Ultrasound basics

1. The generation and use of
ultrasound

2. Piezo-electric effect
• Key Points
• Ultrasound – is sound waves above the range of human
hearing 20kHz.
• Ultrasound – is produced by a piezo-electric
transducer.
• The transducer converts electrical energy into sound
energy.
• Piezo-electric transducers make use of piezo-electric
crystals such as quartz.

3. • Quartz is made up of a
large number of silicate
units.
• Which comprised of
positively charged
silicon ions and
negatively charged
oxygen ions.

4. • When a potential
difference is applied to the
crystal . The resulting
electric field will cause
forces on the positive and
negative ions.
• The ions are not held
rigidly and so the ions will
slightly displace.
• Depending on the direction
of the electric field, this
displacement of the ions
will either make the crystal
become slightly thinner or
slight thicker.
+ + + +
- - - -
The opposite surfaces of
the crystal are silvered to
act as electrodes.

5. • When an alternating voltage is applied to the crystal, the crystal
will alternate between, becoming thinner and becoming thicker,
with a frequency that matches the applied potential difference.
• The amplitude of the vibrations is usually very small. However, if
the frequency of the ac voltage matches the natural frequency
of the crystal, then resonance occurs and the amplitude of
vibrations will be much larger.
• These vibrations can generate sound waves of high frequencies
(greater than 20kHz) i.e. ultrasound.
• The reverse also happens as well, i.e. if the crystal is subjected
to an alternating force such as a ultrasound wave. Then this will
cause the ions to displace slightly and as such will create a
potential difference across the ends of the crystal.

6. Piezo-electric transducer/receiver
When an alternating voltage is applied the crystal will produce
an ultrasound wave
When the crystal receives an ultrasound wave, the crystal will
produce a potential difference.
So the transmitter can also act as a receiver of ultrasound
waves.