Passive Time Alignment

The Allpass Network consists of 2 coils and 2 capacitors.

When more than one loudspeaker is used in a box, the problem arises that the loudspeaker units have very different features.
Look at a three-way system. The midrange speaker may have a diameter of 13 cm, while the cone is 2 cm deep.
The corresponding doom tweeter has a bulb cap that is completely at the front, usually sticking out a bit in front of the mounting plate.

Three possibilities to improve the phase behavior:
  • a cascaded front plate
  • a slightly sloping front plate
  • or by using an allpass network

If the two loudspeakers are mounted together on a flat plate, the air movements produced by the tweeter will arrive at the listening position rather than the vibrations delivered by the woofer.

With correct dimensioning, all frequencies are passed unchanged, while a constant time delay is obtained in a given frequency range.

An allpass network, however, has only a limited frequency range about which it good works.
The network must therefore also be adapted to the range that is required for a particular application.
With such an allpass network you are always stuck with a certain combination of delay time and frequency range.
A large time delay gives a fairly small frequency range, while a small time delay increases the frequency range.

This can be solved by putting multiple networks in series.
However, it is not necessary to make a loudspeaker box phase-correct over the entire frequency range. The speakers still have their phase deviations and also the crossover caused phase deviations.

An intermediate solution is then to dimension a delay network so that it provides the desired delay in the acquisition range between two speakers (let the time delay run to certainly an octave above the separation frequency).
The advantage of this method is that with a delay network a greater time delay is possible and that the calculated separation filter can do its job well (the phase behavior between the speakers is much better).

The input resistance of the all-pass network is always exactly the same as the terminating resistance (speaker resistance), so that nothing changes in the operation of a crossover.

Calculate Time Delay, Capacitor, Induction Coil
and Frequency Range

Distance between Speakers :  cm
Loudspeaker DC Resistance [ Re ] :  ohm

Delay : microseconds ( µs )
Filter Values
Capacitor [ C ] :  µF
Induction Coil [ L ] :  mH
The time delay has dropped to 95% of its nominal value at  Hz
The time delay has dropped to 90% of its nominal value at  Hz

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