Two-Port Network Analysis and Modeling of a Balanced Armature Receiver
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Hearing Research 301 (2013) 156e167 Contents lists available at SciVerse ScienceDirect Hearing Research journal homepage: www.elsevier.com/locate/heares Research paper Two-port network analysis and modeling of a balanced armature receiver Noori Kim*, Jont B. Allen Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green Street, 2137 Beckman Institute, 405 N. Mathews, Urbana, IL 61801, USA article info abstract Article history: Models for acoustic transducers, such as loudspeakers, mastoid bone-drivers, hearing-aid receivers, etc., Received 20 September 2012 are critical elements in many acoustic applications. Acoustic transducers employ two-port models to Received in revised form convert between acoustic and electromagnetic signals. This study analyzes a widely-used commercial 9 January 2013 hearing-aid receiver ED series, manufactured by Knowles Electronics, Inc. Electromagnetic transducer Accepted 7 February 2013 modeling must consider two key elements: a semi-inductor and a gyrator. The semi-inductor accounts for Available online 26 February 2013 electromagnetic eddy-currents, the ‘skin effect’ of a conductor (Vanderkooy, 1989), while the gyrator (McMillan, 1946; Tellegen, 1948) accounts for the anti-reciprocity characteristic [Lenz’slaw(Hunt, 1954, p. 113)]. Aside from Hunt (1954), no publications we know of have included the gyrator element in their electromagnetic transducer models. The most prevalent method of transducer modeling evokes the mobility method, an ideal transformer instead of a gyrator followed by the dual of the mechanical circuit (Beranek, 1954). The mobility approach greatly complicates the analysis. The present study proposes a novel, simplified and rigorous receiver model. Hunt’s two-port parameters, the electrical impedance Ze(s), acoustic impedance Za(s) and electro-acoustic transduction coefficient Ta(s), are calculated using ABCD and impedance matrix methods (Van Valkenburg, 1964). The results from electrical input impedance measurements Zin(s), which vary with given acoustical loads, are used in the calculation (Weece and Allen, 2010). The hearing-aid receiver transducer model is designed based on energy transformation flow [electric/ mechanic/ acoustic]. The model has been verified with electrical input impedance, diaphragm velocity in vacuo, and output pressure measurements. This receiver model is suitable for designing most electromagnetic transducers and it can ultimately improve the design of hearing-aid devices by providing a simplified yet accurate, physically motivated analysis. This article is part of a Special Issue entitled “MEMRO 2012”. Published by Elsevier B.V. 1. Introduction eddy-current diffusion (the ‘skin effect’). In 1989, Vanderkooy demonstrated that, at high frequencies, the behavior of the A transducer converts energy from one modality to another. impedance of a loudspeaker changes from the behavior of a A hearing-aid receiver is an electromagnetic loudspeaker that normal inductor to that of a semi-inductor, because of the eddy converts an electrical signal to acoustical pressure. It is referred to current diffusing into the iron pole structure of the loudspeaker. as an electromagnetic transducer in part, because small magnets Using a Bessel function ratio, Warren and LoPresti (2006) repre- are involved. Since these miniature loudspeakers are widely used in sented Vanderkooy’s semi-inductor model as a ‘diffusion ladder modern hearing-aids, and remain one of the most expensive network,’ a combination of resistors and inductors. In 2010, Weece components of the hearing-aids, a detailed understanding of them and Allen used this representation in a bone driver model.pffiffi After is therefore critical to optimize their design. demagnetizing the bone driver, they established the s behavior There are two poorly understood elements of special interest and determined the ladder network elements from the measured in the electromagnetic transducer: the semi-inductor and the gy- electrical impedance of the transducer. Thorborg et al. (2007) also rator. The semi-inductor component is required to account for introduced a loudspeaker model with lumped circuit elements, including a semi-inductor. * Corresponding author. In 1946, McMillan introduced the anti-reciprocal component as E-mail address: [email protected] (N. Kim). a network element. Two years later, Tellegen (1948) coined the 0378-5955/$ e see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.heares.2013.02.007 N. Kim, J.B. Allen / Hearing Research 301 (2013) 156e167 157 term ‘gyrator,’ and categorized it as a fifth network element, along with the capacitor, resistor, inductor, and ideal transformer. Other than Hunt’s 1954 publication, we remain unaware of any publica- tions which implement anti-reciprocity in their electromagnetic transducer model using a gyrator. Fig. 1. Schematic representation of an electro-magnetic transducer and the equivalent Referring back to the original formulation of Wegel (1921), Hunt factored ABCD matrix representation. The electrical and mechanical sections are (1954) proposed the basic 2-port impedance matrix which converts coupled with a gyrator. Eq. (1) is the equivalent impedance matrix representation electrical signals to mechanical force, (Hunt, 1954). EðuÞ Z ðsÞTðsÞ IðuÞ models allows for an intuitive yet accurate interpretation of physical ¼ e ; (1) FðuÞ TðsÞ zmðsÞ VðuÞ properties. For example, when using a gyrator to represent the me- chanical and electrical transformation, stiffness can be represented where E is the voltage, I is the current, F is the force and V is the as a capacitor and mass as an inductor in the series combination. particle velocity. The direction of I and V is defined as into the Given the mobility (dual) network, it is necessary to swap the E inductor and capacitor, placing them in parallel combination. Thus network. This matrix involves an electrical impedance Ze ¼ j ¼ , I V 0 we feel that this dual network is less intuitive for understanding the F a mechanical impedance z ¼ j , and an electromagnetic system. m V I¼0 F E transduction coefficient T ¼ j ¼ j . We call Z , z , and T 1.1. A balanced armature receiver I V¼0 V I¼0 e m the Hunt parameters,1 which are functions of the Laplace (complex) The Knowles Electronics2 ED series receivers shown in Fig. 2(a), frequency s ¼ sþju. When the two off-diagonal elements of Eq. (1) including the ED7045 and ED1913, are balanced armature receivers are equal, the system is reciprocal; if they are opposite in sign, the (BAR),3 widely used in hearing-aids. The ED receiver is system is anti-reciprocal, which is a necessary condition of the 6.32 Â 4.31 Â 2.97[mm] in size. These receivers consist of a coil, electromagnetic transducer, due to Lenz’s law. Eq. (1) is equivalent armature, two magnets, and a diaphragm. Unlike the alternative to the ‘factored’ ABCD (transmission) matrix of Fig. 1. moving-coil drivers, the coil of the BAR has a fixed position (does The impedance matrix (Eq. (1)) is useful when making mea- not move) (Jensen et al., 2011), thereby reducing the mass and surements. For instance, system’s electrical input impedance and providing more space for a much longer coil. As the result of the output acoustic impedance (or output mechanical impedance) can lower mass, the BAR frequency response is higher, and due to the be represented with the impedance matrix elements, Z and Z (or e a greater coil length, the sensitivity (T) is greater. The armature used z ). The ABCD matrix representation (Fig. 1) is useful for network m for the ED7045 is an E-shaped metal reed (Bauer, 1953), whereas a modeling, but then may be transformed into an impedance matrix U-shaped armature is widely used for the telephone instruments for experimental verification. (Mott and Miner, 1951). Both shapes have advantages and disad- One may convert between an impedance matrix to an ABCD vantages. For example, the U-shape armature has better acoustic matrix using the relationship: performance (i.e., wide band frequency response) while the E- shaped armature lowers the vibration of the body more effectively. AB 1 Ze DZ ¼ ; (2) The armature is placed through the center of the coil and in be- CD Ta 1 Za tween two magnets, without touching them. The movement of the 2 armature is directly connected to the diaphragm through a thin rod where D ¼ ZeZa þ T ,and Ta is the acoustic transduction imped- Z a (Fig. 2 (a)). ance (Van Valkenburg, 1964). Inverting the acoustical and electrical variables in Eq. (2) gives the inverse ABCD system: 1.2. Sensitivity analysis of ED series SPICE model ðuÞ À ð Þ ð Þ ðuÞ P ¼ 1 D s B s E ; ðuÞ D ð Þ ð Þ ðuÞ (3) U ABCD C s A s I Fig. 3 shows the Knowles Electronics commercial SPICE circuit model (Killion, 1992). This SPICE model contains a gyrator and where DABCD ¼ DA þ BC ¼1, due to anti-reciprocity. Eq. (3) allows is meant to be equivalent to the physical system, but does not one to calculate the Thevenin pressure or Norton velocity in terms accordingly represent the system in an one-to-one physical manner. of the electrical input. In order to fully understand each component, we implemented In the electromagnetic transducer models of both Weece and the Knowles PSpice model in Matlab using transmission matrices. Allen (2010) and Thorborg et al. (2007), an ideal transformer was Unlike PSpice, Matlab provides a more flexible platform for a matrix used to convert electrical current into mechanical force (or acoustical model manipulation. Matlab does not critically depend on the pressure) of the transducer. As described in Beranek (1954),the user’s operating system (Knowles’ PSpice model is inflexibly tied to mobility strategy (along with the impedance or admittance analogy) both the Cadence Orcad Schematics and Capture, and Windows is appropriate to represent electrical to mechanical transduction XP). PSpice requires a DC path to ground from all nodes, thus R1, when modeling anti-reciprocal electromagnetic transducers using RK512, RK513, and RK514 components have been added for this an ideal transformer.