Experimental Study On Measurement And Compensation Of Acoustic Pickup From Vibration Of Ossicular Chain

Objective:In order to systematically analyze the feasibility of inside acoustic. pickup from the vibration of ossicular chain, experimental piezoelectric microphones were manufactured, which worked as an implantable microphone to be coupled with ossicular chain in cat ear, as well as in fresh temporal bone. And the measurement and signal processing of the acoustic pickup were performed.Methods and Materials:①ANSYS software of finite element analysis was utilized to build the vibration simulating models of PT300-10 piezoelectric ceramic bimorph element (PCBE) under the unrestricted single cantilever beam.②Two sizes,4.5 mm by 1.5 mm by 1.2 mm, and 5.0 mm by 1.5 mm by 1.2 mm, of experimental piezoelectric microphones (Pz-Mics) with rectangular section were manufactured. Twelve Pz-Mics (each size; 6) were respectively glued on the wind cap surface of one testing speaker to mimic acoustic pickup from ossicular chain in vitro.③Pz-Mics were respectively glued on the malleus, incus, or replaced in tympanic cavity without any touch on ossicular chain in cat ears (8 cases with 13 ears) to pick up the testing signal (pure tone, sweep signal and speech signal); control group:acoustic pickup of Hy-M30 microphone (made by HYUNDAI Corp, Korea) at the site of cat ear.④Pz-Mics were respectively glued on the malleus, short process of incus, or replaced in tympanic cavity without any touch on ossicular chain in fresh temporal bones (4 cases with 7 ears) to pick up the testing signal (pure tone, sweep signal and speech signal); control group:acoustic pickup of Hy-M30 microphone (made by HYUNDAI Corp, Korea) at the site of cadaver ear.⑤The acoustic pickup from the vibration of cat malleus was processed to compensate and modify its distortion and loss from testing signal.Results:①The simulating model shows that the optimal size of PCBE is 2.5 or 3.0 mm by 1.0 mm by 0.3 mm, under vibration with single cantilever beam and its resonant frequency is about 20 kilo Hertz.②the Pz-Mic, size of 5.0 mm by 1.5 mm by 1.2 mm and mass of 38 milligram, performed better in the ex vivo acoustic pickup experiment than the other size one. It can sensitively responsed all the human hearable frequencies signal with high level signal to noise ratio (SNR) and flat frequency response curve (FRC), thereby to be used in implantation experiments.③The Pz-Mic picked up most frequencies of all testing signals when glued on the handle of cat malleus. The pickup signal had the capable of stimulation to demonstrative electrode array of totally implantable cochlear implant. However, there were strong noise and huge losses of below 1000 Hz frequencies in the pickup, in spite of good performance at speech signal frequencies. Its pickup from cat incus was very weak.④The Pz-Mic's performance of pickup from fresh cadaver malleus was wholely better than that from cat malleus, especially on the low frequencies. As the same consult, its pickup from fresh cadaver incus was weak.⑤The performance of acoustic pickup from fresh cadaver is better than those from cat (for malleus group, there was a more flat FRC in human fresh temporal bone; for incus group, the acoustic pickup from fresh temporal bone is stronger than from cat ear). The major reason for above difference lies in the mass match, according to the different mass,17 milligram of cat malleus,28.4 milligram of fresh cadaver malleus and 38 milligram of Pz-Mics. The overload might disturb the normal reply vibration of ossicular chain to testing signal stimulation, especially at the low frequencies. Therefore, there is always significant loss of low frequencies acoustic signal.⑥After signal processing of bandpass filter from 145 Hz to 9288 Hz firstly, and then selectively amplifing with wide dynamic range compression program for the acoustic pickup from cat malleus, it can be compensated to a great extent, and output different gains at different frequencies.Conclusions:①The serial experiments on Pz-Mic's acoustic pickup in and ex vivo demonstrate safely the feasibility of picking up and processing the acoustic signal from human ossicular chain.②For the restrictive of manufacturing Pz-Mic, the optimal size and mass have not been reached, yet. The collective experimental data could not also meet the pratical demand. ③The promising advance of technology in micro electromechanical system (MEMS), higher efficient piezoelectric ceramic, the Pz-Mic and its fixed device might be improved both in size and mass. The more efficient software might be also developed to process the acoustic pickup. The excellent strategy of acoustic pickup from ossicular chain will be improved to perform for the totally implantable cochlear imlant (TICI) system in the near future.