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Research Papers

Characterization of the Dominant Structural Vibration of Hearing Aid Receivers

[+] Author and Article Information
B. R. Varanda

Department of Mechanical Engineering,
State University of New York at Binghamton,
4400 Vestal Parkway East,
Binghamton, NY 13902
e-mail: brenno.varanda@gmail.com

R. N. Miles

Department of Mechanical Engineering,
State University of New York at Binghamton,
4400 Vestal Parkway East,
Binghamton, NY 13902

D. Warren

Knowles Corporation,
1151 Maplewood Drive,
Itasca, IL 60143

Contributed by the Technical Committee on Vibration and Sound of ASME for publication in the JOURNAL OF VIBRATION AND ACOUSTICS. Manuscript received January 6, 2016; final manuscript received June 14, 2016; published online August 16, 2016. Assoc. Editor: Jeffrey F. Rhoads.

J. Vib. Acoust 138(6), 061009 (Aug 16, 2016) (8 pages) Paper No: VIB-16-1009; doi: 10.1115/1.4033950 History: Received January 06, 2016; Revised June 14, 2016

Results are presented of an analysis and characterization of the mechanical vibration of hearing aid receivers, a key electroacoustic component of hearing aids. The function of a receiver in a hearing aid is to provide an amplified sound signal into the ear canal. Unfortunately, as the receiver produces sound, it also undergoes vibration which can be transmitted through the hearing aid package to the microphones, resulting in undesirable feedback oscillations. To better understand and control this important source of feedback in hearing aids, a rigid body model is proposed to describe the essential dynamic features of the system. The receiver is represented by two hinged rigid bodies, under an equal and opposite dynamic moment load, and connected to each other by a torsional spring and damper. A method is presented to estimate the parameters for the proposed model using experimental data. The data were collected from translational velocity measurements using a scanning laser vibrometer of a Knowles ED-series receiver supported on a complaint foundation. Excellent agreement is shown between results obtained using the analytical model and the measured translation and rotation of an independent receiver. It is concluded that a dynamic model of the receiver must account for both rotation and translation of the structure in order to properly describe its motion due to an input current.

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References

Figures

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Fig. 1

A possible structure-borne feedback path from a miniature behind-the-ear hearing aid patented package [5]

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Fig. 2

Balanced armature receiver main components schematic. (Courtesy of Knowles Corporation.)

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Fig. 3

Noninverting voltage to current converter schematic. A Texas Instruments TL074BCN Op-Amp was used, the load represents the coil in the receiver, Rs is a 0.01% 1 kΩ resistor, Vin is the input voltage signal, and Vout is the referenced current.

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Fig. 4

Test setup schematic. A total of six locations were measured: five velocity measurements of the top surface of the receiver and the receiver's diaphragm velocity directly on top of the drive-rod. Each velocity measurement was referenced to the current applied to the receiver.

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Fig. 5

The receiver's motion is dominated by rotation. Measured velocity magnitude and coherence are shown for one of the ED-26805 units tested at five different surface locations. It is clear that the magnitudes at the locations 1, 5, and 2 increase up to 6 dB as they get closer to the terminal edge of the receiver due to a body rotation. The parallel measurements across the width of the receiver (1, 4) and (2, 3) are in agreement indicating there is no significant side-to-side rotation.

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Fig. 6

Schematic of the coordinate transformation to translational and rotational motion. (a) An arbitrary coordinate system, or, having its origin at the top surface location where the structure and armature are hinged. (b) A measurement, Hzsi, at the top surface of the receiver, treated as a plane, can be represented as a vertical shift, zo, an angular tilt around the y-axis, θs, and an angular tilt around the x-axis, ϕs.

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Fig. 7

Frequency response for the least squares transformed, planar rotational, and translational movement derived from five surface measurements at specific locations of a modified ED-26805-000 receiver on a cotton foundation. The level of the angular movement around the x-axis, ϕs, is within and below the laser vibrometer noise floor and becomes too noisy to properly illustrate at frequencies higher than 3.3 kHz.

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Fig. 8

Rigid body model representation of the receiver

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Fig. 9

Arbitrary coordinate system used as the center of rotation when extracting mass properties from the CAD assembly. The final location for the center of rotation is later z-shifted in the calculations.

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Fig. 10

The representation of the receiver as a pair of coupled, rotating rigid bodies captures the essential dynamic response. Results obtained using the characterized parameters model are in excellent agreement with the measured data obtained for an independent modified ED-26805-000 unit except for a slight difference in resonance, an expected behavior within manufactured batches of receivers. The vertical translation defined as zo has a dB reference of μm/mA, while the rest of the angular measurements are referenced to rad/A or mrad/mA.

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