Manual Macrom 45.40 (page 10 of 31) (English, German, French, Italian, Spanish)

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THEORY OF OPERATION

CONTINUOUS PHASE ADJUSTMENT

The effects of phase shift. The unavoidable consequence of the configuration is the phase

shift, which we will not comment at length as it goes far beyond the boundaries of this

discussion. It is enough to say that any crossover network affecting the signal amplitude also

affects the phase.

What is the phase? The phase is the time reaction between two signals. You probably

encountered this basic problem in connecting two speakers to a stereo system. If the two

speakers are “out of phase”, the bass frequencies are output only partially giving as a result a

“LEAN” sound. On inverting the wires of one speaker the problem can be solved because the

phase of the signal that goes to that speaker is inverted. This is an example for a phase shift

of 180° between two speakers of the same section (low pass); with crossovers it is possible to

have phase shifts also between different sections; they can reach from 0° and change

gradually to 360°.

Phase shift caused by crossover filters affects the following characteristics:

1) the ability of the crossover/speaker

system to reproduce the wave form

2) the flatness of the combined acoustic

output of two or more speakers (low and

high)

3) the radiation angle of the speaker.

Various crossovers produce different

outputs with different radiation angles

where the combined output of several speakers reaches a peak. The figure explains the

concept of radiation. The change of radiation angle is caused by the phase shift that is

sensitive to the frequencies at the crossover point. The fundamental concepts for the choice of

a crossover curve and of the cutoff characteristics are based upon the following:

a) considerations about the speaker system

b) the effect of the phase shift of the crossover and the positioning of the speakers with regard

to the frequency response of the global system.

Phase alignment the crossover and the

speaker: the best way to allow a speaker

system to accurately reproduce the pulses

of a wave pattern is the positioning of the

speakers in such a way that the wave

front coming from different sources

reaches the ears of the listener in exactly

the same time fraction; the improvement

of this characteristic is called phase

alignment or time alignment. This concept is better explained by the figure. Sound travels at a

speed of approx. 1100 ft./sec. through air; therefore, a distance of 12 inches stands for a delay

of one millisecond at 1000 Hz. If two speakers are not well aligned the waves forming the

wave front reach the ear with a delay one with respect to the other. This problem in phase

alignment is particularly acute in the near regions of the cutoff frequencies because all the

speakers contribute to the overall output of the system; any delay between the sound sources

(woofer, tweeter) gives as a result an unbalanced frequency response in the region around the

cutoff frequency. The best way to minimize this problem is to physically correct the vertical

alignment of the different speakers until the perfect alignment of their single acoustic center

points is obtained. Due to this adjustment the distance between the speaker and the listener is

the same for all the speakers within the system thus arranging them on the same acoustic

level.

O/M 45.40 (5T) 25/06/02 19:25 Page 10

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Macrom 45.40 Manual

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