February 1953 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
While the wavelengths involved
are a bit longer (ok, a lot longer) with audio than with radio signals, the effects
of constructive and destructive interference due to the multipath phenomenon are
fundamentally the same. Author Crowhurst goes into detail about phasing and the
dissociation effect. Dissociation has to do with being in a region of ambiguity
regarding the angle of arrival of a wavefront due to a combination of signal phase
and amplitude combinations, and the listener's location relative to the source.
Again, these are the same issues that cause problems with radio receivers,
particularly on moving platforms like cellphones while travelling in a car or walking
down the street. Radars and electronic warfare systems do a lot of number crunching
trying to sort out angle of arrival information. Oh, and you can also learn something
worthwhile about how to arrange your home entertainment system speakers by reading
this as well.
Speaker Phasing and Dissociation Effect
By N. H. Crowhurst
Fig. 1 - Generalized diagram of the movement of air particles around
Keeping the apparent sound source in the correct place calls for careful phasing checks
It is evident from correspondence the author has received since his article "Loudspeaker
Crossover Design" appeared in the July issue of Radio-Electronics, that many people have
recognized the phenomenon called "dissociation effect" without fully understanding its
To understand the behavior of sound waves we must have the relative wavelengths of
audio frequencies clearly in mind. Acoustic waves travel approximately 1,100 feet per
second in free air so the length of one wave at a frequency of 1,000 cycles is a little
over 1 foot. Lower frequencies have longer waves, while the wavelengths at higher frequencies
Our Sense of Direction
Next we must see how it is possible for us - equipped with only two ears - to determine
the direction from which a sound originates. A single ear can give only a limited sense
of direction because the spiral communicating channel between the outer ear and the mechanism
of the inner ear eliminates the external directivity. Directional sensitivity must be
a function of the interpreting faculty of the brain derived by comparing the nerve impulses
received from both ears.
Is this ability to discriminate based on the intensity relationship or on the phase
relationship between the two ears? The difference in intensity between sounds reaching
the two ears from a given direction in free space depends on the obstructing effect (diffraction)
of the head. This effect increases with frequency, so the intensity on one side is greater
at high frequencies than at low frequencies. The phase difference is also greater at
high frequencies, because low-frequency waves are much longer than high-frequency waves
and change less in the short interval between their times of arrival at the two ears.
So the fact that we are more sensitive to the direction of origin of sounds at higher
frequencies can be explained by either the intensity-difference or the phase-difference
theories. Our subconscious probably utilizes both effects to some degree, but the dissociation
effect makes it quite evident that the phase relationship between sounds received by
our two ears is the more important of the two.
To prove this, we need to understand a little more about sound waves and their propagation.
It is well known that a cone loudspeaker working without any kind of baffle or cabinet
loses its effectiveness at low frequencies because air escapes around the edge of the
cone. (When the cone is moving forward, air particles around the rim rush backward into
the partial vacuum behind the speaker.) But what happens to other air particles farther
away from the speaker?
Fig. 2 - Plan of auditorium that presented a serious problem in acoustics.
Proper phasing of speakers solved it.
Fig. 3 - Another speaker layout that calls for a special phasing technique.
Fig. 1 is a diagram of air-particle movement (somewhat exaggerated) at various points
surrounding the speaker. Particles along the cone axis move back and forth along straight
lines radiating from the source. On either side of the axis the particles spin in elliptical
paths which grow shorter and narrower as the distance from the speaker increases. At
extreme distances these ellipses flatten to straight lines which also radiate from the
center of the cone. Note, however, that at points along the plane of the cone the particles
move at right angles to the radius line, so that the sound at these points seems to come
from left and right instead of from directly in front of the listener. (Under ideal conditions,
the sound waves from left and right would cancel, so that no sound would be heard along
the plane of the cone.)
How does this affect our sense of direction? Try listening to a speaker from different
positions. You will find that anywhere - except for a small region near the plane of
the cone, the source of sound is easily identifiable with the speaker unit. In the plane
of the cone, however, the dissociation effect becomes noticeable and it is almost impossible
to say where the sound comes from. When the dissociation effect is greatest, you get
the impression that, instead of having a single speaker unit in front of you, there are
two similar units, one on each side.
The article in the July issue gave as an example two identical speakers mounted side
by side with the listener standing on the center line facing the two units. When the
speakers are connected in phase the sound seems to come from a point midway between them;
but when they are out of phase, the sound seems to come from one side or the other. What
does this tell us? With two identical speakers and with the listener at equal distances
from both, it is obvious that both ears will receive sounds of equal intensity. But in
one case the apparent source is readily identified as being in front of the listener,
while in the other case the apparent source is somewhat indefinitely identified as being
on both sides of the listener. If intensity were the only factor responsible for our
sense of direction, we could not detect this change in phasing. This experiment shows
that relative phase at the two ears is the important factor.
A similar effect can be noticed if the loudspeakers are mounted some distance apart,
and the listener is an even greater distance away on the center line. If connections
to one speaker are reversed and the listener moves off center, the phase patterns from
the two speakers will gradually fall into line and cancel. At a greater distance off
center there should be another anti-phase position, but by the time this position is
reached, the intensity of the sound from the nearer speaker is sufficiently greater than
that from the more distant one so as to nullify the dissociation effect, and the nearer
speaker now seems to be the source.
Phasing in PA Work
Having investigated the matter so far, we can ask the question, "Is loudspeaker phasing
important for PA work?" The answer is definitely yes. The author remembers one job where
phasing played an important part. Fig. 2 shows the layout of the installation. The auditorium
was a long, narrow rectangle, with the stage at one end.
The only points where speakers could be mounted were at the sides of the stage and
immediately above it, at the ends of the narrow sections of the gallery. An engineer
who did not realize the possible consequences had simply connected the four speakers
in parallel without regard to phasing. The hall was acoustically poor due to a natural
echo, but it was symmetrical, and he could not understand why it was extra bad at certain
spots especially along the right-hand side. We suggested that two of the speakers be
disconnected, and observations of the type described above be conducted on the center
line. Similar tests were then made with the other speaker pair. We found that one speaker
on the right-hand side had been connected out of phase with the other three. Reversing
the offending speaker not only improved the bad spots, but made listening considerably
better everywhere at the back of the hall. The natural echoes were still evident, but
not to such a degree as to render sound almost unintelligible. The incorrectly phased
speaker had introduced some echo effects of its own, which made listening even more difficult,
except where the listener was comparatively close to one speaker unit.
Extended investigation on other installations has shown that it always pays to check
speaker phasing. It may be thought that where speakers are arranged as in Fig. 3 correct
phasing between symmetrical pairs would be important, but not between other units at
different distances from the front of the hall. Tests show that there is one really effective
method of connection and this is invariably with correct phasing. The explanation seems
to be that when a listener hears sound from two sources, one of which is nearer than
the other - as must happen in some positions with an installation of this type - the
nearer source gives the impression of a direct sound, while the sound from the more distant
source is like an echo. Where the echo arrives long enough after the direct sound to
be distinguishable from it, phasing is unimportant, but there are always some positions
where the two sounds arrive so close together that the ear cannot distinguish them as
separate sounds. At such positions, phasing can make an important difference.
Fig. 4 - Side-by-side speakers can create problems unless carefully
Fig. 5 - Crossover networks for dual-speaker systems. See text for
Another type of installation is shown in Fig. 4; correct phasing is very important
here. Walk around the back of the room while sound is being broadcast: when nearer to
one speaker the sound seems to come from the vicinity of this speaker; at a point equidistant
from two speakers, if the two are in phase, the apparent source of sound seems to pass
smoothly from one speaker to the other; but if they are incorrectly phased there will
be an area of confused sound where the building echo seems emphasized, often to the point
Dissociation effect can also occur with dual speaker units fed from an electrical
crossover network, but the effect is slightly different from the previous examples. You
get the impression that one part of the frequency spectrum has a source different from
the remainder of the spectrum. In large dual-speaker installations, such as in movie
theaters, this dissociation effect will be swamped by the natural reverberation of the
auditorium. However, the effect can be quite disconcerting in home equipment, giving
the sound an unnatural quality that many listeners have complained of.
Fig. 5 shows two typical loudspeaker crossover networks, and Fig. 6 gives their phase
characteristics. Although the two networks have identical schematic configurations, the
one shown in Fig. 5-a has values chosen to provide constant resistance, while the network
of Fig. 5-b uses typical wave-filter-derived values. To make the distinction between
the two types clearer, component values have been marked in terms of their reactances
at the crossover frequency, Xo being a reactance equal to the characteristic
impedance at the crossover point.
The top and center "A" curves in Fig. 6 show the phase responses of the high-frequency
and low-frequency sections of the constant-resistance-type network. These have a constant
phase difference of 270° over the entire frequency range, as indicated by the solid line
"A" at the bottom. On the other hand, the high-frequency and low-frequency sections of
the wave-filter-derived network have a phase difference of 270° only at the extreme
limits of the frequency range, while the difference between them increases to almost
450° at the crossover frequency (curve "B" at the bottom).
Fig. 6 - Crossover-network phase relations. "A" curves are for constant-resistance
networks; "B" curves for wave-filter type. Top and center curves show high- and low-frequency
shifts, respectively; curves at bottom show phase differences between high- and low-frequency
units over entire range.
With this type of crossover network, no matter how the h.f. and l.f. units are connected
in an attempt to maintain constant phase difference between them at or near the crossover
frequency - there will always be a rapid deviation from the constant-difference condition
near the crossover point. As a result, some component frequencies of the reproduced sound
will have their apparent sources shifted to one side or the other, away from the general
apparent source of the speaker combination.
If we are trying to reproduce a musical tone which contains a series of harmonics
extending through the crossover frequency, this type of network will move the apparent
sources of some of the harmonics to positions a small distance away from the common source
of the others.
Before concluding it is perhaps well to emphasize one point on the question of phase
difference that seems to confuse a number of readers. In electrical circuits, phase difference
is essentially a time difference, measured in degrees over the duration of one cycle
at the frequency considered. The acoustic effect on which our ears base their directional
deductions is better understood as the slope of the wave in space, at any particular
instant in time, and is thus a kind of spatial phase difference. This distinction may
help some who find it difficult to see why two interacting acoustic fields which differ
in phase can produce effects noticeable to the ear, even though electrical phase differences
of much greater extent are not normally detectable.
Posted September 6, 2018