The ALLISON:TWO
loudspeakers system has performance identical with that of the ALLISON:ONE
system down to 50 Hz. below that frequency its power output is 2 dB less. The
cabinet is 30% smaller but has the same shape and similar proportions. The
crossover network, mid-range and tweeter units are identical. Thus the same
dispersion of high-frequency energy, power-handling capability, and placement
flexibility of the larger system are provided at a substantially lower price.
two 8-inch woofers are used in the TWO rather than the 10-inch woofers in the
ONE. yet the systems are so audibly alike that a small difference can be
detected only with music containing the lowest fundamental frequencies, and
even then only infequently.
As with all ALLISON
loudspeaker systems, the grille panels are formed of sturdy perforated ABS
plastic - a cover material which doeas not absorb extremely high-frequency
energy, even far off the tweeter axis. It is far more acoustically transparent
than cloth or open-cell foam.
The mechanism by
which this effect occurs can be understood as follows. Consider a typical box
loudspearer system positioned in a room so that its woofer cone is about two
feet from each of the three nearest room
surfaces-say, the floor and two intersecting walls. When the speaker is
radiating a very low frequency the cone moves relatively slowly and over a
relatively long distance. If the readiated frequency is 40 Hz, for example, it
takes 1/40 second (25 milliseconds) for the cone to execute one complete
forward-and-backward cycle. Each half of the cycle takes 12,5 milliseconds.
As the cone begins a
forward movement it generates the start of a compression wave. This impulse
travels at the speed of sound (1,130 feet per second) to each of the three room
boundaries and is reflected back toward
the woofer cone, arriving there some 3,5 milliseconds after it left, while the
woofer is still generating the compression half of the sound cycle. The
reflected waves increase the instantaneous pressure seen by the woofer and
enable it to radiate more power than it could in free space - a maximum of 9 dB
more power at extremely low frequencies, for which the reflected pressure is
virtually in perfect phase coincidence with the woofer's motion.
But as the woofer
tries to radiate higher frequencies, it must reverse its motion more quickly.
At 140 Hz, for example, the cone reverses direction every 3,5 milliseconds. it
begins its inward half-cycle of motion (attempting to create a rarefaction) just
as the three compression - wave reflections begin to arrive back from the room
boundaries two feet away. In this case the reflected pressure is completely out
of phase with the cone motion, decreasing its radiation efficiency some 11 dB
below the anechoic output. That is the worst case: a 20-dB variation in power
output (from +9 dB to -11 dB), when the woofer is equidistant from the three
nearest room surfaces, from a loudspeaker system which measures flat in an
anechoic chamber.
Usuallly the
boundaries are not equally distant from the woofer and the effect is not as
intense. Typically, the variation in power delivered by the speaker to a
listening room is 6 to 12 dB within the woofer range. These effects simply do
not exist in anechoic chambers, where
loudspeakers are commonly tested, beacuse there are no reflections from
the chamber walls. And measurements made in "live" rooms are
complicated by the standing-wave resonances. Consequently a room's influence on
the actual power output pf a loudspeaker system as a factor separate from other
room effects, has not been well understood until recently.
An uncontrolled
variation in system response of this magnitude would be considered intolerable
if it originated in, say, a phono pickup cartridge or an amplifier. But it is
just as audible when it originates in a loudspeaker. If it could be eliminated,
or of its severity could at least be reduced appreciably, an improvement should
be expected in the accuracy of the reproduced sound field.
How can this be
accomplished?
The most elegant
solution is exemplified in the Allison: Three corner loudspeaker system. The
cabinet is designed so as to place the woofer as close as possible to three
room surfaces (the floor and two intersecting walls), with the result that the
reflections arrive back at the woofer in a very short time (about 1
millisecond). The woofer's operating range is restricted by a crossover network
to an upper limit of 350 Hz; at that frequency, the woofer spends 1,5
milliseconds on each half-cycle in one direction. Therefore the reflected
pressure is essentially in phase with the woofer's motion, and increases its
power output, over its entire operating frequency range. Flat power output from
the system is thereby made possible. Put another way, the woofer's radiation
loding has been stabilized.
An Allison: Three
should be in a room corner. if it were to be moved away from the corner along
one wall, the missing side wall could be replaced by another Three system
placed side-to-side against the first one. the radiated sound pressure from
each system would its own reflected pressure from a corner side wall, and the
performance of the pair along one wall would be the same as that of a single
system in the corner.
The Allison: One
loudspeaker system is exactly that: a pair of model Three systems in one
cabinet, operating as one system, which can be located anywhere along a room
wall except in a corner.
An Allison: Two
system is equivalent in concept to a model One. its size and price are smaller,
and its power output below 50 Hz is 2 dB less than that of a model One.
Otherwise the systems are identical, In practice this compromise is seldom
sudible; the Two is very much a full-range loudspeaker system.
A hole in the woofer
power output curve caused by out-of-phase reflections is avoided in Allison
models One, Two, and Three by placing the frequency at which it occurs above
the woofer crossover point. But in order to do this the crossover frequency
must be quite low - not above 400 Hz 0r s0 - even with a woofer as close as
possible to the room-surfsce intersection. This requires a three-way
loudspeaker system design with separate mid-range and tweeter units, because it
is not practical to operate ahigh-quality tweeter down to a 400 Hz crossover
point. Consequently, in a less expensive two-way system with the woofer
operating up to 1 or 2 kHz, the reflections from nearby room surfaces will be
out of phase with the woofer motion at some frequencies within its range. There is no way to avoid this.
The voice-coil
diameter of Allison tweeter's convex is only 1/2 inch; the side of the cone
curvesinwardly, and the outside edge is fastened securely, at a diameter of
1-1/16 inch, to a mouting plate. Because ther is no compliant suspension at the
outer edge the entire cone surfsce is forced to flex as the voice coil moves axially. As it does
so, each point on the surface of the cone moves with a velocity that has an
in-phase component of motion perpendicular to the voice-coil direction as well
as a component parallel with it.
The tweeter thus
simulates the motion of a pulsating hemisphere to a remarkable degree. there
are other tweeters which generate cylindrical wave fronts; they have excellent
dispersion in one plane (the horizontal) But no other design even approaches
the Allison tweeter's dispersion at all angles, vertical and horizontal, in the
forward hemispere.
This uniformity of
output in all directions is not merely a technical tour-de-force of no audible
signifcance. It provides a convincing illusion of space around a reproduced
sound source, without distortion is not merely a technical tour-de-force of no
audible significance. It provides a convincing illusion of space around a
reproduced sound source, without distortion of its natural size, and it
generates a stereo image that is maintained even for listeners located outside
the area definied laterally by the speakers. The tweeter is used in all Allisin
louspeakers systems and is unique to them alone.
Specifications
Speaker complement:
Two 8" (203 mm)
woofers;
Two 3½" (89 mm) Convex Diaphragm mid-range
units;
Two 1" (25 mm)
Convex Diaphragm tweeters
Crossover
frequencies: 350 and 3,750 Hz
Crossover network:
LC half-section at
both crossover frequencies. Air-core chokes and non-polar computer-grade
capacitors are used. Three-position control switch (accessible from front)
supplied for selection of system acoustic power response, from nominally flat
to concert-hall balance slope.
Impedance: 8 Ω
nominal (7 Ω minimum at any setting
of balance switch)
Efficiency: better than 0,7% when placed at floor-wall
intersection.
Minimum amplifier
power: 30 watts per channel to produce
100 dB sound pressure level in most domestic room environments.
Acoustic power
output capability: 0,5 acoustic watt
minimum , over full frequency range, with 70 watt peak input.
System resonance
frequency: 52 Hz, nominal
Effective system
Q: 1.0. Low-frequency response -3 dB at
41 Hz, -6 dB at 34,5 Hz
Enclosure: Stabilized Radiation Loading sealed acoustic
suspension design.
Outside
dimensions: 36" high by 16"
wide by 9⅜" front to back (914 mm by 406 mm by 238 mm).
Internal volume: 1,775 cubic inches
(29,1 liters).
Material: High density particle board veneered with
walnut, oiled finish
Weight: 57 lbs (25,8 kg)
Full Waranty for
Five Years
