Nowadays, hearing impaired people can be treated with a multitude of different devices and surgical interventions that are selected in function of the type and degree of hearing loss of the patient. These known therapies cover the major part of possible hearing losses except severe to profound mixed hearing loss. The present thesis results from an industrial project that aims to develop a dedicated hearing therapy that treats this kind of hearing loss based on direct acoustic cochlear stimulation (DACS). A semi-implantable investigational hearing device is used in a clinical trial with four patients to validate this novel therapy approach. The development of this investigational hearing implant presents a considerable challenge due to the numerous medical and engineering aspects that have to be considered. The conceptual design process is therefore of central importance because it incorporates all the relevant aspects and defines the future device characteristics and performances. Fundamental design aspects that are not considered at this early design stage can only be implemented with difficulties and increased efforts later on. This is the motivation to develop within the scope of this thesis a conceptual design methodology that supports and facilitates this essential design phase. A conceptual design methodology for acoustic hearing implants is therefore presented at the example of the DACS investigational device. The methodology consists of a four-step-procedure and a system diagram tool. The four steps of the procedure are: Step 1: determination and analysis of the human body functions that shall be emulated by the system Step 2: determination of the preliminary system architecture Step 3: determination of the user and system requirements Step 4: conceptual design The system diagram tool guides the engineer through all four steps and helps visualizing and structuring the system environments, functions, interfaces and, most important, all interactions between the system elements and environments. The diagram drawing rules ensure that all requirements coming from the environments are correctly linked with the concerned system functions and therefore ensure that the system meets all these requirements. Finally, the system diagram is a valuable tool for the conceptual design step. Because of the clearly represented system elements and interactions, it supports the development of integrated solutions. The output of the presented methodology is a design concept that cannot be directly quantified to assess the benefit of the methodology. Only once the resulting device is successfully verified against the system requirements and validated against the user requirements, it can be concluded that the underlying concept is adequate. The outcome of the clinical study shows that the four implanted patients benefit all from the DACS device, which is therefore well suited to treat severe to profound mixed hearing loss. This confirms that device meets the user requirements and implies that conceptual design methodology is adequate. The realized investigational device consists of an externally worn audio processor, a percutaneous connector, and an implantable actuator. The hermetically sealed balanced armature actuator provides a maximal equivalent sound pressure level of 125 dB over the frequency range between 100 Hz and 10'000 Hz with a limited 1-mW power supply. A lumped parameter model of the actuator dynamics and a finite element model of the electro-mechanic actuator are used for the detailed actuator design.