What is stereo?

There are now two system of high fidelity, monophonic (monaural) and stereophonic. Monophonic is a system that starts from one microphone and is fed through a single high fidelity set. Stereophonic is a double system. Two separate microphones are placed at different sides of the orchestra and two different systems are used to keep the two signals or channels separated. Two separate speakers are used, placed on different sides of you room. Stereo is much like 3-D photography, two slightly different sound reach your ears giving you a new dimension in sound.

H.H. Scott '59

Allison Model TWO (1976)

 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.

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 9" 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

Nessun commento:

Posta un commento