# Another Time and Phase thread

Hello guys I found this post on another site.Please explain as this stuff seems to get more and more confusing to me.Which is it time and phase or not.This was posted by an AA member.

I see much stuff about phase and time with confusion. If there are two drivers mounted on a baffle, say a midrange and a tweeter, then it would be nice if the acoustic radiation from the two drivers were in phase. Linkwitz Reily 24dB/octave accomplish this when the two drivers are in the same acoustic plane (voice coil alignment is very close to this with an offset baffle) In this case, for one octave above and below the crossover, the electrical signal applied to the drivers are 360 degrees out of phase. For continuous signals applied to the two drivers through the crossover and at the crossover frequency the motion of the midrange is one cycle behind the motion of the tweeter. This allows the main acoustic radiation axis to stay on the same axis as the individual drivers. The problem with the LR crossover is that half the energy applied for those two octaves around the crossover is thrown away by the form of the crossover. The transfer function has 4 terms plus a constant. Only the first term and constant appear in the acoustic output.
As far as that minimum phase stuff. Everyone seems to forget that the drivers must acoustically sum (low and high add together) somewhere in front of the speakers in the acoustic environment. With out of phase drivers that summing point starts down (midrange below the tweeter with applied signal of midrange lagging tweeter signal) and then moves up relative to the axis of the speaker depending on frequency. If you do this in a circuit, the summing is literally a point and so no such physical axis even exist. Speakers are not points and are not circuits though. A 6dB/octave crossover has a phase of plus 45 degrees for the tweeter and minus 45 degrees for the midrange at the crossover point. This is why the crossover is -3dB. With the two drivers 90 degrees out of phase, cancellation must occur. In this case. Half the energy is canceled out by the destructive interference from the two drivers at the crossover frequency. Also, if the voice coils are aligned as before, at the crossover frequency the acoustic center of radiation for the tweeter has moved forward in phase (effectively may be thought about as moving forward in space for analysis purposes) and the acoustic center of the midrange has moved back. The axis of radiation where the two drives sum and are in phase has been tilted down. The angle of tilt is directly related to the distance between the two drivers and the crossover frequency. If the drivers are more than one wavelength apart at the crossover frequency, then the tilt is so much that a second radiation axis occurs. This axis point way up with a acoustic radiation null between these two axis. Wave length in inches is equal to 13500/frequency of interest. This part about radiation is all basic physics) physics 102 from radiation from multiple sources.