Crossover design for multiple woofers

[Mike Menke]

Hello All,

I am new to building speakers, and am struggling a bit in the basics of crossover design, regardless of what order the design is.
I want to create a 3-way design that has four speakers. 1x tweeter, 1x mid driver, and 2x small woofers. To do this do I actually have to build a 4 way design with parallel low pass filters, or can I wire the two speakers in either series / parallel and then adjust the impedance accordingly in the calcs to find my crossover components.

i.e can I calculate for a 4ohm nominal impedance for two 8ohm woofers in parallel on a single low pass filter, or will that mess up the crossover point.

Apologies in advance, us mech E guys didn't pay attention in EE classes.

 

[everettT]

You might want to try a much less complicated design for your first project. Even seasoned diy guys topical dont tackle large 3 ways often.

 

[Mike Menke]

You might want to try a much less complicated design for your first project. Even seasoned diy guys topical dont tackle large 3 ways often.

My first project was a basic 3-way center channel speaker that turned out beautiful. I am not scared of something more complicated. worst thing I do is waste some money on some components, let the smoke out, but have fun learning something I didn't know before. I did take the time to look at the manufacturer specs and find actual impedance at my crossover points as well as adding l-pad attenuators to even out the SPL levels on the project I Just finished. Now I am addicted!!

 

[everettT]

Congrats its very rewarding. You might start here

https://www.amazon.com/Loudspeaker-Design-Cookbook-Vance-Dickason/dp/1882580478

 

[Mike Menke]

Congrats its very rewarding. You might start here

https://www.amazon.com/Loudspeaker-Design-Cookbook-Vance-Dickason/dp/1882580478

Purchased and on the way !

 

[Swerd]

Welcome to Audioholics Mike, especially to a DIY builder.

As mentioned above by everettT, a 3-way crossover is complicated. A 2-way is much more rewarding for a beginner. Your methods may have been sound on your previous 3-way center channel speaker, or you may have had beginner's luck.

To directly answer your question, wire two woofers (each one nominally 8 ohms) in parallel to achieve a nominal 4 ohms. Treat them equally while designing the crossovers. Don’t complicate things with a 3½-way or 4-way.

A hint: Variable L-pad attenuators may seem like a good idea, but they always seem to oxidize and fail soon. In the long run, it works better to do some trial and error with several different fixed value L-pad resistors until you like the balance across a crossover point. But this is among the last steps in making a good crossover.

How are you estimating the values of the crossover components? Are you using a crossover software application, or are you using text book type equations?

You do seem to understand that impedance in a speaker is not a constant value, it varies with the frequency. In the example below, a SEAS Prestige ER18RNX, 4 traces are shown for that 6½" woofer. Three of them are frequency response curves measured on-axis (dark trace) and two angles off-axis (probably 15° and 30°). The driver’s impedance is graphed below them and uses the impedance vertical axis on the right side. It's the curve with the large peak between 30 and 40 Hz.

The manufacturers’ impedance curves are usually measured with the driver mounted on a large open baffle, not mounted on a cabinet. Madisound suggests some cabinet alignments:
Sealed box of 0.25 ft³ for an F3 of 97Hz.
Vented box of 0.5 ft³ with a 2" diameter vent × 6" long for an F3 of 53Hz. (I'd go with the vented box.)

If you want to use two of these woofers, build a test box where the woofer chamber is 1.0 ft³, where the front baffle is wide enough (>9") to mount two woofers one above the other, with two vents 2"×6". Mount the woofers and measure their impedance wired in parallel in that cabinet.

A properly designed crossover takes the variable impedance data into account rather than assumes one constant value. This is why I recommend using a crossover software application. This will also require a measuring rig.

You should do similar frequency response and impedance measurements for the mid range and tweeter as they are mounted on your test cabinet. The cabinet will have a noticeable effect on their responses as well.

 

[Mike Menke]

Welcome to Audioholics Mike, especially to a DIY builder.

As mentioned above by everettT, a 3-way crossover is complicated. A 2-way is much more rewarding for a beginner. Your methods may have been sound on your previous 3-way center channel speaker, or you may have had beginner's luck.

To directly answer your question, wire two woofers (each one nominally 8 ohms) in parallel to achieve a nominal 4 ohms. Treat them equally while designing the crossovers. Don’t complicate things with a 3½-way or 4-way.

A hint: Variable L-pad attenuators may seem like a good idea, but they always seem to oxidize and fail soon. In the long run, it works better to do some trial and error with several different fixed value L-pad resistors until you like the balance across a crossover point. But this is among the last steps in making a good crossover.

How are you estimating the values of the crossover components? Are you using a crossover software application, or are you using text book type equations?

You do seem to understand that impedance in a speaker is not a constant value, it varies with the frequency. In the example below, a SEAS Prestige ER18RNX, 4 traces are shown for that 6½" woofer. Three of them are frequency response curves measured on-axis (dark trace) and two angles off-axis (probably 15° and 30°). The driver’s impedance is graphed below them and uses the impedance vertical axis on the right side. It's the curve with the large peak between 30 and 40 Hz.

The manufacturers’ impedance curves are usually measured with the driver mounted on a large open baffle, not mounted on a cabinet. Madisound suggests some cabinet alignments:
Sealed box of 0.25 ft³ for an F3 of 97Hz.
Vented box of 0.5 ft³ with a 2" diameter vent × 6" long for an F3 of 53Hz. (I'd go with the vented box.)

If you want to use two of these woofers, build a test box where the woofer chamber is 1.0 ft³, where the front baffle is wide enough (>9") to mount two woofers one above the other, with two vents 2"×6". Mount the woofers and measure their impedance wired in parallel in that cabinet.

A properly designed crossover takes the variable impedance data into account rather than assumes one constant value. This is why I recommend using a crossover software application. This will also require a measuring rig.

You should do similar frequency response and impedance measurements for the mid range and tweeter as they are mounted on your test cabinet. The cabinet will have a noticeable effect on their responses as well.

That info I certainly useful!! Thank you. I did use an online crossover calculator, but only after I was comfortable doing it manually and has an understanding of the different filter orders and methodologies. I used erseaudio to get my final values. I did also use fixed resistance for my l-pad, but have not done any trial and error to see if I can balance things out additionally.

 

[Swerd]

Do you have any drivers in mind for your 3-way design?

Avoid crossing the woofer-to-mid range below roughly 400 Hz. As the crossover frequency gets lower than that, inductor coils and capacitors get larger and much more expensive.

If you can find a 4" mid range driver that behaves nicely in the range of 400 to 4000 Hz, you might have a good speaker.

Crossovers with 2nd order slopes usually require very well behaved drivers that can cost more. A crossover with true acoustic 1st order slopes often ends up being very complex and expensive. In general, many people have found Linkwitz-Riley 4th order crossover slopes work well, and as a result, they often get recommended.

I did use an online crossover calculator, but only after I was comfortable doing it manually and has an understanding of the different filter orders and methodologies. I used erseaudio to get my final values.

Most, if not all, of the online crossover calculators assume a fixed value for impedance. If you can measure and record frequency and impedance while the audio frequencies are being swept, you will have better results.

Doing all this does involve some expense in the software, the measuring rig, and the time it takes to learn to use this stuff.

The best software can digitally simulate a functional crossover. If your computer has a dual-duplex sound card, you can run the simulated crossover through a test version of the speaker, hear and record the sound, and see the resulting frequency response curve on the computer screen at the same time. You can alter details of the crossover network and hear & see the results right away. So you can quickly do a lot of trial & error while sitting at the computer.

 

[Mike Menke]

I do have a few drivers in mind.

For tweeters I'm wanting Morel ET 338 1 1/8in
For mid, Faitalpro M5N8-80
Dual woofers using Aurum Cantus AC-200MKII 8"

All are 8ohm nominal.
The mods are a bit hot and will need a decent amount of attenuation, but that is what I used in my center channel and LOVE the sound.

Pardon the butchered post from mobile!

 

[Swerd]

I've heard some nice Morel tweeters (other less expensive models) in the distant past, but I'm not at all familiar with your midrange or woofers.

Tweeters Morel ET 338 1 1/8in

One thought. Those tweeters have a fairly low Fs of 700 Hz. That means they can easily crossover as low as 2-3 times the Fs, or 1400 to 2100 Hz. If you use LR4 crossover slopes, 1400-1500 Hz should be doable. In a 3-way you may not need tweeters that go that low. And if not, you can save some money with tweeters that don't have to go low. It depends on how high those Faital mids can go without break up.

For mid, Faitalpro M5N8-80

99 dB sensitivity . You're not kidding, kind of hot. From a quick look at their frequency response curve it looks like they can easily do 2000 Hz, but I'm not sure about 3000 Hz. Higher than 3000 looks iffy to me. That's what having a test rig is good for.

Dual woofers using Aurum Cantus AC-200MKII 8"

Those seem like they ought to cross to the mids at ~500 Hz without problem.

What kind of bass cabinet dimensions & vent size do you plan for two of those woofers?

Parts Express suggests a vented cabinet 0.22 ft³ with a vented F3 of 83 Hz. They don't say what vent size. Double the cabinet volume and vents for two woofers. But a vented F3 of 83 Hz seems rather high if these speakers have a resonant freq (Fs) of 25 Hz. I'd call PE Tech Support and ask about that. For two 8" woofers, I'd like to see an F3 at least as low as 30-35 Hz.

 

[Mike Menke]

To be honest, I have not gotten to trying to design the cabinet dimensions yet. I was hoping to sit down this weekend, start building the prototype of the crossover for each band and experiment with how I liked the cross points, padding, etc using just my ears. Once i was happy with the crossover design I was going to print the circuit design, etch some board and build them, then figure out a mock up of my box in some MDF. For my center channel prototype I made all the chambers larger than I was expecting final size to be and played around with adding scraps to reduce the volume. For the final box I built to the learned dimensions. I do think this go around I am going to get some testing equipment. I am sure my wife would Looooove me having more geeky tech toys around!!

 

[Swerd]

To be honest, I have not gotten to trying to design the cabinet dimensions yet. I was hoping to sit down this weekend, start building the prototype of the crossover for each band and experiment with how I liked the cross points, padding, etc using just my ears.

There's a good reason why you need to know cabinet dimensions, especially how wide the front baffle is, before you design the crossover. Depending on how wide the cabinet is, diffraction from the edges of the cabinet could affect the low end of the tweeter, or with a wider cabinet, as you might have, the mid range. The best way to account for this is to observe it when the drivers are mounted in the cabinet. The following explanation is copied from this page http://www.salksound.com/blogtopic.php?id=5

Here is the frequency response of a typical 1" dome tweeter, unfiltered, in an 8" wide cabinet. Its intended to be used with a 6½" woofer in a 2-way design.

"Things are in pretty good shape from 3,000 Hz on up. But notice the dip and peak below that point. All tweeters will have a similar profile in this box. As the sound waves at those frequencies reach the edges of the baffle, some will be reflected back and either cancel or augment new waves just leaving the tweeter. These 'diffraction effects' are also evident in the slight dips much further up. But what’s important is that even excellent tweeters will not have a smooth natural roll off in the types of cabinet baffles that are normally used."

That tweeter can easily go below ~1500 Hz, but in that cabinet, diffraction creates a peak at ~1700 Hz and a dip at ~2700. To get around that, use 4th order slopes crossed at ~2400 Hz.

Once i was happy with the crossover design I was going to print the circuit design, etch some board and build them, then figure out a mock up of my box in some MDF.

You're getting too far ahead of yourself.

First determine the volume and dimensions of the cabinet. Play with the T/S parameters of your woofers and online calculators, such as here http://www.mh-audio.nl/spk_calc.asp#mnu_drivers, until you are satisfied with the predicted bass response. The math for predicting cabinet volume and bass response works well. Keep total Q no higher than 0.7.

Once you have a cabinet design, build one (unfinished is fine), mount the drivers, and run the 3 pairs of wires out the port vent. Measure the frequency response and impedance of the woofers, mid range, and tweeter separately and unfiltered. To protect the tweeter from unfiltered low frequencies, use a capacitor ~5-10 µF in series with it. Then you can start designing the crossover.

 

[Swerd]

I could go on…

You've selected some expensive high performance drivers. And you seem to know some of what's needed to build a good sounding speaker. It would a shame to waste those drivers with a poor design.

 

[Mike Menke]

Swerd, you are right. The advice you are giving is exactly why I joined this forum. I certainly want to learn the right approach. I started reason the loudspeaker design book I picked up from parts-Xpresss tonight. Tons ! Of good info in there ( thanks everettT)

I will do some work on the box design this weekend rather than electrical design.

When I think I have a design I'll report back.

 

[everettT]

Swerd, you are right. The advice you are giving is exactly why I joined this forum. I certainly want to learn the right approach. I started reason the loudspeaker design book I picked up from parts-Xpresss tonight. Tons ! Of good info in there ( thanks everettT)

I will do some work on the box design this weekend rather than electrical design.

When I think I have a design I'll report back.

You might want to verify the T/S parameters. Even some high end drivers are considerably different than their published specs.

 

[everettT]

John Krutke's website has quite a few tested drivers and projects.

 

[Swerd]

John Krutke's website has quite a few tested drivers and projects.

Very good idea. @Mike Menke If Krutke's T/S data is different from that of the manufacturer, trust Krutke (Zaph Audio).

Aurum Cantus AC 200 MkII http://www.zaphaudio.com/8test/

Comments: Nice looking cast frame and carbon fiber sandwich cone. Relatively smooth response though the breakup is fierce at 4 kHz. Both test samples had a severe resonance between 600 and 700 Hz which limits this driver to usage in the bottom of a 3-way, preferably crossed LR4 below 400 Hz. This resonance is apparent in the harmonic distortion plot but not the frequency response. Harmonic distortion is relatively high above the problem frequencies anyway. Consistency is good, but the 600-700Hz issue lowers the value and usability ratings. Tested June 2009.

Compare Krutke's measured frequency response to Aurum Cantus's
http://www.zaphaudio.com/8test/compare.html

T/S Parameters - Krutke http://www.zaphaudio.com/8test/compare.html

T/S Parameters - Aurum Cantus
https://www.parts-express.com/pedocs/specs/296-420-aurum-cantus-ac-200mk2-specifications-45628.pdf

 

[Swerd]

@Mike Menke A good web forum for DIY speaker builders is the Parts Express Tech Talk Forum.

You'll find a lot more experienced DIY speaker builders there than here.

 

[Mike Menke]

Ok, so I have had some time to do some more research. I am going to use some drivers I already have and build a less expensive setup to test my abilities before I go buy expensive drivers.
I am going to use the following:
1x Morel Cat 308 Tweeter
1x FaitalPro M5N8-8
2x Dayton Audio DS175-8

I intended to build the size enclosures as compartments in an overall larger tower.

Woofer enclosure info

Box info for Mid

Crossover design with Zobel + L-Pad

Overall SPL curve

 

[Mike Menke]

Actually changed the crossover type and points to flatten the curve