Active acoustic devices

Details Active acoustic devices Active acoustic devices

1999-01-20

2003-02-18

Kuntz, Curtis (Department: 2643)

Electrical audio signal processing systems and devices

Electro-acoustic audio transducer

Driven diverse static structure

C381S423000, C381S426000, C381S431000

Reexamination Certificate

active

06522760

ABSTRACT:

DESCRIPTION
Field of the Invention
This invention relates to active acoustic devices and more particularly to panel members for which acoustic action or performance relies on beneficial distribution of resonant modes of bending wave action in such a panel member and related surface vibration; and to methods of making or improving such active acoustic devices.
It is convenient herein to use the term “distributed mode” for such acoustic devices, including acoustic radiators or loudspeakers; and for the term “panel-form” to be taken as inferring such distributed mode action in a panel member unless the context does not permit.
In or as panel-form loudspeakers, such panel members operate as distributed mode acoustic radiators relying on bending wave action induced by input means applying mechanical action to the panel member; and resulting excitation of resonant modes of bending wave action causing surface vibration for acoustic output by coupling to ambient fluid, typically air. Revelatory teaching regarding such acoustic radiators (amongst a wider class of active and passive distributed mode acoustic devices) is given in our patent application Ser. No. 08/707,012; and various of our later patent applications concern useful additions and developments.
BACKGROUND TO THE INVENTION
Hitherto, transducer locations have been considered as viably and optimally effective at locations in-board of the panel member to a substantial extent towards but offset from its centre, at least for panels that are substantially isotropic as to bending stiffness and exhibit effectively substantially constant axial anisotropy of bending stiffness(es). Aforementioned patent application Ser. No. 08/707,012 gives specific guidance in terms of optimal proportionate co-ordinates for such in-board transducer locations, including alternatives; and preference for different particular co-ordinate combinations when using two or more transducers.
Various advantageous applications peculiar to the panel-form of acoustic devices have been foreshadowed, including carrying acoustically non-intrusive surfacing sheets or layers. For example, physically merging or incorporating into trim or cladding is feasible, including as visually virtually indistinguishable. Also, functional combination is feasible with other purposes, such as display, including pictures, posters, write-on/erase boards, projection screens, etc. The capability effectively to hide in-board transducers from view is enough for many applications. However, there are potential practical applications where it could be useful to leave larger, particularly central, panel regions unobstructed even by hideable transducers. For example, for video or other see-through display use, pursuit of translucence, even transparency, of panel members is not worthwhile with such in-board intrusions of transducers, though a panel-form acoustic device would be highly attractive if it could afford large medial areas of unobstructed visibility.
SUMMARY OF THE INVENTION
According to one device aspect of this invention, there is provided a panel-form acoustic device comprising a distributed mode acoustic panel member with transducers located at a marginal position, the arrangement being such as to result in acoustically acceptable effective distribution and excitement of resonant mode vibration. Existence of suitable such marginal positions is established herein as locations for transducer, along with valuable teaching as to judicious selection or improvement of one or more such locations. Such judicious selection may advantageously be by or as would result from investigation of an acoustic radiator device or loudspeaker relative to satisfactorily introducing vibrational energy into the panel member, say conveniently by assessing parameters of acoustic output from the panel member concerned when excited at marginal positions or locations. At least best results also apply to microphones.
From the relevant background teaching as of the time of this invention, availability of successful such marginal locations is, to say the least, unexpected. Indeed, main closest prior art cited against patent application Ser. No. 08/707,012, is the start-point for its invention and revelatory teaching, namely WO92/03024 from which progress was made particularly in terms of departing from in-corner excitation thereof. Such progress involved appreciating that distributed resonant mode bending wave action as required for viable acoustic performance results in high vibrational activity at panel corners; as is also a factor for panel edges generally. At least intuitively, and as greatly reinforced by practical success with somewhat off-centre but very much in-board transducer locations, such high vibrational activity compounds strongly with panel margins self-evidently affording limited access, thus likely available effect upon, panel member material as a whole; this compounding combination contributing to previously perceived non-viability of edge excitation.
For application of this invention, a suitable acoustic panel member, or at least region thereof, may be transparent or translucent. Typical panel members may be generally polygonal, often substantially rectangular. Plural transducers may be at or near different edges, at least for substantially rectangular panel members. The or each transducer may be piezo-electric, electrostatic or electro-mechanical. The or each transducer may be arranged to launch compression waves into the panel edge, and/or to deflect the panel edge laterally to launch transverse bending waves along a panel edge, and/or to apply torsion across a panel corner, and/or to produce linear deflection of a local region of the panel.
Assessment of acoustic output from panel members may be relative to suitable criteria for acoustic output include as to amount of power output thus efficiency in converting input mechanical vibration (automatically also customary causative electrical drive) into acoustic output, smoothness of power output as measure of evenness of excitation of resonant mode of bending wave action, inspection of power output as to frequencies of excited resonant modes including number and distribution or spread of those frequencies, each up to all as useful indicators. Such assessments of viability of locations for transducers constitute method aspects of this invention individually and in combination.
As aid to assessment at least of smoothness of power output, it is further proposed herein to use techniques based on mean square deviation from some reference. Use of the inverse of mean square deviation has the benefit of presenting smoothness for assessment according directly to positive values and/or representations. A suitable reference can be individual to each case considered, say a median-based, such as represented graphically by a smoothed line through actual measured power output over a frequency range of interest. It is significantly helpful to mean square deviation assessment for the reference to have a be normalised standard format; and for the measured acoustic power output to be adjusted to fit that standard format. The standard format may be a graphically straight line, preferably a flat straight line thus corresponding to some particular constant reference value; further preferably the same line or value as found naturally to apply to a distributed mode panel member at higher frequencies where modes and modal action are more or most dense.
In this connection it is seen as noteworthy that whatever function is required for such normalising to a substantially constant reference is effectively also a basis for an equalisation function applicable to input signals to improve lower frequency acoustic output. It is the case that viable distributed mode panel members as such, and with preferential aspect ratios and bending stiffness(es) as in our above patent application, may naturally have acoustic power output characteristics relative to frequency that show progressive droops towards and through lower frequencies where resonant modes and modal action are less dens

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