MIDLAND CHUTES ENGINEERING REPORT

Vibration Tests on Chute Segments

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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

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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

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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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