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Old 15th December 2003, 11:12 PM   #21
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AM: The commercial evidence suggests that smooth folds are not necessary - PMC, et al. But, you have put your driver at one of the bends and I don't know if that will create a problem vs. straight pipe, so I asked the question. It may be a partial answer to your question about the effects of folding a line. As I said, my rationale for smoothing the bends is purely my own a... retentiveness. From all the other experiments that have been documented on the web, it's likely that your design will perform as calculated.

SR and GM: You are right that I haven't yet done all the math about the harmonics of the TL. But I do remember something from having actually played music. Everyone talks about tuning TLs as if they only produce the frequency matched to the resonance of the driver, and a sine wave at that. While that is important if that happens to be the note and waveform being played into the speaker, that is almost never the case. So in reality, we have a line reproducing sound which almost never matches the resonance of the driver or the tuning of the line. I have yet to find the definitive paper on why the 1/2, 1/3 or other harmonics matter for all these complex non-sinus waves, other than to match the driver parameters. Maybe you can point me to those.

:)ensen
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Old 15th December 2003, 11:29 PM   #22
sreten is offline sreten  United Kingdom
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Sorry AM,

but all the literature on the subject is very confusing, so its not
easy to say - yes that should work or no I don't think it will.
Or yes it will work but this will be better.

I'm old enough to remember when such designs were used for
normal speakers not subwoofers, they seemed to have problems
in the higher bass more than the lower bass, which mostly was
well received.

The best received designs seemed to have great bass extension,
but low efficiency and not particularly high power handling.
Of the (normal not subwoofers) ones I've heard there is certainly
bass extension (and how in some cases) but not the speed
which comes with integration of the bass with rest of the range.

So IMO organ pipes yes, rock bass drums no, but JMO.

I'd say yes it will work as a subwoofer. I don't know whether if it
would be better with added angled corner sections. I don't know
how you intend to build it but adjusting the stuffing to taste could
be a little awkward if you don't have a removable front baffle.

If I was building a subwoofer or a speaker I'm of the opinion you
can get better efficiency/ bass extension/power handling trade-
off out of a given box size with reflex loading, but the sound is
different to t/l loading.

sreten.
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Old 16th December 2003, 12:57 AM   #23
GM is offline GM  United States
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Hey A-M:

I haven't had time to do too much with this, and you haven't stated your performance goals so I just did two sims, one a max flat reverse taper where Fb = Fs, and a reverse tapered EBS alignment tuned to 18Hz. With no Xmax specs or room info, I don't have a clue how much SPL can be generated with either. Also, who knows how accurate the published specs are, and tapered pipes being what they are, if the specs are off much then the alignment goes to '**** in a handbasket' and all you can do at that point is tune it aperiodically or with a vent.

WRT your folding scheme, if drawn to scale it won't work as predicted due to the discontinuity of the flare rate through the bends.

Anyway, here's some numbers to punch into mathcad to view:

max flat reverse taper:

L = 64.43"
So = 247.6"^2
SL = 24.76"^2
Xo = 0.3544
density = 0.5lbs/ft^3

reverse tapered EBS:

L = 91"
So = 247.6"^2
SL = 24.76"^2
Xo = 0.3251
density = 0.5lbs/ft^3

Once in-room, either design may have too much gain around Fb due to room/boundary gain. Great for HT or organ music, but may be a bit overpowering for other types of music. If so, then either use digital EQ or tune for flattest response with stuffing in the vent.

HTH,

GM
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Old 16th December 2003, 01:24 AM   #24
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Default Thanks guys

I expect that the TL's will have a character altogether different to a bass reflex design. As for what they will sound like when done, there is only one way to find out. I was attracted to the design by the idea that a TQWT design can extend low bass by manipulating the rear wave such that it is in phase with the front wave at Fs. Various articles I read indicated that even with a less than ideal pipe configuration, Fs can be somewhat manipulated with stuffing, or even mass loading ie., using a port. One or two articles suggested that large TQWT cabinets in subwoofer applications can be more forgiving of inferior drivers. I think I have to agree with Sreten's comment that many of these suggestions may be somewhat controversial and suggestive.

All that being said, I have heard some amazing bass (isn't that a song? ) coming from small drivers in TL cabinets. So, given that I have invested very little in the driver, and that the cabinet I already had was almost exactly the right configuration, port length and diameter, as called for by the manufacturer, and it performs reasonably well, it can't hurt more than a weekend's work and a sheet or two of plywood to try a TL!

I plan to build the box from 5/8" construction grade plywood. I'll leave one side removable for tweaking. I'll then test the system, and if I can get satisfactory performance through tweaking with stuffing at various densities, then I'll finnish the cabinet by laminating 1/2" MDF panels and hardwood corner inlays and a harwood top to the outside of the plywood box. Pipe pannels will be braced with oval oak dowels I salvaged from an old bannester and I'll laminate asphault shingles, and close pile carpeting to minimize cabinet resonances. I expect only approximatel .25lb/ft2 of stuffing throughout the pipe would be necessary (works well in MathCad).

One concern I have is Port Noise at Terminus. The best configurtion I could get with MathCad was with a small end XS of .5 of the driver area. That works out to approximately 2.3"X16" effectively slot loading the driver. The Xmax is only 7.5 mm, so I don't think I'll get much air moving to create turbulence, even with an underdamped driver!

But I might be in for a surprise.
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Old 16th December 2003, 02:28 AM   #25
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Default Rethinking

Hi GM,

My last post was composed before your post. Thanks so much for the info. I punched those figures in except for X0, what does that represent and where does it get plugged in to MathCad?

The SPL graphs look quite good, even with the shorter pipe. The 91" pipe though had a troublesome peak at 75 to 100 hz. I have quite a strong room gain at 80hz, so much so that I have my sub 180* out of phase with my mains to tame it by creating a null at the xo point. I fooled around with the pipe length and found that 80" gave me the best response. The peak was pushed up above 80hz, and low end roll off was just 6db down from 20 to 80hz. 80" will certainly fit much easier into my cabinet volume constraints than 144"!

Thanks a bunch.

The project will have to wait until after Christmas, but I'll keep you all informed.
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Old 17th December 2003, 03:57 AM   #26
GM is offline GM  United States
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Evening!

>Hi GM,

My last post was composed before your post. Thanks so much for the info.

====

You're welcome!

====

> I punched those figures in except for X0, what does that represent and where does it get plugged in to MathCad?

====

Some (many?) of us use the original version of the worksheet. Xo is the driver location WRT the top as a percentage. For the later worksheet it would be 64.43*0.3544 = ~22.83" and 91*0.3251 = ~29.58" down from the top.

====

>The SPL graphs look quite good, even with the shorter pipe. The 91" pipe though had a troublesome peak at 75 to 100 hz.

====

Not really. This design is preferred if there's more room and/or corner gain that boosts the lower BW more. Possibly where you placed the driver makes more of a rising response than my locations.

How does it look with the driver at my recommended point?

====

> I have quite a strong room gain at 80hz, so much so that I have my sub 180* out of phase with my mains to tame it by creating a null at the xo point. I fooled around with the pipe length and found that 80" gave me the best response.

====

OK, I didn't realize you'd mapped your room. This info would have been helpful. At this length, the driver's optimum location is at ~26.85" down from the top.

====

> The peak was pushed up above 80hz, and low end roll off was just 6db down from 20 to 80hz. 80" will certainly fit much easier into my cabinet volume constraints than 144"!

====

This is a truism, and you've learned a bit more about how to design them. I just hope the specs are close enough.

Anyway, looking forward to your thoughts on its performance.

GM
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Old 17th December 2003, 04:38 AM   #27
GM is offline GM  United States
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Hey PP,

>SR and GM: You are right that I haven't yet done all the math about the harmonics of the TL. But I do remember something from having actually played music. Everyone talks about tuning TLs as if they only produce the frequency matched to the resonance of the driver, and a sine wave at that.

====

Oh really?! I disagree! Anyway, the performance requirements of a musical instrument are ~180deg out of phase with that of a signal reproducer (speaker).

====

>While that is important if that happens to be the note and waveform being played into the speaker, that is almost never the case.

====

True, but you seem to missing the 'big picture'.

====

> So in reality, we have a line reproducing sound which almost never matches the resonance of the driver or the tuning of the line.

====

This can be true, but it's irrelevant per se. What's important is the cab, be it sealed, BR, TL, etc., damps the resonance of the driver and boosts or suppresses the driver's BW as required within the design's limitations up to its mass roll off point (~(Fs*2)/Qes) to achieve a desired alignment.

====

> I have yet to find the definitive paper on why the 1/2, 1/3 or other harmonics matter for all these complex non-sinus waves, other than to match the driver parameters.

====

Because they impact the driver's overall performance, often to beyond its mass roll off point by modulating the diaphragm if not suppressed. You can see this from these undamped plots:

http://www.geocities.com/rbrines1/im...ppening_DP.gif

http://www.geocities.com/rbrines1/im...ppening_FR.gif

Do you really want these upper harmonics 'coloring' your system's music signal?

====

>Maybe you can point me to those.

====

There's probably some out there, but I'm not aware of them. MJK's various docs are a good read, and this site has some good info on the physics of pipe harmonics, etc.: http://hyperphysics.phy-astr.gsu.edu...clocol.html#c1 Indeed, this is a large and excellent site. Unfortunately, it doesn't have a reverse tapered pipe so you'll have to look at a conical pipe and mentally reverse the image.

GM
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Old 18th December 2003, 07:16 PM   #28
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Default Looks like more reading...

I haven't yet had a chance to really take apart Bob Brines work and have only read the theory at Martin King's site. It's easy to see why a lot of people like the big baffle speakers as it's closest to basic theory. I would dread having to work out both the acoustics and the crossovers for a two-way in the same cabinet so for now I'll stick with opamps, as it is much easier.

Thanks for the help so far and good luck to you AM.

:)ensen
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Old 19th August 2004, 04:35 PM   #29
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Thumbs up Finally!!

Hi guys, I don't know if any of those who posted on this thread will get notified of this update, but I have finally gotten around to building my sub.

The project was made much easier by my stumbling upon a prefab bathroom vanity cabinet that had almost the perfect dimensions for the design. This gave me a "skeleton" to make it a breeze to put together a quick prototype to try out and see if it would be worthwhile completing. I ran the small variations in cabinet geometry through MathCad and came up with the attached graph.

It cost me all of $36.00 Cdn to buy the cabinet and a near complete sheet of MDF at a local reclaimed lumber store run by Habitat for Humanity, so I'm even contributing to a good cause while saving money, how cool is that!!

I just had to strip out the hardware, cut out two 19.5 X 31.5" panels for the front and back and a 21" X 18" panel for the pipe. 3 hrs later it was ready for prelim listening tests, unstuffed and without any bracing. My expectations were that it would be very colored but that I'd get a sense of whether the bass extension was anywhere near the MathCAD curve.

I was pleasantly surprised with the outcome. Listening to the sub without the main speakers in playing back stand up bass, I could hear some cabinet colouration in the uppermost octaves of the bandpass ~80-100hz, but I was very impressed to hear all the bass tones reproduced without any obvious peaks. I was extremely surprised when I listened with my main speakers in (Angstrom Reflexions).

My Angstroms are "full range" floor standing speakers that can reproduce acceptable bass down to 40hz or so, but even without a sub in, bass always sounded muddy d/t a troublesome room peak at 80hz. Therefore, I have always run the sub 180* out of phase to attenuate this peak by creating a null at the crossover point. This worked to tame the peak, but unfortunately, my bass reflex sub produced alot of cabinet harmonics at these octaves so things still sounded muddy. Further, my old sub's low bass extension rolled off only an octave or so lower then the mains, so I had to turn it up for movies and back down for music, and it was never very musical.

Not so for the TL. Thanks to it's faithfull reproduction of midbass at the crossover point, the muddiness is all gone. Better yet, the low bass is far more coherent and musical. I am very impressed with the performance of this inexpensive Jensen Sub. It has a relatively low Xmax at 7.29mm, which I think may be an advantage, as most of my reading suggests that low Xmax subs tend to have better transient response. This seems to hold true in that I found the sub to match the 8" Vifa drivers of the Reflexions quite nicely.

I listened to several of my standard reference CD's which started with Holly Cole's version of "I can see clearly now", that has alot of bass energy at the crossover point. I could never enjoy this track before because of my 80hz room gain, but with this sub, it no longer sounded so thuddy and I suddenly found myself enjoying the Angstroms again. I then put Sarah McLauchlan's Freedom Sessions in. There is very low bass in this recording that sounds horrible in slow, large driver subs and it sounded very smooth with very little discernable change in output from the higher bass notes, down to the lowest. I listened to several other tracks and was very pleased to hear the variation in tonal qualities of base notes in each track, rather then hearing the sub's colouration making it sound like every musician is using the same bass notes. This should only get better with bracing and stuffing!

Now for the looowww bass extension. I watched Finding Nemo with my family and was stunned, as was my wife, by just how low this big box could go. Scenes such as the sliding ship were reproduced with such violent, dynamics that we all held our breath and gasped. Most impressively, bass heavy scenes did not all sound like a one note rumble track was playing. There was a distinct difference between the sound of the rushing ocean current and the sound of the deep water angler fish chasing the heroes. The speed of the sub was apparent in how well it reproduce chest ripping bass sounds without any sense of strain. In fact, I've got the sub amp's gain control down several db from the previous setting. Most impressive, was the whale belly scene which litterally shook our house off it's foundation. This was the only time I heard the cabinet rattle. I suspect this is where the 15hz peak in the MathCAD curve may have been excited.

All in all I am very happy with this simple project. It is quite literally, the best sub I've ever heard in a home installation, and it isn't even finnished yet. Not bad for less than $100.00 total investment (not including the $60.00 Sanyo Amp that powers it)!

I'll take some photos of it when I stuff and brace it, but I don't think I'll be finnishing it until I move my Home Theatre into our basement rec room this fall (now a necessity if I'm ever to watch Movies late at night).
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Old 19th August 2004, 04:47 PM   #30
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