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MIDLAND CHUTES ENGINEERING REPORT
Vibration Tests on Chute Segments
Volume
1
H.L. Blachford, Inc Acoustics laboratory
H. L. Blachford, Inc Troy, Michigan
M I D L A N D C H U T E
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Table of Contents
Introduction 1
Results 2
Theory and Measurement Procedures 4
Exhibit I 8
Equipment 9
Exhibit 2 10
Figure 1 11
Figure 2 12
Figure 3 13
Figure 4 14
Figure 5 15
Figure 6 16
Figure 7 17
M I D L A N D C H U T E
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Results
The six samples were rank ordered using figure 1, which lists the loss
factors from 31.5 to 4000 Hz. Human perception of air-born noise
generated by vibration below 125 Hz is limited. Hence loss factors from
figure 1 for frequencies above this value were used in ranking the
samples. Clearly, three samples were clustered near the same loss
factor region, while the other three samples were clustered near the
same loss factor region, while the other three samples differed enough
to warrant separate rank categories. The following list shows the best
ranking.
1. (tied) Helix seamed w/Aquaplas
Straight seamed w/Aquaplas
2. Helix seamed w/Mastic
3. Helix seamed sample
4. Straight seamed w/Mastic
5. Straight seamed sample
These results confirm that damping material and fabrication technique
both play a part in improving the loss factor of the chute segments. The
helix seamed samples apparently had greater structural support which
enhanced the loss factor when compared to the straight seamed
samples. This premise is supported by the fact that both of the lowest
ranked samples were straight seamed chute segments. Further support
ranked samples were helix seamed samples.
Although the helix seam is effective in improving the loss factor of the
samples, it alone is not as effective as the damping materials in
improving loss factor. The three top ranked samples were treated with
damping materials. The untreated helix seamed sample lies just below
Chapter
1
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this group but above the single seamed mastic treated sample. This
suggests that the fabrication techniques appear better at improving loss
factor that mastic damping material.
Two out of the top three samples were treated with Aquaplas. The third
required mastic damping material with a helix seam to accomplish
equivalent damping qualities. The Aquaplas appeared to be significantly
better at improving the damping qualities than mastic material.
During the tests, it was noticed that the Aquaplas was applied in a
thickness thinner than recommended by the test facility. The
manufacturer recommends that when the thickness equals the
thickness of the metal, results are good, When the ratio of thickness of
Aquaplas to sheet metal is 1.5 to 1, the results are very good, When the
ratio is 2 to 1, the results are excellent. The Aquaplas was applied
approximately as thick as the sheet metal in these samples.
Figures 2 through 7 are spectra of the resonant modes for each of the
six samples. Each of the spikes in the spectra represent a mode, and by
reading the corresponding point on the abscissa, the frequencies for
each mode is identified. It is interesting to note the presence of an
unusually large number of modes for the less well damped samples. The
relative height of each mode indicates the magnitude of its contribution
to the total vibration of the sample.
Figure 2 is a spectra of the modes for the helix seamed sample with
Aquaplas. There are less than 10 modes for this sample. Their relative
intensities are small, especially when compared to figure 4. It
represents the helix seamed sample with no damping material. The
relative magnitude of each of these peaks in figure 4 is greater than in
figure 2, and there are more modes.
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THEORY AND MEASUREMENT PROCEDURES The quality of air-born sound produced when the chute segments are
used in high rise building is directly related to their vibrational
characteristics. The coupling relationship between vibration and air-
bourn noise is an intricate and complex subject. Extensive noise tests
at numerous positions around the samples would be required to
determine how these phenomena interrelate. The location of the modes
around the cylinders is time dependent, and the measurement of its
corresponding air-borne frequency component must be measured from
numerous positions over an infinite number of minute time intervals.
Hence, the measurement of how the
vibrational activity of the samples couples with air-borne phenomena,
although considered, was believed to be beyond the scope of this study.
This test program studied in detail the vibrational aspects of the six
samples provided. A vibrational transducer, an accelerometer, was
affixed to the outside of the cylinder approximately half way down its
side. Using piezoelectric signals which were amplified and analyzed.
The analysis equipment converted acceleration at any given moment
into decibels (db), referenced to 10-6g (g=9.8 m/s2). The
following equation illustrates the relationship; where (a) is
the acceleration:
Db=20 log a/10-6g
An accelerometer may be compared to a microphone in its application.
The difference being that a microphone measures the vibration of air
particles by sensing sound pressure variances. Also, an accelerometer
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is attached to a solid object while a microphone is directly coupled to a
fluid—air. They both output electrical impulses which are converted into
decibels. However, the use of the units of “decibels” in each case is
greatly different as they are referenced to different physical constants.
Vibration, like air-borne sound, can be broken down into frequency
components. The human range nominally lies between 20 and 20,000 Hz
(Hz=cycle/sec) for air-borne sounds. Acoustical experts believe that the
significant frequency for human perception lies between 500 and 4000
Hz. Which is referred to as the “speech range”. Certainly sounds below
125 Hz are of lesser importance as they are seldom perceived as readily
as frequencies above this range.
Acoustical engineers customarily reduce the frequency components of
sound into octave bands or 1/3 octave bands. An octave is a category of
frequencies lying between f1 and f2, (where f2=2f1). A further
subdivision of these frequencies traditionally is applied by using 1/3
octaves. The American national standards institute (ANSI) has
recommended center frequencies defining 1/3 octave bands. They
include 63, 125, 250, 500, 1000 Hz and etc. up to 20,000 Hz (20 KHz).
Following the shock of an impact or an excitational force, most objects
will continue to vibrate. The duration of latent vibration for a given
object if dependent on its stiffness and manner of clamping. Limp
objects quickly lose vibrational energy due to great intermolecular shear
losses. Their overall decay rate, vibration measured over all
frequencies, is measured in dB/sec and is usually large. Well clamped
rigid objects on the other hand, experience sustained vibrations and
show a low decay rate. The decay rate of materials can be altered by
using damping materials, or by designing objects to have greater
strength, which inhibits vibrational amplitude.
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As stated earlier, the vibration of a material or sample after an
excitational force is applied may be broken down into various frequency
components. All objects have characteristic resonant frequencies, or
frequencies of natural vibration called “modes”. These modes are
dependent upon the dimensions of the sample and stiffness, and are
relatively independent of placement of the excitation or magnitude. In
some cases the modes can be deviated slightly by altering greatly the
application of the excitation.
The loss factor (n) of a material or sample is a more descriptive
acoustical term than decay rate. It reports the ability for a sample to
dissipate vibrational energy at a specific frequency (fg), and is related to
decay rate (D in db/sec) by the following equation:
N=(D/27.3) (fn)
It is therefore possible to determine the loss factor of a sample by
measuring the decay rate at a given frequency and using the above
equation. This was the technique followed in this study.
Six samples were studied in this project. Half of the samples were
fabricated from a single sheet of 18 ga. Steel by wrapping it around to
form a cylinder with a seam running the length of the cylinder. These
will be referred to later in this report as “straight seamed samples”. The
remaining samples were formed from a piece of 18 ga. Steel wrapped
around in a helix fashion to form a cylinder of equal diameter and length
as the other samples. The latter samples will be referred to tater in this
report as “helix” seamed samples, (see exhibit 1)
Two of the samples of each fabrication technique were not treated with
damping materials. One helix seamed and one straight seamed sample
was treated with mastic damping material, and finally, one helix seamed
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and straight seamed sample was treated with Aquaplas, a water based
damping material manufactured by H. L. Blachford.
The samples were suspended during the tests. Preliminary studies
showed that most of the samples resonated in 20 or more modes. The
higher frequency modes tend to dissipate more quickly than do the
lower ones, which resulted in a nonlinear overall decay rate. Graphic
level recorder displays showed steep slopes at the beginning of the
decay process, and moderate decay rates near the end of the process.
Parallel nonlinearity between the samples did not exist. This
compounded the problems associated with attempting to measure
overall decay rate.
An alternative approach achieved satisfactory results for measuring
decay rates. A 1/3 octave band filter was placed between the amplified
signal from the accelerometer and the graphic level recorder. This
enabled the decay rate for a narrower band of frequencies to be
measured. The decay rate for the various 1/3 octave signals was
consequently linear, and was measured from 31.5 through 4000 Hz. By
using the equation above, the loss factor (n) for each sample in the
designated 1/3 octave bands was determined. Figure 1 shows the test
results for each of the six samples.
Figures 2 through 7 show the resonant modes for each of the six
samples. These spectra were generated using a Rockland narrow band
analyzer and an X-Y plotter. The amplified signal from the accelerometer
was evaluated by the analyzer. Generally the technique employed
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involved impacting the suspended sample with a mallet and observing
the vibration over a given frequency for a short duration. To improve
resolution, 6 or 7 spectra were generated by repeatedly impacting the
specimen, These spectra were stored and then averaged. The resulting
spectra were printed as shown in figures 2 through 7. The spectra were
examined from 0 to 1 KHz. Higher frequencies were also examined, but
the significant vibrational activity was below 1 KHz, so higher
frequencies were omitted in the figures.
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EQUIPMENT*
• Bruel & Kjaer B&K 4367 accelerometer
• Bruel & Kjaer B&K 1612 1/3 octave filter
• Bruel & Kjaer B&K 2107 frequency analyzer
• Bruel & Kjaer B&K 2305 graphic level recorder
• Houston Instruments 200 X-Y Plotter
• Rockland 512/F FFT analyzer
*See exhibit 2 for equipment setup.
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