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Last Updated: 20070417, mei P14 - Microphones and Loudspeakers - 1Monday, May 7, 09:00 — 13:00 Chair: Thomas Gmeiner, AKG Acoustics GmbH - Vienna, Austria P14-1 Refinements of Transmission Line Loudspeaker Models—Juha Backman, Nokia Corporation - Espoo, Finland Simple waveguide models of loudspeaker enclosures describe well enclosures with simple interior geometry, but their accuracy is limited if used with more complex internal structures. A refinement of a transmission line loudspeaker model is discussed, presenting one-dimensional waveguide approximations for bends and corners. Bends and corners are represented as area changes in the line, approximated as one-dimensional line segments with parameters adjusted to match the exact solutions for sharp (rectangular) corners in a waveguide. Besides modeling, the paper discusses the sound transmission characteristics of commonly used bend types and the applicability of the results to folded horns. Convention Paper 7071 (Purchase now) P14-2 The Use of Negative Source Impedance with Moving Coil Loudspeaker Drive Units: An Analysis and Review—Michael Turner, University of Leeds - W. Yorkshire, UK and Switched Reluctance Drives Ltd., Harrogate, N. Yorkshire, UK; David Wilson, University of Leeds - W. Yorkshire, UK The effect of negative source impedance on the frequency response and pole-zero pattern of a moving coil loudspeaker drive unit is explored from first principles, and closed-form expressions for the transfer function and system poles are developed. Direct control of motor velocity via the substantial cancellation of voice coil impedance is discussed. Implementation using positive current feedback is analyzed, considering loop gain, damping, and stability from a control theory perspective. Pole placement techniques are shown to be effective in controlling theoretical system behavior at high frequencies. Modeled and measured results are presented. A selection of previous papers and applications concerned with operation of loudspeakers from negative source impedances is briefly reviewed. Practical issues and some possible applications are discussed. Convention Paper 7072 (Purchase now) P14-3 Effects of Acoustical Damping on Current-Driven Loudspeakers—Rosalfonso Bortoni, THAT Corporation - Milford, MA, USA; Sidnei Noceti Filho, UFSC - Federal University of Santa Catarina - Santa Catarina, Brazil; Homero Sette Silva, Selenium Loudspeakers - Nova Santa Rita, Brazil Previous works show the benefits of exciting loudspeakers with current sources, but they do not present a study showing the behavior of this technique when acoustical damping is applied to the diverse types of loudspeakers. This paper presents theoretical and practical analysis of the frequency response of acoustically damped current-driven loudspeakers installed in closed box, vented box, and 4th and 6th order band-pass systems. Also, it presents a subjective analysis comparing a closed box system excited by voltage and current sources. Convention Paper 7073 (Purchase now) P14-4 Development of a Highly Directive Endfire Loudspeaker Array—Marinus Boone, Delft University of Technology - Delft, The Netherlands; Wan-Ho Cho, Jeong-Guon Ih, Korea Advanced Institute of Science and Technology (KAIST) - Daejeon, Korea Control of the directivity of loudspeaker systems is important in applications of sound reproduction with public address systems. The use of loudspeaker arrays shows great advantages to bundle the sound in specific directions. Usually the loudspeakers are placed on a vertical line and the directivity is mainly in a plane perpendicular to that line although the radiation direction can be adapted with filter techniques, called beamforming. In this paper we present results on the applicability of a loudspeaker line array where the main directivity is in the direction of that line, using so-called endfire beamforming, resulting in a “spotlight” of sound in a preferred direction. Optimized beamforming techniques were used, which were developed for the reciprocal problem of directional microphone arrays. Effects of the design parameters of the loudspeaker array system were investigated, and we found that the stability factor can be a useful parameter to control the directional characteristics. A prototype constant beamwidth array system was tested by simulation, and measurement and the results supported our findings. Convention Paper 7074 (Purchase now) P14-5 Mass Nonlinearity and Intrinsic Friction of the Loudspeaker Membrane—Ivan Djurek, Faculty of Electrical Engineering and Computing - Zagreb, Croatia; Antonio Petosic, Danijel Djurek, AVAC – Alessandro Volta Applied Ceramics - Zagreb, Croatia Vibration of the loudspeaker’s membrane was analyzed in the regime of comparatively low driving currents (I0 < 100 mA) in terms of mass nonlinearity Meff and intrinsic friction RM. The latter contributes to the damping term of the differential equation of motion and depends on the elongation of vibration. RM is the sum of intrinsic friction Ri of the membrane and friction Rv coming from air viscosity on its surface. Independent measurements of flexural strength of the membrane were performed and correlated to experimental observations of the vibrating system. Experiments were also performed with membranes additionally reinforced by application of materials with higher Young modules. Convention Paper 7075 (Purchase now) P14-6 Modeling of an Electrodynamic Loudspeaker Using Runge-Kutta ODE Solver—Antonio Petosic, Ivan Djurek, Faculty of Electrical Engineering and Computing - Zagreb, Croatia; Danijel Djurek, AVAC – Alessandro Volta Applied Ceramics - Zagreb, Croatia The modeling of low frequency (<100Hz) electrodynamic loudspeaker is presented as one degree of freedom nonlinear damped oscillator described by an ordinary differential equation of motion. The model has been compared to an equivalent LRC circuit model, and it was shown that differential the equation approach is more suitable for calculations that include nonlinearities occurring in an electrodynamic loudspeaker, as well as couplings of different vibration modes, particularly those coming from vibrating air and the loudspeaker itself. The nonlinear differential equation of periodically driven anharmonic oscillator was solved numerically, and calculated amplitude frequency dependence and electric impedance have been compared to the experimental data. Calculations included different working regimes of the loudspeaker being operated in an evacuated space and air. Convention Paper 7076 (Purchase now) P14-7 Chaotic State in an Electrodynamic Loudspeaker—Danijel Djurek, AVAC-Alessandro Volta Applied Ceramics - Zagreb, Croatia; Ivan Djurek, Antonio Petosic, Faculty of EE and Computing - Zagreb, Croatia An electrodynamic loudspeaker has been operated in a nonlinear regime when the k-factor strongly increases with displacements. For driving AC currents up to 2 A the vibration spectrum contains high frequency harmonics of the classic von Kármán type; for currents in the range 2.6 to 4 A doubling of driving period appears; and for currents in the range from 4 to 4.8 A multiple sequences of subharmonic vibrations begin with 1/4f and 3/4f. An application of currents higher than 4.8 A results in a white noise spectrum, which is a characteristic of chaotic state. Convention Paper 7077 (Purchase now) P14-8 An Improved Electrical Equivalent Circuit Model for Dynamic Moving Coil Transducers—Knud Thorborg, Tymphany A/S - Taasrup, Denmark; Andrew Unruh, Tymphany Corporation - Cupertino, CA, USA; Christopher J. Struck, Independent Consultant - San Francisco, CA, USA A series combination of inductor and resistor is traditionally used to model the blocked electrical impedance of a dynamic moving coil transducer, such as a loudspeaker driver. In practice, semi-inductive behavior due to eddy currents and “skin effect” in the pole structure as well as transformer coupling between the voice coil and pole piece can be observed, but are not well represented by this simple model. An improved model using only a few additional elements is introduced to overcome these limitations. This improved model is easily incorporated into existing equivalent circuit models. The development of the model is explained and its use is demonstrated. Examples yielding more accurate box response simulations are also shown. Convention Paper 7063 (Purchase now) |