The current experiment shows that ultrasound radiation force excitation can be used
as a noncontact method for modal excitation as an alternative to a standard mechanical shaker for objects as
large as a guitar. By using both the
ultrasound excitation and Polytec vibrometer, it is possible to perform completely noncontact modal analysis!
It has now been demonstrated that the ultrasound radiation force can be used for non-contact excitation of objects ranging in
size from atomic force microscope microcantilevers and other MEMS devices 300μm in length,
to hard drive suspensions and centimeter-scale cantilevers, to an object as large as an
acoustic guitar. The exact same ultrasound transducer was used for non-contact excitation of all of these objects,
ranging from less than 100 Hz for
the guitar, to over 200 kHz for MEMS microcantilevers.
The guitar used was a Cordoba 45R classical guitar. It was hanging from a guitar stand near the top of the neck at the nut; a piece of felt was inserted in the strings to damp the string vibration. To allow for comparison
between mechanical and ultrasound excitation, the back was in contact with the shaker stinger at a single point.
To perform non-contact excitation, a pair of ultrasound
transducers with carrier frequencies of about 500 kHz were driven with a dual-sideband
suppressed carrier waveform that was stepped through difference frequencies of 70 Hz to 500 Hz.
The ultrasound radiation force results in excitation at this difference frequency of 70 Hz to 500 Hz.
The ultrasound transducers were highly focued to a spot roughly 2mm in
diameter, with almost no excitation outside of that focus spot.
For comparision with conventional base excitation,
a mechanical shaker (Bruel-Kjaer 4810)was placed in contact with the back of the guitar with a small piece of wax on the end of the stinger. For base excitation measurements, the mechanical shaker was driven with a pseudo-random frequency waveform that included frequency components from 70 Hz to 500 Hz.
For both the ultrasound exciation and base excitation methods, the vibrometer beam was pointed at the scan points illustrated below, and the response was measured at each point.
For measurements using ultrasound radiation force excitation, the displacement was much smaller than that obtained
using a mechanical shaker. Therefore a Stanford Research SR830 lock-in amplifier was used to measure the magnitude and
phase instead of the built-in ADC on the vibrometer conroller.
The graph below shows the velocity measured at a single point on the surface as a function of the
The mechanical shaker was used for excitation for graph (a), and the ultrasound radiation force
was used for excitation for graph (b).
The resonance frequencies measured using both mechanical base excitation and non-contact ultrasound excitation were
The vibrometer software determined the deflection shape based on the
magnitude and phase measured at each scan point. The video files for these are available below.
Note: Each video file is about 1MB, so they may take some time to download