efzauner Posted November 4, 2006 Share Posted November 4, 2006 Guys There is a debate going on in some of the Car Audio forums about putting subwoofers in series. The issue is putting same model subs in series either in same box or different identical boxes. Some manufacturers say never to put subs in series, although it is OK to put Double Voice Coils in series on the same sub. The reason give is that slight differences in the subs cause the back emf of one to modulat the other sub. I have been in audio hobby for 30 years and have never heard of this. I have searched for some technical based research or credible source to back this claim up and have not found anything. I see many high end speakers with 4 woofers that I presume are simply 8 ohm speakers in series/parallel config. I have not read any reviews on speakers where a manufacturer claims that putting woofers in series is bad. Do any of you know anything about this subject? Thanks.. Quote Link to comment Share on other sites More sharing options...
Guest " " Posted November 4, 2006 Share Posted November 4, 2006 Series meaning electrically in series and not acustically in series (multiple chambers)? Voice coils have impedance and inductance, when put in series, they sum, most folks factor this into design. You can avoid these issue sby using an autoformer such as the Atlas AF140, or Crown CT170 . The autoformer allows impedance to be stepped up or stepped down. Quote Link to comment Share on other sites More sharing options...
efzauner Posted November 4, 2006 Author Share Posted November 4, 2006 Speakerfritz Yes we all know that 8+8 =16. That was not the queston. The question was if there were any other factors, such as does one VC's back emf adversely affect the other's movement etc. It is mainly with woofers or subwoofers. Quote Link to comment Share on other sites More sharing options...
Al Klappenberger Posted November 4, 2006 Share Posted November 4, 2006 Guys, Two woofers in sereis would couple together since the amp connected between the two will look like a direct connection since it is very low impedance. So try it.. Just connect two drivers to each other (not amp) and tap on one cone see if you hear it in the other. I bet you will, slightly, but what would that hurt? I don't know. In parallel this could not happen, so to err on the side of caution, just connect them in parallel! Al k. Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 4, 2006 Share Posted November 4, 2006 The graph below is a SPICE simulation of two identical woofers in series. Each woofer is modeled using the standard equivalent circuit with lumped parameters determined from T/S parameters. The frequency range passes thru the Fs (~28Hz). THe voltage source is 10V max. amplitude. The top plot is the vc current and phase of the first woofer, the bottom plot is the second woofer. The two plots are identical. Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 4, 2006 Share Posted November 4, 2006 A series configuration. Again the vc current and phase of the current are identical BUT current is higher in both so the system is louder. Quote Link to comment Share on other sites More sharing options...
djk Posted November 4, 2006 Share Posted November 4, 2006 "tap on one cone ....but what would that hurt? " Doesn't mean anything, never happens in actual use. Series woofers must be identical, and they must be in the same box with a common air cavity. If you believe that high DF means something (I don't), then series will yield a DF of 4X higher than parallel. Quote Link to comment Share on other sites More sharing options...
efzauner Posted November 4, 2006 Author Share Posted November 4, 2006 Thanks for the responses. The Spice simulations are nice but what if you have slighlty different T/S parameters? Does your model allow for back emf caused by a mechanical excitation of one woofer by another? (BTW where did you get an equivalent eletrical model of the T/S paramters.. i tried finding it a while ago and gave up I would like to simulate that too! ) What I am trying to understand is this: to same model woofers, but with slightly different TS parameters because if slightly different mass, compliance winding etc. Just off by a few percent due to manufacturing tolerances of the winding, paper cone, rubber surround etc. What if a transient drum beat excites a 29hz resonance in one woofer and 31Hz resonance in the other. Will the emf caused by one coil resonating at 29 hz cause the other woofer's coil also to move at 29hz as well as its own resonance of 31 hz causing intermodulation. I am trying to find a problem that we normally overlook that made JL audio recommend not to put them in series. I did email JL and hope to get a response. I may be searching for something that does not exist, but the guys on the car audio forum are very stubborn and do not have the same level of background, tools, and experience as here, and tend to stick with their own ideas regardless of the fact because they do not have much technical education or knowledge. Many struggle with ohms law... Quote Link to comment Share on other sites More sharing options...
Al Klappenberger Posted November 5, 2006 Share Posted November 5, 2006 Guys, I don't think the SPICE models are relivant. I don't think they consdier the back EMF at all, just the factors that influence impedance. I don't think the coupling from driver to driver in a sereis configuration means anything either, not even if the amp has infinite damping factor and looks like a direct connection from driver to driver. The coupled back emf has no amplification and would be totally overwhelmed by the power from the amp. AL K. Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 5, 2006 Share Posted November 5, 2006 Thanks for the responses. The Spice simulations are nice but what if you have slighlty different T/S parameters? doesn't matter Does your model allow for back emf caused by a mechanical excitation of one woofer by another? SPICE preserves phase information, so answer is yes. I may be searching for something that does not exist, but the guys on the car audio forum are very stubborn and do not have the same level of background, tools, and experience as here, and tend to stick with their own ideas regardless of the fact because they do not have much technical education or knowledge. Many struggle with ohms law... Then, perhaps you've answered your own question(?). Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 5, 2006 Share Posted November 5, 2006 Al- The equivalent circuit considers the effects of moving mass and suspension effects reflected through the magnetic domain. It is a very accurate representation of what an amplifier "sees". I have a treatment on my website, if interested. In fact, I suspect that your software will work it nicely. jw Quote Link to comment Share on other sites More sharing options...
Duke Spinner Posted November 5, 2006 Share Posted November 5, 2006 tWhat if a transient drum beat excites a 29hz resonance in one woofer and 31Hz resonance in the other. Will the emf caused by one coil resonating at 29 hz cause the other woofer's coil also to move at 29hz as well as its own resonance of 31 hz causing intermodulation. I am trying to find a problem that we normally overlook that made JL audio recommend not to put them in series. I did email JL and hope to get a response. I may be searching for something that does not exist, JBL uses different woofers in the same box frequently some are in series Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 5, 2006 Share Posted November 5, 2006 If you examine the SPICE predictions of the impedance of two slightly different woofers in series and in parallel you get this graph below. The woofers are modelled using equivalent circuits. I do not have experimental data to compare it against. To Al's point, there is no "back-emf" elements in the model. But the equivalent circuit is a complete model of a loudspeaker in free air so why would additional elements be required? The arrows are the "in-circuit" impedance response of each driver (note that they are identical). The light green plots are the total impedance of each system. Assuming that the total impedance is verified by experiment, that would indicate that the individual driver responses are also accurate. Quote Link to comment Share on other sites More sharing options...
m8o Posted December 1, 2006 Share Posted December 1, 2006 From what I recall from a few dozen years back is I think it has to do with dampening factor of the driving circuit and how that affects the contol of the bass driver. One cone on the amp terminals sees the output impedance of the amp. Two cones in series causes the output impedance of the driving circuit to follow the curve of the other bass speaker. What -real- effect that has on the electro-mechanical properties of both cones and the acoustical effects it produces is something I can't speak about though other then I think that can result in more resonance when the speaker impedance is high. Quote Link to comment Share on other sites More sharing options...
brennyE Posted December 2, 2006 Share Posted December 2, 2006 I always thought that Back EMF was eventually overcome once overloaded? or does this only apply to a inductor being constantly fed power? Quote Link to comment Share on other sites More sharing options...
WMcD Posted December 2, 2006 Share Posted December 2, 2006 I believe John W. is absolutely correct. No surprise there. I've never seen it said in as few words but believe that this back emf theory is really there to say that a model using only a resistor is inaccurate because it does not consider the back emf created by inductors and capacitors. Once you include them, then you do have the back emf. It is the back emf which causes the variations in impedance we see in the L C R model. Some of this becomes a little more clear if we stick doggedly to the fact that impedance is not a single thing. Rather it is the voltage / current ratio which a source experiences or "sees" when trying to drive a load. If there is another source of voltage or current hidden in the load, the impedance seen will vary. The inductors and capacitors do short term supply energy just as the moving mass and squashed springs in the actual speaker do. And these can absorb energy too, which is where they get the energy in the first place. Sorry no diagrams. Let's work with just resistance and batteries. Suppose we have an 8 volt battery feeding an 8 ohm resistor. The current is one amp. Now suppose we put two 8 volt batteries in parallel to feed the same resistor. The second battery represents the back emf. Now each battery supplies 0.5 amps while still putting out 8 volts. But what is the impedance seen by each battery? Each is supplying 8 volts at 0.5 amps. By ohms law we get Rimpedance = 8 / 0.5 = 16 ohms. Gee wizz, the actual resistor hasn't changed in physical properties, but the second battery has altered what we call impedance seen by the first. We can equate that to an a.c. circuit in the bass driver at resonance. The mass is moving back and forth and the spring squashing and unsquashing. This mechanical resonance drives the voice coil and magnet motor to form a generator (the second battery). Therefore the impedance seen by the amp (first battery) increases. This is why we see an impedance peak at resonace in a raw driver or a driver in a sealled box. = = = It seem to me that the warning against series circuits really goes to those where the two speaker systems do not have identical parameters. Then things get very unpredictable. This is because the a.c. impedance curves do not match. Gil Quote Link to comment Share on other sites More sharing options...
Jay481985 Posted December 2, 2006 Share Posted December 2, 2006 well the thing that jumped out at me and that no one else really touched on for woofers was the fact that even though they may be "identical" and made from the same factory line, speakers just like any other tool is still inherit to the conditions at the factor and alot of other factors. That is why engine horsepower for cars is rated on the average of usually 5 different cars since horsepower and torque are a bit different from every engine/car. That is why certain speakers tweeters I have seen go for I believe 300 dollars more because they were tested and certified to be matched compared to the same line that was not matched. So can one dictate another, I think it can be possible. Quote Link to comment Share on other sites More sharing options...
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