List of Experiments for Advanced Vibrations and Noise Engineering
1 Study of vibrations of a box stationed on flexible springs.
2 Study of absorber system and its tuning for a fixed fixed beam.
Study of free and forced vibration using universal vibration
3
machine.
Estimation of internal damping of different beam specimen by regression
4
analysis.
To find the natural frequencies and mode shapes of a free free beam
5
experimentally and verify the same analytically.
6 Noise mapping of a machine using sound intensity probe
7 To verify the inverse square law for sound
Study of various function of Sound Level Meter and use it for field
8
measurements
9 To get the transmission loss of a panel (e.g. door).
10 To study the sound of musical instruments.
Experiment no. 1
Aim : Study of vibrations of a box stationed on flexible springs.
Equipments: Motor , Box, Springs, Variator, Tachometer , etc.
Specifications:
Spring constant : 10 Kg/cm each
Motor Weight : 3.25 Kg
Unbalance mass : 20 gm
Box Weight : 5 Kg
Spring length : 62 mm
Spring diameter (inner) : 35 mm
Spring diameter (Outer) : 42 mm
Variator :Dimmerstat
Load 8 Amps
I/P 230 V 50 to 60 Hz
O/P 0-275 V
Assumptions:
1. Properties of the structure are isotropic in nature and constant.
2. The system is linear one
Usefulness of experiment:
Most of the practical structures like automotives; machine tools can be modeled like a rigid
mass on springs.
Procedure:
1. Find the weight of the box, spring stiffness in vertical (K) and lateral directions.
2. Calculates all the six natural frequencies.
3. Experimentally find the resonant frequencies by exciting the system with unbalanced
masses on the motor.
4. Correlate the analytical results with the experimental results found. Discuss the
discrepancies if any.
Questions:
1. What is the significance of unbalance mass in this experiment?
2. Locate the modes which you practically seen ? how this modes are different?
3. Explain the reasons for errors occurred in Experiment ? Why there is difference
between practical and theory ?
Experiment no. 2
Aim : Study of absorber system and its tuning for a fixed fixed beam.
Equipments: Motor , Tuned damper , Variator, Auxiliary mass ,Tachometer , etc.
Specifications:
Motor : Full Horse Power
50 Watt, 50 Hz
Max. rpm 4000 rpm
Variator :Dimmerstat
Load 8 Amps
I/P 230 V 50 to 60 Hz
O/P 0-275 V
Usefulness of the Experiment :
Many times it is not possible to avoid resonance condition .In that the vibration of the main
system can be eliminated by coupling a properly designed auxiliary system to the main
system. Actually the excitation gets transmitted to the auxiliary system, bringing the main
system to rest.
Undamped dynamic vibration absorbers are extremely effective for constant speed
machines but they loose their effectiveness with any changed in speed of the machines. The
coupled system has to new natural frequencies. So, any change in speed of the machine might
bring the system close to these natural frequencies and then whole purpose of designing the
absorber system gets defeated.
Procedure:
1. Find the resonant frequency of main system analytically and verify it by observation.
2. Consider that the main system is distributed with excitation frequency equal to
resonant frequency . Design the absorber system ( i.e. position of given mass on
cantilever beams) for reducing the vibrations of main completely.
3. With main and absorber system (designed ), study the vibrations of the system.
4. Calculate the two natural frequencyies of the designed system and verify them by
observations.
5. Repeat step 2 & 3 for distributinbg frequencies equal to half the resosant frequency ,
and equal to the resosant frequency.
Questions:
1. Explain the reasons for errors occurred in Experiment? Why there is difference
between practical and theory?
2. State the Assumptions you made in experiment?
3. Why the total system after coupling auxiliary mass shows two frequencies?
4. Give at least two application of such system where this type of system can be
implemented to reduce the vibration?
Experiment no. 3
Aim : Study of free and forced vibration using universal vibration
machine.
Equipment : Speed controller, motor, disc, tachometer, spring , damper, drum. Etc.
Specifications :
Weight of unbalance mass : 28.9 gm
Spring constant (Upper) : 600 gm/cm
Spring constant (lower) : 172 gm/cm
Weight of motor : 7 kg
Gear ratio 28:1
Center distance of disc up to unbalance : 60 mm
Drum speed 3 rpm
Drum circumference : 292.1 mm
Centre distance of disc upto unbalance : 6 cm
Motor rpm : 200 rpm
Volts : 240 V
Assumptions :
System is linear
All properties are isotropic
Usefulness of the Experiment:
Natural frequency and damping factor of a system should be known as their ignorance
may lead to serious disaster. At resonance i.e. when excitation frequency matches with the
natural frequency of the system, amplitude of vibration becomes excessive for small damping
and decreases with increase in damping. Practically, the system may go into destruction much
before resonance if the damping is low. So it is very important that the working/excitation
frequency should be well away from the natural frequency and if it can’t be done, then
adequate damping should be provided so that the amplitude of vibration should be within safe
limit.
Logarithmic decrement and half- power method give a good idea about natural
frequency and damping of the system. If the amplitude of the vibration is not perceivable in
that case the phase plot (Phase vs. Frequency) will give indication about the resonance.
Procedure:
1. Disturb system by hand and trace the decaying vibration of the rotating drum.
2. Determine the logarithmic decrement from the log decrement curve.
3. Determine the damping factor .
4. Calculate damped time period, d .
5. Calculate the natural frequency of the system.
B
1. Excite the system by unbalance weights attached to the disc coupled with the motor.
2. Running motor at a speed (resonant speed) giving maximum amplitude, the
displacement trace on rotating drum and phase angle trace on the rotating disc are
taken.
3. For speeds (at least four) below and above the resonant speed (giving maximum
amplitude) , the displacement and phase angle traces are obtained.
4. Plot the amplitude vs. frequency and phase angle vs. frequency curves. Find the exact
resonant speed and damping factor from bandwidth of resonance curve.
5. Compare the results obtained by two methods.
Questions:
1. Explain the reasons for errors occurred in Experiment? Why there is difference
between practical and theory?
2. Plot a graph of response of motor system on cantilever
a) speed Vs phase angle
b) speed Vs Amplitude
Experiment no. 4
Aim : Estimation of internal damping of different beam specimen by
regression analysis.
Instruments used: Beam specimen of Steel, Plastic, Bakelite, Aluminum,
Accelerometer, Charge amplifier, Oscilloscope.
Specifications :
Charge Amplifier :
Oscilloscope :
Accelerometer :
Assumptions :
System is linear
All properties are isotropic
Usefulness of the Experiment:
Structural vibration of instrument panels, aircraft structures etc have to be within
limits always. So these structures have to be of materials which have high internal damping.
Vibration levels can be kept well within either using passive means or active means . Passive
means include adding dampers, using viscoelastic materials , changing dynamic
characteristics by modifying mass & stiffness etc. Active means employ external source of
energy to dampen vibration.
Procedure:
1. Clamp one specimen in the rig as shown in fig.
2. Place an accelerometer near the beam tip.Input accelerometer output into the charge
amplifier . Input charge amplifier output into digital oscilloscope.
3. Displace beam tip by 2 mm . Observe the logarithmic decrement of vibratory response
on the digital oscilloscope.
4. Using the formula of logarithmic decrement determine damping ratio and the natural
frequency.
5. Save the beam response in a floppy disc.
6. Using data record in the floppy disc , fit following equation on the data using
regression analysis and thus find out the damping ratio and natural frequency.
This can be easily done by Sigma plot
7. Now fit following equation the envelope of response:
For this step, you will have to manually determine peaks of the vibration cycles and
the corresponding time. Take about 10 peak points for this step.
8. Compare results of step 3,6 & 7
9. Repeat above steps for all the experiments.
Questions :
1. Explain the reasons for errors occurred in Experiment? Why there is difference
between practical and theory?
2. Why the damping of each material is different?
3. What is the effect of placing Accelerometer on measurement?
4. plot the graph of Amplitude Vs Damping ratio ?
Experiment no. 5
Aim :To find the natural frequencies and mode shapes of a free free beam
experimentally and verify the same analytically .
Instruments used: Vibration exciter, free-free beam, Oscillator, Amplifier.
Specifications:
L = Length of beam : 765 mm
W= Width of beam : 38.6 mm
H = Height of beam : 26 mm
T= Thickness of beam : 2.66 mm
Beam Material : Aluminum
ρ = Density : 2500 Kg/m^3
E =Young’s modulus : 71.7 GPa
Charge Amplifier : 0–5A
: 0 – 25 V
Assumptions :
System is linear
All properties are isotropic
Usefulness of the Experiment:
In real life aeroplanes, missiles, rockets, space vehicles , satellites , submarimes etc
are modeled as free–free mechanical systems. Also in real life vibration problemns of
structures, nodal points can be determined by sprinkling sand on the structure and then by
seeking where sand accumulates. Sand accumulates at the nodal points.
Procedure:
1. Excite the beam at different frequencies sweeping the range from low frequencies
(about 30 Hz.) to high frequencies(upto about 2000Hz.) Note the points of resonance
and thus determine natural frequencies of the beam.
2. For each natural frequency sprinkle sand on the beam and locate nodal points.
3. With the help of above formulae and table, find out analytically the natural
frequencies and modes of vibration of the given beam.
4. Compare analytical and experimental results.
5. Record significant observations, if any , regarding excitation of second and third
modes experimentally.
Questions:
1. Explain the reasons for errors occurred in Experiment? Why there is difference
between practical and theory?
2. Why some modes are not observed?
3. Why at high frequency sand particle does not accumulates at the mode point?
Experiment no. 6
Aim : Noise mapping of a machine using sound intensity probe
Instrument: Sound Intensity probe and Larson Davis FFT analyzer
Theory:
Octave analysis provides information on frequency content of sound. An Octave is
a frequency band where highest frequency twice the lowest frequency. There are standard 1/1
and 1/3 octave bands. A third octave covers a range where the highest frequency is 1.26 times
the lowest frequency.
Experimental setup:
Procedure:
Setting up the Model 2900B for Acoustic Intensity Measurements
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1. Press SYSTEM to enter the main menu of the 2900B.
2. Select FILTER [G] to enter the filter menu.
3. Then choose 1/3oct [B] to activate the 1/3 octave band digital filters. Press short [G]
to select the short filter.
4. Press EXIT to get back to the System menu.
5. Press INPUT [K] to select the high and low pass analog filters.
6. Then select 20-10k [N] to activate the 20 Hz and 10KHz filters.
7. Press the SAME[O] key to assign the 20-10 kHz to channel 2.
8. Press EXIT twice to return to the MAIN menu.
Selecting An Exponential Detector
------------------------------------------------------------------------------------------------------------1.
To set these values, first press EXIT. Then choose DETECTR [H] to enter the
detector or averaging menu.
2. Select EXP [C] to activate the exponential detector.
3. Then press AV.TIME [H] (averaging time).
4. Next, select 1/8 [D] sec.
5. Press EXIT to return to the MAIN menu, then press the SYSTEM hardkey.
Creating A Measurement Data File
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1. Press FILES [O] to enter the file menu.
2. Next, press create [A].
3. Using the alpha keys above and below the display, create a file name.
4. Then press EXIT to enter the file name. Press EXIT again.
5. Select INTENSY [E] to activate the Acoustic Intensity analysis measurement option.
6. Press EXIT to leave the SYSTEM menu and enter the Acoustic Intensity display
menu.
Normalization Of Input Channels
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1. Press pr/t/sp [L] to edit and enter the values for static pressure (in millibars),
temperature (degrees Celsius), and the spacer length (in millimeters).
2. After you have established the values of these parameters, press EXIT to enter the
changes.
Naming A Job, Part, And Area
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1. Press job [I] to enter the name creation field for the job name.
2. Press the various alpha soft keys to build the job name that you desire. Then choose
EXIT to enter the job name.
3. Press part [J] to enter the name creation field for the part name.
4. The part would be a subcomponent of the job. Again, press the various alpha soft keys
to create the part name that you wish. Then choose EXIT to enter the part name.
5. Press area [K] to enter the name creation field for the area name.
6. The area would be a subcomponent of the part. Once again, press the various alpha soft
keys to create the area name.Then choose EXIT to enter the area name.
7. After you enter the area name, you will be prompted to enter the “area” (in square
meters) of the test area. Type in this value, and then press EXIT to enter this value.
Selecting A Linear Detector
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1. Then, press R/S to begin a measurement. Adjust the input range of the 2900B using the
up and down arrow hard keys.
2. The proper input range would be at the range setting one or two steps above the setting
where the 2900B overloads. Starting from a high range, step down (using the down
arrow key), the range one 10 dB increment at a time until OVER appears on the
screen. Then arrow up one or two 10 dB steps. You are now range optimized.
3. Press R/S to stop the measurement. The autorange feature of the 2900B can also be
used but usually more time consuming than the manual method.
4. Set the detector for linear single averaging by first pressing DETECTR [H].
5. Then choose LIN.S[A].
6. Next, press AV.TIME [H].
7. Enter a DETECTOR TIME of some value between 20 and 60 seconds. In general,
you will get a more accurate measurement with a longer averaging time. Press EXIT
to enter this value.
8. Press EXIT to return to the MAIN menu.
Performing A Measurement
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1. Press KEY A on the probe (or the R/S hard key) to initiate the measurement. Either
“scan” the probe about the measurement surface (in an orthogonal fashion) or fix the
probe at a single point (orthogonal to the surface) until the end of the averaging time is
reached. The first measurement is now complete.
2. Press the CURSOR hard key, then press the left or right arrow key to move the cursor
to the various 1/3 octave bands.
3. Press SPL [D] to view the averaged sound pressure level that was measured.
Experiment No. 7
Objective: To verify the inverse square law for sound
Instruments: Frequency generator, speaker and sound level meter
Introduction:
The sound pressure level varies inversely as the square of the distance from the
source. The general rule of thumb is that, with no reflecting surfaces or other background
sound or interference (free-field conditions), a sound level drops 6 dB for every doubling of
the distance from the source
E.g. If the distances from the sound source are d1 and d 2, then the difference ‘ d’ in dB at
these points is:
d = 10 log (d2/d1)2 = 20 log (d2/d1) = 6.02dB
Setup:
Fig 1. Measurement points at a distance d, 2d, 3d……
Procedure:
1. Set the setup as shown in figure.
2. Switch on the noise source
3. Measure the SPL at location ‘1’ at certain distance‘d’ from the source.
4. Measure the SPL at locations 2, 3, 4….
Experiment No. 8
Objective : Study of various function of Sound Level Meter and use it for
field measurements
Instrument: Integrating Sound level Meter Type 2239A, Calibrator
Theory:
A sound level meter is an instrument designed to respond to sound in
approximately the same way as the human ear and to give objective, reproducible
measurements of sound pressure level. There are many different sound measuring systems
available. Although different in detail, each system consists of a microphone, an processing
section and a read-out unit.
The microphone converts the sound signal to an equivalent electrical signal. The
most suitable type of microphone for sound level meters is the condenser microphone, which
combines precision with stability and reliability. The electrical signal produced by the
microphone is quite small and so it is amplified by a preamplifier before being processed.
Several different types of processing may be performed on the signal. The signal
may pass through a weighting network. It is relatively simple to build an electronic circuit
whose sensitivity varies with frequency in the same way as the human ear, thus simulating
the equal loudness contours. This has resulted in three different internationally standardized
characteristics termed the "A", "B" and "C" weightings.
Calibration:
Sound level meters should be calibrated in order to provide precise and
accurate results. This is best done by placing a portable acoustic calibrator, such as a sound
level calibrator or a pistonphone, directly over the microphone. These calibrators provide a
precisely defined sound pressure level to which the sound level meter can be adjusted. It is
good measurement practice to calibrate sound level meters immediately before and after each
measurement session. If recordings are to be made of noise measurements, then the
calibration signal should also be recorded to provide a reference level on playback.
Procedure:
1. Understand the use and details of Sound Level meter.
2. Use the instrument to measure sound pressure levels at different locations:
i. Engine generated noise in IC engine Lab.
ii. Machinery generated Noise in central workshop.
iii. Compressor generated noise in Vibration Instruments Lab.
iv. Noise generated due to passing of an Aircraft.
v. Noise generated by a car or any other vehicle passing by main road.
Experiment No. 9
Objective: To get the transmission loss of a panel (e.g. door).
Instruments: Sound Level meter, Signal generator, amplifier, speaker.
Figure 1
SLM
Amplifier
Signal
Generator Door
Figure 2
Procedure:
1. Arrange the setup as shown in figure. near a door.
2. Use signal generator to generate the signal at different frequencies.
3. Measure the SPL in front of the speaker.
4. Now close the door and measure the SPL behind closed door.
5. Find the change and plot it against the frequency.
Experiment No. 10
Objective: To study the sound of musical instruments.
Instruments: Musical Instruments (Tabla, guitar), microphone, amplifier, FFT Analyzer
Introduction:
For guitar, the plucked string produces specific sound. There are 5 strings of different
diameters. The frequencies of normal modes of vibrations of the plucked string are dependent
on the mass per unit length, tension in the string, and length of the string. The frequency
content of the sound generated due to vibrations of the string depends on the initial excitation
position. For plucked string like in the case of guitar, pluck-position plays important role in
exciting some modes while suppressing the other modes. Obviously, wherever you pluck,
some modes of vibrations are not excited.
In this experiment, sound generated due to vibrations of guitar strings will be studied.
The string is excited at specific locations and then the sound generated is recorded using
microphone. The output of microphone is fed through amplifier to the FFT analyzer to study
the frequency content.
For the first string, pluck at different positions along the length and observe the effect
on the frequency content and record the observations. Repeat the same procedure on one
more string. Now for the same position of plucking (center of the supported length of the
string), press the string at some distance away from one of the ends so as to alter the effective
length of the string and estimate the natural frequency from the observed frequency pattern of
the sound generated. Observe the variation of natural frequency with length. Now repeat
these readings for some changed value of the tension level in the string. Check for the
fundamental mode frequency for all the five strings and compare the frequencies. Record and
infer from the observations.
For tabla, measure the frequency content of the sound by hitting the membrane at
different positions. Observe if these frequencies are close to the estimated frequencies based
on the jmn values for different modes of vibration of membranes.
Normal mode natural frequency of a stretched string: fn = n*c/(2L)
Normal mode natural frequency of stretched membrane: fmn=jmn *c/(2*π*a)