10
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
