Experimental characterization of piezoelectric and elastic operational field-dependent non-linearities of multilayer benders for resonant driving of bone-conduction hearing aids via a dual-actuators side-by-side device

Abstract

A soft piezoceramic multilayer (ML) bending thin and thick dual-actuators side-by-side device, for use in bone-conduction hearing aids (BCHA), was characterized experimentally. For this purpose, first, quasi-static (0.1 Hz square wave) transverse displacements (deflections) of each ML bender were measured using a laser vibrometer (LV) under unimorph and bimorph drivings at various non-amplified peak (p) voltages (1Vp–13Vp). The resulting peak displacement–voltage curves were found clearly nonlinear, indicating a pronounced piezoelectric (PE) field-dependent nonlinearity (FDNL). Then, experimental electric impedance analyses were conducted for each ML bender, using an impedance analyzer, under 1Vp unimorph driving for the whole audio frequency range (20 Hz–20 kHz). The identified three modes’ superposed impedance magnitudes and phases showed their increasing deviation to the frequency scale left with increasing the modes’ order, indicating a possible softening elastic FDNL, as confirmed later by harmonic frequency response LV measurements at 1Vp. Finally, the harmonic frequency responses, for the above audio frequency range, were measured for each ML bender using the LV under unimorph and bimorph drivings at various non-amplified voltages (1Vp–10Vp). Here, beside the PE FDNL, the frequency response functions showed clearly a pronounced softening elastic FDNL. Consequently, with increasing input voltages, the identified two modes’ displacement amplitudes were nonlinearly increasing, while resonant frequencies and corresponding quality factors were nonlinearly decreasing. The three parameters-voltage curves were found to fit well (R2 ≈ 1) with high-order polynomials. These experimental results contradict the widely assumed displacement–potential linearity of middle ear or BCHA implantable PE vibrators of open literature available types (with or without shim bimorphs, unimorphs or diaphragms, and stacks) under similar targeted driving voltage and frequency ranges. The ML benders type quasi-static, frequency response and electric impedance characterizations, as well as the dual actuators side-by-side device, are proposed here for the first time for hearing aid applications.