17 foot pipe sub 12-230 Hz ±5dB

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The sub has two parts, a base holding a 15-inch, 1965 Stephens 150W, and a 17x17x72-inch box mostly ¼ inch plywood weighing about 45 lbs, containing three tubes (with two abrupt bends) with sound exiting from the top.

The first pix shows an average of 3 mic positions near my chair. No EQ or crossover filter. Can't say for sure if bottom bass is as it should measure, but that's 12-230 Hz ±5dB (which I ordinarily cross over around 130 Hz). Nice.

The second and third pix shows near field sound outputs at the driver and at top exit. In each, you see the very lightly absorbent versus final stuffed (roughly the familiar .5 lb per cu foot). You can see the weird pipe effects. Something's cooking at multiples of 8.5 Hz.

The fourth pix has five impedance curves. The top and bottom are 47- and 7-Ohm resistors to benchmark the curves. The big bump at 26 Hz is the free-air driver. The smaller bump is in the lightly stuffed. The all-but-flat final curve near the middle is the stuffed pipe.

B.
 
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Here are the missing pix.
 

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Some construction pix. You can see the exit port at the near end in the first pix and reversed in second pix. Yes, mostly ¼-inch plywood but with some strengthenings. The interior partitions are celotex which is a light-weight cellulose board with acoustic properties.
 

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Using REW for measurement? Let's see what the THD looks like at 2.83V... :)
Thanks freddi and Brian Steele.

Mic placed near field sort of midway between the ends. About 90dBC at mic but ranges from 80 to 96 in the near field... long pipes are different. 2.8 volts.

BTW, I never work with REW calibrated since there are so many control points in the measurement path that touching any (like to keep the mic from overloading the mic pre-amp) destroys the calibration. So please ignore those axis numbers on mic charts.

The distortion results, presented in normalized-to-fundamental format, are pretty reasonable for a sub, esp. the low bass where you generally see worse than 10% figures (AKA -20dB) and unusual to see any that play anywhere as low. Must be that ¼-inch plywood construction or maybe I should firmly attach the pipe structure to the base... just sitting on it now.

B.
 

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About 90dBC...

That's 90dBC measured at 70 Hz. So the vertical axis that seems to say about 67dB at 70 Hz really should be 90 and 13dB added to the scale, if you want to know the absolute sound level at that near field spot.

As far as the listening results, that would be the swell first curve of post #2, not near field, particularly given the spatial and tuning anomalies of a 17-foot pipe when too close.

I judge my treble quality by listening to the orchestral anvil (no kidding) in band 7 of the Holst Band Suites. The less it sounds like an ordinary bell the more it sounds like an untuned, dull massive metal anvil.

Curiously, listening to the big orchestral side drum* in the same piece with this pipe sub is instructive too. The sound is now "duller" and more rumbly in the sense, I think, of being less peaked to the tunings of the enclosure.

B.
*ironically, electric RTA of this piece has very big peak at 40 Hz but some little rumbles at 12 and 18 Hz. Another good bass recording is Bach's old T&F in d, Murray/Organ Blaster, has substantial content below 30 Hz even below 20 Hz (see picture... you can see the individual pipes, eh). Anybody ever do an RTA of Dark Side of the Moon?
 

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quite impressive and a pretty build - a plan would be nice sometime and estimate of what modern drivers it may like

Thanks. Sorry to be so long in replying.

And sorry too that I'm an empirical guy and have to leave it to others to suss out the model. I couldn't say how my pipe behaviour relates to TL models. But here're some notes.

1. The three pipe sections are different cross-section shapes (that was intentional in following my taste for heterogeneity in audio design) but each is about 85 sq inches and that's about half the surface of the 15-inch driver and quite different than the Official TL size. Sure saves space if you're aiming for a long pipe.

2. I think the stuffed pipe is acting much like an IB. So there's no need for extra stiffness in the cone. Likewise, fairly efficient so no need for clumsy heavy VC.

3. I don't know why but there's good output below Fs particularly with the pipe standing in a room corner (and both driver and exit door are in the corners of the corner). So maybe no need for extra low resonance, unlike any other enclosure I can think of.

4. The ¼-inch walls* sure vibrate in a way that would make you smirk for a sealed box. Not sure if the forces in a pipe are large or that the vibrations are too much of a shortcoming. For sure, the distortion and other specs within the sub passband are noticeably nice (distortion is spectacular below 35 Hz), so it can't be so terrible but then I'm biased.

B.
*an old guy had to be able to carry it around, eh.
 
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This will be a "just do it" / trial, error & correct-project as I can not predict the result. The idea rose from Pass El-Pipe-O due to space problems in my narrow living room. I'm also investigating other possibilities, but I see that the design will benefit from lowering the transition closer to woofer i.e. the effective pipe lenght will increase.
I also got a glimpse that it could be used in other ways, to example turn it upside down, implementing a 8" woofer and make a Linkwitz LX-Mini / Pluto look-alike.
 
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Being a dipole person and having done a lot of room REW sweeps, part of my thinking was to separate the driver from the pipe exit. For a long pipe with enough stuffing to suppress everything except the lowest tones (which is why we are building subs and only a long pipe can have enough stuffing), I figured the two emitting sources would complement themselves as they waft into the room.

Also, a well-stuffed long pipe can be pretty "aperiodic" and minimally resonant. That's the ideal for all us with a liking for dipole open sound. The alternative for seriously low full bass would be a very large sealed enclosure with a driver with a resonance at 15 Hz... although comparable to the size of a long pipe.

I can say the construction bears out my thinking in that the driver emits the full bass range but the exit port emits just the lowest notes. But I have no evidence if my separation theory is correct since rooms mix-and-match sounds in funny ways.

B.
 
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Thanks for asking. Speaker continues to perform wonderfully well and great to have that dull extra-low visceral bass thump from the side drum.

Prolly would have been wiser to use the word "labyrinth" instead of "pipe".

Labyrinth is the term used by Stromberg-Carlson* in 1950s before the peculiar term "transmission line" came into use for the same thing years later. As I posted later, the better SQ way to think about such devices is as sequestering the rear wave, not tuning it as per TL theories. Funny how tuned boxes often appeal like "perpetual motion" machines to folks on the sub forum.

Long pipe to sequester rear wave - diyAudio

B.
* I had one of their 6L6 amps with many special features.... one of the best audio engineering companies at the time
 
I just need a starting point. I’m more of a Visual person as to reading the numbers
I will have my wood shop up and running in a few more weeks and when I do I will post what I’ve done
I have the test mics and software for the numbers and will be more then happy to share what I’ve done
 
Four pieces, 18 x 72 inches or so. I used quarter-inch mahagonny-faced plywood. Make a base as in the picture. You need to divide the interior into three pipes of equal cross-section area. I used celotex a cellulose board with some sound absorbent qualities as the interior dividers.... and the odd piece of plywood for strength.

Fill the pipes with lots of pillow stuffing, medium-fluffy. I used rolls of fibreglass window screening formed into 6 foot long stretches with multiple pockets to hold the stuffing in place.

The key design goals were (a) a long labyrinth with (b) the driver and the exit door as far apart as possible and (c) with both benefiting from sitting in corners of the room (in my case, floor and ceiling).

Hope that helps.

B.
 
Not sure I have the knowledge to give the best answer. For most sequestering rear wave mountings, once you have the driver, "it is what it is".

There is the foundational idiocy that subs today always have their resonance within their passband. So, the lower the driver resonance the better. And the stronger the motor (BL) the better. To argue about gaining something by worsening either of those parameters because the sim told you to do so, makes no sense to me.

As far as other parameters, they might have some slight relevance to some fantasy-tuned boxes. But I've never been interested in those kinds of boxes.

B.
 
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