Biomedical Engineering Reference
In-Depth Information
7.5
SONAR-BASED SYSTEMS
Sonar systems operate by transmitting a brief pulse of ultrasound from a transducer and
then listening for an echo. As the speed of sound in air is reasonably constant (about
340 m/s), the round-trip time gives the range to the reflecting surface, and because the sound
is radiated and detected over a narrow angular field, the beam pattern of the transducer,
this also gives a good measure of direction to the reflecting surface.
For short-range applications, most ultrasonic transmitters are either made from thin
wafers of piezoelectric (PZT) material or use electrostatic transducers. A typical piezo-
electric transducer similar to the unit shown in Figure 7-7, such as the SensComp 40LT16,
has the following characteristics (SensComp, 2004a):
•
Center frequency: 40
+
/
−
1 kHz
•
Bandwidth: 2 kHz
•
Transmit sound pressure level (SPL): 120 dB min at 40 kHz; 0 dB relative to 0.0002
μ
bar per 10 V root mean square (RMS) at 30 cm
• Maximum drive voltage: 20 V RMS
• Beam angle (6 dB): 55
◦
The matched receiver for this transducer is the 40LR16, which has similar specifications:
• Center frequency: 40
+
/
−
1 kHz
• Bandwidth: 2.5 kHz
• Sensitivity:
−
65 dB at 40 kHz relative to 0 dB
=
1V/
μ
bar
Electrostatic transducers are typically larger than the PZT variety, as can be seen in
Figure 7-8, and thus have narrower beamwidths. Because they are not resonant, they also
exhibit a much wider bandwidth, as seen in the abridged specifications for the SensComp
600 series reproduced as follows (SensComp, 2004b):
•
Center frequency: 50 kHz
•
Bandwidth:
≈
40 kHz
•
Transmit SPL: 110 dB min at 50 kHz; 0 dB relative to 20
μ
Pa at 1 m (300 VACpp
150 VDC bias)
Beam angle (6 dB): 15
◦
•
FIGURE 7-7
Principles and structure of a PZT ultrasound transducer. (a) Operational
principles of the bimorph element. (b) Transducer diagram showing impedance matching
cone and electrical connections. (c) Photograph of transducer. [Adapted from (Brooker,
2008).]
