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I am an audio DIYer and have become intrigued with the idea of nested sonotube subwoofers because of their small footprint that can be tucked into a corner. I'll finalize my design later, but one possibility is a tapered Transmission Line design.
The application will be two-channel listening (non-electronic music) and supplementing a front loaded horn system. A bandwidth of 30-80 Hz would be fine. I love the clean sound of the bass on my corner horns, so the sub would need to integrate with that as a priority.
I have been reading up on these, but for the life of me I can't find a clear, plain & simple description of the relative strengths and weaknesses of the sound quality of a TL design when used as a subwoofer.
In simple terms, what do they sound like?
Thanks ,
-Tom
Follow Ups:
Thank you for your collective thoughts
Mixing different types of bass cabinets can only lead to grief.
The phase shift on the low end is different, due to the differing slope of the bass roll-off. Thus, the bass energy from the different types of bass cabinets will not sum, and often will cancel out at certain frequencies.
Trying to mix horn loaded bass cabinets with T-line loaded bass cabinets is just about the worst combination, due to the completely different roll-off characteristics. Most horn-loaded bass designs try to extend the horn action down as low in frequency as possible, resulting in one of the steepest roll-off slopes around (approx. 36 dB/oct), while the T-line approximates a 6 dB/oct roll-off for the first portion of it's roll-off, gradually converting to a 12 dB, then 24 dB. The net phase shift throughout the bass roll-off region is completely different, and will not sum except at a few certain frequencies.
Mixing horns with different cut-off frequencies will also be a problem, as will mixing bass reflex cabinets with different tuning frequencies, etc. Same thing: phase responses that do not track with each other.
If you like the sound of your existing horns, then the best way to increase the quantity of the bass is to add identical models on each channel. This will also extend the low frequency horn cut-off a little lower, due to mutual coupling.
There really isn't a good way to extend it significantly lower without adding a crossed-over sub-woofer, and even then, you still have to try and match the sub to the horns(not a trivial task) or the results will be less than satisfying.
Jon Risch
Hi Tom,
I'm a DIY loudspeaker guy too. I have built several subwoofers---active, passive, transmission line and sealed (never been a ported guy for subs, but some will say that a transmission line is just a glorified "ported box"---I disagree).
OK, sound wise?
Well, again I'm only interested in sealed and transmission line designs so I have compared them back and forth. When I am designing, it really comes down to "how much room do I have to work with" for starters. A transmission line design will take up far greater space then a sealed design, and sealed designs are much more impervious to "design errors", but I digress.
Sound wise, all else being equal (that's the hard part!!), what I hear in a transmission line sub versus a sealed sub is a wee-bit more bass "pitch definition" in the t-lines, and a bit more dynamic sound overall, which I have even measured with a fast-acting meter just to be sure I wasn't hearing things. Transmission line subs typically sound a wee-bit "cleaner" to me overall then a sealed sub. However, I also like many sonic aspects of the sealed subs over the transmission lines. Typically a fuller and richer midbass, with more "slam" and weight overall. Just a fuller, slightly rounder bass sound.
Done correctly EITHER a sealed or t-line design will provide excellent results for what you are doing. And once you step into the world of room correction on your subwoofers like I and many have (IMHO, the only way to achieve accurate bass in any given room) then you mitigate those descriptive terms I used above ten-fold. Plus you will get a MUCH better blend with your "sats" as a bonus.
My opinion for mating with the horns you're using is to give it a try and go t-line if you have the room. Transmission line bass with horns might make for a magical combo.
Regards,
Joel
The biggest strength of the TL style enclosure is that the cabinet is inherently very stiff. Physics - wise there are better designs... Duke, below, notes the inherent issues. That leads to hybrid designs such as a damped TL or TL designs terminated either with a port, aperiodic "vent" or PR. One could argue for a given cabinet volume there are other ways to get good low frequency response. Usually the TL design has been used for designed that extend above the Subwoofer frequency range, where the characteristics of the driver are better chosen for bass and perhaps lower midrange, and the TL loading is chosen because the loading parameters of the loudspeaker are not optimized for a reflex or sealed loading - within the other design constraints of the system.
Of course, the marketing power of "Transmission Line" loudspeakers have led to a following in High End, leading to designers specifying their product as TL, when it actually is just a vented box.
I've neither built nor measured a TL sub, but have built maybe fifty or so TLs that weren't subs.
The output from the end of a TL emerges in-phase with the frontwave at the frequency where the path length is 1/2 wavelength. If the woofer is well chosen and the line isn't too narrow, we can hope for good bass output down to the frequency where the line length is 1/4 wavelength. However at the frequency where line length is equal to 1 wavelengh, the output from the end of the line emerges 180 degrees out-of-phase with the frontwave, causing a cancellation dip. Stuffing the line to reduce the magnitude of output at this 1 wavelength frequency will also reduce the beneficial output down lower (though not as much). There is a technique that results in good bass extension with minimal cancellation dip, but it also results in a fairly low efficiency box and suitable woofers are rare.
You can see this dip in frequency response measurements of TL-type speakers, such as models from PMC and the Zu Druid (the Druid isn't exactly a TL, but it's still a resonating tube). You will also see this dip, and usually a sequence of higher dips, in the frequency response measurements of bass horns. The energy that causes the higher dips is normally absorbed by the stuffing in a transmission line, but they'd be there in an unstuffed line.
So my suggestion is to take the anticipated 1 wavelength dip into account and expect a TL sub to cover a bit less than 2 octaves.
Duke
Me being a dealer makes you leery?? It gets worse... I'm a manufacturer too.
Regarding the original question about the "signature sound" of a TL, so far we have:
1. Don't criticize an IMF, apparently this is a sensitive issue.
2. Tapped horn, I am aware of these, but there is no definition of what its own "signature" might be.
3. Low frequency extension. Well I am not sure, based on measurements, that a TL will extend much lower than a bass reflex.
4. Tricky to build and a bad implementation may obscure any advantage.
5. To be honest, I don't know what "voluptuous" means.
Getting closer and thanks.
And you're capable of looking up the meaning of a word. Maybe.
Some designers use transmission loading - or variants - for midrange and tweeter drivers. That doesn't have to do with extending the frequency response of those drivers downwards....
In the vein of transmission line mids, Bud Fried considered the mid-range line more important than the bass line. The music is mainly in the mids and he felt a proper line was a better loading for mids than a simple closed box that he believed strangled the mids.
I recall the Fried model G, a large 3 way with a version of the variovent for the bass but a line mid-range. The early one had a tube going from the front to the back of the main box. But later ones had a longer 3 fold line. I was around when he was comparing the two lines to decide whether to use the 3 fold one. It was obviously more open and detailed and went into later production.
So I guess you don't see a compelling reason to use a TL for a subwoofer.
There is plenty of info about the sound of a TL for the rest of the spectrum, but what is special about a TL as a subwoofer?
You know, all the possible reasons listed previously.
You probably should pursue a bass reflex design, though.
Better group delay than bass reflex, slightly smoother bass impedance characteristic, maybe better handling in terms of not re radiating the backwave through the driver. In theory.Those are the main pluses and minuses.
I think they do extend the low end and give it a nice voluptuous quality when done properly.
Edits: 12/21/09
If you're willing to build a folded enclosure, you might want to consider a tapped horn - look it up. They're supposed to be the latest and greatest thing. I'm not sure they're easy to design, though.
The tapped-horn guru Tom Danley posts at avsforum.com. They have a model that sits flat on the floor and you can put a couch on top of it, or it can stand up against a wall. I think that model may be offered as a flat-pack kit.
TLs are like anything else; some some good and some sound bad. I thought the IMFs sounded muddy and tubby but the later Fried Model H sounded deep and tight.
I think it has more to do with execution than type.
nt
"When the going gets weird, the weird turn pro." -HST
I owned IMF Studios and I knew Bud well for decades and IMFs were quite full in the midbase. The later Frieds were tighter.
The problem with the IMFs was the mounting of the B110. Later, removing it (and the HF drivers) into another enclosure with a narrower front dimension. Crossover adjustments to suit.
Kal
I'm not sure what you're saying. The B110 was crossed at 375 Hz way above the mid-bass and so should have no affect. The Studio didn't use a B110( early ones were modded EMIs and later were a modded Danish driver and they had similar bass. I suspect that the combination of woofers chosen and damping in the line led to the sound: they needed more damping.
1. Yes, the B110 was crossed over too high and the B139 affected the midrange nearly up to 1kHz! This was necessitated by the limited enclosure size granted to the B110. Building a much more generous (if narrower) B110 enclosure permitted me to drop the crossover to a bit above 100Hz to great advantage.
2. Sure. I never had/used the smaller Studios.
Kal
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