Useful for beginners

1. Sensors for Engineering Applications
UNIT-3
Prepared by
Dr. Jami Venkata Suman
Department of ECE

2. Unit III
Sound, Ultrasound and Infrasound sensors
Principles, Audio to electrical sensors and transducers:
moving iron microphone, moving coil microphone, capacitor
microphones. Microphone problems, frequency and
wavelengths. Electrical to audio transducers: moving iron
transducer, moving coil transducer, Capacitor transducers.
Ultrasonic transducers, Infrasound sensors.

3. SOUND
Principles: Any sound is associated with a mechanical
vibration at some stage.
Many sounds are caused by the vibration of solids or gases,
and the effect of a sound on the hearer is to vibrate the
eardrum.
The sound wave is the waveform caused by a vibration and
which in turn causes an identical vibration to be set up in
any material affected by the sound wave.
The mechanical vibration need not necessarily cause any
sound wave, because a sound wave needs a medium that
can be vibrated, so that there is no transmission of sound
through a vacuum.

4. A sound sensor is defined as a module that detects sound waves through its
intensity and converting it to electrical signals.
Sound waves propagating through air molecules
Sound detection sensor works similarly to our Ears, having diaphragm which
converts vibration into signals. However, what’s different as that a sound sensor
consists of an in-built capacitive microphone, peak detector and an amplifier
(LM386, LM393, etc.) that’s highly sensitive to sound.
With these components, it allows for the sensor to work:
 Sound waves propagate through air molecules
 Such sound waves cause the diaphragm in the microphone to vibrate, resulting in
capacitance change
 Capacitance change is then amplified and digitalized for processing of sound
intensity

5. Three components are needed for sound to be heard: A source – where the
sound is made. A medium – something for the sound to travel through.
A receiver – something to detect the sound.
This sensor includes three pins which include the following.
• Pin1 (VCC): 3.3V DC to 5V DC
• Pin2 (GND): This is aground pin
• Pin3 (DO): This is an output pin
Specifications:
• The range of operating voltage is 3.⅗ V
• The operating current is 4~5 mA
• The voltage gain 26 dB ((V=6V, f=1kHz)
• The sensitivity of the microphone (1kHz) is 52 to
48 dB
• The impedance of the microphone is 2.2k Ohm
• The frequency of m microphone is16 to 20 kHz
• The signal to noise ratio is 54 dB

7. The perception of sound by the ear is a much more complicated
issue.
Objective measurements of sound waves can make use of the
intensity, measured as the number of watts of sound energy per
square metre of receiving surface, or of the wave quantities of
pressure amplitude or displacement amplitude.

8. Features
• The features of the sound sensor include the following
• These sensors are very simple to use
• It gives analog o/p signal
• Simply incorporates using logic modules on the input area
Applications
• Security system for Office or Home
• Spy Circuit
• Home Automation
• Robotics
• Smart Phones
• Ambient sound recognition
• Audio amplifier
• Sound level recognition

9. Audio to Electrical Sensors and Transducers

10. Audio to Electrical Sensors and Transducers
Audio Sound Transducers include both input sensors that
convert sound into electrical signals such as a microphone,
and output actuators that convert the electrical signals back
into sound such as a speaker.
Link: https://www.electronicshub.org/sound-transducers/

11. A microphone converts sound into a small electrical current. Sound waves hit a
diaphragm that vibrates, moving a magnet near a coil. In some designs, the coil moves
within a magnet. Other microphones, such as condenser microphones, work on the
principle of capacitance.

14. Each of the three primary types of microphones—dynamic microphones,
condenser microphones, and ribbon microphones—has a different
method for converting sound into electrical signals.

15. A ribbon microphone is a unique type of dynamic microphone that is based
around a thin, corrugated strip of metal (often aluminium) or film suspended
between two magnetic poles. Unlike traditional moving-coil dynamic mics,
the ribbon element responds to variations in the velocity of air particles,
rather than the pressure. As the ribbon vibrates within its magnetic field, it
generates a tiny voltage that corresponds to these changes in velocity.

16. Moving Coil Microphone Diagram
In dynamic microphones (aka moving-coil
microphones), a coil of wire surrounds a
magnet and is connected to a diaphragm
which vibrates in response to incoming
sound waves. When sound waves hit the
diaphragm, the coil oscillates back and forth
past the magnet, generating a current which
creates the audio signal.
A moving coil microphone has three main
parts: a diaphragm, a moving coil and a
permanent magnet. The diaphragm is a thin
piece of metal, plastic or aluminum that
vibrates when it is struck by sound waves.

17. Moving Coil Microphone Diagram
The moving coil microphone uses a constant-flux magnetic circuit in
which the electrical output is generated by moving a small coil of wire
in the magnetic circuit (Figure).
The coil is attached to a diaphragm, and the whole arrangement is
usually in capsule form, making this pressure operated rather than
velocity-operated., The electrical output is at 90degrees phase angle to
the sound wave.

18. That linearity is excellent for this type of microphone.
The coil has a low impedance, and the output is
correspondingly low, but not so low that it has to
compete with the noise level of an amplifier.
The low inductance of the coil makes it much less
susceptible to hum pick-up from the magnetic field of
the mains wiring.
it is possible to use hum-compensating (nonmoving)
coils, known as hum buckers, in the structure of the
microphone to reduce hum further by adding an
antiphase hum signal to the output of the main coil.

19. CAPACITOR MICROPHONES
The amount of electric charge
between two surfaces is fixed,
and one of the surfaces is a
diaphragm that can be vibrated
by a sound wave. The vibration
causes a variation of capacitance
that, because of the fixed charge,
causes a voltage wave.

20. The output impedance is very high, and the amount
of output depends on the normal spacing between
the plates- the smaller this spacing, the greater the
output for a given amplitude of soundwave. The
construction of the microphone ensures that it is
always pressure-operated.
The two main objections to the capacitor microphone
the need for a high-voltage supply and the hum pick-
up problems of the very high impedance.

21. The high-voltage supply (called a polarizing voltage)
was needed to provide the fixed charge; this was done
by connecting the supply voltage to one plate through a
very large value resistor.

22. The high impedance made it difficult to use the microphone
with more than a short length of cable.
The capacitor microphone can be very linear in operation and
can provide very good quality audio signals without the need
for elaborate constructional techniques.
The revival of the capacitor microphone came about as a
result of a the electret.
An electret is the electrostatic equivalent of a magnet, a piece
of insulating material that is permanently charged.
The principle states that if a hot plastic material (in the
broadest sense of a material that can easily be softened by
heating) is subject to a strong electric field as it hardens, it
will retain a charge for as long as it remains solid. Materials
such as acrylics (like Perspex) are electrets, and the idea was
once considered for the manufacture of monoscope tubes.

23. Very simple construction of a
capacitor microphone, consisting
only of a slab of electret metallized
on the back, a metal (or metallized
plastic) diaphragm, and a spacer ring
(Figure), with the connections taken
to the conducting surface of the
diaphragm and of the electret.
This is now the type of microphone
that is built into cassette recorders,
and even in its simplest and cheapest
versions is of considerably better
audio quality.

24. Another form of capacitor microphone uses a light
film of pyroelectric material whose polarization
(separation of charge) is also changed by strain.
Using such a film, metallized on one side, as one
plate of a capacitor whose other plate is a perforated
metal sheet creates a simple microphone whose
output can be larger than that of most other
capacitor types.