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In Reply to: RE: Summary article on importance of transients over pitch posted by hahax@verizon.net on September 16, 2014 at 21:09:02
I looked at the crossover of the Froy MTM 2 way and it is 1st order on the midrange/bass and 3rd order on the tweeter. The circuits in the midrange crossover are dedicated to flattening the FR and the impedance of the midrange. It is a series notch filter and and impedance adjustment parallel to the mid/bass drivers.
That attention to impedance probably gets the speaker to be in phase in the midbass to the top of the midrange at the 2.2 khz LP. That is most of the critical range and should give you what you experience. If you use the subwoofer at a low enough freq then you will have no phase effects in the critical portion. In a 2 way, even an LR4 has no phase well into the pass band.
In a bandpass the LR4 is truly vicious with the phase, running the entire 360 deg. from the LP to the HP.
Follow Ups:
It's 4th order acoustically. I'm a personal friend of the designer and if he says it's 4th order, I believe him.
Thanks for that. I wasn't saying that it wasn't so acoustically and electrically combined, just that I see the midwoofers at an electrical 1st order on the schematic.
I was just speculating that in the context of the 250-6000 hz range that seems most critical to transient perception the midwoofers are likely to have low/no phase shift for all but the top octave of the range.
What matters is the total crossover according to my friend and that's the combination of electrical and acoustic characteristics. By the way even though it's 4th order an LR4 crossover properly done is very well damped(Q = 0.49) which is a big factor in low overhang.
You're incorrect with you analysis of the componetry of the Froy crossover. The midbass filter has no (specific) components for impedance adjustment. There's a notch filter consisting of the LC and the CR network forms the 4th order acoustic rolloff in conjunction with L1.
It's incorrect to label the LP and HP as 1st-order and 3rd-order respectively. The electrical networks look that way, but the acoustic design of the Froy is an LR4 design at 2.2khz. All LR4 crossovers have a 360 degree phase rotation at the crossover frequency. It's a defined characteristic of this design.
The bass roll-off would follow the typical vented-box characteristic. Your statement should read "If you use the subwoofer at a 'high' enough freq then you will have 'minimal' phase effects."
There's nothing wrong with the Froy design with respect to phase response. It's as expected.....for what it is.
Cheers,
Dave.
I understand the acoustic and electrical rolloffs should add up to 4th order. Otherwise the MTM arrangement does not work well.
I see that the CR and LCR can be both part of the notch filter, but the low steady impedance of the speaker outside the crossover region and driver resonance seems not to have a high order electrical phase shift. I have not run the values so I don't know. The drivers are that reactive that phase would get to 360?
The MTM arrangement does not require any particular crossover. In fact, the original D'Appolito scheme did not use a 4th-order acoustic network. It was designed as a 3rd-order network to facilitate a less abrupt change in power response. I've seen the MTM arrangement implemented as everything from 1st-order to 4th-order. :)
The MTM arrangement has some disadvantages but the primary advantage outweighs many of those.....IMO. The twin woofers add to a 6db sensitivity increase which generally matches that of many stock tweeters. Thus, you can usually design a crossover network with minimal/no attenuation on the tweeter and have system sensitivity approximately 6db higher than an MT version with the same drivers.
On the Froy design, the CR is not part of the notch filter and the system impedance curve follows that of a typical MTM implemented this way. There is no impedance leveling network employed on the Froy design. If there were you would see an LCR series network connected in shunt with the input terminals.
All LR4 crossovers exhibit a 360 degree phase shift. It has nothing to do with the drivers unless the crossover point is fairly close to the driver native acoustic roll-off point. You can see an excellent example of this with the Froy design.
Cheers,
Dave.
I should have said that it is better executed with a steep XO (acoustic and electric combined) slope and has a narrow vertical sweet spot. My friend with the Focal Utopia has to choose chairs with the right height to match the angle of the speakers.
Thanks for correcting me but I still don't see the 4rth order network in the Froy schematic.
It's 4th order acoustic, not electric. You have to take driver characteristics into account. And if it were 4th order electric in this case it wouldn't be really 4th order. It would be higher order. You need a good computer program to get the transfer function you want. If you just take nominal driver values and do a textbook crossover, you get nothing like what you think you are.
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