Fiber optic lever displacement transducer

Spatial Multiplexing Technique for Improving Dynamic Range of Speckle Correlation based Optical Lever

Speckle correlation based optical levers (SC-OptLev) possess attractive characteristics suitable for sensing small changes in the angular orientations of surfaces. In this study, we propose and demonstrate a spatial multiplexing technique for improving the dynamic range of SC-OptLev. When the surface is in its initial position, a synthetic speckle intensity pattern, larger than the area of the image sensor is created by transversely shifting the image sensor and recording different sections of a larger speckle pattern. Then, the acquired images are stitched together by a computer program into one relatively large synthetic speckle pattern. Following the calibration stage, the synthetic speckle intensity pattern is used to sense changes in the surface's angular orientation. The surface is monitored in real-time by recording part of the speckle pattern which lies within the sensor area. Next, the recorded speckle pattern is cross-correlated with the synthetic speckle pattern in the computer. The resulting shift of the correlation peak indicates the angular orientations of the reflective surface under test. This spatial-multiplexing technique enables sensing changes in the angular orientation of the surface beyond the limit imposed by the physical size of the image sensor.

Implementation of a speckle-correlation-based optical lever with extended dynamic range

A speckle-correlation-based optical lever (SC-OptLev) is constructed for the measurement of small changes in the orientation angle of a surface. The dynamic range of SC-OptLev is found to be twice that of a conventional OptLev for the same experimental configurations. Different filtering mechanisms are implemented, and the correlation results are compared. Two types of computer-automated SC-OptLevs, an open-source-based computing system with a low-cost image sensor and a commercial computing system, are presented with assistive computational modules.

Femtosecond laser enabled selective micro-holes drilling on the multicore-fiber facet for displacement sensor application

We experimentally demonstrate a femtosecond laser enabled selective micro-holes drilling technique on the multicore-fiber facet. The precise position of individual cores at the seven-core fiber facet is initially locked by the image processing algorithm, and then six micro-holes are successfully fabricated after the pulse energy of femtosecond laser is optimized. Meanwhile, the use of fabricated seven-core fiber for the application of reflective intensity-modulated fiber optics displacement sensor (RIM-FODS) is comprehensively investigated. By using the beam propagation method (BPM), we theoretically investigate the effect of micro-hole depth on the RIM-FODS performance, in terms of both dead zone and measurement range. We identify that, with the increase of micro-hole depth, the dead zone range can be substantially reduced at the expense of measurement range reduction. However, multiple micro-holes with a successive depth difference can overcome such problem. When the micro-holes with depths of 5, 10, 15, 20, 25, 30 μm are fabricated on the seven-core fiber facet, and the dead zone range can be substantially reduced from 150 μm to 20 μm, together with an extension of measurement range from 250 μm to 400 μm.

Output characteristics of two-circle coaxial optical fiber bundle with regard to three-dimensional tip clearance

A two-circle coaxial optical fiber bundle is a type of optical sensor which has been widely used in non-contact radial displacement measurement applications. This paper has focused on output characteristics of a two-circle coaxial optical fiber bundle used as a measurement unit in a probe designed to measure a three-dimensional tip clearance. First, a model that could calculate intensity point-by-point on each receiving fiber with a simplified algorithm of overlap area was established, an aperture angle compensation algorithm was then proposed to improve the performance of the model. In order to test the reliability of the model, an experiment was done with different three-dimensional displacement of the reflector. Comparison between experimental and simulation results indicates the model built in this article could better describe how three-dimensional displacement of reflector affects the output of the bundle than previous studies.

Tip-Clearance Measurement in the First Stage of the Compressor of an Aircraft Engine

In this article, we report the design of a reflective intensity-modulated optical fiber sensor for blade tip-clearance measurement, and the experimental results for the first stage of a compressor of an aircraft engine operating in real conditions. The tests were performed in a ground test cell, where the engine completed four cycles from idling state to takeoff and back to idling state. During these tests, the rotational speed of the compressor ranged between 7000 and 15,600 rpm. The main component of the sensor is a tetrafurcated bundle of optical fibers, with which the resulting precision of the experimental measurements was 12 µm for a measurement range from 2 to 4 mm. To get this precision the effect of temperature on the optoelectronic components of the sensor was compensated by calibrating the sensor in a climate chamber. A custom-designed MATLAB program was employed to simulate the behavior of the sensor prior to its manufacture.

Displacement sensor based on Kerr induced phase-modulation of orthogonally polarized sinusoidal optical signals

We present a novel short, medium, and long range displacement sensor using a Kerr phase-interrogator. Displacement induces relative phase variation between two orthogonally polarized sinusoidal optical signals. The Kerr phase-interrogator converts the phase variation into power variation through Kerr induced phase-modulation. Displacement sensing over a range of 12 mm with micron level resolution around the quadrature points is demonstrated.

An optical fiber bundle sensor for tip clearance and tip timing measurements in a turbine rig

When it comes to measuring blade-tip clearance or blade-tip timing in turbines, reflective intensity-modulated optical fiber sensors overcome several traditional limitations of capacitive, inductive or discharging probe sensors. This paper presents the signals and results corresponding to the third stage of a multistage turbine rig, obtained from a transonic wind-tunnel test. The probe is based on a trifurcated bundle of optical fibers that is mounted on the turbine casing. To eliminate the influence of light source intensity variations and blade surface reflectivity, the sensing principle is based on the quotient of the voltages obtained from the two receiving bundle legs. A discrepancy lower than 3% with respect to a commercial sensor was observed in tip clearance measurements. Regarding tip timing measurements, the travel wave spectrum was obtained, which provides the average vibration amplitude for all blades at a particular nodal diameter. With this approach, both blade-tip timing and tip clearance measurements can be carried out simultaneously. The results obtained on the test turbine rig demonstrate the suitability and reliability of the type of sensor used, and suggest the possibility of performing these measurements in real turbines under real working conditions.

Understanding the effects of Doppler phenomena in white light Fabry-Perot interferometers for simultaneous position and velocity measurement

In static tests, low-power (<5 mW) white light extrinsic Fabry-Perot interferometric position sensors offer high-accuracy (μm) absolute measurements of a target's position over large (cm) axial-position ranges, and since position is demodulated directly from phase in the interferogram, these sensors are robust to fluctuations in measured power levels. However, target surface dynamics distort the interferogram via Doppler shifting, introducing a bias in the demodulation process. With typical commercial off-the-shelf hardware, a broadband source centered near 1550 nm, and an otherwise typical setup, the bias may be as large as 50-100 μm for target surface velocities as low as 0.1 mm/s. In this paper, the authors derive a model for this Doppler-induced position bias, relating its magnitude to three swept-filter tuning parameters. Target velocity (magnitude and direction) is calculated using this relationship in conjunction with a phase-diversity approach, and knowledge of the target's velocity is then used to compensate exactly for the position bias. The phase-diversity approach exploits side-by-side measurement signals, transmitted through separate swept filters with distinct tuning parameters, and permits simultaneous measurement of target velocity and target position, thereby mitigating the most fundamental performance limitation that exists on dynamic white light interferometric position sensors.

Fiber optic displacement sensor with a large extendable measurement range while maintaining equally high sensitivity, linearity, and accuracy

This paper presents a fiber optic displacement sensor composed of a transmissive grating panel, a reflection mirror, and two optical fibers as a transceiver. The proposed fiber optic displacement sensor guarantees a stable reflected signal acquisition for application in real industrial fields. Through a parametric study of the grating pitch of the transmissive grating panel, the signal-to-noise ratio, linearity, resolution, accuracy error, and sensitivity of the proposed sensor were investigated. The measured bidirectional movement demonstrated a peak to peak accuracy of 10.5 μm, high linearity of 0.9996, resolution of 3.1 μm at the full bandwidth, signal-to-noise ratio of 27.7, and high sensitivity of 31.8 μm/rad during a movement of 16,004.0 μm using the transmissive grating panel, which had a grating pitch of 200 μm. Even for an extended measurement range, the proposed scheme enables the same accuracy, linearity, and sensitivity to be maintained when compared with conventional laser displacement sensors and fiber optic displacement sensors.

Using a validated transmission model for the optimization of bundled fiber optic displacement sensors

A variety of intensity-modulated optical displacement sensor architectures have been proposed for use in noncontacting sensing applications, with one of the most widely implemented architectures being the bundled displacement sensor. To the best of the authors' knowledge, the arrangement of measurement fibers in previously reported bundled displacement sensors has not been configured with the use of a validated optical transmission model. Such a model has utility in accurately describing the sensor's performance a priori and thereby guides the arrangement of the fibers within the bundle to meet application-specific performance needs. In this paper, a recently validated transmission model is used for these purposes, and an optimization approach that employs a genetic algorithm efficiently explores the design space of the proposed bundle sensor architecture. From the converged output of the optimization routine, a bundled displacement sensor configuration is designed and experimentally tested, offering linear performance with a sensitivity of -0.066 μm(-1) and displacement measurement error of 223 μm over the axial displacement range of 6-8 mm. It is shown that this optimization approach may be generalized to determine optimized bundle configurations that offer high-sensitivity performance, with an acceptable error level, over a variety of axial displacement ranges. This document has been approved by Los Alamos National Laboratory for unlimited public release (LA-UR 11-03413).

Biomedical photoacoustic imaging

Photoacoustic (PA) imaging, also called optoacoustic imaging, is a new biomedical imaging modality based on the use of laser-generated ultrasound that has emerged over the last decade. It is a hybrid modality, combining the high-contrast and spectroscopic-based specificity of optical imaging with the high spatial resolution of ultrasound imaging. In essence, a PA image can be regarded as an ultrasound image in which the contrast depends not on the mechanical and elastic properties of the tissue, but its optical properties, specifically optical absorption. As a consequence, it offers greater specificity than conventional ultrasound imaging with the ability to detect haemoglobin, lipids, water and other light-absorbing chomophores, but with greater penetration depth than purely optical imaging modalities that rely on ballistic photons. As well as visualizing anatomical structures such as the microvasculature, it can also provide functional information in the form of blood oxygenation, blood flow and temperature. All of this can be achieved over a wide range of length scales from micrometres to centimetres with scalable spatial resolution. These attributes lend PA imaging to a wide variety of applications in clinical medicine, preclinical research and basic biology for studying cancer, cardiovascular disease, abnormalities of the microcirculation and other conditions. With the emergence of a variety of truly compelling in vivo images obtained by a number of groups around the world in the last 2-3 years, the technique has come of age and the promise of PA imaging is now beginning to be realized. Recent highlights include the demonstration of whole-body small-animal imaging, the first demonstrations of molecular imaging, the introduction of new microscopy modes and the first steps towards clinical breast imaging being taken as well as a myriad of in vivo preclinical imaging studies. In this article, the underlying physical principles of the technique, its practical implementation, and a range of clinical and preclinical applications are reviewed.

One-dimensional single-mode fiber-optic displacement sensors for submillimeter measurements

We demonstrate the working principle of a one-dimensional intensity-based fiber-optic displacement sensor. The sensor consists of one receiving fiber, which is moved laterally in the optical field emitted by an emitting fiber. It is shown numerically that the sensor response is highly linear (nonlinearity error of 0.1 to 2%) for a wide range of travel (2.24 to 860 microm). The sensor response is also simulated experimentally using a highly precise robot, the results of which correspond very closely to numerical ones. Linearity, travel, and sensitivity are experimentally determined for different gaps between the emitting and the receiving fibers (10 microm to 10 mm). A design chart that includes the nonlinearity error (0.5% to 2%), the travel (2.78 to 860 microm), the sensitivity (0.032 to 0.37 dB/microm), and the gap distance (1 to 10 mm) is finally proposed.

Plethysmographic arterial waveform strain discrimination by Fisher's method

Plethysmography has been used for over 50 years to measure gross change in tissue blood volume. Over the cardiac cycle, perfused tissue initially expands as the blood flow into the arterioles exceeds the flow through the capillary bed. Later in the cardiac cycle, the accumulated blood drains into the venous vasculature, allowing the tissue to return to its presystolic blood volume. Specific features in the plethysmographic waveform can be used to identify normal and abnormal perfusion. We are developing a Doppler strain-imaging technique to measure the local pulsatile expansion and relaxation of tissue analogous to the gross measurement of tissue volume change with conventional plethysmography. A phantom has been built to generate plethysmographic-style strains with amplitudes of less than 0.1% in a tissue-mimicking material. With Fisher's discriminant analysis, it is shown that normal and abnormal plethysmographic-style strains can be differentiated with high sensitivities using the Fourier components of the strain waveforms normalized to compensate for the variance in the strain amplitude estimate.

Mechanics of the mammalian cochlea

In mammals, environmental sounds stimulate the auditory receptor, the cochlea, via vibrations of the stapes, the innermost of the middle ear ossicles. These vibrations produce displacement waves that travel on the elongated and spirally wound basilar membrane (BM). As they travel, waves grow in amplitude, reaching a maximum and then dying out. The location of maximum BM motion is a function of stimulus frequency, with high-frequency waves being localized to the "base" of the cochlea (near the stapes) and low-frequency waves approaching the "apex" of the cochlea. Thus each cochlear site has a characteristic frequency (CF), to which it responds maximally. BM vibrations produce motion of hair cell stereocilia, which gates stereociliar transduction channels leading to the generation of hair cell receptor potentials and the excitation of afferent auditory nerve fibers. At the base of the cochlea, BM motion exhibits a CF-specific and level-dependent compressive nonlinearity such that responses to low-level, near-CF stimuli are sensitive and sharply frequency-tuned and responses to intense stimuli are insensitive and poorly tuned. The high sensitivity and sharp-frequency tuning, as well as compression and other nonlinearities (two-tone suppression and intermodulation distortion), are highly labile, indicating the presence in normal cochleae of a positive feedback from the organ of Corti, the "cochlear amplifier." This mechanism involves forces generated by the outer hair cells and controlled, directly or indirectly, by their transduction currents. At the apex of the cochlea, nonlinearities appear to be less prominent than at the base, perhaps implying that the cochlear amplifier plays a lesser role in determining apical mechanical responses to sound. Whether at the base or the apex, the properties of BM vibration adequately account for most frequency-specific properties of the responses to sound of auditory nerve fibers.

Development of a differential, optical, reflective displacement sensor by use of a multilayered waveguide

A noncontact and compact optical displacement sensor is proposed and demonstrated. The principle of this system is based on the differential optical-fiber displacement sensor [Appl. Opt. 38, 1103 (1999)]. The waveguide of the sensor consists of three thin plate glasses. This approach can miniaturize and lighten the system. The performance of the sensor is geometrically analyzed. The linearity and working range of the sensor are significantly improved compared with those of the optical fiber.

Development of a differential optical-fiber displacement sensor

A novel optical-fiber displacement sensor is proposed and demonstrated. It consists of a laser diode light source, an optical-fiber probe, and two photodetectors. The bundling of the probe is sectioned into three parts: a centrally positioned fiber in the bundle for illumination, the first-neighbor fibers for receiving (part I), and the remaining fibers for receiving (part II). The ratio of the difference to the sum of the output signals from the part I and the part II receiving fibers can eliminate the variation in the sensitivity of the sensor to reflectivity of the target. The performance of the sensor is geometrically analyzed. The working distance is determined by the distance from the centered illuminating fiber to the boundary between the part I and the part II receiving fibers. The experimental measurements made with three different reflectivity targets confirm that the sensor performance is independent of the three reflectivities, as predicted by the analysis.

Fiber-optic sensor system for rapid positioning of a microelectrode array

A fiber-optic intensity sensor has been developed for vertically positioning microelectrode arrays above the retina of a live frog. Closely spaced fibers illuminate and collect reflections from the retinal surface, and the output is electronically processed to drive an automated positioning circuit. Experimental and theoretical evaluations of fiber types and separation for both specular and diffuse reflectors, in vitro and in vivo, are presented, and multimode fibers on 125-mum centers are chosen for retinal experimentation. The sensor has applications in assessing spatial selectivity of stimulation of a multielectrode array and may be adaptable for lateral positioning.

Vibration monitoring by using a dynamic proximity sensor with interferometric encoding

An optical sensor for the dynamic measurement of proximity is presented. The sensor combines an extrinsic geometric transducer with interferometric encoding for high vibration sensitivity. Static calibration showed a unique variation in interference contrast over at least 60 fringes, leading to a measurement range of 20 mum and a resolution of at least 0.033mum. Dynamic excitation by using low-amplitude vibrations at 3.6 kHz showed a similar contrast variation, verifying fringe discrimination up to the sixth order. With verification of dynamic performance over all 60 fringes, the sensor should offer a low-cost approach to vibration monitoring in electrical switchgear.

Geometrical analysis of an optical fiber bundle displacement sensor

The performance of a multifiber optical lever was geometrically analyzed by extending the Cook and Hamm model [Appl. Opt. 34, 5854-5860 (1995)] for a basic seven-fiber optical lever. The generalized relationships between sensitivity and the displacement detection limit to the fiber core radius, illumination irradiance, and coupling angle were obtained by analyses of three various types of light source, i.e., a parallel beam light source, an infinite plane light source, and a point light source. The analysis of the point light source was confirmed by a measurement that used the light source of a light-emitting diode. The sensitivity of the fiber-optic lever is inversely proportional to the fiber core radius, whereas the receiving light power is proportional to the number of illuminating and receiving fibers. Thus, the bundling of the finer fiber with the larger number of illuminating and receiving fibers is more effective for improving sensitivity and the displacement detection limit.

Optical fiber bundle displacement sensor using an ac-modulated light source with subnanometer resolution and low thermal drift

An optical fiber bundle displacement sensor with subnanometer order resolution and low thermal drift is proposed. The setup is based on a carrier amplifier system and involves techniques to eliminate fluctuation in the light power of the source. The achieved noise level of the sensor was 0.03nm/√Hz. The stability was estimated by comparing the outputs of two different sensors from the same target for 4 ks (67 min). The relative displacements between the fiber bundle ends of the two sensors and the target surface varied in the area of 400 nm depending on the ambient temperature variation at 2 °C. However, the difference in output between the two sensor systems is within 2 nm for more than 1 hour of measurement. It is expected that it would be reduced to within the area of 0.1 nm if the ambient temperature were controlled to within ±0.1 °C. It is concluded that the stability of the sensors is sufficiently good to be used with nanotechnological instruments.

Effect of lymph composition on an in vitro preparation of the alligator lizard cochlea

The effects of different artificial lymphs on the cochlear duct of the alligator lizard were studied in an in vitro preparation. The duct was dissected and cemented to the glass floor of a chamber that had been filled with an artificial lymph. The vestibular membrane was removed and latex beads (1-5 microns in diameter) were allowed to settle on the endolymphatic surface of the duct. During perfusion with an artificial lymph solution, the positions of beads were measured and video images of the duct were obtained. Artificial lymphs were isosmotic and included artificial endolymph (AE), artificial perilymph (AP), Leibovitz's L-15 culture medium, an AE solution whose calcium concentration was the same as that of AP, and AE and AP solutions in which gluconate was substituted for chloride ions. Results obtained in AE were consistently different from those in other lymphs. The displacements of beads, the projected area of the papilla, the occurrence of blebs, and direct observation of cells in the duct all indicated that the tissue swelled in AE (with or without 2 mmol/l Ca) but showed no consistent shrinking or swelling in any of the other artificial lymphs. Thus for the solutions we used, the presence of both potassium and chloride was required to elicit the swelling response to isosmotic artificial lymphs. There were some regional differences in the swelling response: the swelling of the endolymphatic surface of the tissue in a direction orthogonal to the basilar membrane surface was smaller on the free-standing region of the basilar papilla than either on the tectorial membrane or on the hyaline epithelial cells. The preparation was osmotically stable in AP and in both AE and AP solutions in which gluconate was substituted for chloride ions. After exposure to these solutions for as much as 300 min, the preparation showed no gross signs of deterioration visible with the light microscope, and continued to exhibit a highly specific osmotic response to the composition of the bathing medium.

Fiber optic displacement sensor employing a graded index lens

A novel intensity modulated fiber optic displacement sensor based on the relative motion of an optical fiber and a graded index lens is described. The technique allows the use of either single- or multimode optical fibers in either single or dual fiber configurations. Displacement sensitivities of 25 A have been measured using 50-microm core diam multimode fibers, with a light emitting diode source. Theoretical sensitivities of the order of 0.01 nm are predicted with the use of single-mode fibers.

Functional role of the olivo-cochlear bundle: a motor unit control system in the mammalian cochlea

A fiber optic lever is applied to the measurement of the motion of the basilar membrane motion in guinea pigs. In response to intense tones from either ear, the motion includes a substantial summating shift in the mean position in addition to a travelling wave originally described by von Békésy. His stroboscopic technique and most techniques used since have been concentrated upon measuring vibrations of the basilar membrane synchronous with the stimulus and have been insensitive to variations in the baseline position such as a summating component of motion analogous to the extracellular summating potential. In addition to the role of the outer hair cells in providing normal hearing sensitivity, they evidently play a role in regulating the mean position of the basilar membrane. For a fixed frequency, the polarity of the mean position varies systematically with sound level and place and summates with time since onset. Since these cells are the target cells for the olivocochlear bundle, homeostasis in the cochlea would appear to be linked efferent function and involve cochlear mechanics. The negative damping hypothesis asserts that hair cell activity is necessary for low thresholds. The results presented here demonstrate that OHC activity exists independent of neural thresholds. The discussion develops the concept that threshold losses are due to a mismatch of opposing tonic forces which normally maintain the mean position of the basilar membrane. Structure is examined in relation to function and the group of outer hair cells innervated by a single medial efferent neuron is identified as a motor unit. Implications of central control of individual motor units include peripheral involvement in selective attention tasks.

Fiber-optic displacement sensor with temporally separated signal and reference channels

A fiber-optic displacement sensor with temporally separated signal and reference channels is proposed. The sensing technique used is based on determining the relative amplitude of the signal and reference pulses that together form a double pulse. The relative amplitude of the pulses in the double pulse is a function of the displacement. A setup to generate the double pulse with individual pulses of 5-ns duration and 10-ns delay is described. A novel signal processing technique, used to determine the relative amplitude of such pulses in the double pulse by analyzing different portions of the signal spectrum, is explained. Experimental data are also provided.

Effects of middle ear pressure changes on umbo vibration

Effects of middle ear pressure changes on umbo vibration were studied in 5 fresh human temporal bones taken from cadavers. Umbo vibration to a constant sound pressure of 120 dB SPL at the tympanic membrane was measured with an MTI 1000 Fotonic Sensor. The results showed that there was a loss of umbo vibration at lower frequencies below 2 kHz; 5.0 +/- 1.0 dB loss at -100 mmH2O, 9.2 +/- 1.3 dB loss at -200 mmH2O, and 13.1 +/- 1.8 dB loss at -300 mmH2O. In contrast, there was a slight increase in umbo vibration at around 2-3 kHz. The effect was considered to be mainly due to increased stiffness of the tympanic membrane and decreased air volume in the middle ear cavity.

Fiber-optic diaphragm-curvature pressure transducer

A novel fiber-optic sensor that determines pressure from diaphragm curvature is discussed. This sensing technique exhibits a high degree of sensitivity and linearity and is less susceptible to environmental perturbations than previous intensity-modulating techniques. Experimental results are presented for a miniature pressure transducer suitable for in vivo medical applications.

Quantitative analysis of methods for reducing physiological brain pulsations

Normal movements of the mammalian brain, caused by the arterial and venous pressure fluctuations of each cardiac and respiratory cycle, have made obtaining stable intracellular recordings from neurons difficult. This study quantitated the movements of the cats' brainstem and examined the effects of traditional neurophysiological techniques used to reduce pulsation. Two components of brain movement were recorded: (1) an arterial component--relatively low amplitude (110-266 micrometers) and short duration (330-400 ms) excursions corresponding to the pressure wave of each cardiac systole [A-wave]; and (2) a pulmonary component--slower (10-12/min), high amplitude plateau-like displacement (300-950 micrometers) lasting for a time (2.4-5.1 s) corresponding to the inspiration of each respiratory cycle [P-wave]. Pneumothoraces and mechanical ventilation combined with elevating the animal's head reduced the pulmonary component by an average of 68% and the arterial component by 40%. Cerebrospinal fluid drainage could reduce the P-wave component of movement by as much as 50%. To reduce arterial pulsations below 100 micrometers, the mean arterial pressure (MAP) had to be lowered to less than 40 mm Hg, which was not compatible with maintaining normal brainstem auditory evoked responses. Residual movements at MAPs greater than 50 mm Hg were still sufficient to make stable intracellular penetration of small neurons difficult. The authors suggest the solution to this problem is the development of a cardiopulmonary bypass system which generates a non-pulsatile flow of oxygenated blood, described in a companion paper.

Comparison of acoustically coupled and mechanically coupled speech

Phonetically balanced work lists were mechanically coupled onto the ossicular chain of anesthetized guinea pigs by piezoelectric-type drivers and the resulting cochlear microphonic (CM) recorded on magnetic tape. Similar recordings of the CM resulting from free field tympanic membrane stimulation by hi-fi speakers were also obtained. The recordings were compared by conventional discrimination testing. In discrimination testing of all the raw recordings, listeners achieved essentially perfect scores. Addition of masking noise sufficient to reduce mean discrimination scores to 65-70% revealed no significant discrimination differences. When piezoelectrically initiated, CM-derived lists were compared with similar lists passed through hearing aids in an anechoic chamber, the preference of a panel of listeners for the quality of mechanically coupled speech was significantly higher. Mechanical-acoustical displacement equivalency at normal physiological levels and freedom from mechanical-electrical artifact were demonstrated by measurement of ossicular chain displacement by a fiber optic lever displacement transducer.