Electrical audio signal processing systems and devices – Including frequency control
Reexamination Certificate
1999-08-18
2003-02-18
Kuntz, Curtis (Department: 2643)
Electrical audio signal processing systems and devices
Including frequency control
C381S431000
Reexamination Certificate
active
06522758
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a frequency response correction system, and, more particularly, to a system utilizing a combination of circuit stages configured to phase-interact with one another and compensate for the frequency response of a planar diaphragm speaker.
A variety of planar diaphragm loudspeakers have been developed in recent years using differing materials and having differing constructions and configurations. In general, such planar loudspeakers typically include a relatively stiff and substantially planar diaphragm that is coupled at its rear surface to a loudspeaker driver. The driver presses on the rear surface of the diaphragm and causes sufficient vibration of the diaphragm to efficiently produce sound. Generally, the frequency response of a planar loudspeaker is determined by the type and density of the material used for the diaphragm, and the area, thickness and contour of its sound producing region, as well as the type, position and configuration of the driver. Each of these parameters is chosen in an attempt to achieve an acceptable degree of fidelity in the reproduction of sound in both the low and high frequency ranges.
Some of the advantages provided by planar loudspeakers over other types of loudspeakers include greater dispersion of sound and economy of manufacture. A further advantage of certain planar loudspeakers is that the front surface of the diaphragm can be molded or finished to take on the appearance of a relatively large acoustic tile, permitting unobtrusive installation of the loudspeaker in ceilings of commercial structures formed of like-appearing acoustic tiles as part of a distributed sound system. Alternatively, the front surface of certain planar loudspeakers can be molded smooth and flat and installed in an architectural ceiling or wall in such a manner that the front surface of the planar diaphragm is flush with the front surface of the ceiling or wall. This type of installation of planar loudspeakers in walls or ceilings enables a common decorative finishing material to be applied to the diaphragm and surrounding ceiling or wall surface, thereby making the loudspeaker non-visible from the exterior side of the wall or ceiling. A number of such diaphragms can be joined together in a contiguous and seamless array to create a sound screen upon which video images can be projected as part of a home theater as shown and described in U.S. Pat. No. 5,007,707, which is assigned to the same assignee as the present application.
To comply with building and safety codes, the individual planar diaphragm loudspeakers of a distributed sound system may have to be surrounded on the rear side by a sealed metal enclosure or box. Whenever installed in an architectural wall or ceiling, whether or not in a separate sealed enclosure, there is usually a severe limitation in the depth of air space behind the planar diaphragm relative to the surface area of the diaphragm, which creates unusual and adverse acoustic conditions. These conditions typically result in an unacceptably high system resonant frequency (F
r
), as well as an unacceptably high system resonant Q (Q
f
). As a consequence, a response peak typically occurs in a mid-bass region, and low bass frequency response is typically deficient. For example, the response peak for a planar diaphragm loudspeaker in an air chamber having a limited depth dimension might be in the range of 125 to 200 Hz., whereas preferably it would be in the range of 25 to 50 Hz.
The degree to which F
r
and Q
f
parameters are non-optimal varies with specific planar diaphragm speaker design characteristics and the air chamber behind such speaker. In general, a product line might include several planar diaphragm speakers having different size diaphragms, and each of those speakers may have several different metal enclosures or boxes from which to choose depending on where the speaker assembly is installed. It would be desirable, therefore, if the signal compensation for non-optimal F
r
and Q
f
parameters could be calibrated to the specific planar diaphragm speaker/air chamber combination.
Another characteristic of planar diaphragm speakers mounted in air chambers with a limited depth dimension is that they often exhibit an integrated power response decline in a mid-treble region (e.g., about 5 kHz.) and an integrated power response rise in a high-treble region (e.g., above 10 kHz.), which in turn degrades mid-range and treble reproduction accuracy. Again, the degree to which such mid-treble and high-treble responses are non-optimal varies with specific planar diaphragm speaker design characteristics and the associated air chamber. Signal compensation for non-optimal mid-treble and high-treble characteristics preferably should also be calibrated to the specific planar diaphragm speaker/air chamber combination.
One known way of compensating for the frequency response characteristics of loudspeakers involves use of graphic and parametric equalizers. However, such equalizers require intricate and painstaking alignments at multiple frequency points since the adjustment of one frequency band tends to interfere with other frequency band adjustments, making it difficult to set relatively sharp frequency cut-offs. Moreover, such equalizers are relatively expensive. Consequently, the use of such equalizers is not considered to be a very convenient or desirable solution to the problem of compensating for the above-described frequency response characteristics of planar diaphragm speakers mounted in air chambers with a limited depth dimension. This is particularly so for a distributed system of planar diaphragm speakers in which there might be a variety of different planar diaphragm speaker/air chamber combinations, each with its own compensation requirements.
Another way of compensating for the frequency response characteristics of planar diaphragm loudspeakers is described in co-pending application Ser. No. 09/099,049. This system incorporates cascaded equalization circuits and includes, among other elements, a multi-section switch in a resonant circuit to enable single-control selection of pre-set amplitude (A), frequency (F) and bandwidth (Q) parameters corresponding to various enclosure depths. As a practical matter, however, this system provides frequency compensation characteristics that are more suited to a home theater application than to distributed sound applications of planar diaphragm speakers.
Accordingly, there is a need for a method and apparatus for compensating for one or more of the above deficiencies in the frequency response of planar loudspeakers when mounted in air chambers with a limited depth dimension that can be calibrated for a specific planar diaphragm speaker/air chamber combination in a simple and cost effective manner. The present invention fulfills these and other needs.
SUMMARY OF THE INVENTION
Briefly, and in general terms, the present invention resides in a novel system for compensating the frequency response characteristics of a planar diaphragm speaker mounted in an air chamber with a limited depth dimension. The system may include one or more unconventional frequency compensation stages or circuits for processing an audio source signal applied to a planar diaphragm speaker/air chamber combination. The system also may be implemented in a manner that easily and economically allows calibration or adjustment of the frequency compensation characteristics of the system to accommodate a variety of different planar diaphragm speaker/air chamber combinations.
More specifically, the present invention provides electronic compensation, in an unconventional manner, for unacceptably high system resonance frequency and system resonant Q parameters of a planar diaphragm speaker mounted in an air chamber having a relatively small depth dimension. The present invention also may provide electronic compensation for a decline in integrated power response in a mid-treble region and a rise in integrated power response in a high-treble region of a planar diaphragm speaker.
In a presently preferr
Lao Lun-See
Sheppard Mullin Richter & Hampton LLP
Sound Advance Systems, Inc.
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