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Originally posted by billfitzmaurice
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I usually invert the polarity of the tweeter if the phase is out by 180 degrees, is this wrong?
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It's not wrong, but this is the deal: a 3rd order highpass eliminates the phase issue, increases the tweeter power handling and reduces distortion, and all for the cost of one capacitor, which is 5 bucks or less.Comment
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Doesn't the extra cap shift the phase by 90 degrees? At this point, won't the two drivers always be out of phase? What am I missing?Comment
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The drivers will always be out of phase, even if there's no crossover, as they have different phase responses. It doesn't matter. The only time you can hear it is when the phase of two different sources is at or near 180 degrees apart where their pass bands overlap, which is at and near the crossover frequency. If they're 135 degrees apart you can't hear it, if they're 225 degrees apart you can't hear it. Perhaps what's confusing is the misuse of the term phase, when the correct term is polarity. Laymen more often than not use the former when they mean the latter.Originally posted by Billet View PostComment
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I guess I meant to say out of phase at the crossover frequency. And if I understand correctly, changing the polarity results in a 180 degree phase shift. I'm still trying to learn if phase is important to the overall sound. I have a friend who tries to time align everything, I think that is great if you always sit on axis
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Time alignment is a little different than phase. Drivers can be in-phase, yet not time-aligned. Time alignment takes into account the relative delays, including the difference in time-of-flight to the measuring position. There's a lot more to be said, here is something to chew on: https://sound-au.com/ptd.htm
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I'll add, 90 degrees difference at the crossover is called phase quadrature. Many famous speakers were designed that way back when Butterworth crossovers were the rage. As Bill said it's not audible per se, although the drivers will sum differently in quadrature vs in-phase: both must be -3dB instead of -6dB at the crossover to achieve a flat response. Once again, some light reading: https://www.ranecommercial.com/legacy/note160.htmlFrancisComment
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Regarding time align, just as with crossovers it can be critical if the distance between the driver acoustic centers is in the vicinity of 1/2 wavelength at and near the crossover frequency. Eliminating that possibility is the reason why the Altec A-7 horn loaded the woofer. It wasn't about response, it was about moving the woofer acoustic center to match the acoustic center of the HF driver. There was even a procedure to measure the system response to determine the ideal position of the HF horn when mounted above the cabinet, moving it back and forth until it was perfect. Back in the days of direct radiating woofers with horn loaded HF horns crossed at 500Hz, where 1/2 wavelength is about 13 inches. time align was a major concern. With all direct radiators crossed over much higher time align isn't that big a deal. Another concern with time align is when the acoustic center of the HF section is physically behind the LF. That results in the highs being perceived as muted. Preventing that is the reason for touring pro-sound PA systems delaying the LF signal. With all direct radiating drivers the HF acoustic center is closer to the listener than the LF. This makes the high frequencies more prominent, due to precedence effect, and is usually responsible for what's often referred to as forward sounding.Comment
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Time alignment is still a big deal for those of us who use horns. I use digital delay on the woofers to time align them with the horn acoustic center, which is about 17" behind the woofer acoustic center. I can hear the difference a few mm of delay makes.FrancisComment
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OK. MANY thanks to those who have helped so far. I have played around with a bunch of different driver combinations in the interim. I have also "built" existing designs in Vituix in an effort to try replicate the designers outcomes and see what effect changing components has each design. I am attaching my current iteration of a RS150P and RST28F MTM. Maybe not the most exciting driver choices, but it seems to be working with a low component count. I would greatly appreciate if anyone could quickly review this for glaring flaws. I have made sure all component values are available in real life and added the actual impedance numbers in for the inductors. The dip at 4600Hz is bugging me, but I have not yet figured out how to get rid of it.
I am currently stuck at adding the driver layout. I always thought that these were real world physical distances, but now I am reading that I need to run the drivers through Response Modeler to come up with the correct numbers. Although maybe I am conflating driver layout and driver alignment? Seems like more research is necessary.
Thanks again.Comment
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The dip at 4.6k is in the tweeters response. There's no reason to attempt to correct it. Looks like you are crossing well above 3k. I'd suggest a lower xo point. Did you use target slopes?Originally posted by Sebreaux View Post
Will the tweeters be at ear level? Location can be important.Comment
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Cross is at around 3100 I think. It is just where it fell as I was playing with the components. Would there be a better target to shoot for? Would a lower crossover point help with off axis response?
I think that the lowest I can manage getting the tweeters is 6-8" above ear level. Main listening position is about 16 feet from the face of the speakers if that makes any difference.Comment
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The lower you cross, the less critical the height will be. If you built a tm instead of an mtm, the tweeter could be at the bottom of the baffle with the woofer above it. From 16', it may be just fine even if it's a little bit high. I cross many of my speakers below 2k. Not all tweeters sound good that low. If you play the system very loud, the xo point may need to be a little higher than if you don't.
Based on the specs, I'd maybe try to sim the x-over at 1.8k 4th order.
Take a look at just the woofer summation at 1m on axis, and then alter the listening position to 3m or so, and 10" below the tweeter axis. If you are 10" lower than the tweeter center line, the woofers will have some issues summing as the frequency goes higher. I suspect that at 2k, all will be fine even if your ears are 10" lower. The sim should help you see the potential issue. It may not be an issue at all at that distance. At 8' it might be more of an issue. It's just a geometry thing. The distance to your ears will be longer to the top woofer than to the bottom woofer..Comment
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Your XO looks "okay" (for starters). Should get you into the "tweaking" range. I see about +1 to +2 dB of baffle-step, which should be fine.Comment
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The impedance is getting pretty low. Try a bigger coil, and smaller cap on the woofer portion, and see if that helps.Originally posted by Sebreaux View PostComment
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I'm going to again suggest that in the special case of an on-wall speaker (or a slightly proud sitting in-wall speaker as the case may be), you need to include both the baffle diffraction effects and the rear wall boundary gain effects in your sims in order to approach some measure of accuracy in your sims.
So for this I'm using Jeff B's Diffraction and Boundary Simulator. I've chosen a 4.5" cone diameter on a 15"x30" baffle with a 1.5" radius with the driver placed closer to the bottom of the cabinet at 7.5"x6" and with the cabinet sticking out only 3" from the wall.
The 1st pic shows the baffle diffraction effects. The 2nd shows the rear wall gain and the 3rd shows the effects of both combined. Last is the combined effects added to the FR of RS150P-8 (black trace).
So note that the 6dB of baffle step loss is cancelled out by the 6dB of gain from the proximity to the rear wall. Also note that because the driver is so close to the wall, that rear wall gain continues right up almost to 1000Hz so that the combined effect you are getting is close to a 6dB increase in the FR between about 100Hz and 1200Hz which you have not taken into account in your simulations. Additionally, the effect can be manipulated by changing baffle dimensions, driver placement and/or distance to the rear wall. Of course changing the driver may alter things as well. For its intended on-wall application, there is a reason that those slim Peerless drivers have a FR that looks different from most other drivers. Not that the Dayton looks unworkable - I haven't made the attempt but it looks like a very broad notch filter aimed at the midrange hump might do the trick to flatten out the FR.
You should also note that in VituixCAD (as well as XSim), when you use 2 drivers in parallel, the program displays just 1 of the driver responses below the summed response which means that it is actually 6dB down from the real combined response of the 2 drivers. Therefore, the actual xo point is not going to be where the (single) woofer FR crosses the tweeter FR but instead be at a point 6dB higher on the tweeter FR. So for example, in your 2nd sim in post 28, it looks like the xo is at about 1900Hz where the 2 FR's cross at about 86dB, but in reality the xo is 6dB higher than that at about 92dB which puts the actual xo point closer to something like 2600Hz.
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