rfg@hound.UUCP (R.GRANTGES) (10/17/85)
[] TL = Transmission Line ? If so the principle is something like this. If I error, I'm sure that I will be corrected. One basic problem with speakers is that they have two sides. What happens to the sound coming from the back side? If you do nothing about it it will sneak around front and cancel out most of what's coming out there. One school of thought says: get rid of it. That's not as easy as it seems. You can build a big box, stuff it full of lamb's wool and hope for the best, you can do lot's of things. One thing you can do is try to build a transmission line to carry the back sound wave away, far far away. If you succeed, you have gotten rid of it. So you build this transmission line and you try to make it very lossy so that if anything bounces back from the other end, which really isn't far, far away after all, it will be very weak by the time it gets back and won't cause any bad feelings. A variation is to figure on taking advantage of the delay in the back wave as it moseys along down the TL as a form of phase shift. At just the right point you suprise the wave into radiating into the room in just the right phase to aid the front side speaker radiation instead of messing it up. If that is pretty hard to do right (more like impossible) then at least your lossy TL will hopefully have weakened it so its too wimpy to matter anyhow. Presumably while the TL may not work perfectly in practice, it will beat the stuffed box as a better "infinite baffle." Unfortunately, it also loses the opportunity to use the sealed volume of air in the box as a <much> more linear spring than speaker mfrs know how to build with their voice-coil supporting "spiders." This is the secret invented by Ed Vilchur and used in the AR speaker. The AR uses the air as an "acoustic" suspension. It uses the box stuffed with lambs wool to lose the sound it doesn't want to use for springing. No question it works mighty good at low frequencies where the wool doesn't absorb much but you need the spring action. the english used to have a TL speaker which sounded very good. I forget its name. I <think) a modern version is called a "Time Window", but mebbe I'm wrong. . -- "It's the thought, if any, that counts!" Dick Grantges hound!rfg
bdw@drutx.UUCP (WelkerB) (10/17/85)
Hmmm... funny you should mention time windows. I had a recent opportunity to get a pair of DCM "Time Windows" for about $450, and was just nerving myself to ask about them here. Anybody got anything to say about them? Each is a tall (about 4-4.5 foot), skinny (maybe 10 inches) sort of columnar affair. They have a pleasing sound (round and clear (grin)) to my ears, and are said by the sales staff to be a good "jazz" spkr (whatever that is...). They are second hand. Tips on or experiences with are hereby appreciated in advance. If more than two people would also like to know, i will post any mail i get. Thanks, Bruce Welker The Denver Works: AT&T-ISL ihnp4!drutx!bdw
mohler@drune.UUCP (MohlerDS) (10/20/85)
I believe the famous English TL speaker you may be refering to is the WEBB TLS or the IMF RSPM IV. Both used a KEF B139/II in a tapered (and wool damped) TL box design. David S. Mohler AT&T - ISL @ Denver drune!mohler
rdp@teddy.UUCP (10/21/85)
In article <1417@hound.UUCP> rfg@hound.UUCP (R.GRANTGES) writes: >[] >TL = Transmission Line ? > Yes... >Unfortunately, it also loses the opportunity to use the sealed volume >of air in the box as a <much> more linear spring than speaker mfrs >know how to build with their voice-coil supporting "spiders." This is >the secret invented by Ed Vilchur and used in the AR speaker. The >AR uses the air as an "acoustic" suspension. It uses the box stuffed >with lambs wool to lose the sound it doesn't want to use for springing. The issue of who invented the enclosure system you describe is not settled. There are patent descriptions long before Vilcur invented the phrase "acoustic suspension". In any case. A study of that period of speaker development is most enlightening. The problem with direct radiator loudspeakers at that time was, in fact, hooribly high distortion because of, among other things, the non-linearity of mechanical suspensions. Yes, the AR-1 did have low distortion in the bass, but why?. Well, as it turns out, Vilchur did MANY things. He put the driver in a box whose "compliance was significantly lower than that of the driver. To do that, he had to invent a new kind of edge suspension (the "half-roll"). He also used a voice-coil/magnet assembly that was inherently more linear (long voice coil in short magnetic field means the BL product is pretty independent of position, but that also means that only a small portion of the voice coil is ever really doing any work). Now, these last two devices contributed as much, if not more, to the low distortion of the system as the concept of acoustic suspension. Now, given his box size, in order to get any reasonable low frequency bandwidth, he had to make his cone VERY heavy, resulting in low efficiency. Now, other manufacturers (before and since) have utilized the devices Vilchur used in linearizing driver motion. Using these inherently more linear drivers in the bass reflex and IB systems also resulted in severely reduced distortion, along the same magnitude that the acoustic suspension systems achieved. Add to the fact that one can optimize a bass reflex system to reduce cone motion at resonance (the big offender for distortion because cone motion is at a max there), then distortion in such systems at low frequencies can be significantly better than acoustic suspension. Now, if the hype for acoustic suspension where true, then the TL lines I spoke of should have higher distortion at a given frequency and level than an acoustic suspension system. But, in real measurement, they do not. In fact they often have significantly less because the drivers I used are better than the ones AR and KLH and ADVENt and the like where using in contemporary systems. As an aside, has anyone ever seen or heard an AR-1? It was one of the most unbelievably BAD speakers around. The bass was OK, but the top end! The "tweeter" consisted of a 8 inch (12 inch?) very cheap Altec-Lansing full range PA driver. It was truly wretched! How AR ever got started on THAT thing is beyond me. (Maybe because everybody else was using that same PA driver as the WHOLE system?) I have a very deep philosophical difficulty with the whole "Boston" hi-fi scene. It is not the innovative area many people attribute it to be. Most of the Audio companies that have come and gone have suffered from real bad cases of "blinder vision" design. All of the famous old time companies come to mind, AR, KLH, Advent, Allison, etc. A classic example is Roy Allison and the speakers form Allison Acoustics. His goal was to minimize the interactions of the woofer with the room boundary by placing the woofer as close to these bondaries as possible. THis, I will admit, he achieved in his first series of speakers to a remarkable extent. However, he ignored other things which were, to me and others, sonically more significant. For example, the undamped panel resonances in the Allison 1 and 2 where absolutely horrible. The system sounded like a wooly cardboard mailing carton that coupled very well to the room. The tweeter was atrocious, and you could here the crossovers as plain as day. But it was successful because it solved one problem, at the complete ignorance of many others. Dick Pierce