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Question about crossovers and phase

J

judgeschreber

Well-Known Member
I am in the process of designing some crossovers (my first attempt) for a pair of speakers I recently acquired. They are going to be simple, 1st order crossovers. I have a neat little "virtual crossover box" that allows you to try out different value components and switch them around on the fly--a really cool tool. Using this in combination with some software to plot frequency response, I've come up with the crossover points, and it looks pretty good on paper and sounds pretty good with the virtual crossover box. However, I have a question about phase.

The virtual crossover box lets you switch the phase of the tweeter and midrange on the fly. With the tweet and mid in the "-" position, the sound is nice and open, but I can hear each driver distinctly, and the sound seems to be coming from three distinct points--the midrange in particular really pops out. When they are in the "+" position, the speaker sounds more constricted and a bit less pleasing, but it also sounds closer to a point-source. I cannot hear the distinct drivers, and the sound seems more coherent.

Does anyone have any insight into which is the correct way to wire the drivers? I assume that the more coherent and point-source-sounding it is, the closer it is to being in phase, but then why do the speakers then sound more constricted and stuffy? Could it be that my ears are just not properly adjusted, and that I am mistaking the out-of-phase interference patterns for spaciousness and openness of sound? Unfortunately I only have one virtual crossover box, so I can't test stereo imaging until I actually build the crossovers (parts coming early next week!).
 
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I presume the drivers are opperating with in an acceptable range and not straining per say. 1st order xover points do overlap considerably! If you used -12db per octave then phasing would be much more critical. You would want the mid out of phase in that case. How may octaves apart are xover point set? 4 octaves or more apart is preferred. Did you calculate the correct driver ohm values at the xover frequency point?
DC
 
You'll never get it "right" for all conditions.

With low-slope crossovers, there's major overlap of driver response in the crossover region(s). Horizontal displacement (front to rear, primarily) of the driver acoustic centers is also a significant factor. There are phase interference issues no matter the connection, and having the same absolute phase of all drivers may not be optimum.

The best compromise will be determined at normal listening distance and height, and an RTA analysis from there will show you immediately what's going on with respect to phase. Even then, however, the findings may not be definitive, in which case you'll just have to go with what sounds best to you. Move the speakers or listen from another location, and the results will likely be different.

It's one reason first order has largely been abandoned. The woofer-mid transition is more significant than mid/high, typically. Install phase inversion switches on the mid and high drivers, and diddle with them forever, maybe.... :p:
 
you did not state the type of cab....direct radition...horns and folded horns...domes...panel technology...etc, there is an implied requirement to invert the phasing as the path lenth increases relative to the path lenth of other drivers....example...woofers in folded horns...mids/tweets directly aligned.

In lower order xovers, the need to invert phasing is rare.
 
"and that I am mistaking the out-of-phase interference patterns for spaciousness and openness of sound?"

That's quite possible, when two loudspeakers are wired out of polarity they make a diffuse sound, when wired in polarity the image will snap back. You can observe this effect with some home theater set up discs.

I was gonna look in Dickason and see what he has to say but then I had to go install a ceiling fan in my wife's sewing room. Everything that could go wrong did (the people that call themselves engineers nowadays) and now I'm tired and pissed and Dickason has to wait. ;)
 
Thanks for the advice everyone. I guess it's not as exact a science as I had thought. From what you guys are saying, thought, it looks like the mids and tweeters do not need to be wired out of phase.

As for the type of cab, it is an older GE cab that looks much like an EV Aristocrat. It is closed in back, and has a distributed port--about a dozen or so 1" holes in the baffle. The woofer and mid are cone drivers (a Jensen alnico 12" and some 5" whose name escapes me), and the tweeter is a small Olson horn with about 2" diameter circular opening and 2" deep.
 
Tom: what you describe about the image "snapping back" is exactly what I heard when I changed the polarity back and forth. Thanks--now I think I know better what to listen for.

Maybe since I was only listening to one speaker in mono, I thought the disperse sound of the drivers wired out-of-phase was nicer and fuller, and the tighter, focused image seemed a little dull. Hopefully this will be rectified when I listen in stereo.
 
Hey Tom,
Have you seen that subwoofer which is a fan?You could kill two birds with one stone!:D
 
Out of curiosity - how is the mid mounted? Does it have a separate box around it to prevent interaction with the woofer?
 
The mid isn't isolated. That's actually something I've been thinking about modifying. Would there be any reason NOT to isolate the mid, or is it always a pretty good idea?
 
If the mid and woofer aren't isolated, they act as pressure driven radiators for each other. The mid especially will feel the effect of the bass from the woofer, somewhat defeating the idea of the crossover to isolate the high-excursion bass frequencies from the mid. With this set up, the analysis and design is really complex, so I'm not surprised that you aren't sure which phasing sounds better. If out of phase, the mid assists the movement of the woofer, which is equivalent to having a bigger box. At the same time, the cones are radiating out of phase into the room. Driving in-phase, you have the opposite effect. Either way tunes the bass differently from an isolated box. Not sure if the woofer has such a great effect on the frequency response of the mid, but it could cause some intermodulation distortion at high levels, I think.

[warning - working from limited knowledge here - happy to hear comments from others who have experience with un-isolated vs. isolated set-ups]

Edit - I was a little too brief - the mid cone, if not driven, makes the box equivalently bigger for the woofer. When the mid is driven, you get an aiding or opposing effect on top of that, depending on the phasing and amount of drive and therefore depending on the crossover.
 
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Iolating the Mid/Woofer, please consider the bass bin volumn (FWIW) since you mentioned the cabinet was enclosed.
Please let us know your impressions of the mod if you decide to follow through.
I tried it with a Bozak 302A mid (also sealed) and discovered it to be a waste of time, throwing all of the positive design (smoothness) out the window. Internal bracing did tighten things up and improve things by a small margin.
Getting Mr. Tobin's XO's was a BIG Positive move.
Good Luck.
 
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If the mid is open back, it's essential that it be isolated. That means a small sub-enclosure behind the driver, which doesn't have to be a big deal. RTV a 5' or 6" deep heavy plastic flower pot onto the baffle behind it, or a closed-off length of Sonotube, and stuff it completely with fiberglass, except for enough room for the rear of the mid driver, of course. Seal the hole where your wires go through with RTV, as well. Gotta be absolutely sealed.

I have no experience with the distributed port thing, but it'd certainly be interesting if you'd measure the resultant box tuning. It's hard to know what would be optimum for that particular woofer without its T/S parameters, but knowing the actual result would give some indication as to whether the distributed port approach is actually working. :yes:
 
Hmmm, it's getting more and more complex....

Basically, these speakers were homebrews with really nice GE cabinets that I picked up from a yardsale. The person who constructed them was an audio hobbyist and retired electrical engineer, so I figured I wouldn't mess with the woofers, since I imagine he did his homework in tuning the ports, and I have no idea of what the T/S parameters are for the woofers (they are Jensen A12-PM, and I've been unable to find any solid info on them).

Seeing that the mids were never isolated, though, now I'm wondering how much to trust the original builder. So far here's what I'm planning to do: (1) Build the crossovers at 500 and 3500Hz crossover points, which work well on paper according to the measurements I did of each driver while in the box. Also, those points measured well when testing out the speaker with the virtual crossover box. (2) because I measured some peaks and valleys in common with both the mid and the woofer, I figure there may be some cabinet resonances, so I will be adding some bracing and damping material to the cabinets. (3) I will add a sub-enclosure to the mid.

So, how would I go about measuring the box tuning with the distributed ports? I, too, would like to know whether the distributed ports are working or not...
 
judgeschreber said:
So, how would I go about measuring the box tuning with the distributed ports? I, too, would like to know whether the distributed ports are working or not...
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http://audiokarma.org/forums/showthread.php?p=1145510#post1145510

You measure the woofer impedance in the box. There are two peaks in the bass region, below ~100 Hz, typical, one from the port resonance, and one from the driver resonance. A close approximation of the box tuning frequency is given by the point of minimum impedance between the two peaks.

Fb, the box frequency, can be more accurately determined, but the calculation method involves combining the above determinations with one made with the box closed, not a good option with your distributed ports. I guess you could cork all the holes for that, but it's probably not worth the effort. Fmin impedance will tell you the box tuning within a couple of Hz of actual, typically.

I believe your suspicions may be correct regarding the quality of design engineering incorporated in your speakers. Remember that much of the current science of loudspeaker application was unknown at the time they were built. Manufacturers did their designs empirically, with lots of experience using particular drivers. Homebrews were hit or miss, depending upon the available design resources. Both are frequently wrong, to varying degrees.

The Dickason book also describes how to measure the T/S parameters of your drivers, though that's a bit more of a chore....
 
Cool, Thanks!

I actually discovered that my MLS software can also measure impedance, and then extrapolate the T/S parameters too. So I guess I will make two measurements: with the woofer in free air to determine T/S, and with it in the enclosure to determine the box frequency. I assume that I should isolate the mid driver before taking impedance measurements, so that it won't act as a passive radiator, correct?
 
Is THIS the Jensen driver? I'm also thinking all of those small ports might act as a 3rd order port, which is the equivalent of a regular port stuffed full of soda straws. Also called a, "resistive" port. Not to be confused with an aperiodic. Crossovers get really complicated. If the woofer rolls of at 500 Hz. say, and you use a coil set for 500 Hz. then you are looking at a 3rd order acoustic crossover. It's the total acoustic roll off that has to be considered. Then there is the baffle radiation, and reflection, impedance of the drivers change with all of the frequencies etc. and the vertical and horizontal distances between the drivers. Yes sealing the midrange is a must. I'm surprised that it would even work after the beating it took in the woofer cabinet.

I wish I had better news! :)
 
Yup, that's the driver. Hmmm, seeing how much one of those sold for, if the crossover rebuild and cabinet bracing don't work out like I'm hoping, maybe I can sell the drivers and use the money to start over with a new baffle and a simpler, 2-way system. Whatever the case may be, these cabinets are too nice to not be put to good use.

(and if nothing else, even if the project ends up a failure, I'm at least starting to get my feet wet with DIY speaker design--something I've been wanting to get into for a while now).
 
judgeschreber said:
Yup, that's the driver. Hmmm, seeing how much one of those sold for, if the crossover rebuild and cabinet bracing don't work out like I'm hoping, maybe I can sell the drivers and use the money to start over with a new baffle and a simpler, 2-way system. Whatever the case may be, these cabinets are too nice to not be put to good use.

(and if nothing else, even if the project ends up a failure, I'm at least starting to get my feet wet with DIY speaker design--something I've been wanting to get into for a while now).
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Yes when I first bought the Cookbook, I was shocked at how much there was to it. And my math skills......Ha! That ruled me out, so I bought Linearx's LEAP and LMS. Now HERE is a link to a lot of older T/S parameters. If you get almost any new driver they will be listed, and I can give you some big hints as to what type of alignment to use.
 
judgeschreber said:
I'm at least starting to get my feet wet with DIY speaker design--something I've been wanting to get into for a while now.
Click to expand...
:thmbsp:

Yes, you can do this. It's just not as simple as you may have assumed.

Once you know the free-air Fs of the Jensen woofer and its other parameters, you'll have a better idea of what is possible. I suspect it's more suitably a mid-bass driver, which is fine, of course, but then you're talking a 4-way.

It sounds like you want to work with these cabinets. What is the calculated internal volume?
 
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