ysics and Technical
Characteristics of
Ultrasonic Sonar
Systems
camera-design specifications determine the often un-
usual classroom behavior of these systems. System
components were designed to meet SX-70 camera
constraints: the system reliably determines ranges from
about 0.4 m11 m, operates on battery power (low-
voltage, high-peak current), is simple to use, and is
rugged and inexpensively mass-manufactured. The
Polaroid system consists of two main components: a
single transducer that acts as both the ultrasonic signal
source (loudspeaker) and detector (microphone), and
a small circuit board or ranging module that generates
and processes transducer signals into standard digital
logic signals.
2
The transducer is manufactured from a 0.07-mm
thick piece of kapton plastic film vacuum coated with
a thin layer of gold. The resulting flexible, conductive
foil (imagine a piece of aluminized mylar balloon) is
stretched on a circular frame held directly before a
rigid grooved circular aluminum plate. The two as-
Dan Maclsaac
O
ne of the most striking examples of tech-
nical innovation in elementary mechanics
pedagogy in the last 20 years has been the
widespread adoption of the ultrasonic
sonar ranger.
1
Hence, the underlying physics,
measurement limitations, classroom behavior, and
standard pedagogy for these systems is useful profes-
sional knowledge for all introductory mechanics
instructors. Most sonar systems for physics pedagogi-
cal use in North America (Vernier, TI, PASCO, etc.)
are assembled from ultrasonic ranging system compo-
nents patented and manufactured by the Polaroid
Corporation.
History and Physics of the Polaroid
System
Polaroid developed this system in the late 1970s for
use as an automatic focusing system for the Polaroid
SX-70 Sonar OneStep Land Camera, and original
Dan MacIsaac and Ari Hämäläinen
Dan MacIsaac is an active member of the AAPT and a frequent contributor to The
Physics Teacher. He coordinates the physics and physical science teacher preparation pro-
grams at Northern Arizona University, where he also teaches college physics for nonma-
jors. MacIsaac studies and writes on physics learning and teaching.
Department of Physics & Astronomy
Northern Arizona University
Campus Box 6010
Flagstaff AZ 86011-6010;
danmac@nau.edu THE PHYSICS TEACHER

Vol. 40, January 2002
40
Ari Hämäläinen
Ari Hämäläinen received his M.S. (1986) and Ph.D. (1998) degrees in physics from the
University of Helsinki. After two years as a schoolteacher, he is now a lecturer at the
University of Helsinki in the teacher education program. He teaches the laboratory
course and researches the utilization and implementation of microcomputer-based labo-
ratory techniques in physics teaching.
Department of Physics
University of Helsinki, Finland;
aohamala@cc.helsinki.fi
change in transducer capacitance, and the ranging
module reports to the computer via a digital signal
that a signal echo has been received. Upon receipt of
this digital signal by the computer, the elapsed-time
clock is stopped. The elapsed time ( t) is used along
semble as the two plates of a parallel plate capacitor,
where the aluminum plate is quite rigid and the kap-
ton foil plate is highly flexible (Fig. 1).
When a high-voltage signal (a 49.4-kHz square
wave at 300 to 400 V) is applied to the transducer
assembly, the foil is driven back and forth by electro-
static forces creating ultrasonic pressure waves in the
air. These waves are directed outward from the trans-
ducer in a characteristic beam pattern (Fig. 2) to strike
the target being ranged. The transducer is also operat-
ed as a microphone; returning reflected pulses physi-
cally move the foil, and this motion can be detected as
a change in transducer capacitance. This foil capacitor
arrangement is one of several systems capable of
producing and detecting ultrasound ordinary au-
dio-speaker cones cannot mechanically respond at
these frequencies.
The ranging module requires a 5-V supply capable
of short (1-ms) current surges of up to 2.5 A. The
ranging module transmitting circuitry accepts a digital
command signal from a computer or calculator to
transmit a pulse, then creates a pulse train of sixteen
5-V 49.4-kHz square-wave pulses. These low-voltage,
high-current pulses are then sent through a step-up
transformer to supply the transducer with low current,
300- to 400-V pulses. The transducer foil mechanical-
ly moves, converting the electrical pulse train to ultra-
sound and transmitting it outward from the front of
the transducer housing. After transmitting the pulse
train, the transmitter circuitry shuts down, and receiv-
ing circuitry is turned on to monitor the capacitance
of the transducer. The interface or calculator that is
running the transducer starts an elapsed-time clock
simultaneous to pulse transmission.
At the target, the ultrasound pulse train is partially
reflected and partially absorbed, then returns (greatly
attenuated) to the transducer. The receiver detects a
Fig. 1. Polaroid electrostatic-transducer assembly.
(Original art permission of Polaroid Corp.)
Fig. 2. Polaroid electrostatic-transducer transmission
beam pattern in 2D. Note the central lobe defines a 30º
wide detection cone for the ranger.
(Original art permission of Polaroid Corp.)
note: db normalized to on-axis response 41
THE PHYSICS TEACHER

Vol. 40, January 2002
with the known speed of sound v
s
to generate a round-
trip travel distance using the relation d = v
s
t. Half
this round-trip distance (d/2) is the range to the target.
At a pressure of 1 atm, the speed of sound is given by
v
s
= (331.0 + 0.6 T) m/s, where T is temperature in de-
grees Celsius; so in a 20 C room typically v
s
= 342
m/s. For example, when a computer measures t
equal to 10 mS, this corresponds to a range of about
1.71 m, calculated as: d = v
s
t = (342 m/s) (10 x 10
-3
s) = 3.42 m. Then the one-way range is r = d /2 =
(3.42 / 2) m = 1.71 m. A handy off-the-cuff figure for
this kind of problem is that sound travels about a third
of a meter every millisecond, or one meter every three
milliseconds.
Blind Spots and Buzzing Sounds
Immediately after transmitting the pulse train via
the transducer, the transmitter circuitry shuts down.
The receiving circuitry for this same transducer is then
turned on, but only after a short time delay. This is
necessitated by the fact that the foil, after transmitting
the pulse train, continues to vibrate (or ring) for a
brief period and must be allowed to self-damp suffi-
ciently so that its motion is less than that produced by
the reception of an echo pulse. Otherwise, residual
ringing from the previous transmission would be mis-
takenly detected as a return pulse. (Due to atmospher-
ic absorption, partial target reflection, and the inverse
square law, the returning ultrasonic echo is attenuated
by a factor of more than 10
6
compared to the trans-
mitted signal)
2
. This problem was solved in the origi-
nal camera design by simply not turning on the receiv-
ing circuitry until a delay of 2.38 ms passed, which is
plenty of time for the foil to mechanically damp down
under most circumstances. This is a very inexpensive
solution that in no way interfered with the original
camera design closer focus was not possible with
SX-70 cameras.
This 2.38-ms blanking interval corresponds to a
round-trip distance of 81 cm by the calculation d =
v
s
t = (342 m/s) (2.38 x 10
-3
s) = 0.81 m = 81 cm.
Half this number (40.5 cm or about 1.3 ft) corre-
sponds to a one-way minimum range below which the
standard Polaroid ranging module cannot detect a re-
turning pulse. The standard system is therefore
blind for the first 2.38 ms after pulse transmission or
within the closest 40.5 cm directly before the trans-
ducer. This blind spot presents considerable pedagogi-
cal difficulties if working with an object that
approaches from a greater distance to one less than 41
cm before the transducer. The object apparently dis-
appears or jumps on a computer-generated distance
plot. This effect is magnified when looking at data
derived from position data (velocity and acceleration)
that are sensitive to position discontinuities.
Over the last 20 years, advances in foil manufacture
and the use of a continuous 150 to 200 V dc bias volt-
age on the transducer plates have greatly improved the
damping of the Polaroid transducer foil. Although the
default setting for the blanking interval between trans-
mission and receiver activation is still 2.38 ms for all
Polaroid ranging modules to ensure compatibility,
newer hardware versions can achieve shorter blanking
intervals. Polaroid has added a digital signal input on
the ranging module that allows the module purchaser
to add external circuitry to control the blanking inter-
val, and reducing this interval below 2.38 ms reduces
the size of the blind spot. It is possible to reduce the
blind spot to about 15 cm (6 in) by adding such tim-
ing circuitry. This is the present limit possible with
these components, and some manufacturers have re-
cently released such modified rangers (e.g., PASCO).
Hence, the standard Polaroid ranger blind spot can be
reduced but not eliminated by extra effort, and this
improvement is becoming a standard design element
by physics education apparatus suppliers.
Because of different ranges and target characteristics
that occur in typical use, there are tremendous varia-
tions in return acoustical signal strength. This requires
that the ranging module use