order to determine if a small driver can be used without a sub in a small bedroom or tv (NOT HT) system.
4. The enclosures will be BIB inspired pipe/horns to achieve maximum bass potential and will be sized such that the enclosures strech the limits of the drivers I have on hand but if this proves to be the case, at least 2 other affordable driver options are readily available, which may prove more suitable.

THE DRIVERS
I already have the fostex 108e sigma and 126e but if the box proves to be too large for these, the 127e or pioneer a11 will definitely work. I won't be buying the 127 or a11 if one of the drivers I have onhand works well.

Recommended BIB mouth sizes for these drivers are as follows:
108 - 33 inches
126 - 63 inches (could be happy with less)
127 - 63 inches (could be happy with more)
a11 - 60 inches (could be happy with more, probably a good bit more)
- all drivers have ~70 hz fs, except the 108, which is 77

THE ENCLOSURES
- BIB inspired pipe/horn - inverted and back firing
- line length - 95.5 inches (this is a stretch for the 108 but all other drivers will find it almost ideal)
- flare - conical expansion starting from a point
- mouth size as built - 81 inches (this is a big stretch for the 108, less so for some of the others)
- driver position - .417 x line length, centered about 42 inches up from the floor

THE COMPETITION
I have been using a buddy's fonken style ported box to break in the 108's. It's hardly a fair fight, but these serve perfectly for a reference point. The box is 5 liters and response is pictured below (corner loaded for easy reference with the corner loaded pipe/horn below)

Click the image to open in full size.

And just for reference, this is the new pipe/horn assuming an end loaded driver (the 108). Hornresp can't sim the driver position I use so the graphs don't mean much but certain info can be inferred. Like the fact that this has WAY more gain than the ported box above. (And less excursion too) But it's also almost 10x larger.

Click the image to open in full size.

STAGE ONE
I only have enough clamps to make one at a time, so initial impressions consisted of a/b testing the 108 in the ported box vs the 108 in the first completed but UNSTUFFED pipe/horn.
The ported box sounds like a decent (but bass shy) mini should. Maybe a bit bright. On the other hand the pipe/horn had WAY more (sometimes almost unbelieveable) bass, but muddy midbass/low mids. Definitely not bright anymore. Kind of hollow or boxy sounding. Clearly a lot of stuffing needed. And unfortunately I'm already doubting the ability of the 108 to produce satisfying spl without a sub. Not really a surprise though.

STAGE TWO
A few hours later, the other box was ready, so I added it to the system with the first pipe/horn and added stuffing to both - maybe about 225 cubic inches of fibreglass insulation stuffed through the driver hole and spread around as well as possible.
With stuffing the overall tone improved greatly, but still muddy sounding in places. The bass was still incredible (for a 4 inch driver, that is) and bass response is flatter, but not quite strong enough for my taste (until around fs, where performance is still pronounced), the box sounds like it's tuned too low for this driver. And the max spl is clearly not enough for even a secondary small system without a sub. (IMO) Almost but not quite.
I could try to improve the overall sound with the 108 but displacement is the main issue here. The 108 is out.

STAGE THREE
Sub in the 126. For some reason I did not think the 126 would require as much stuffing so I removed half of what was in there.
Immediately the sound was much better, the 126 matched the size and tuning of the boxes very well (as far as I can tell so far). Almost surprising how natural, balanced and musical it sounds, no more obvious midrange bloat, and it probably does not need any more stuffing. Time will tell, and I was mainly interested in testing max bass spl in the single short listening session I have had so far.
The 126 did seem to have much more max bass potential than the 108. It seemed substantial. It is actually incredible considering the fact that the driver has only 65 sq cm of cone area, but at the same time I am not 100 percent sure yet if the amount of bass justifies the size and weight of the boxes. They are fairly large. And costly too, if premium material is used. OTOH, if this can be overlooked, these now seem to be very close to being everything I need from a small secondary system in terms of bass extension and max spl.
As I said though, I only had about 1 hour with these speakers with the 126 so far, so things could change at any time.

STAGE FOUR
Has not happened yet. Next up I need to listen a lot more and adjust stuffing to taste by ear. Once that is done, I need to measure how low these actually go. At the same time, I can determine if the 126 is ideal or if one of the other remaining 2 drivers (which prefer larger enclosures) would be a happier match. If there is a significant bump (increased spl) at the tuning frequency (fundamental harmonic) then I can safely use one of the other drivers, and maybe even get a bit more bass. The pioneer a11 has WAY more rated xmax, but not sure now much more in real life, since the 126 can obviously go way more than it's rated xmax.

CONCLUSION
(So far) At first glance, it seems that the 108 can't be stretched as far as I'd like. And there are displacement issues.
When using the 126 I am very surprised at how little stuffing it seems to need. And how balanced it sounds in terms of frequency response.
When using the 126 the bass output is phenomenal when compared to other optional enclosures for the same driver. But is it worth the size and weight? Could a frugal horn do just about the same thing in half the size? Not sure yet.
At this point it is undetermined whether I would be satisfied with this speaker without a subwoofer in a small secondary system over the long term. But I am absolutely certain that the 8 inch 206 in a BIB inspired pipe/horn could easily be enough, and might even be good enough to pull it's weight in a nice primary stereo system. I was worried that BIB and other style pipe horns might have unacceptable frequency response through the lower midbass to upper mids. It doesn't seem to be a concern with my second choice of drivers (the 108 was not so good, but probably due to the box more than anything else)
I see no reason not to proceed with the 206 project next, as I would already consider this project a success, although it needs much more scrutiny.


Pics and more enclosure details to come, maybe tomorrow.
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Old 6th August 2008, 04:17 AM   #2
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I really like your thinking on this. I am also looking into some bibs for the A11 as well as a few other small drivers. Please keep us posted on your progress.

I do have a few questions though.

1.) Are the cabs positioned in corners?

2.) Are you optimising positions after each speaker swap?(or is it more of an enclosure thing than a driver behaving in an enclosure thing? I don't know)

3.) What kind of basis are you judging "adequate" bass performance? I know this is subjective, but just try and get me in the ballpark. You remarked that a 206 "may" perform well enough for a main system. Were you thinking in terms of output or bass quality? What kind of system are you benchmarking this against?

4.) Wow, there were a lot of questions in that last one. Sorry. Rest for this round.

5.) How did you arrive at the bib sizes? Just using the bib calculator on the A11(using 70Hz for Fs, 3.66l for Vas and .58 for Qts) I come up with a CSA of 47 sq. in. Is this part of stretching the limits?

I can say that I am very interested in what you find. Thanks for posting this.
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Old 6th August 2008, 05:54 AM   #3
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Quote:
(1.) Are the cabs positioned in corners?

2.) Are you optimising positions after each speaker swap?(or is it more of an enclosure thing than a driver behaving in an enclosure thing? I don't know)

3.) What kind of basis are you judging "adequate" bass performance? I know this is subjective, but just try and get me in the ballpark. You remarked that a 206 "may" perform well enough for a main system. Were you thinking in terms of output or bass quality? What kind of system are you benchmarking this against?

4.) Wow, there were a lot of questions in that last one. Sorry. Rest for this round.

5.) How did you arrive at the bib sizes? Just using the bib calculator on the A11(using 70Hz for Fs, 3.66l for Vas and .58 for Qts) I come up with a CSA of 47 sq. in. Is this part of stretching the limits?
1. Not only are they in corners right now, but the corners are in a recessed alcove of a very large room. It's a great spot for bass support but not so much for good sound.

2. Not too concerned. Just stick drivers in, make sure they are in phase and somewhat near a corner and they're good to go.

3. In this instance "adequate" levels of bass are purely subjective. It has to be entirely subjective anyway, since I can't acurately measure it. I knew I would need about 50 hz at decent spl to consider it satisfactory and that's why the line length was chosen to be 95.5 (close enough). Ideally I'd like them loud enough to compare favorably with the Technics 3 way ported boxes with 10 inch woofer that used to be in these corners.
When I mentioned the 206 would probably perform well, it was the midrange I was worried about, not the bass. I want speakers with decent sound quality, so this first experiment was to see if I like this style of pipe/horn or not before commiting to the expense of a much larger 206 version. I have no benchmark other than the fact that subjectively I don't think I would mind having the 206 version in my main system, probably mostly without a sub except for HT.

5. Where did you get your specs? I'm looking at the partsexpress a11 page and it says fs 70, qts .35, vas .31 cubic feet (~8.8 liters IIRC). Put these numbers into the BIB_166_GM_v2 calculator gives mouth size 60 inches IIRC.
For the other 3 drivers, BIB mouth size was taken from recommended size from zillaspeak.com.
If you look at the designs at zillaspeak you can see that several of the drivers have 2 recommended sizes, the size the calculator outputs and the "more bass" version which is usually about 30 percent larger (mouth). Extrapolating from this, I figure the a11 can handle a mouth size between about 60 - 90 square inches, 90 being the "more bass" version.
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Old 6th August 2008, 01:48 PM   #4
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Quote:
B]Originally posted by just a guy[/B]
Where did you get your specs? I'm looking at the partsexpress a11 page and it says fs 70, qts .35, vas .31 cubic feet (~8.8 liters IIRC).
They have the measured specs on a link on that page. At the bottom of this page. Aside from the Fs, I belive that these are the specs that have been accepted as closer to "true" though I'm not 100% on that.
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Old 6th August 2008, 02:44 PM   #5
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Yeah, the Fs of 70Hz is way off on the A11, though it has been proven that they can drive a BiB with a line length of 120" just fine.

Kensai
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Old 6th August 2008, 04:11 PM   #6
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I just had an observation on maybe 'cleaning up' the 108 you have.

You mentioned putting the stuffing all around the driver in the peak area. What I've done is pretty much ignore the BIB suggested
stuffing and put some up in the peak and a thin absorption material
behind the driver. I've used billiard felt or a thin piece of foam
in some of my setups. And I think I have to credit GM for this.

The chamfer on the speaker cutout I do every time now.

I have one bib that is slapped together with a 50 cent Pioneer in
the max size cabinet for the FE127e. The specs on this 50 cent
are really low. What is surprising is how much bang for the buck is there. To clean up the low end on it I'm considering doing a 'finished build' to see if that last bit of improvement appears. From the specs, the cabinet size would be dramatically smaller.

You are right about the wood. Top end stuff is a serious investment.
My finished build for the cheapie will be made of scraps.
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Old 6th August 2008, 04:15 PM   #7
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Free air resonance on the FE126E averages out at 90Hz or more. Most manufacturers claims are about as trustworthy as a politician presented with the opportunity of increasing their power. So I wouldn't put much faith in them generally.
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Old 6th August 2008, 04:16 PM   #8
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Quote:
Originally posted by loninappleton
What I've done is pretty much ignore the BIB suggested
stuffing and put some up in the peak and a thin absorption material behind the driver.
Ignore the suggested damping? You've just described the ~'standard' initial damping strategy for these cabinets. Being chamberless corner horns, they all need a degree of tailoring to room, system & personal preference after that -some will be happy as-is, others will need a light layer on the base etc.
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Old 6th August 2008, 04:46 PM   #9
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Thanks for the responses guys. I DID realize the specs for the fostex were inaccurate due to conversations here in the past. I DID NOT realize the a11 specs at partsexpress were wrong, and while I am happy that you guys pointed it out I'm not too concerned I am confident that it will like to be in a larger box than the 126 (and hopefully won't be needing it anyway).

Quote:
Yeah, the Fs of 70Hz is way off on the A11, though it has been proven that they can drive a BiB with a line length of 120" just fine.
I'm sure it can drive that line length, and probably more, but the real issue here is displacement. Like any other speaker system, the lower you tune it the higher displacement requirements get. A ~ 40 hz tuning for a 4.5 inch driver is pushing it a bit. Can it do it? Yes. Can it do it loud? NO.

Lon, I wasn't sure exactly what to do with the stuffing, so I shoved it through the hole and simultaneously shoved it up and over the divider and down into the peak, and just generally spread it everywhere and made a mess. It's probably mostly fallen down into the tip by now, not sure, but it doesn't seem to need much (on first impresssion). More construction details coming when I get the pics.
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Old 6th August 2008, 05:12 PM   #10
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I should probably explain why I made the box so big and seemingly ignored the published recommended mouth sizes.

1. The wood was already cut professionally for an abandoned project so I wanted to use these panels without changing (and messing up) the straight, professional cuts.

2. I like bass and the point of the project is to see if a small driver can be used without a sub, so anything to boost the bass a bit is welcome.

3. Hornresp shows some general trends when it comes to oversizing the box a bit.
a. Excursion goes down.
b. The fundamental harmonic gets stronger.
c. Every harmonic above the fundamental is LESS pronounced.
Put these 3 together and you get more bass with less excursion and less midrange ripple.

4. Putting the driver at ~40 percent line length has some advantages but there are disadvantages as well. Generally weak bass is one of the disadvantages. Making the box bigger can compensate for this somewhat. MJK's alignment tables (attachment B IIRC) shows this brilliantly with pictures.

5. In the end I'd rather be too big than too small. Even if my boxes ended up being too big for any of the 4 drivers of interest, there are countless other 4.5 inch drivers with higher vas and qts and inevitable one of them is going to work. Alternatively, a low q and vas 5.25 could be used for extra spl.

So that's why so big. So far the 126 seems to be a good fit, so maybe all the assumptions here were not too far off.
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