Noncontact Modal Analysis of a Pipe Organ Reed using Airborne Ultrasound

Stimulated Vibrometry

May 25, 2004Acoustical Society of America Meeting

Thomas M. HuberPhysics Department, Gustavus Adolphus College

Mostafa Fatemi, Randy Kinnick, James GreenleafUltrasound 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. 2003 ASA) Mechanical driving of reed is indirect - driver on shallot shakes

systemDoes 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 f1 = 32.4 kHz

Frequency of second swept from f2 = 32.4kHz to 33.2 kHz

Difference frequency fAudio =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 PointsMeasured on Surface

Face-On ViewOf Vibrating Reed

Rotated ImageShowing Displacement

Modal Shapes of organ reed: Ultrasound Stimulated Vibrometry

1st Cantilever 726 Hz

Torsional2.95 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

ConclusionsDemonstrated 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

http://physics.gustavus.edu/~huber/asa2004/