The VOTT represented nearly as great an improvement over the then state-of-the-art Shearer Horn as that speaker did over the previous Western Electric systems. However, its market impact was arguably greater. While Altec Lansing was already the preeminent manufacturer of movie loudspeakers, the Voice of the Theatre was singularly responsible for Altec Lansing's rise to market dominance. That dominance would reach near monopolistic levels through the 1950's and 1960's until challenged by their greatest rival - JBL.
The Shearer Horn had become the movie standard of the past decade. Virtually every loudspeaker manufacturer in the industry was offering a derivative of this basic concept, including Altec Lansing. Both Hilliard and Lansing, being the primary developers of that system, were in the best position to recognize areas for improvement. In 1944, they set about the task of documenting existing deficiencies and then developing a concept to address them.
The principal deficiency identified by Lansing and Hilliard was referred to as a lack of presence. This was attributed to a number of factors. It included a dip in the mid-bass response between 250-500hz, radiation from the rear of the speaker system, phase discrepancies due to a long, folded horn path, and cabinet resonance.
The primary solution to these problems lay in the design of a radically new bass horn enclosure. For the first time, a bass cabinet was developed that fully enclosed the drivers and eliminated rear radiation. The enclosure also front loaded the bass drivers in a direct, flared horn as opposed to the previous folded designs. This eliminated the mid-bass dip caused by dissipation of higher frequencies in the horn folds. However, the front horn was not large enough to propagate bass frequencies below 100hz. The ingenious solution was to port the cabinets to allow it to act as a bass reflex enclosure down to 50hz. Finally, the cabinet was comprehensively braced to maximize rigidity and minimize unwanted resonances. The enclosure was given the model number 210 and two systems were developed based on different configurations. The A4 utilized a single 210 with attached wing walls to reinforce the bass response. The A2 used 2-210's, side-by-side, also with wings.
The drive units in the VOTT were the brand new 288 compression driver and 515 bass driver. However, these units were more of an evolutionary development compared to the significant advance of the 210 bass horn. The 288 was actually a permanent magnet version of the field coil 287 with only a few refinements. The 515 bass driver was a legacy of the previous 604 duplex design. The need to mount a horn throat inside the cone's voice coil resulted in the use of a 3" diameter coil for the bass driver. This design was carried over into the new 515 woofer since the larger diameter brought a number of advantages in efficiency and power handling. This driver, along with the cone of the 604, were the first examples of 15" drivers using flat-wire coils.
Overall, the most significant advance represented by these drivers was the first use of permanent Alnico V magnets in a theatre system. This greatly reduced operating costs and increased reliability. The drivers were also more efficient and had higher power ratings than equivalent field coil units. This allowed increased output with no increase in amplifier requirements.
The final system met or exceeded all of the original design goals. The short, direct horn path allowed the high frequency driver to be mounted on the exact plane of the bass driver. Where the Shearer Horn had reduced phase discrepancies to 1 millisecond, the VOTT eliminated it entirely. The overall system efficiency was between 2-8db more efficient than the Shearer derivatives due to the new bass horn and more sensitive drivers. Bandwidth was extended at both extremes and, most importantly, the overall response was much more linear.
The A4 and A2 versions of the VOTT were introduced in 1945 to much acclaim. The Academy of Motion Picture Arts and Sciences immediately began tests of the new system that confirmed the sonic improvements. By 1955, the Academy adopted the VOTT as the industry standard for motion picture playback. The VOTT's were to remain in production at Altec into the 1990's.
© 2000 Don McRitchie