Measuring and modeling basic properties of the human middle ear and ear canal. Part I: Model structure and measuring techniques

摘要

This is the first of a three part paper dealing with the acoustical and mechanical properties of the middle ear. As a functional description of the middle ear cannot be separated from that of the ear canal, the external ear is also considered, but regarding only the sound field inside the ear canal, not outside the head: The cochlea also has to be taken into account to provide a complete description, but only with respect to its input impedance, because it has the function of a load impedance of the middle ear. All the measurements (the original data are always complex transfer functions depending on frequency) were taken using fresh human temporal bones. An attempt was made to obtain all the data necessary to develop a one-dimensional model representing the "basic" functions of the middle ear and ear canal, i. e. those functions that are predicted by the one-dimensional model. In reality many fluctuations caused by varying vibrational modes occur. The measurements indicate that it is actually reasonable to assume a basic function which is superimposed by fluctuations. Thus a fairly comprehensive understanding of the "basic" effects of the middle ear and ear canal can be achieved and described by a one-dimensional model. Most elements of the simple model can be quantified on the basis of the measurements. Some elements (the joint impedances and some elements of the stapes and cochlea) are estimated on the basis of the measurements, or chosen from within a plausible range. The model reflects our present knowledge, but may be subject to changes if further measurements are performed in the future. The organization of the three parts is as follows. In this first paper (I) the "philosophy" of modeling and measuring is presented. Modeling and measuring together form a whole in that the model comprises solely measurable elements and all the measuring procedures refer to the model. The basic model structure, which is then extended to include more detailed circuits in the following parts, is given. Particular attention is paid to the "drum coupling region" at the end of the ear canal, because this notion considerably influences our measuring techniques. The equipment and its application is described in part I. Part II [1] gives a complete survey of all the parts of the middle and external ear which are considered to contribute to the basic function. The corresponding blocks of the model are investigated separately, assuming a one-port (measuring an impedance) or a two-port (measuring a chain-matrix). The equipment used consists of an acoustical measuring tube (sound pressure, volume velocity and acoustical impedance), a mechanical measuring head (force, velocity, and mechanical impedance), a fiber-optic displacement sensor and a hydrophone for picking up the vestibular sound pressure. The transfer functions and impedances were measured by means of a four-channel spectrum analyzer, mostly in a frequency range from 160 Hz to 16 kHz. However, due to noise and systematical errors the upper frequency limit is often lower, usually about 10 kHz, but in some cases even about 5 kHz. The measurements comprise the ear canal(external radiation impedance, propagation losses, and the "drum coupling region"), the tympanic cavity and the antrum, the "kernel" (the "heart" of the middle ear, consisting of the drum, the malleus, and the incus including their suspensions), and the stapes and cochlea. In part III [2] the external and middle ear are considered as a whole, i. e., the subsystems presented in part II are put together. This is done by calculation (using the model) and by measurement. In this way frequency responses were measured which are redundant with respect to the measurements presented in part II. The functions are eardrum impedances measured using different conditions and transfer functions of the middle ear in both directions. Actually all the measurements of parts II and III together were used to derive the element values of the model.