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Correction Circuit and Method for Improving the Transient Behavior of a Two-Way Loudspeaker System

US Patent: 5,185,801
Date Filed: July 18, 1991
Date Issued: February 9, 1993
Inventors: John Meyer, Paul Kohut
These two breakthrough patents describe technology that enables a two-way loudspeaker system (the HD-1 high-definition audio monitor) to approach the theoretical ideal of a single point source covering all but the lowest half octave of the audio bandwidth.

The original impetus behind the patents stemmed from John Meyer's search for an accurate, full-range reproducer to use as an in-house laboratory instrument. On many occasions the engineering staff required various acoustic signals (including voice and music) for testing. However, generating such signals "live" in the lab using human subjects and/or musical instruments was impractical and playing back recorded sounds through a conventional speaker produced unreliable results. Thus began a five-year search to achieve near-perfect impulse response in a full range speaker system. Although some previous methods had been described and even patented, all dealt with a single point in space. The Meyer Sound engineering team set out to accomplish this feat over a reasonably broad listening area.

These milestone patents address several interrelated problems in achieving such an impulse response that many others in the field had previously considered essentially beyond practical solution, as evidenced by papers in professional journals.

No single transducer able to achieve significant output can achieve reasonably flat amplitude response across the full audio bandwidth; at least two transducers are required. However, splitting the audio signal into two bands with a crossover inevitably introduces phase errors, and adding additional amplitude correction circuits to achieve absolutely flat response can further compound these errors. Then there's the basic problem inherent in the physical mass of the transducers, which is that each transducer has a certain amount of inertia that effectively stores energy, but the amount of energy stored is uneven across the audio spectrum. Consequently, all previous two-way speaker systems were intrinsically incapable of reproducing an accurate impulse response. The click (pulse) fed to the input did not match the click coming out.

Co-inventors John Meyer and Paul Kohut realized that the desired results could be achieved only through an integrated system wherein all elements - transducers, cabinet, amplifiers and control electronics - were designed in concert to achieve simultaneously flat amplitude and phase response. Essentially, the patented circuits employ a combination of phase offset and amplitude correction techniques to compensate for the energy storage of the transducers, the differing responses of the two transducers in the crossover region, and for any frequency-dependent amplitude anomalies that may be introduced by earlier phase correction. The technology in the original patent was incorporated into the HD-1 studio monitor, first introduced in 1989.

Interestingly, the extreme accuracy of the HD-1 subsequently generated considerable controversy regarding the purpose of a recording studio monitor. Should a monitor accurately reproduce the signal present at its input? Or should it be representative of what the typical consumer would hear through conventional loudspeakers? The answer is subjective, of course, but it is worth noting that this question generated little discussion until the HD-1 made virtually absolute accuracy an option for the first time.

The second patent is closely related to the first, but deals with specific issues related to horn-loaded high frequency drivers. The original patent was concerned solely with direct radiating systems.