Noncontact Modal Analysis of a Pipe Organ Reed using Airborne Ultrasound Stimulated Vibrometry May 25, 2004 Acoustical Society of America Meeting Thomas M. Huber Physics Department, Gustavus Adolphus College Mostafa Fatemi, Randy Kinnick, James Greenleaf Ultrasound Research Laboratory, Mayo Clinic and Foundation

Overview  Introduction  Organ reed pipes  Introduction to ultrasound stimulated vibrometry in air  Comparison of ultrasound stimulation to other techniques  Conclusions

Two primary goals for this experiment  Demonstration of audio-range ultrasound stimulated vibrometry in air  Interference of ultrasound causes noncontact excitation of object  Ultrasound can be focused at point – little excitation of other areas  Doesn’t depend on electrical or other properties  Has been demonstrated in water, but not in air  Study vibrational modes of organ reed pipes  Torsional and other modes using mechanical shaker (Nov ASA)  Mechanical driving of reed is indirect - driver on shallot shakes system Does it yield actual modes?  Does the addition of a mechanical driver perturb the system?  With ultrasound stimulation, direct excitation of reed with no contact

Ultrasound Stimulated Vibrometry  Pair of ultrasound beams directed at object, in this case organ reed  One ultrasound transducer differs from other by audio-range frequency  Beat frequency between the ultrasound beams causes vibration of reed  Vibrations are detected using a laser vibrometer

Example of Ultrasound Stimulated Vibrometry  Two low-cost ($5) 32.8 kHz ultrasound transducers directed at organ reed  Frequency of one kept at f 1 = 32.4 kHz  Frequency of second swept from f 2 = 32.4kHz to 33.2 kHz  Difference frequency f Audio =0 to 800 Hz causes excitation of reed  Vibrations detected using Polytec PSV-300 scanning laser vibrometer  Reed resonance at 580 Hz clearly seen in vibrometer spectrum

Different ultrasound methods used in this study  Two separate ultrasound transducers, such as 32.8 kHz w/ 1kHz bandwidth  Somewhat difficult to align these to converge at same spot on reed  Confocal transducer ($5k); 550 kHz broadband, 30mm focal length  Annulus where inner and outer ring driven at two different frequencies  Single transducer driven in AM mode: Dual sideband-suppressed carrier  Both frequencies emitted from single transducer  Requires only one transducer & RF amplifier  However, both frequencies are combined in transducer; some audio emitted

Comparison of ultrasound stimulation and other excitation methods Ultrasound stimulation  Produces very clean spectra  Observed modes in good agreement with theory Mechanical Shaker  Placed in contact with shallot.  Can cause vibration of other portions of system (supports, clamps) Speaker  Placed 10 cm from reed  Frequency response limits range of excitation frequencies

Modal analysis using scanning vibrometer  Reed was excited using ultrasound or mechanical shaker  Scanning vibrometer deflects laser beam across vibrating surface  Uses Doppler shift to determine amplitude and phase of velocity at each point  Software plots 3-D deflection shape for each peak in spectrum Scan Points Measured on Surface Face-On View Of Vibrating Reed Rotated Image Showing Displacement

Modal Shapes of organ reed: Ultrasound Stimulated Vibrometry 1st Cantilever 726 Hz Torsional 2.95 kHz 2nd Cantilever 4.54 kHz  Mode shapes similar to shaker excitation; consistent with theory  Ultrasound spot size is 1mm; vibrating reed section is 9 mm by 5 mm

Selective excitation using ultrasound stimulation  With a focused ultrasound source, can control where excitation occurs  Ultrasound focused at reed surface, so only the reed itself is excited  The shaker vibrates the entire structure, including clamps, supports, etc.  Low-frequency peaks in shaker spectrum due to clamps/supports As evidence that these are due to supports: A 100g weight was added to one clamp, which shifted the resonance

Conclusions Demonstrated Ultrasound Stimulated Vibrometry in Air  Completely noncontact for both excitation and measurement  No mass loading of object  Excitation bandwidth of ultrasound transducer  Might enable mechanical excitation of objects at 20 kHz or higher  Selective – focused at surface, so no backgrounds due to clamps/supports  Single-point transducer (1 mm spot) excited modes of extended objects  Vibrations in excess of 5μm (4mm/s) at 145 Hz for 36mm x 6mm reed  May be applicable in industrial or commercial settings, such as MEMS Vibrational Modes of Reed Organ Pipe  Validation of prior measurements showing torsional and higher-order modes  Identical modal frequencies for shaker and ultrasound excitation  Consistent mode shapes for both excitation methods  Indicates that mechanical shaker technique valid for this system