In the class of hearing aids generally referred to as implantable hearing instruments, some or all of various hearing augmentation componentry is positioned subcutaneously on, within, or proximate to a patient's skull. Generally, implantable hearing instruments are divided into two sub-classes, namely, semi-implantable and fully implantable. In a semi-implantable hearing instrument, one or more components such as a microphone, signal processor, and transmitter may be externally located to receive, process, and inductively transmit an audio signal to implanted components such as a transducer. In a fully-implantable hearing instrument, typically all of the components, e.g., the microphone, signal processor, and transducer, are located subcutaneously. In either arrangement, an implantable transducer is utilized to stimulate a component of the patient's auditory system (e.g., tympanic membrane, ossicles and/or cochlea).
By way of example, one type of implantable transducer includes an electromechanical transducer having a magnetic coil that drives a vibratory actuator. The actuator is positioned to interface with and stimulate the ossicular chain of the patient via physical engagement. (See e.g., U.S. Pat. No. 5,702,342). In this regard, one or more bones of the ossicular chain are made to mechanically vibrate causing stimulation of the cochlea through its natural input, the so-called oval window.
As may be appreciated, implantable hearing instruments that utilize an implanted microphone require that the microphone be positioned at a location that facilitates the receipt of acoustic signals. For such purposes, such implantable microphones are most typically positioned in a surgical procedure between a patient's skull and skin, often at a location rearward and upward of a patient's ear (e.g., in the mastoid region). Because the diaphragm of an implantable microphone is covered by tissue (e.g., skin), ambient acoustic signals are attenuated by this tissue. Accordingly, it is desirable that the acoustic sensitivity (e.g., pressure sensitivity) of an implanted microphone be enhanced to allow for detection of low amplitude/magnitude ambient acoustic signals.