Laser Doppler Vibrometer System Research Articles

Vibration analysis of piezoelectric materials by optical methods

This study provides two noncontact and realtime optical measurement methods to assess the displacement, natural frequencies, and mode shapes of a vibrating piezoelectric material. The methods are carried out using amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) and laser Doppler vibrometer (LDV), which are full-field and point-wise displacement measurement, respectively. Because the fringe patterns measured by AF-ESPI appear as a clear picture at the natural frequency, both natural frequencies and mode shapes of the vibrating material can be successfully obtained. In the LDV system, a swept-sine excitation signal from the function generator to the beam can result in a corresponding peak in frequency response curve at natural frequencies. From the frequency response curve, the natural frequencies are thus acquired. Measured results by both methods are seen to be in good agreement with theoretical predictions by the Galerkin method and finite element method.

Objective evaluation of nasality using laser Doppler vibrometer

Objective measurement of nasality is very important in terms of clinical evaluation. But there is no definite methodology of objective evaluation of nasality. We already reported that new methodologies of the Laser Doppler Vibrometer (LDV) and Sub-miniature Electret Microphone (SEM), both of which could detect the nasal/non-nasal difference of sustained phonemes, seemed valid to evaluate nasality [Kumada et al., 140th ASA Meeting (2000); Kumada et al., 141st ASA Meeting (2001)]. LDV, used recently in an industrial field as well as medical/biological field, is the generator and the detector of the laser to detect the velocity of the object’s vibration by Doppler’s effect. LDV seemed a powerful tool for speech physiology by which vibration of a body wall can be measured during speech and singing, without any contact receiver. The objectives were to evaluate the degree of nasality of phonemes in words by LDV and SEM, and to discuss the possibility of the clinical use of these methodologies. The subjects were 5 healthy Japanese males (Tokyo dialect speakers). The materials were Japanese words /shinkansen/ and /shimbunshi/. Both LDV and SEM systems could detect a nasal/non-nasal difference in spoken words, and seemed valid to evaluate nasality in speech. Further study is planned using patients (e.g., cleft-palate) as well as healthy subjects.

Laser Doppler vibrometer and subminiature electret microphone: New methods for objective evaluation of nasality

It is very important to evaluate nasality objectively in terms of both intersubject (severity of nasality) and intrasubject (validity of treatments, i.e., operation, speech therapy etc.). But there is no definite methodology for an objective evaluation of nasality. Our objectives were to evaluate the degree of nasality with a laser Doppler vibrometer (LDV) and a subminiature electret microphone (SEM) (using an accelerometer as a known standard), and to discuss the possibility of clinical use of these methodologies. LDV, used recently in the industrial field as well as in the medical/biological field, is a generator and detector of lasers to detect the velocity of the object’s vibration by the Doppler effect. The subjects were five healthy Japanese males (Tokyo dialect speakers). Used were sustained phonations (about 2 s) of /m/ and five Japanese vowels (/i/, /e/, /a/, /o/, /u/), and the Japanese words, shinkansen and shimbunshi (six repetitions for each phoneme). By both LDV and SEM systems, a significant difference was found beween /m/ and each vowel, which means that both systems can detect nasal/non-nasal differences and seem valid to evaluate the severity of pathological nasality (e.g., a cleft palate). Further study is planned for not only sustained phonation and word but also for sentence level, using patients (pathology) as well as healthy subjects (physiology). Furthermore, LDV seems to be a powerful tool for speech physiology where vibration of the body wall can be measured during speech and singing, without any contact receiver.

Acoustic role of the buttress and posterior incudal ligament in human temporal bones

Objectives: In middle ear surgery using intact ear canal wall techniques, the buttress, which is the bony bridge at the medial end of the posterior-superior bony ear canal, is commonly retained during posterior tympanotomy. In some cases, the surgical exposure may be improved by resectioning the buttress, and this requires sectioning the posterior incudal ligament. To date, the acoustic effects of removing the buttress with sectioning of the attached ligament have not been studied. Method: Using a laser Doppler vibrometer system, 15 human cadaver temporal bones were measured with 80 dB sound pressure level at the tympanic membrane over the 0.1 to 10 kHz frequency range. Result: The resection of the buttress and sectioning the posterior incudal ligament had no effect on stapes footplate velocity. Conclusion: These results suggest that the posterior incudal ligament does not play a significant role in the acoustic function of the ossicles. (Otolaryngol Head Neck Surg 2001;124:274-8.)

On field validation of non-invasive laser scanning vibrometer measurement of damaged frescoes: experiments on large walls artificially aged

Past experiences demonstrated that the study of surface vibrations could be used to locate defect positions and sizes in frescoes. At present a non-invasive diagnostic system is under development and the aim of this work is to present the results obtained on large painted walls. After initial measurement set-ups based on accelerometers and impact hammers, a novel system based on laser vibrometers and acoustic stimulation was assembled. Full remote and contactless investigation of typical defects of frescoes, detachments, cracks and delaminations is thus possible with a very high accuracy. For the present investigation we employed a commercial scanning laser doppler vibrometer (SLDV) system, a horn loudspeaker and bass reflex enclosure to fully cover the audio frequency range. This paper will present experimental results gathered from large samples made by LRMH and compare them with those extracted by traditional investigation techniques.

Evaluation of a Cement Incus Replacement Prosthesis in a Temporal Bone Model

In reconstruction of the ossicular chain for a damaged incus, it is important that the incus replacement prosthesis (IRP) length is ideal in order to provide optimal tension between the tympanic membrane or malleus and stapes head to achieve the best post-operative hearing result. Even though the length of commercially available IRPs can be adjusted, it still may be difficult to achieve clinically. We describe experiments in a human temporal bone model using dental cement as an IRP after removal of the incus. This cement IRP (CIRP) hardens in situ and becomes the length of the gap to be spanned so that tension should be ideal. Two different CIRPs were studied; one was a conventional rod-type CIRP connecting either the umbo or mid-malleus handle to the stapes head. The second was a Y-shaped CIRP (Y-CIRP), connecting two sites on the malleus to the stapes head. The wide Y-CIRP connected the malleus head and umbo to the stapes head, while the narrow Y-CIRP connected the malleus neck and mid-handle to the stapes head. The acoustic performance of these experimental CIRPs was studied using a laser Doppler vibrometer system in 12 fresh human temporal bones. The CIRP demonstrated better acoustic performance than conventional IRPs studied previously in the same model. While all the CIRPs showed similar function below 2.0 kHz, the narrow Y-CIRP appeared best above 3.0 kHz. A prosthesis of this type may have an acoustic advantage over conventional IRPs.

A fiber optic dual‐beam laser Doppler vibrometer for measurement of electrostrictive and piezoactive response of thin films

A two‐beam laser Doppler vibrometer (LDV) is described, designed to measure the d33 electromechanical coupling coefficient for electrostrictive and piezoactive thin films. Optical fiber couplers are used to split the light into multiple beams and optical fibers are used to simultaneously illuminate the same spot on both sides of the film. The LDV measures the normal displacement on each side and the change in thickness of the film is determined from the algebraic sum of the two LDV signals. It is found that large bending motion of the film can lead to a significant error in the measured d33 coefficient. This is illustrated with data for PVF2 films and sources of this error are identified. The application of the LDV system to measure the large electrostrictive effect in polyurethane films is discussed. [Work supported by the Office of Naval Research.]

Measurement of evanescent waves on a cylindrical shell

Calculations are made of the reflection coefficient matrix at the end of a cylindrical shell for various end boundary conditions. The calculations show mode conversion on reflection at the end of the shell, including conversion from propagating to evanescent waves. The evanescent waves are studied experimentally using the laser Doppler vibrometer (LDV) described previously [H.-G. Kil etal., J. Acoust. Soc. Am. 93, 2389(A) (1993)]. The LDV system is used to measure the amplitude and phase of the particle velocity at a large number of points on the shell surface. The data are analyzed in the frequency-wave number domain using Prony decomposition. Data will be presented for a shell 0.94 m long and 1.6 mm thick, freely suspended in air and excited radially by a shaker at approximately twice the ring frequency. [Work supported by ONR.]

Laser-doppler vibrometer measurements of acoustic to seismic coupling

An atmospheric sound wave can couple with the poroelastic ground, resulting in ground motion. This is the phenomenon of acoustic to seismic coupling. Previously, ground velocity was measured using geophones. We report vertical ground velocity measurements made with a modified off-the-shelf laser-doppler vibrometer (LDV). The measurements are difficult because high atmospheric sound pressure levels of ≈ 100 dB relative to 20 μPA are necessary for good signal-to-noise in the acoustic-to-seismic coupling ratio measurements; the LDV also couples with the sound field above the ground. We report good agreement among geophone and LDV measurements of vertical ground velocity for an impulsive atmospheric sound source. A continuous wave sound source was used to measure the transfer function of the LDV system for frequencies in the range 100–500 Hz.

Measurement of head/disk spacing modulation using a two channel fiber optic laser Doppler vibrometer

A two-channel laser Doppler vibrometer system is described for studying spacing modulation of the head/disk interface in a magnetic recording disk file. Three types of sliders, namely a standard two-rail slider, an experimental negative-pressure slider, and a cross-cut slider, are studied. In comparing the results from the single-channel and dual-channel laser Doppler systems, excellent agreement is observed for the resonant frequencies of the air bearing. The single-channel system, however, exhibits problems with offset in the velocity and displacement signals that are absent with the two-channel system. >