Herbert Hudde

Measurement of the eardrum impedance of human ears

The determination of an acoustical impedance requires measurements of pressure and volume velocity. As no direct method is available for measuring velocity in an ear canal, a technique was developed which is based on pure pressure measurements. The ear canal is used as a measuring tube, the area function of which is also deduced from the pressure measurements. High-frequency measurements in living subjects involve many sources of errors. A criterion for deciding if a good measurement has been made is given. The technique of measurements is described, regarding both the use of probe tube microphones and the computer aided data recording. Finally, the results are presented, and some comments are given. A reliable interpretation of the results seems to be impossible because of lack of our knowledge of the middle ear function at high frequencies.

Scattering matrix of a discontinuity with a nonrigid wall in a lossless circular duct

An abrupt change of cross section in a circular lossless duct is considered. The analysis includes a nonrigid wall at the interface of both parts of the duct. Whereas previous investigators used iterative algorithms to calculate the behavior of the discontinuity, here a straightforward method is developed which avoids any convergence problems. In particular, large discontinuities can also be examined even if an arbitrary number of higher wave modes can propagate. The solution is given in form of a scattering matrix. For the plane wave mode a simple equivalent circuit is derived. The results are compared to an approximate solution given by Karal [J. Acoust. Soc. Am. 25, 327–334 (1953)] and to a measurement. At low frequencies all data agree excellently. Some scattering parameters for different cases (different size of discontinuities, different wall admittances, difference frequency ranges) are represented. It is demonstrated that the magnitude of certain scattering parameters can exceed unity, if the corr...

Determination of the Shape and Inertia Properties of the Human Auditory Ossicles

This paper describes a method to accurately determine the shape of the human middle ear ossicles. If the density and the shape of a body are known, the inertia properties can be calculated using commercially available software. The inertia properties have been calculated for the stapes and for the incudomalleal unit. Different rectangular views of the stapes and the incudomalleal unit are shown to depict the position of the center of gravity and the position of the principal axes of inertia.

The acoustical input impedance of excised human lungs—Measurements and model matching

The input impedance at primary bronchi of excised human lungs was measured in the frequency range from 2-5000 Hz. For the measurements, a self-developed acoustic impedance head and a narrow-band measuring system with sinusoidal excitation were used. The lungs were inflated and deflated by using an arrangement called respiratory state controller. The impedances were thus measured at different states of lung inflation. An already existing mathematical model was developed further to cover not only fairly inflated lungs, but also deflated ones. The parameter sensitivity of this model is investigated. The acoustomechanical parameters of the model were fitted to match the impedances measured. It turns out that some of these parameters are hardly calculable. The values given in this paper were chosen to agree with the measurements and to be physically reasonable. Although the measurements were performed at primary bronchi, the model is able to predict also impedances at the top end of the trachea (at different respiratory states). This impedance is useful for speech signal processing applications. The model prediction of the trachea impedances agrees well with previous results of other authors.

A Finite Element Model of the Human Head for Auditory Bone Conduction Simulation

In order to investigate the mechanisms of bone conduction, a finite element model of the human head was developed. The most important steps of the modelling process are described. The model was excited by means of percutaneously applied forces in order to get a deeper insight into the way the parts of the peripheral hearing organ and the surrounding tissue vibrate. The analysis is done based on the division of the bone conduction mechanisms into components. The frequency-dependent patterns of vibration of the components are analyzed. Furthermore, the model allows for the calculation of the contribution of each component to the overall bone-conducted sound. The components interact in a complicated way, which strongly depends on the nature of the excitation and the spatial region to which it is applied.

The propagation constant in lossy circular tubes near the cutoff frequencies of higher‐order modes

The propagation constants of higher‐order modes in lossy circular ducts are investigated with particular interest to the frequency range around the corresponding cutoff frequencies. The analytic expressions for the propagation constants published up to now contain poles at each cutoff frequency. In this article, it is shown that no poles exist. The transition between the frequency range of evanescent and propagating modes is rapid but continuous. An explicit formula for the propagation constant is derived. The theory is experimentally checked with an indirect measuring method that is able to give results in close vicinity to the cutoff frequencies. The measured attenuation due to the transmission line losses is partially somewhat higher than theoretically expected. The deviations are less than about 30%.

Accuracy of acoustic ear canal impedances: Finite element simulation of measurement methods using a coupling tube

Acoustic impedances measured at the entrance of the ear canal provide information on both the ear canal geometry and the terminating impedance at the eardrum, in principle. However, practical experience reveals that measured results in the audio frequency range up to 20 kHz are frequently not very accurate. Measurement methods successfully tested in artificial tubes with varying area functions often fail when applied to real ear canals. The origin of these errors is investigated in this paper. To avoid mixing of systematical and other errors, no real measurements are performed. Instead finite element simulations focusing on the coupling between a connecting tube and the ear canal are regarded without simulating a particular measuring method in detail. It turns out that realistic coupling between the connecting tube and the ear canal causes characteristic shifts of the frequencies of measured pressure minima and maxima. The errors in minima mainly depend on the extent of the area discontinuity arising at the interface; the errors in maxima are determined by the alignment of the tube with respect to the ear canal. In summary, impedance measurements using coupling tubes appear questionable beyond 3 kHz.

A Functional View on the Peripheral Human Hearing Organ

The human hearing organ is a signal processor par excellence. Its amazing abilities are often described in terms of psycho-acoustic models. However, in this chapter the focus is laid on the physical background, particularly on the physics of the peripheral hearing organ. The peripheral system can be looked at as a signal conditioner and preprocessor which stimulates the central nervous system. It comprises acoustic, mechanic, hydro-acoustic, and electric components which, in total, realize a sensitive receiver and high-resolution spectral analyzer. For daily life it is extremely important that the hearing organ can also work under adverse conditions. This includes the need for general robustness and low sensitivity with respect to varying external and internal working conditions. In the hearing organ several strategies are found which noticeably differ from technical solutions.

Acoustical higher‐order mode scattering matrix of circular nonuniform lossy tubes without flow

In this paper, the acoustical behavior of circular ducts with nonuniform area functions is considered in terms of a corresponding scattering matrix. The elements of the scattering matrix denote the portions of reflected and transmitted waves of the mode order k when a wave of mode order n is incident. This way, the approach is able to treat wave modes of arbitrary order. When the tube under consideration consists of sections of constant cross‐sectional area, the method of calculation can be applied directly. Otherwise, the (continuous) area function has to be approximated by a corresponding step function; i.e., the tube is modeled by a certain number of contiguous sections (stepped duct approximation). In contrast to other publications, the area function is not restricted to be slowly varying along the tube axis. Losses as well as liners are simply taken into account by altering propagation constants and wave impedances, correspondingly. After derivation of the numerical algorithm, some examples of nonuni...

Key Features of the Human Middle Ear

The human middle ear was investigated using a generalised circuit model which can simulate the spatial vibrations of the ossicular chain. The behaviour was observed for acoustic excitation via the normal air conduction path and for mechanic excitation due to shaking the complete temporal bone. More insight into the functionality of the middle ear was obtained by also considering abnormal conditions such as stiffenings. It turned out that the mammalian middle ear is superior to a columella ear due to its particular design. The placement of comparably heavy bones (malleus head and incus body) outside the main transmission path between the manubrium and the stapes footplate in combination with a very flexible ossicular chain creates several favourable properties.