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post #1 of 12 Old 11-04-10, 06:22 PM Thread Starter
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Question Speaker Impedance Wiring and Crossovers

Hi all,

Most of my experience building speakers comes in the form of the many different subwoofers that have spent time in the back of my vehicles. I've been installing car systems for years, but the only things that require any sort of thinking are tuning the subwoofer enclosure and setting up the amps with correct gain and crossover settings. I think it's all oodles of fun, but I thought that I could apply some of the talent to building a good-sounding set of speakers for the the home.

I have an old-school home theater setup with less-expensive gear: a 12 years-old Sherwood receiver with Pro-Logic II, KLH 3-way towers with 15" woofers, a KLH bookshelf for the center channel, and 2 small Pioneer bookshelf speakers for surrounds. It's alright and will shake the light fixtures, but the system is lacking.

To restrain myself from divulging ALL of my HT plans (like upgrading the receiver, adding a powered sub, etc...), I will start with something that is confusing me (but shouldn't) that ya'll, I'm sure, will be able to straighten out. It goes a little something like this...

Whenever I hooked up multiple subwoofers or DVC subs in a car, I had to wire them in series, parallel, or series-parallel to get the right impedance for my amp. But when selecting components for HT speakers (LCR, Surround) things don't seem as easy I thought they were.

First of all, if I were building a 2-way system and were to wire (2) four ohm speakers (a woofer and a tweeter), I would expect to get 8 ohms if wired in series, 2 ohms if wired in parallel. But whilst dissecting my bookshelf speakers I came across my first confusion. That is, the total load for the speakers were 8 ohms (well, 7.4 with the old multi-meter, but I think they normally read a little low for speaker impedance). Yet, when I pulled things apart, both the woofer and tweeter were 8 ohms each. So, in my brain, that tells me that the total impedance should be 4 ohms since the + terminals of each speaker were wired to the + spring terminal and the negative terminals likewise. I thought that I read somewhere that if the crossover point is the same for the tweeter and woofer, impedance stays the same. But the one set only had a 1st order crossover on the tweeter - no coil on the woofer. The other bookshelf had a 2 caps and a coil, meaning...? I forget... So why is the impedance the same? Is it because the tweeter was not wired directly to the woofer, then to the source? I am obviously missing something here.... Please explain...

Secondly, a while back, I ordered some 5.25" woofers and 1" tweeters to build some bookshelf/satellites out of. They are each 4 ohms. Originally, I was going to wire them in series to get an 8 ohm load, which what most HT receivers like. Then, I realized that it would be somewhat difficult to wire them in series and cross over the woofer since I have to wire the negative terminal of the one to the positive terminal of the other. How do I physically wire crossovers (I was thinking of using 2nd order's on both woofer and tweeter, or 2nd order tweeter so that I could set the frequency a little lower and 1st order on the woofer), especially if the speakers are run in series to up the impedance to 8 ohms?

Third, I was toying with the idea of using 2 woofers and 1 tweeter per speaker in an MTM design similar to some of the designs on partsexpress and other places. But I noticed that they use (2) 8 ohm woofers wired in parallel crossed over before getting to the speaker so you only need one crossover; and (1) 4 ohm tweeter. But that means they would have to wire them in series to get back to 8 ohms and my dilemma rests with my second question above. My third question is really about the necessity of having the same impedance for each driver - is it necessary? Even if the effective impedance of the woofers is 4 ohms, the power will be split between the two of them, right? So you would need twice as much power to drive the woofer to the same excursion and spl as single woofer. But since there are twice as many woofers, you get the same spl? And the fact that the woofers are 8 ohms each doesn't throw a wrench in to it? What if I were to wire (2) 4 ohm woofers and (1) 8 ohm tweeter to get a 4 ohm load? Would that shape my sound differently or cause any other issues? And how would I magically use (2) 4 ohm woofers and an 8 ohm tweeter to get an 8 ohm load like in question 1? That would be ideal since I would only need to get 3 more tweeters of the 8 ohm variety and 2 woofers to make a 5 channel surround system. Is it taboo to use drivers with different impedances? If so, how to you set up a 3-ways system without 12 ohm speakers?

Fourthly, I understand that crossover digrams are just that - diagrams... But it looks to me like the woofer and tweeter are always wired in parallel, which tells me that the center channel design for Dr. K's MTM on the partsexpress website would really run in 2 ohms (8 ohm + 8 ohm woofers = 4 ohms, 4 ohms woofers + 4 ohms tweeter = 2 ohms...). So I guess this goes back to my question about the physical wiring of the drivers and setting up a crossover on a series connection...

In my particular case, I have 6 woofers and 3 tweeters. I was originally going to build 2-ways, but they aren't loud enough for me, so I decided on an MTM design with the possibility of a HPF around 60-80hz on the MTM's (aside from the above-mentioned crossover), plus a 12" powered sub tuned ~20Hz with a 300 watt bash plate amp for low-end duties. I prefer to use the same woofers and tweeters for the front sound stage to match the sonic signature up front, but if I just need to ditch the current plan and start over from scratch... well, I'll have to study up on my wife-e-nomics to convince the powers that be that it would be a "smart decision"...

Sorry for the novel, but I'm transitioning my mode of thinking. Pardon my noob-ness... I have plenty more questions for anyone who get's bored. But for now, this will suffice. Thanks!

Last edited by Blenton; 11-04-10 at 07:13 PM.
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post #2 of 12 Old 11-04-10, 07:28 PM
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Re: Speaker Impedance Wiring and Crossovers

Blenton,

Where do I begin to help you? First off, if you measure with an ohmmeter, you are measuring resistance (also known as DC resistance, or DCR). You are right on one point. if you place two eight ohms in parallel, it's 4 ohms. Two eight ohms in series is 16. Now it happens that speakers are rated in a nominal impedance. Let me attempt to explain:

AC current flow is constantly changing, as is your audio signal. While your speaker has a DC resistive component, the coil is formed into an inductor, which 'impedes' the inrush of CURRENT when a swiftly rising signal is applied. The turns are in close proximity separated by insulation, which also form a capacitor, which 'impedes' the rising of that VOLTAGE.

Without introducing you to ELI the ICE man, take it at this point on faith that these effects interact and change with frequency. This complex opposition to alternating current is known as impedance, and is different from resistance in that it has three components. You might have seen an impedance curve if you have shopped for woofers. The published impedance for a speaker is usually a nominal value.

Get ready for the next piece. The art and science of putting together multiple driver speakers involves crossover networks that add resistance, capacitance and inductance to intentionally direct certain frequencies to each component, hopefully giving both a leveled frequency response, and at the same time, presenting a relatively constant load to the amplifier.

If you really want to understand all of this, a good book from the library on introductory AC circuits should get you on your way. Then dive into some of the many volumes about speaker design.

Luckily, there are a a lot of calculators on the web to crunch a lot of complex math for you, that will let you build some workable units. But, when the result is 'almost' what you want, a good understanding of what's going on will help you make informed tweaks.

Have a good time as you pick up the bits and pieces of knowledge that make life fun.

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post #3 of 12 Old 11-04-10, 08:25 PM Thread Starter
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Re: Speaker Impedance Wiring and Crossovers

Thanks for the reply, torceador. Nominal impedance vs. actual impedance makes sense - hence, the impedance curves for the drivers. So is there a quick and dirty way to guesstimate the nominal impedance of the system before adding the crossover network? Or is that like asking the gender of the baby before it's conceived? I know that a Zobel network is sometimes used to flatten out the impedance curve the drivers (not that I've ever built one...); so does the crossover often act as a Zobel network, or do I need to work one in to the crossover? Or does it just turn out with the desired impedance when you add capacitors, inductors, and resistors?

I think that if, for this project, I get my hands on a set of 8 ohm tweeters (I have 4 ohmers), it will be easier to calculate things in long run. Does that sound like a reasonable idea? (using (2) 4 ohm woofers and (1) 8 ohm tweeter to build an 8 ohm speaker?) Or should I just 'run what I brung' and compensate for the impedance with the network?

I haven't really studied electrical theory - just part of an upper-level physics class before I changed degrees from mechanical engineering. Though I'm no mathematician, those calculus and linear algebra classes better be good for something in the end... I've got a copy of The Loudspeaker Design Cookbook by Vance Dickason kickin' around my place, but I started reading it a few years ago in the middle of my schooling. So it got trumped by other reading material.

I'll see what I can dig up for AC circuits and try to find some decent freeware to start me on my way...
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post #4 of 12 Old 11-05-10, 08:30 AM
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Re: Speaker Impedance Wiring and Crossovers

Blenton,

Wow, for not really studying electronics, to mention a Zobel network (also look up Boucherot cell on the same topic), you show a desire to go deeper. A Zobel network is indeed to match impedance, (used a lot by the phone companies) but with everything in speaker building, there are multiple things to balance. You could add passive components to a speaker system until it looked like only a resistor. That would satisfy the amplifier and guarantee maximum transfer of power, but where would that power go? Would it be wasted in the passive components, or would it produce sound? The inevitable sacrifice of adding passive components is adding lead and lag to currents and voltages actually ending up at the terminals of the drivers themselves.

Luckily, we are the beneficiaries of many millions of hours of crunching numbers and 'what if?' calculations on slide rules, RPN calculators, spreadsheets, and now JAVA apps that have all been distilled down do digestible bites that can be managed by the hobbiest.

I can't write a book on the forum, but ask away on specifics and I'll help where I can.

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post #5 of 12 Old 11-05-10, 09:02 AM
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Re: Speaker Impedance Wiring and Crossovers

Blenton,

Sorry, but one thing I meant to tell you and forgot. When you ask about what you can do with the speaker components you have and how they could be assembled, make sure and state where you bought them, and what part number. If any Thiele-Small (T/S) parameters have been supplied, list them. I had this stuff painfully explained to me by Richard Heyser, along with Don and Carolyn Davis in the 70's when it was new and confusing. Now days, it's a fast track to speaker design and you'll get a lot of help if you get specs out on the table. There have been thousands of good and bad DIY designs executed using tools based on T/S parameters, some rivaling the big boy toys.

After the AC circuits introduction, don't forget to add Acoustics and Electroacoustics to your reading plan. If you struggled through Laplace and Fourier like I did, rejoice! You can use it in audio all day long to toggle between time and frequency domains.

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post #6 of 12 Old 11-05-10, 12:22 PM
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Re: Speaker Impedance Wiring and Crossovers

You've jumped into speaker design without any background, so things will be very confusing. The normal best advice for newbies is to build a proven design, that is, one where a knowledgable designer has characeriaced drivers and designed a custom network to achieve good performance.

If you want to use the drivers you have, there are 3 options:
- learn speaker design, get a measurement rig, characterize drivers and design your XO
- ask on other forums if anyone's used these drivers before; maybe someone's already done the design
- use what little you know about the drivers to pick an off-the-shelf XO and live with the (likely poor) results

The first can be learned by following links here (start with explanatory links at teh top):
http://www.pvconsultants.com/audio/frdgroup.htm

The second can be best done on the Parts Express or HTGuide forums as they do a lot more speaker design. You know enough to avoid the third.

Have fun,
Frank
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post #7 of 12 Old 11-09-10, 06:25 PM
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Re: Speaker Impedance Wiring and Crossovers

What's involved with diy speakers? https://sites.google.com/site/undefi...-provendesigns

Otherwise go with something already designed...http://techtalk.parts-express.com/sh...82&PID=2777698

Michael
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post #8 of 12 Old 11-09-10, 08:37 PM Thread Starter
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Re: Speaker Impedance Wiring and Crossovers

Blast... I posted a reply this weekend, but it looks like it didn't show up. Anyways, torceador - thanks for straightening me out with the Zobel network. It has come up in my reading every now and again, but I was hesitant to try and implement one, and it sounds like for good reason. I would just be wasting power pouring it in to the network and not in to the speakers.

Here is some information about and the T/S parameters for the speaker I've got. I purchased them all from madisound.
I have (6) Silver Flute W14RC25-S 5 1/2" Shielded wool cone woofers, 4 ohms each. The specs are:

5-1/2"
Wool Cone
Cast Frame
25mm VC
Vented Pole Piece
Rubber Surround
F3 of 52Hz in 0.35 ft3 ported enclosure

Re 3.7 Ohms
Sensitivity 89 dB
Power 60 watts
Fs 48.2 Hz
Qms 4.06
Qes 0.46
Qts 0.41
Vas 13.95 Ltrs (.4926 ft^3)
Mms 8.41 g
Cms 1297.9 mM/N
Sd 0.0087 M2 (.0936 ft, 13.48 in)
BL 4.53 TM
X-max 3.5 mm
Le at 1kHz 0.49 mH
Frame 142 mm
Cutout 122 mm
Depth 75 mm
Krm 1.100 mW
Kxm 7.405 mH
Erm 0.819
Exm 0.690
Box Rec.: 10 ltrs, 1.5" V x 4.4" L

Whilst shopping for something to start with 2 years I picked these out, but had to put them on the back shelf due to my schooling. The reason I chose them was (among other things) an F3 of 52 hz in a smallish ported enclosure, which is about as low as I'd ever want them to go. I've got a 12" powered sub in the works to replace my 15" towers from high school, so they don't need to go super-low. But 52 hz sounds about right to me. The Qts value of .41 denotes that it is best suited for a ported box, and I prefer the sound of ported enclosures (that extra octave down low). Granted, I have to make sure they aren't driven super low or the woofer will unload and, well, I guess what ensues next could be termed 'fun'... The woofer also has a 3.5mm xmax (yeah, nothing compared to a bigger woofer) means they will be able to move some air and be considered almost full-range speakers (when combined with the tweeter). I don't like the smaller separates/satellites because they sound too tinny and hollow to me and often have troubles reproducing deeper voices, let alone sound effects or music. So I'm trying to make some full-bodies sounding speakers. Also, if I remember correctly, the speaker had a frequency response from 45 - 5000hz.

The tweeters (of which I have 3) are Vifa DX19TD05-04 3/4" silk dome (I think) tweeters, 4 ohms. Specs are:
Fres: 834,77 Hz
Qm : 2,47
Qe: 0,9
Qt : 0,66
Vas : 0 Liter
Sd : 4,55 cm2
B*l : 1,89 T*m
Rm : 0,47 Ns/m
Re : 2,8 ohm
Md : 0,22 gram
SPL : 89 dB 2,83V/1m (B&K 2012

The SPL matches on both the tweeter and the woofer, which would help to keep one from overpowering the other. Also, a graph of the tweeter's response shows a flat response down to 1khz and a -3db cut off of ~700hz. yeah, crazy low for a tweeter. So my reasoning was that to blend the tweeter and woofer, I would have 2 octaves (1khz - 2khz, 2khz - 4khz) to use without stressing the components. If I set the xover point at ~2khz and used a second order filter (at least on the tweeter, maybe just a first order on the woofer), I thought that they would work well together.

Originally, I figured that since 4 ohms + 4 ohms = 8 ohms from wiring subwoofers, I should order each speaker in 4 ohms. Then, as mentioned in my first post, I found out that most 8 ohm nominal bookshelf speakers consisted of 2 8 ohm drivers. But one of the designs that I've been looking at recently is on parts express - Dr. K's MTM speakers. They use 2 8 ohm woofers and 1 4 ohm tweeter. So could I get away with using 2 4 ohm woofers and 1 8 ohm tweeter to achieve the desired 8 ohm nominal load? That would necessitate me finding some more tweeters, but I'm sure I could find uses for the tweeters that I've got.

fbov and buggers - unfortunately I suffer from hardened-brain-case-syndrome, which means I have to do things the hard way. I just have this thing about making something my own - even if it is as simple as slapping a sticker on my bumper that fits my personality so that my car is just a tad bit different from all the others the road... I figured this would be a good learning experience (hence, the 2 years it's already taken me to leap from tuning the port on my car subwoofer down to <30hz (cars don't need much more than that) to building a set of LCR speakers. I'm not saying I have to come up with some radical new, wild and crazy design. In fact, Dr. K's MTM's look just right to me. So while working with something already designed, I can figure things out as I go, too.

I've got a copy of some speaker design software on my machine (I can't remember what it is exactly since I have to boot up my Windows VM to run it), so I'm working on learning the program. Thanks for all the advice - keep it coming!
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post #9 of 12 Old 11-10-10, 04:03 PM Thread Starter
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Re: Speaker Impedance Wiring and Crossovers

My apologies if this get's posted multiple times...

I posted a reply this weekend, but it looks like it didn't show up. Anyways, torceador - thanks for straightening me out with the Zobel network. It has come up in my reading every now and again, but I was hesitant to try and implement one, and it sounds like for good reason. I would just be wasting power pouring it in to the network and not in to the speakers.

Here is some information about and the T/S parameters for the speaker I've got. I purchased them all from madisound.
I have (6) Silver Flute W14RC25-S 5 1/2" Shielded wool cone woofers, 4 ohms each. The specs are:

5-1/2"
Wool Cone
Cast Frame
25mm VC
Vented Pole Piece
Rubber Surround
F3 of 52Hz in 0.35 ft3 ported enclosure

Re 3.7 Ohms
Sensitivity 89 dB
Power 60 watts
Fs 48.2 Hz
Qms 4.06
Qes 0.46
Qts 0.41
Vas 13.95 Ltrs (.4926 ft^3)
Mms 8.41 g
Cms 1297.9 mM/N
Sd 0.0087 M2 (.0936 ft, 13.48 in)
BL 4.53 TM
X-max 3.5 mm
Le at 1kHz 0.49 mH
Frame 142 mm
Cutout 122 mm
Depth 75 mm
Krm 1.100 mW
Kxm 7.405 mH
Erm 0.819
Exm 0.690
Box Rec.: 10 ltrs, 1.5" V x 4.4" L

Whilst shopping for something to start with 2 years I picked these out, but had to put them on the back shelf due to my schooling. The reason I chose them was (among other things) an F3 of 52 hz in a smallish ported enclosure, which is about as low as I'd ever want them to go. I've got a 12" powered sub in the works to replace my 15" towers from high school, so they don't need to go super-low. But 52 hz sounds about right to me. The Qts value of .41 denotes that it is best suited for a ported box, and I prefer the sound of ported enclosures (that extra octave down low). Granted, I have to make sure they aren't driven super low or the woofer will unload and, well, I guess what ensues next could be termed 'fun'... The woofer also has a 3.5mm xmax (yeah, nothing compared to a bigger woofer) means they will be able to move some air and be considered almost full-range speakers (when combined with the tweeter). I don't like the smaller separates/satellites because they sound too tinny and hollow to me and often have troubles reproducing deeper voices, let alone sound effects or music. So I'm trying to make some full-bodies sounding speakers. Also, if I remember correctly, the speaker had a frequency response from 45 - 5000hz.

The tweeters (of which I have 3) are Vifa DX19TD05-04 3/4" silk dome (I think) tweeters, 4 ohms. Specs are:
Fres: 834,77 Hz
Qm : 2,47
Qe: 0,9
Qt : 0,66
Vas : 0 Liter
Sd : 4,55 cm2
B*l : 1,89 T*m
Rm : 0,47 Ns/m
Re : 2,8 ohm
Md : 0,22 gram
SPL : 89 dB 2,83V/1m (B&K 2012

The SPL matches on both the tweeter and the woofer, which would help to keep one from overpowering the other. Also, a graph of the tweeter's response shows a flat response down to 1khz and a -3db cut off of ~700hz. yeah, crazy low for a tweeter. So my reasoning was that to blend the tweeter and woofer, I would have 2 octaves (1khz - 2khz, 2khz - 4khz) to use without stressing the components. If I set the xover point at ~2khz and used a second order filter (at least on the tweeter, maybe just a first order on the woofer), I thought that they would work well together.

Originally, I figured that since 4 ohms + 4 ohms = 8 ohms from wiring subwoofers, I should order each speaker in 4 ohms. Then, as mentioned in my first post, I found out that most 8 ohm nominal bookshelf speakers consisted of 2 8 ohm drivers. But one of the designs that I've been looking at recently is on parts express - Dr. K's MTM speakers. They use 2 8 ohm woofers and 1 4 ohm tweeter. So could I get away with using 2 4 ohm woofers and 1 8 ohm tweeter to achieve the desired 8 ohm nominal load? That would necessitate me finding some more tweeters, but I'm sure I could find uses for the tweeters that I've got.

fbov and buggers - unfortunately I suffer from hardened-brain-case-syndrome, which means I have to do things the hard way. I just have this thing about making something my own - even if it is as simple as slapping a sticker on my bumper that fits my personality so that my car is just a tad bit different from all the others the road... I figured this would be a good learning experience (hence, the 2 years it's already taken me to leap from tuning the port on my car subwoofer down to <30hz (cars don't need much more than that) to building a set of LCR speakers. I'm not saying I have to come up with some radical new, wild and crazy design. In fact, Dr. K's MTM's look just right to me. So while working with something already designed, I can figure things out as I go, too.

I've got a copy of some speaker design software on my machine (I can't remember what it is exactly since I have to boot up my Windows VM to run it), so I'm working on learning the program. Thanks for all the advice - keep it coming!
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post #10 of 12 Old 11-24-10, 03:50 PM Thread Starter
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Re: Speaker Impedance Wiring and Crossovers

So after reading through the Loudspeaker Design Cookbook, I think I've got a game plan to turn these speakers in to LCR's... First of all, I don't have any measuring equipment (yet), though I'm sure eventually down the road I will end up with some So I will have to do the math and then listen for dips and peaks in the response. Here's what I'm thinking:

I'll ditch the Vifa tweeters (and use them on another project), which saddens me a bit since I was excited to listen to them. Instead, I'll purchase (3) Dayton DC28FS-8 1-1/8" Silk Dome Shielded Tweeters. They are rated at 8 ohms nominal (5.6 actual) and match the SPL of the woofers. I'll keep the Silver Flute woofers and wire 2 of them in series for an 8 ohm load. That way, both the woofer and tweeter portion of the speakers will be 8 ohms each.

I want to use a Linkwitz-Riley alignment for the crossover so that the slope will be steep enough to protect the drivers, I'll have a flat sum at the xover point, and have a 180 degree phase shift which can easily be compensated for by reversing polarity of the tweeter. Since the Fs of the tweeter is ~800hz, two octaves above that would be 3200 hz. But the woofer has a nice little 4 db peak in response starting just below 5kHz, so I'm thinking 2500 hz (an octave below the start of the peak) will make a good xover point for this combination.

Using the L/R alignment for woofers at 7.4 ohms (Re = 3.7 *2) and the tweeter at 5.6 ohms, my components turn out to be C1 = 5.69 uF, L1 = .71mH, C2 = 4.2 uF, and L2 = .94 mH. Unless I decided to wind my own coils, I'll just have to match them to the closest components available.

3 questions I have about this setup... 1) How do I calculate the inductance of the two woofers wired in series? Due to reactive rise of the woofer voice coil impedance, I've been told it's best to use a Zobel network (or conjugate filter, as Vance Dickason refers to it) to flatten the impedance curve when test equipment is not available. The capacitor is equal to Le/Re^2, and the resistor is 1.25 * Re for the no-brainer calculations, or C = 1/(2Pi*Freq*Re) and R = 1.25*Re. Since I'll be wiring the woofers in series, the Re value is 7.4 (2 * the RE each woofer, 2*3.7 = 7.4). But for the value of Le, do I just double the Le value of one woofer (which is .49mH) since I'm wiring them in series? If I remember correctly, inductance goes up in series and down in parallel. But by how much, I'm not sure.

Question 2) To compensate for time alignment of the woofer and tweeter if they are both flush mounted to the baffle, I can reverse the polarity of the tweeter? Does the mean that after reversing it once to put the speakers in phase after compensating for the 2nd order xover, do I reverse it AGAIN to fix the time alignment on the tweeter, making it normal polarity? I could just listen for a huge dip in the frequency response at the xover point after hooking everything up, but if ya'll have any 'rules of thumb', I'd appreciate them. Perhaps to unconfuse myself, I will built the enclosures so that the woofer protrudes from the box so that the drivers' acoustic centers will be aligned.

Question 3) Since I'll be using two woofers with 89 db sensitivity each, will combining them cause a shift in my reference efficiency causing them to over power the tweeter? Or will the fact that I have 2 drivers be cancelled out by the fact that the power that each receives will be divided in half since they share the power source? In the LDC, it mentioned something about this, but I thought it was total output that it was modeling. Meaning, if you have two woofers using the same source, in phase, they sum to +6db of the total volume or loudness (or whatever the technical term is, measure in dB) that a single one of them would have produced.

I know that this setup will only get me close to a flat frequency response across the board and may cause some weird things to happen that I won't likely be able to compensate for without the proper equipment, but close. As mentioned in early posts, I'll be building MTM's in .7 ft^3 ported boxes (2 separate chambers).

Last edited by Blenton; 11-24-10 at 04:06 PM.
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