Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Driven diverse static structure
Reexamination Certificate
1999-02-19
2001-10-02
Kuntz, Curtis (Department: 2743)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Driven diverse static structure
C381S162000, C381S335000, C381S421000, C381S431000, C181S173000
Reexamination Certificate
active
06298140
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention relates to electroacoustic transducers for accurate reproduction of sound. Preferred embodiments of the invention use a layered assembly of thin films, baffles and atmospheric air, as an oscillating medium.
2. Description of Prior Art
Transducers for accurate or high-fidelity reproduction of sound available on the market today, when defined by the kind of oscillating medium they use, are of rigid-diaphragm type or stretched thin-film type:
1. As a rule, rigid-diaphragm transducers are employed in the construction of transducers with enclosures. They mostly consist of a cone-shaped radiating area with an electromagnetic driven area (voice coil) positioned at the apex of the cone. The front of the radiating area is fully exposed, whereas at the back of such area there is a magnet assembly and supporting hardware.
2. Stretched thin-film transducers do not employ any enclosure. They are mostly rectangular shaped with either an electrostatic or electromagnetic driving (motor) structure distributed over the entire front or back (or front and back) of the driven area.
I will first examine deficiencies common to all transducers. Then I will cover deficiencies or problems typical of each of the two kinds of transducers with emphasis on stretched thin-film transducers. Examined deficiencies are mostly confined to physical properties and phenomena causing audible irregularities in frequency response and tonal quality (timbre) of reproduced effective output.
A. Deficiencies Common to All Transducers, Affecting Perceived Tonal Quality
A first deficiency common to all transducers is the erratic acoustic power response as a function of frequency. Such erratic response is the result of irregularities in radiated output caused by characteristic mode patterns, known as normal modes of oscillation, resulting from standing waves at resonance frequencies of an oscillating medium. Such mode patterns are determined by oscillating sections of maximum displacement known as antinodes that are delimited by lines of zero displacement known as nodes. The areas of any adjacent antinodes of an oscillating medium have the tendency to be equal in size. Such tendency is a function of the geometry of the oscillating medium boundary in the sense that the closer to a symmetric geometry such boundary is, the more pronounced such tendency is. Each antinode moves out of step or with 180 degrees phase difference with any adjacent antinode. Moreover, the acoustic power radiated by any section of an oscillating medium is a function of the average displacement amplitude of such section. Thus, for each pair of adjacent antinodes, separated by a nodal line, the resulting minimum in the average displacement causes a drop in the effective output. By extrapolation, for each resonance (characteristic) frequency of an oscillating medium producing a mode pattern with an even number of antinodes, there is a minimum average displacement giving an audible dip in effective output. Such dip in effective output is defined as an antiresonance minimum. For a mode pattern with an odd number of antinodes, there is an audible peak in the effective output determined by a maximum from a remaining single (not paired up) antinode. Such peak in effective output caused by a mode pattern with a maximum average displacement is defined as a resonance maximum.
The irregularities in effective output of resonance-antiresonance minima and maxima become particularly audible in the low end of the acoustic spectrum where mode patterns occur at wider spaced resonance frequencies of the oscillating medium. The sparser such resonance frequencies are, the fewer mode patterns occur per unit frequency range, and the more pronounced the effect of such irregularities is, as perceived by the hearing mechanism. One more reason for strong and audible resonance-antiresonance irregularities from mode patterns is the stronger coupling with air of the intrinsically larger antinode areas occurring at low resonance frequencies. Consequently, not only the acoustic power response is erratic but also tonal quality (timbre) of radiated sound deteriorates because dips and peaks, unrelated to the spectrum at source, occur in the reproduced complex sound such as music or speech. Such resonance-antiresonance irregularities are also particularly pronounced under transient conditions at all frequencies.
A second deficiency common to all transducers relates to the tonal quality or timbre of reproduced sound as affected by, correspondingly, size and position of the driven area with respect to size and boundary of the total oscillating area. To a large extent, tonal quality is determined by the structure of the spectrum as derived from a reproduced complex waveform. In turn, a reproduced waveform is affected by the coexistence of superposed standing waves of an oscillating medium at any instant. Therefore such deficiency is reduced to relating tonal quality directly to the content in modes of oscillation being simultaneously excited in the oscillating medium.
A disturbance at a point of an oscillating medium will excite simultaneously a number of modes in proportion to the amplitude associated with each mode pattern at that particular point. One extreme possibility would be for a disturbance, applied at a point of maximum displacement—the centre of an antinode, giving rise to a pronounced associated mode. Another extreme possibility would be for a disturbance, applied at a point of minimum displacement—a nodal line, not being able to excite at all the associated mode. In practice, a disturbance spans at least two adjacent antinodes and interferes with the formation of associated mode patterns because the applied forces act in conflict with the adjacent out of step displacements.
Established and prevalent driving configurations in transducers involve:
(a) Concentration of exerted forces only on a symmetrically positioned, with respect to boundary, central and small section of an oscillating medium, such as in rigid diaphragm transducers;
(b) Distribution of exerted forces throughout the entire area of an oscillating medium, including the central section, such as in thin-film transducers.
Either way, at lower frequencies, a centrally driven section of an oscillating medium is crossed by nodal lines delimiting relatively large adjacent antinodes. The out-of-step displacements of adjacent antinodes conflict with forces applied by a disturbance and restrict the full development of associated mode patterns. Hence, a waveform reproduced by a centrally driven oscillating medium will have spectrum poor in low frequency overtones, causing tonal quality irregularities in effective output. At higher frequencies, progressively denser nodal lines delimit progressively smaller antinodes, and the effect of disturbances conflicting with out-of-phase antinode displacements becomes only statistically significant throughout the entire oscillating medium.
A third deficiency common to all transducers is related to back-to-front-wave-leakage phase cancellations whereby a back compression or a back rarefaction leaks around the edge of the transducer and catches up, respectively, with a front rarefaction or a front compression. The back-to-front-wave-leakage phase cancellations are an additional cause of irregularities in effective output, phenomenon particularly pronounced at low frequencies.
A fourth deficiency is related to all transducers using more than one transducer units in order to extend the frequency range of reproduced sound. Filters (crossover networks) employed in distributing frequency bands to dedicated transducer units, are circuits with their own resonances and losses that affect the original waveform, therefore affecting the perceived tonal quality reaching the listener's ears. A complex problem to solve is the integration of two adjacent frequency bands of two transducer units at frequencies delimiting such bands. Because both such units radiate sound at such delimiting frequencies, the transition from on
Kuntz Curtis
Ni Suhan
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