Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/3/84; site teddy.UUCP Path: utzoo!linus!decvax!ucbvax!ucdavis!lll-crg!seismo!harvard!talcott!panda!teddy!rdp From: rdp@teddy.UUCP Newsgroups: net.audio Subject: Re: subwoofers and xovers Message-ID: <1454@teddy.UUCP> Date: Fri, 18-Oct-85 14:47:13 EDT Article-I.D.: teddy.1454 Posted: Fri Oct 18 14:47:13 1985 Date-Received: Mon, 21-Oct-85 05:02:15 EDT References: <1395@teddy.UUCP> <30200019@siemens.UUCP> Reply-To: rdp@teddy.UUCP (Richard D. Pierce) Organization: GenRad, Inc., Concord, Mass. Lines: 306 In article <30200019@siemens.UUCP> jar@siemens.UUCP writes: > >I'm very interested how TL works (in fact I have speakers which have TL and >sound great but I don't know how it works). If it is not of general interest >perhaps you could mail to me > >Thanks in advance Juergen Several people have requested me to elaborate on my comments on transmission line enclosures enough so that I took the time last night to put this article together. While not en- tirely rigorous in its presentation, it is an informal sum- mary of a long and complete experiment in transmission line enclosures that was performed, I like to think, in a criti- cal, scientific manner. If you want to flame me, the do so for valid technical reasons, not for accusing something of being better than what you have. It is, granted, a somewhat long article, but it could have been much longer (I omitted, possibly wrongly, such things as specific driver parameters and so forth) Much of the early work in transmission line enclosure was done in England during the late '50's and 60's. Notable amongst the researchers was A. R. Bailey [1] who described a non-resonant enclosure utilizing a damped labarynth. Later, companies such as IMF and Bowers and Wilkins had one or more models based on transmission line enclosures. In the IMF models (the "monitor" and the "Studio"), the TL was ostensi- bly used to augment the lowest output of the bass driver, while the B&W (the "DM2") approached the use of the TL as a means of reducing low frequency colorations. Later, IMF in- troduced the concept of the "active" TL, but, for a variety of reasons, this proved unsuccessful. In this country, small companies such as Audionics produced TL systems. The Audion- ics unit was based on further work by Bailey [2]. Basically, two views of the uses of transmission line bass systems emerged. First, a transmission line can be used to augment and extend the low frequency response of a driver. Second, a transmission line is useful for reducing low and mid-bass colorations that arise because of uncontrolled internal reflections in the enclosure. I will attempt to ex- plore both of these views, and point out the pitfalls and advantages of each. In the first approach, factors such as line length and damp- ing were "adjusted" so that the rear acoustic radiation from the bass driver was modified in such a manner as to ensure energy of sufficient intensity and phase was emitted from the end of the TL to augment the lower regions of the audi- ble spectrum. Several phenomenon were used to optimize these effects. The fact that the attenuation of sound by fibrous materials is reduced at lower fequencies supposedly helped in that the output of the line increased with decreasing frequency, thereby reinforcing the bass. Also, the velocity of propogation is reduced at these lower frequencies [3], making the TL appear longer than it really was. This along with careful enclosure design was to help in the production of extended, low-distortion bass output. The manufacturers further noted that the line presented a mass load on the driver, thereby lowering it's fundamental resonance, but line losses prevented the resonance from having to high a Q factor, thereby controlling the resonance peak. Let's take the IMF speakers as a specific example of a com- mercial implementation of TL enclosures. The "Studio" model consisted of a KEF B-200 SP-1014 8 inch Bextrene bass- midrange driver in a transmission line about 7 feet long, folded into a floor standing enclosure about 14" wide, 16" deep and 32" high. At about 500 Hz, this was crossed over to a 4 inch paper midrange (manufactured by EMI), which was then crossed over to a Celestion HF-1300 tweeter. Because of its infamous characteristic of having NO output above about 12 Khz, a super tweeter took over (in this model it was an STC 3/4" "chemex" plastic dome). The crossover had quit a bit of contouring built in to take care of frequency anomo- lies. The speakers were sold in matched, mirror image pairs. The line was (initially) damped with various combinations of fiberglas and lamb's wool, and some cases of teased jute fiber were seen in the end of the line. At the time (1972), the speakers presented a remarkably ac- curate sonic impression, with the lack of "boxiness" in the bass being one of the most noticeable characteristics. The midbass appeared to be especially "dry" and (in some case) "thin", while there was a definite impression of extended deep bass. However, the speakers suffered from some interesting deep- bass and midbass anomolies. Most notable, both in careful live comparisons and in both anechoic and live room measure- ments, was a prominent peak around 70 Hz, and a prominent dip in the response at around 160 Hz or so. These features were not subtle, the 70 Hz peak being as much as 3 db, and the 160 feature being almost 2/3 octave wide and 2 to 3 db deep. At the low end, the speakers measured -3db at about 32 Hz, and dropped steeply below that. Careful investigation revealed that the cause was the same for both problems: extraneous output from the port at the end of the TL. At 70 Hz, the line was exactly 1/2 wavelength long, and two phenomenon were occuring: 1) the line was resonating at this frequency, reducing the loading on the driver and increasing it's excursion, and 2) the line pro- vide exactly 180 degrees of delay, cancelling the out of phase radiation from the rear of the cone. Each effect, by itself, was not severe, but the two together contributed to the problem. At 160 Hz, the line was 1 wavelength long, also resonant, but at a lower Q. More importantly, there was enough output from the end of the line (now delayed by 360 degrees) to cause some cancellation. At the extremely low frequencies, several other things were happening. The line was indeed providing some mass loading, with the result that the measured system resonance did decrease in frequency, with a subse- quent increase in the system Q. This had the effect of under-damping the system, which would normally lead to over-emphasized bass, but this effect was cancelled by the now almost completely out-of-phase signal emerging from the end of the line. What was being gained by one effect was be- ing lost to another. One was still impressed by the seeming lack of "boxiness". Later studies using microphones placed at strategic (and movable) points in the enclosure, and accelerometers at- tached to the cabinet revealed that the high degree of internal bracing provided by the cabinet contruction method reduced dramatically the colorations caused by panel reso- nances, and the many nonparallel internal reflecting sur- faces reduced internal standing waves. IMF later attempted an "active" transmission line approach. This involved a short (3 to 4 foot) and simple line, with an active 8 inch woofer at each end of the line. This product (the "ALS-40") was remarkably unsuccessful because the line was too short to be of any use at low frequencies, and the inclusion of two driver in what was effectively a sealed box meant a higher overall system resonace. Also, polyurethane foam was used for internal damping, and this has proven to be ineffective at low and mid-bass freweuncies. At one point, they attempted changing the fiber orientation in the fiberglas damping in the line, in attempt to first polarize, and then absorb, the sound from the rear of the driver. Un- fortunately, sound, being a longitudinal and not a transverse wave like light, is not polarizable, and the technique had no effect. Another approach to transmission lines was adopted by Bowers and Wilkins in their DM-2a system. This system was slightly smaller in total volume, and the transmission line was sub- sequently shorter, but the line was more heavily damped. The complement of drivers was somewhat similar, except that the bass driver while of similar size, was of proprietary design, and served as a bass-midrange driver, crossing over at 3500 HZ to the same tweeter and supper-tweeter as above. The crossover was markedly more complex, and showed evidence of quite a bit of contouring and level matching. The major difference in sound was that the B&W did not have the impression of very deep bass extension, but the midbass regions sounded more controlled. Measurements showed a sig- nificantly more even response in the 60 to 200 Hz region than the IMF, and the low end was slightly more damped, but did very well in being only 3 db down at about 37 Hz, anechoically measured. Measurements revealed far less out- put from the end of the transmission line at all frequencies compared to the IMF, and internal inspection showed a much greater amount of internal damping tha the IMF. Also, the cabinet was contructed with heavier panels (1 to 1 1/4 inch in the B&W, versus 3/4 inch in the IMF studio). As an aside, the level of finish and choice of veneers on the B&W was, in most cases, much better. Both of these speakers would often suffer in comparisons with such things as AR-3A's and JBL L100's when such com- parisons were done informally, but almost always, careful listening tests revealed that the IMF and the B&W were capa- ble of rendering far less colored, more detailed images of musical recordings in the bass and mid-bas regions. In some cases, the superiority of the speakers in this region were over-shadowed by the absolutely dismal performance of other speakers in the mid range and treble regions! Subsequent to this time, I engaged in several years of research into the action of transmission line bass enclo- sures, and made several interesting discoveries. Several parameters can be varied in a transmission line, such as line length, cross-section, taper rate, size of exit port, damping material and amount, plus all the vaqueries of driver parameters. The most obvious conclusion I came to, as exemplified in the above examples, is" A transmission line enclosure cannot be successfully used to extend or augment the low frequency performance of a speaker without causing other frequency response anomolies. Well, then, what can it be used for? Well, let's see what happens when we change one of the parameters in a line, the damping. The experiment was per- formed on configuration very similar to the IMF speaker, that is, a 7 1/2 foot line, starting with a cross section of about 1 square foot, and tapering relatively evenly to an exit port of about 1/4 square foot. This was driven by the above mentioned KEF B-200 SP-1014 8 inch driver. With no damping, of course, line resonaces dominated the responce. Adding damping had several effects. First, it re- duced the severity of the resonances. Secondly, the lower the frequency of the line resonance, the less the amplitude of the resonance was affected, but the more the center fre- quency of it was lowered. This is probably due to the de- crease in the velocity of sound due to the damping. Another effect was noticed. The mass loading of the driver by the line was most significant when there was no damping, and was reduced as damping was added. As a result, the fundamental system resonance was lower than driver free-air resonance in the undamped line (as much as 1/2 octave!), but as damping was added, both the frequency rose, and the Q decreased un- til at one point, the system resonance and the driver free air resonance were identical. The system Q appeared to be the same as free air Q also. At this point, it appeared the system was providing a purely resistive load on the rear of the driver. Increasing the damping from this point made the line appear more and more like a sealed box, raising both the system resonance and the system Q in a similar fashion. At this point, which I refer to as "critical line damping", very little difference is noted in the performance of the system when the end of the line is opened or closed. Closed, it shows a slight increase in system resonant frequency, be- cause at that point we are close to dealing with a true sealed enclosure. The amount of damping in the line is far greater than that foudn in the old IMF's, and slight more than in the B&W's. At this point, also, other factors don't have as great an effect as one might imagine. For example, parameters such as line tapering, and, to some extent, line length are not as critical as the amount of damping present, especially above about 70 Hz or so. The cross section is important, it seems, only in the amount of space available for damping higher frequency standing waves (those above 200 Hz). Then, it might seem, transmission lines are good for one thing when done properly. They can take the rear radiation of the bass driver, absorb it, and prevent it from contri- buting to coloration of the bass. In no case was I able to in any way extend the performance of the driver beyond its free-air capabilities without introducing problems at other frequencies. Sonically, the result is a drastic reduction in mid-bass and mid-range frequency anomolies. The quest for very smooth response can be achieved in the regions of 50 to 500 Hz us- ing this technique, and the resultant improvement of tran- sient response by the elimination of these anomolies is ap- parent. Most of all (to me), it is done without magic. The disadvantages have their day, though. The enclosures are large (several cubic feet), very heavy (at least twice that of a similarily constructed closed box or base reflex), com- plex, and expensive to build. They are not terribly effi- cient, although no worse so than straight closed boxes. I should also remind you that the effects of stiffer panel walls because of the internal braces should not be ignored [4]. One way to significantly improve mid-bass performance in nearly ANY speaker is to remove the woofer and install as much internal bracing as possible, not ignoring, of course, the fact that any significant redictions in internal volume must be avoided. Dick Pierce Copyright 1985 Permission is granted for the non-profit use of this docu- ment, including its dissemination to interested readers, on the condition that the author's name and this notice is in- cluded. Inclusion of all or part of this document in any publication is prohibited unless express permission is granted by the author, who retains all rights. Violation of this notice will make Dick Pierce very angry and bitter, and he'll do to you what he did to Amar Bose one day in 1973 (heh-heh!). References: [1] A.R. Bailey, "Non-resonant Loudspeaker Enclosure", Wireless World (October, 1965). [2] A.R. Bailey, "The Transmission Line Enclosure," Wireless World, (May, 1972). [3] L.J.S. Bradbury, "The Use of Fibrous Materials in Loudspeaker Enclosures", Journal of the Audio Engineering Society, Vol. 24, No. 3 (April 1976). [4] James K. Iverson, "The Theory of Loudspeaker Cabinet Resonances", Journal of the Audio Engineering Society, Vol 21, No. 3, (April, 1973).