Electrically evoked otoacoustic emissions (EEOAEs) are sounds measured in the ear canal when alternating current (AC) stimulation is passed into the cochlea. These sounds are attributed to the motile responses of outer hair cells (OHCs). The EEOAE has characteristic amplitude, phase, and fine structure. Multicomponent analysis of the EEOAE shows short (SDC) and long delay components (LDC) that are thought to originate from OHCs near the AC stimulating site and from OHCs at more remote locations, respectively. We measured the effects of various loud noise exposures on the EEOAE and the cochlear whole-nerve action potential (CAP) in animals chronically implanted with a scala tympani electrode. Noise exposures that produced permanent (PTS) or temporary threshold shifts (TTS) were associated with frequency-specific changes in CAP thresholds, EEOAE fine structure, and reductions in the amplitude of the LDC. A frequent observation in this study was an increase in the overall EEOAE amplitude after the noise exposure. The increase was correlated with increased SDC amplitude. The SDC was present in animals chemically treated with ototoxic drugs and mechanical damage to the cochlea. The SDC was eliminated after disarticulation of the ossicular chain. The presence of EEOAE fine structure in the postexposure response is an indicator of TTS in advance of CAP recovery. The results suggest that the EEOAE might be used to differentiate the mechanisms associated with TTS and PTS.
A detailed study of one-, two-, and three-impact per period motion of a vibro-impacting, pinned beam is presented involving experimental results, as well as one- and multi-degree-of-freedom theoretical models. The details of the impact event are examined and correlated to the qualitative appearance of the frequency response. In addition, it is noted that the multi-degree-of-freedom model is necessary in order to predict response at high frequencies. This study is unique in that the model system includes a pinned boundary condition, the forcing frequency is considerably lower than the fundamental in-contact natural frequency, and the frequency analysis extends into a range important for acoustic predictions.
Rattle has become an important issue in the automotive and aerospace industries. Design for the prevention and reduction of rattle noise requires that the underlying mechanisms be understood and powerful, flexible numericat tools be developed. In this paper, we focus on the former, developing a basic theoretical and experimental foundation for determining the vibro-acoustic behavior arising from the nontinear dynamics assoeiatd with the rattle prwess. In order to understand the fundamentrd mechanics of rattle, a model problem was formulatd involving a hinged plate rattling against a stiff contact point. me plate was modeled as a flexible beam and the resulting equations of motion were solved explicitly, We found that the calculated accelerations at the tip of the beam quantitatively agree with those measured from an experimental test stand. In addition, the predicted and measured sound pressure levels (SPL) at various points were found to agree qualitatively. Rnatly, the sensitivity of the transition to chaos on base motion amplitude and frequency were investigated experimentally and theoretically,~T
This study investigates the behavior of a vibro-impacting system with a rigid body mode. The model problem has been formulated as both a one-DOF and a multi-DOF piecewise linear system. The behavior of the models for a base excitation frequency range of 50–200 Hz (4.9–19.5% of the fundamental natural frequency of the in-contact case) is shown for constant base motion acceleration and displacement. It is seen that the models do not predict the same motion for the constant acceleration case, but agree quite closely for the constant displacement case. The system is capable of chaotic motion and several examples of Poincaré sections are shown which suggest the presence of a strange attractor. Future work including analytical methods of analysis for both the one- and multi-DOF systems is discussed.
Rattle has become an important issue in the automotive and aerospace industries. In order to understand the fundamental mechanics of rattle, a model problem involving a hinged plate rattling against a stiff contact point was formulated. In a previous study, this plate was modeled as a rigid body and the linearized equations of motion were solved explicitly. The calculated tip accelerations were compared to those measured from an experimental test stand and agreed qualitatively. Currently, the model has been expanded to include flexible modes in order to capture higher frequency behavior. In addition, instrumentation has been added to the test stand to measure sound-pressure level (SPL) and contact forces. The data for accelerations, contact forces, and SPL were compared in the time and frequency domains. Finally, the closed form flexible body solutions were analyzed with regard to stability to predict transitions from periodic to chaotic rattling behavior.
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