This document discusses measurement of vibrations, sound, and acceleration. It describes that vibrations involve repeated oscillations that can be simple or complex, defined by frequency and amplitude. Excessive vibrations can cause component failure, wear, production faults, and human discomfort. Acceleration is measured using piezoelectric or seismic accelerometers. Piezoelectric accelerometers use a crystal that generates a proportional voltage from applied force. Seismic accelerometers measure mass displacement relative to a frame. Sound power is measured in decibels and involves fluctuating air pressure waves detected by the ear. Microphones convert sound pressure to electrical signals using capacitive, piezoelectric or electromagnetic techniques.

1. Metrology & Mechanical
Measurement
Measurement of Vibrations and
Sound

2. Measurement of Vibrations
• Vibrations – repeated cyclic oscillations of a
system
– May be simple harmonic, may be complex
• Caused when acceleration is applied to the
body alternately in two directions
• Frequency measured in Hz
• Amplitude may be displacement, velocity,
acceleration

3. • Effects of excessive vibrations
– Failure of component due to resonance and
fatigue
– Excessive wear
– Faulty production
– Incorrect operation of precision equipment &
machinery
– Human discomfort – motion sickness, breathing &
speech disturbance, loss of touch sensitivity

4. Measurement of Acceleration
• Two types of accelerometers
1. Piezo-electric accelerometer
2. Seismic accelerometer

5. Piezo electric accelerometer

6. • Simplest & most commonly used transducer
• Sensor consist piezoelectric crystal sandwiched
between two electrodes
• It has mass placed on it
• Unit is mounted on component of which acceleration
to be measured
• Can threaded to the base
• Mass exerts force on the crystal, proportional EMF is
generated
• F = M x A
• As mass is constant, F is proportional to A.

7. • Advantages and limitations
– Rugged and inexpensive
– High output impedance
– High frequency response
– High sensitivity
– Sufficient large range
– Sensitive to changes in temp.
– Hysteresis errors

8. Seismic / Displacement sensing
accelerometer

9. • Displacement of mass due to vibration is
measured and calibrated to measure
acceleration.
• Mass is connected by spring and damper with
casing/housing.
• Frame is attached to machine
• When subjected to vibrations, mass moves
relative to frame
• This movement is sensed by electrical
displacement transducer

10. Sound power measurement
• Sound waves are vibrations consisting of a
succession of rapid variation in the air
pressure
• Fluctuating pressure waves
• Small in magnitude
• Get transmitted through medium
• Detected by hearing mechanism
• Measured in decibels [dB]

11. • Noise – unwanted sound is called as noise.
• Sound above 80 dB
• Effects of noise
– Irritation, distraction
– Prolonged exposure causes hearing loss
– Reduces working efficiency
– Masks audible warning or safety signal
– Interferes with communication between people

12. Microphones
• Converts sound pressure into electrical signal
• Generally use thin diaphragm
• Types of microphones
– Capacitive
– Piezo-electric
– Electromagnetic

13. Capacitive microphone

14. • Consist diaphragm of 0.0002 cm thickness
• Diaphragm and back plate forms two plates of
capacitor
• Air as dielectric medium
• Sound wave – movement of diaphragm –
change in capacitance proportional to
intensity of sound
• E = Q t

15. Piezoelectric microphone

16. Electromagnetic microphone