rdp@teddy.UUCP (10/07/85)
[] > I would like to build a sub-woofer for my stereo system. I know how > to make crossover networks for regular speakers, but I can`t figure > out how to make one work for a sub-woofer. The catch is that I want > bass from both right and left channels to driver the sub-woofer. > I can use caps to eliminate the low frequencies sent to my > regular speakers and chokes to eliminate the high frequencies for > the sub-woofer, but how do I combine the left and right low > frequency signals so they can be sent to one speaker? > > Thanks, > Walt Barnes > ...!ihuxn!wwb > First of all, several points. Making a passive crossover network for subwoofer type frequencies (50Hz - 250Hz) is a real bitch. The component values needed are astronomical. We need (say for a 200Hz cutoff frequency) capacitances on the order of 200 uFd and inductances in the neighborhood of 6 mH. While sitting at home and winding such and inductor is possible, the cost of reasonable to high quality caps of that value suitable for audio use is comparable to the ransom paid for kidnapped first-borns. Be that as it may, let's presume that the basic network is designed (no mean feat in itself). There are several approaches that cab be taken The idea of simply summing the outputs of two amplifiers after two networks can be disasterous. Here we have essentially two voltage sources trying to drive each other, and, occasionally, the load. The fact that much information recorded at these frequencies and lower is essentially monaural doesn't help much. One alternative, which, unfortunately, severely restricts your choice of low frequency drivers, is the dual-voice-coil drivers made by several manufacturers (JBL and EV come to mind). Most of these drivers are perfectly wretched, however. Another approach, one which I have quite succesfully used in the past many times, is to have two drivers in a single cabinet of twice the volume of a single cabinet. This has the disadvantage of taking up as much room as two separate enclosures, but ensures total electrical isolation between channels. The method I used was to construct a transmission-line enclosure designed for drivers with the equivalent radiating area of a 12-inch driver, and use two high-quality 8 inch drivers instead. This way, the enclosure could be used as either a common channel subwoofer or a single channel woofer, simply by changing faceplates and drivers. (For the self-appointed TL illuminate out there, this approach works just fine, If you disagree, then you have the wrong idea about what TL's are capable of doing. If you want to know why this is, just ask, but don't flame me, this is an area I spent considerable time, energy, and money researching). In any case, when designing this whole system, make sure you know completely what the various components of the system are really doing, as that severely constrains the crossover configuration. Note especially that your designing a network close to system resonances (a couple of octaves is close to me), and that you have to deal with the attendant load impedance variations. Good luck Dick Pierce
sjc@mordor.UUCP (Steve Correll) (10/09/85)
> > I would like to build a sub-woofer for my stereo system. I know how > > to make crossover networks for regular speakers, but I can`t figure > > out how to make one work for a sub-woofer. The catch is that I want > > bass from both right and left channels to driver the sub-woofer. > > I can use caps to eliminate the low frequencies sent to my > > regular speakers and chokes to eliminate the high frequencies for > > the sub-woofer, but how do I combine the left and right low > > frequency signals so they can be sent to one speaker? > > > > Thanks, > > Walt Barnes > > Making a passive crossover network for subwoofer type frequencies > (50Hz - 250Hz) is a real bitch. The component values needed are > astronomical. We need (say for a 200Hz cutoff frequency) capacitances > on the order of 200 uFd and inductances in the neighborhood of 6 mH... I second rdp's sensible observations on the difficulty of passive crossovers for low audio frequencies. You might instead consider building a little op-amp circuit to drive a mono amplifier to drive the subwoofer. One op-amp can easily sum the left and right outputs of a preamp (or, with appropriate voltage dividers, the left and right speaker outputs of a receiver) and two more op-amps can low-pass filter the result. The money you save by buying a single woofer (as opposed to a dual-voice-coil woofer or a pair of woofers) will help pay for the mono amp. The National Semiconductor Audio/Radio Handbook (contact National Semi at (408) 737-5000 or 2900 Semiconductor Drive, Santa Clara, CA 95051) tells how to build active crossovers (page 5-1 of the 1980 edition) and summing amplifiers (page 6-12). -- --Steve Correll sjc@s1-c.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc
jar@siemens.UUCP (10/10/85)
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 Juergen A. Reimann Research & Technologie Lab. Siemens 105 College Road East Princeton NJ 08540 (609) 734-6527 ihnp4!adrvax!siemens!jar
danz@hplsla.UUCP (danz) (10/10/85)
You mention that a "passive crossover" is hard to design and build for this range of frequencies. Sounds reasonable to me, and it inspires another question. How about active crossovers? Has anyone out there tried one for this frequency range, and if they have, how were the results? Dan (don't accuse me of being an electrical engineer)
seifert@hammer.UUCP (Snoopy) (10/12/85)
Before everyone runs off and builds subwoofers with two 8" drivers, I'd like to point out that this isn't going to be much of a subwoofer. I've seen specs for so-called "subwoofers" that doen't even go down as far as my AR-11s. (-3dB @ ~35Hz) A subwoofer should go down to at least 20Hz, if you're into organ music there's one last note down around 16Hz or so. Producing these frequencies at any sort of volume means pushing LARGE volumes of air around. The AR-9 has two 12" woofers each, and the -3dB point is 28Hz. (sigh) You can go to larger drivers, but transient response will probably suffer. The enclosure is also going to have to be fairly large. Yes, there are subwoofers on the market with a single 12" driver in, say a 14" cube that go down to 20Hz. A friend of mine has one. It won't play very loud before running into problems. Of course you could set up the system in a smaller room, like a walk-in closet... :-) Physics by any other name presents the same restrictions. (BTW, no I don't have a subwoofer. Maybe someday...) Snoopy tektronix!tekecs!doghouse.TEK!snoopy
rdp@teddy.UUCP (10/15/85)
In article <1556@hammer.UUCP> tekecs!doghouse.TEK!snoopy writes: >Before everyone runs off and builds subwoofers with two 8" drivers, >I'd like to point out that this isn't going to be much of a subwoofer. Well, on the contrary, I would like to point out that I have built about a half dozen pair of such beasts, using two KEF B-200 SP-1014's, in properly terminated transmission lines (which I am not to successfully avoiding to elaborate on) that were measured, anechoically, at being -3 db at 25 Hz. >I've seen specs for so-called "subwoofers" that doen't even go down >as far as my AR-11s. (-3dB @ ~35Hz) A subwoofer should go down to >at least 20Hz, if you're into organ music there's one last note down >around 16Hz or so. At the risk of resurrecting the Pierce/Grantges debate again, I should point out that very little energy of consequence comes out of organs at the fundamental, the vast majority (in the case of pedal reeds and open principals, upwards of 80%) of the energy is distributed amongst the harmonics. >Producing these frequencies at any sort of volume >means pushing LARGE volumes of air around. The AR-9 has two 12" >woofers each, and the -3dB point is 28Hz. (sigh) You can go to larger >drivers, but transient response will probably suffer. If the crossover limits the high-end cut-off to something like 200Hz, and any loudspeaker system can get up at least that far reasonably, transient response will suffer only because of the crossover (neglecting the fact that the lower cutoff also affects it as well). > The enclosure is also going to have to be fairly large. No question about that! >Yes, there are subwoofers on the market with a single 12" driver >in, say a 14" cube that go down to 20Hz. A friend of mine has one. >It won't play very loud before running into problems. > >Of course you could set up the system in a smaller room, like a >walk-in closet... :-) > >Physics by any other name presents the same restrictions. No argument here, also. >(BTW, no I don't have a subwoofer. Maybe someday...) > > Snoopy Well, let's note that the poster lives in a small dog house, so bass really isn't an issue :-) (doesn't anyone sign their real names anymore?) Here is an aside that may be of interest to you sub-woofer enthusiasts. Several manufacturers have made an sold subwoofers based on "slot" loading or "floor" coupling. IN the cases I am discussing here, the woofer ends up facing downwards near the floor. Almost universally, I found such systems to have the "won't play very loud before running into problems" syndrome. The reason? Well, the vast majority of woofer/driver/suspension/spider/voice coil/magnet makers make one assumption about drivers; that they will be used in a horizontally-facing orientation. Facing a woofer up or down causes the driver to be offset mechanically from the center of its linear mecahnical operating region. This then severelly reduces the amount of linear mechanical (and electro-magnetic) motion left for the driver to operate with. The result is that the amount of SPL before noticeable distortion occurs is not very great. Once again, the extensive knowledge of the designer overcomes any limitations impossed by reality! Dick Pierce
eve@ssc-bee.UUCP (Michael Eve) (10/15/85)
> > How about active crossovers? Has anyone out there tried one for this > frequency range, and if they have, how were the results? > There was an article in Audio (about May, 1983) which had designs for three home-built subwoofers based on JBL speakers. There was also a design for an active crossover for subwoofers in a companion article. The crossover used about 7 op-amps and looked pretty easy to wire up. The authors claimed it did not degrade the audio information measurably. It could be driven by either a stereo or mono amp. -- Mike Eve Boeing Aerospace, Seattle ...uw-beaver!ssc-vax!ssc-bee!eve
rdp@teddy.UUCP (10/18/85)
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).
seifert@hammer.UUCP (Snoopy) (10/19/85)
In article <1431@teddy.UUCP> rdp@teddy.UUCP (Richard D. Pierce) writes: > [RDP has built subwoofers] that were measured, anechoically, at being > -3 db at 25 Hz. >At the risk of resurrecting the Pierce/Grantges debate again, I should >point out that very little energy of consequence comes out of organs >at the fundamental, the vast majority (in the case of pedal reeds and >open principals, upwards of 80%) of the energy is distributed amongst >the harmonics. And early transister amplifiers measure just as good as the tube ones, and early CD players measure better than LP players. Sure, a subwoofer that extends down to 25 Hz will be an improvement over the 35 Hz of the 'main' speakers, but if I were going to all the time/expense/bother of buying/building a subwoofer, I would want it to go to AT LEAST 20 Hz, preferably 16Hz. Audio is full of examples of "good enough" not really being good enough. >> You can go to larger >>drivers, but transient response will probably suffer. > >If the crossover limits the high-end cut-off to something like 200Hz, >and any loudspeaker system can get up at least that far reasonably, >transient response will suffer only because of the crossover (neglecting >the fact that the lower cutoff also affects it as well). Perhaps I am using the wrong term? A larger driver will have more mass (other things being equal), and will be more sluggish getting started and stopped. I suspect that several 10-12" drivers would sound better than a single large one (18-24-40") for this reason. >Well, let's note that the poster lives in a small dog house, so bass >really isn't an issue :-) Perhaps you have forgotten that the visible doghouse is merely a shelter for the entrance to the underground mansion. :-) >Here is an aside that may be of interest to you sub-woofer enthusiasts. > [ bit about woofer being mounted in the wrong plane ] Agreed. That's probably a significant part of the problem. Picked up a copy of the October Audio yesterday. Not that many systems actually go down to 20Hz or below. Looks like it would take $1-1.5k to get something reasonable. Not including the fork-lift truck to move it. Which is really the main reason I haven't gotten one yet. I'm still moving much too often. <grumble> Snoopy tektronix!tekecs!doghouse.TEK!snoopy
mohler@drune.UUCP (MohlerDS) (10/20/85)
One exception to the slot loaded subwoofers that won't play loud, that I am very familiar with, is the JANIS W1. It uses a driver that employs a voice coil that "falls" into alignment when mounted face down in the enclosure. Also the W1 is available across the country at a price (on the used market) that would make most people question whether the time required to build a good subwoofer is worth it. I bought mine in mint condition for 250.00 in the denver area, they should be even easier to find out east. David S. Mohler AT&T - ISL @ Denver drune!mohler
rdp@teddy.UUCP (10/21/85)
In article <1575@hammer.UUCP> seifert@hammer.UUCP (Snoopy) writes: >>> You can go to larger drivers, but transient response will probably suffer. >> >>If the crossover limits the high-end cut-off to something like 200Hz, >>and any loudspeaker system can get up at least that far reasonably, >>transient response will suffer only because of the crossover (neglecting >>the fact that the lower cutoff also affects it as well). > >Perhaps I am using the wrong term? A larger driver will have more >mass (other things being equal), and will be more sluggish getting >started and stopped. I suspect that several 10-12" drivers would >sound better than a single large one (18-24-40") for this reason. > In fact, if the response time ("sluggishness gettin started and stopped") is very slow, the upper bandwidth of the driver WILL be severely limited. If you have three woofers, an 8 inch, a 12 inch and a 65 foot, and ALL of them can respond linearly to 200Hz, then imposing the 200 Hz upper limit will make their response time quit similar, regardless of what their unfiltered response time is. My memory may not serve me correct, but I recall the relationship between rise time and bandwidth to be something like: Tr = k/F where Tr is the rise time, F is the upper bandwidth (usually measured at the -3db point, and k is some fool constant I remember as being .35 or something. The exact value here is unimportant, the point is the intimate relationship between bandwidth and response time. Note that the equation is for simple critically damped response, but the point remains the same, bandwidth and transient response are intimately related. In fact, one can be viewed as a transform of the other. giving our three examples some hypothetical numbers, and my (possibly) erroneous constant, we might find: UNIT BANDWIDTH Tr ---- --------- -- 8" woof 2000Hz .2 mSec 12" woof 1000Hz .4 mSec 65' woof 500Hz .8 mSec Well, you say, obviously the 8 inch woof will make a better system because it's MUCH faster. Not so, the same drivers in our 200 Hz system will now have the following (approximate) characteristics: 8" woof 200Hz 2.0 mSec 12" woof 200Hz 2.0 mSec 65' woof 200Hz 2.0 mSec These figures are approximate because there will be a slight additional delay because of the fact that the characteristics of the drivers bandwidth does interact somewhat with the crossovers, but the steeper the corssover cutoff, the more dominent it is in determining transient response. (For those reeady to flame, yes I have made some simplifying assumptions here, but the basic relation ship of transient response and bandwidth still hold true) In fact, you might try the following experiment. Take a tweeter, whose response extends from 1 Khz to 20 Khz, and limit it's bandwidth to say 3 kHz. Now, compare it to the 8" woofer above. Which will have better transient response? Which will be "faster"? Which will be slower? Well, the answer is that the 8 inch will be just as fast as the tweeter, but over all, it's transient response will be significantly better. Why? The tweeter has a bandwidth of 1 kHz to 3 kHz, whereas the 8" has a bandwidth of maybe 50Hz to the same 3kHz. It's transient response (more specifically step response) will be vastly superior to the tweeter's, but it will be no faster or slower. >>Well, let's note that the poster lives in a small dog house, so bass >>really isn't an issue :-) > >Perhaps you have forgotten that the visible doghouse is merely a shelter >for the entrance to the underground mansion. :-) > No, I haven't, but I thought I might have some fun anyway. Dick Pierce
rdp@teddy.UUCP (10/21/85)
In article <1459@teddy.UUCP> rdp@teddy.UUCP (Richard D. Pierce) writes: > >(For those reeady to flame, yes I have made some simplifying assumptions >here, but the basic relation ship of transient response and bandwidth >still hold true) > Well, for those still ready to flame, I have researched the subject a bit and have come up with some more defining descriptions of the slow-woofer nonsense. I had, in a previous article, made a hypothetical table of woofers/bandwidths and rise times that looked like this: MODEL BANDWIDTH RISE TIME ----- --------- --------- 8" woofer 2000 Hz ~.2 mSec 12" woofer 1000 Hz ~.4 mSec 65' woofer 500 Hz ~.8 mSec Using this, I then stated that crossing these each out at 200 Hz will effectively give them the same rise times, or "speed". This conclusion is essentially correct. However, in the interest of presenting more accurate and verifiable information, I looked up specific references and have found the following: The total rise time of the system is proportional to the square root of the sum of the squares of rise times of each element in the circuit. As taken from "Electric Designers Handbook", edited by Plandee, Davis, et al, pp. 13-49: 2 2 2 1/2 Tr = (tr + tr + ... tr ) 1 2 n This means that the second table of rise times (those rise times which result from a 200Hz crossover applied to each woofer) can be modified as follows: MODEL BANDWIDTH RISE TIME ----- --------- --------- 8" woofer 200Hz 2.00997 mSec 12" woofer 200Hz 2.03961 mSec 65' woofer 200Hz 2.15401 mSec The point remains the same. As long as the bandwidth of the driver is than the crossover allows it to be, then the rise time is determined by the crossover. "slow" woofers, by definition, have not the bandwidth of "fast" woofers. There is no such thing as a slow, wide bandwidth driver. As for other potential flames, no I do not know who sells 65' woofers. Nor will I reveal this information if I mistakenly stumble across it. Maybe the saga of the Hartley 24" subwoofer would be good material for the "Audio Anecdote of the <interval>? Dick Pierce
tommie@psivax.UUCP (Tom Levin) (10/23/85)
In article <1556@hammer.UUCP> tekecs!doghouse.TEK!snoopy writes: >Before everyone runs off and builds subwoofers with two 8" drivers, >I'd like to point out that this isn't going to be much of a subwoofer. Ahem...the audio-pro B2-50 subwoofer uses (2) 5-1/4" drivers and I believe it is fairly flat down to 20Hz. A friend of mine designed a subwoofer based on the audio-pro idea. He used 2-6" phillips drivers in acoustically isolated, ported, cabinets. One driver faces inward and the other faces outward. They are connected electrically by a motional feedback circuit. The woofer has it's own mono amplifier. My friend measured the frequency responce as down 3db at 20Hz. It sounds VERY GOOD! Go listen to any of the audio- pro subwoofers. You might be surprised at the bass you can get from a moderately sized driver. -- __________________________________________________________________________ Tom Levin {ttidca|sdcrdcf|logico|scgvaxd|bellcore}!psivax!tommie You've seen those car stickers that say "Baby on Board"??? _________ I want one that says: / \ / Psychotic \ / On \ \ Board / \___________/