Sound heads (two
are shown attached to a calipers above) will respond to frequencies
from 5 MHz to 20 MHz. The center frequency is approximately 13 MHz.
The sound heads have a slot and screw so that they can be mounted
on a calipers or any other 1/8 inch thick material. The emitted sound
is a 2 mm wide beam perpendicular to the front face of the head. In
the picture the sound travels from one head to the other. The beam
emerges from near the bottom of the head.
The IDP Pulser Box is one
way to excite the Sound Heads. It puts out a very short, high Voltage
pulse. When connected to the Sound Heads, it results in a short pulse
of ultrasound with a Fourier transform center frequency of around
13 MHz. A second output is a low Voltage pulse which occurs at the
same time as the high Voltage pulse. It can be used as a time reference
or oscilloscope trigger. A 12 Volt external power supply is included.
The Protractor Table is used for the acoustical refraction and diffraction
experiments. It consists of a rotatable support table with a protractor
to measure the angles of the transmitter and receiver Sound Heads,
and a swing arm mounting for the receiver Sound Head.
Many physics experiments can be done with the Sound Heads. Below
is a list of several of them, along with a brief description and a
list of the equipment required. A kit is available from IDP for each
of the experiments.In addition, all of these experiments require the
use of an oscilloscope, BNC patch cables, and some laboratory stands
and rods.
The speed of sound can be measured in liquids, solids, and gases.
The setup is two Sound Heads attached to a calipers, one of them is
driven by the Pulser Box, the other is attached to an oscilloscope.
With the short rise time from the Pulser Box, the time delay from
excitation to reception in liquid can be measured very accurately
on the oscilloscope. The calipers gives the distance involved, and
the ratio is the sound speed.
For a solid sample in water, the difference in time delay, delta(t),
between the solid being in or out of the sound path is used with the
sample length in the formula Vs = 1/(1/Vw – delta(t)/L) to give
sound speed, where Vw is the speed of sound in water, delta(t) is
the time delay difference, and L is the sample length.
Speed
of Sound Equipment
Required:
two Sound Heads (left and right)
one Pulser Box with power supply
Accessories included in the kit:
material samples of aluminum, copper, and polycarbonate
Acoustical Refraction can be measured with a Protractor
Table and swing arm similar to a spectrometer, with Sound Heads
in place of optics. The arrangement is similar to a prism spectrometer
experiment. The aluminum, or other solid, is not acoustically dispersive
as is a prism for light, instead what is measured is the refracted
signal for the longitudinal wave and the shear wave. With these
results, and the density of the aluminum, all of the elastic constants
can be calculated.
Acoustical Refraction Equipment
Required:
two Sound Heads
one Pulser Box with power supply
one Protractor Table
Accessories included in the kit:
material samples of aluminum, copper, and plastic
a petri dish
several thin plate samples for the thin plates part of the experiment
Acoustical Diffraction can also be measured on the Protractor Table.
The acoustical diffraction grating shown in the picture can be placed
in the center of the Protractor Table, oriented perpendicular to
the path of the beam, and the receiver Sound Head used to measure
the emerging field pattern as a function of angle. Likewise, field
patterns of single or multiple slits can be measured.
Acoustical
Diffraction Equipment
Required:
two Sound Heads
one Protractor Table
one burst generator
The signal source for this experiment is a burst generator. IDP
does not supply this generator, but will give suggestions on which
commercially available generators are adequate.
Acousto-Optical devices are usually made of a material like tellurium
because they have a large acousto-optic coefficient; however, water
has an acousto-optic effect large enough to be used in a laboratory
experiment. With the IDP Sound Heads, a simple acousto-optic modulator
can be made with any laser with a beam diameter less than 2 mm,
a spectroscopic tray, Sound Heads, and a CW driver with sufficient
amplitude.
Acousto-optic
Equipment
Required:
two Sound Heads
one spectrometer sample holder.
a source of 13 MHz with sufficient output capability
An acoustic etalon is based on the same principle as an optical
etalon; namely, that multiple reflections add when the spacing is
such that they all arrive at the second boundary in phase. A function
generator is connected to one Sound Head. The frequency of the function
generator must be within the operating range of the transducers
(5 to 20 MHz). The etalon is a block of aluminum in water. Since
the acoustic impedance of aluminum is around 17 Mrayls, and that
of water is 1.5 Mrayls, the reflection of each surface is ~ 85%.
This will not be a high finesse etalon, but good enough to show
the basic characteristics of such a device. The distance between
the peak responses of the receiver Sound Head is the free spectral
range of the etalon, and the width of the peaks gives the finesse.
These two quantities can be compared to calculated values derived
from the length of the etalon and the acoustic impedances.
One Instrument,
Many Experiments
Two-page summary of 5 physics experiments: the Speed of Sound in
Liquids, the Speed of Sound in Solids, Acoustical Refraction, Acoustical
Diffraction and Acoustical Etalon.
B. A. Auld, Acoustic Fields and Waves in Solids, 2nd Edition,
Krieger Pub. Co., 1990
Experiments in Mechanics, Wave Motion and Heat, Laboratory Manual
for 29:011, 29:027, & 29:081, Experiment W1: The Speed of Sound,
Department of Physics and Astronomy, The University of Iowa
A. R. Selfridge, Approximate Material Properties in Isotropic
Materials, IEEE Transactions on Sonics and Ultrasonics, Vol. SU-32,
No. 3, May 1985
Korpel, A., Acousto-Optics, 2nd edition, 1997, Marcel Dekker,
Inc.
Web Sites
www.ndt-ed.org
This is the web site of the Non-Destructive Testing Resource Center.
It has a large amount of instructional information on it. Follow the
“NDT Instructional Material” link to the Ultrasound part.
www.ondacorp.com
Onda has a good reference section on their web site, including a large
table of material properties.
(two
are shown attached to a calipers above) will respond to frequencies
from 5 MHz to 20 MHz. The center frequency is approximately 13 MHz.
The sound heads have a slot and screw so that they can be mounted
on a calipers or any other 1/8 inch thick material. The emitted sound
is a 2 mm wide beam perpendicular to the front face of the head. In
the picture the sound travels from one head to the other. The beam
emerges from near the bottom of the head.
