sae paper 2015-01-2348

AbstractThis paper presents the results of a study to reduce the background noise level within a large Quiet Room located adjacent to other laboratory testing environments and below a mechanical mezzanine which houses an extensive array of mechanical and electrical equipment including banks of low-temperature chiller compressors, air handling units, and electrical switchgear that serves the entire building complex. This equipment was installed atop the concrete mezzanine floor deck without provisions for isolating vibration. As a result, structure-borne noise from that equipment travels through the floor, radiates from the underside of the floor deck, and intrudes into the Quiet Room below. This causes the background noise level within the Quiet Room to be too high for conducting low sound level measurements and studies on vehicles brought into the Quiet Room.

Conclusions of the study are that provisions originally intended to isolate the Quiet Room from the mezzanine floor deck were both insufficient and poorly installed, resulting in excessive noise transmitted into the Quiet Room. Recommended corrective measures include enhancement of the Quiet Room ceiling to improve the sound transmission loss performance of that assembly, and changes made to the mezzanine floor deck to isolate structure-borne noise. This work is being installed in phases and completion of the initial phase has shown very promising results.

IntroductionA Quiet Room was built as part of the original construction of a circa 1990s automotive technical center. The primary user of the Quiet Room expressed concern that the background noise level/spectrum in the Quiet Room was too high to conduct low level vehicle noise investigations. This led to the initiation of a study to determine the options and cost for reducing noise intrusion into the

Quiet Room from nearby mechanical and electrical equipment installed on a mezzanine level floor deck that runs above and also extends beyond the Quiet Room.

Anatomy of a Quiet RoomA Quiet Room is essentially a hemi-anechoic room that does not necessarily meet the free-field requirements of ISO 3745 [1], and has a sufficiently low background noise level/spectrum that is compatible with conducting acoustical measurements on noise sources of interest. The interior surfaces of a Quiet Rooms are generally lined with highly sound absorbing, flat-faced acoustical panels, the thickness of which define the low frequency extent of the free field.

The Quiet Room was site built with all wall and ceiling surfaces faced with thick layers of semi-rigid, glass fiber sound absorption panels that are layered with large airspaces to create a high degree of sound absorption across a broad spectrum of sound. The glass fiber absorption was faced with an off-white colored scrim, and stainless steel hardware cloth. This was all held in place using stick-pins and chromed buttons. This approach for sound absorption is considered to be unconventional and provides a “home built” appearance. See Figures 1 and 2. It is, however, expected to produce an estimated low frequency cut-off of 182 Hz based simply on the total thickness of the absorption relative to a quarter wavelength [2].

Diagnostic InvestigationThe initial work of this study consisted of a detailed inspection of the room construction and its juxtaposition relative to potential noise and vibration sources that could adversely impact the Quiet Room. This proceeded extensive noise and vibration measurements within the Quiet Room, in the return air ceiling cavity above the Quiet Room, and in the mechanical equipment room (MER) above the Quiet Room.

An Automotive Technical Center Quiet Room Improvement Study - Part I

2015-01-2348

Published 06/15/2015

Richard KolanoKolano and Saha Engineers Inc.

CITATION: Kolano, R., "An Automotive Technical Center Quiet Room Improvement Study - Part I," SAE Technical Paper 2015-01-2348, 2015, doi:10.4271/2015-01-2348.

Copyright © 2015 SAE International

Downloaded from SAE International by Richard Kolano, Thursday, June 11, 2015

Figure 1. Photograph of the Quiet Room showing the extent of sound absorption surface finishes.

Figure 2. Close-up of the scrim-and-hardware-cloth-faced glass fiber absorption panels that line the walls and ceiling of the Quiet Room.

Initially, consideration was given to try and directly vibration-isolate the greatest contributors to excessive Quiet Room noise: the temperature chamber refrigeration compressors and the main building electrical switchgear. However, this idea was dismissed due to the difficulty in addressing an extensive array of pipes and service lines that connect to the compressors and the large feeder lines and conduits that connect to the switchgear.

A detailed examination was also made of the original building design drawings showing the building construction including the Quiet Room. This review showed that the top side of the Quiet

Room ceiling was formed by galvanized sheet steel panels spot welded to an angle iron grid that was suspended from the structural steel and underside of the mezzanine floor deck above using tie wires. These ceiling panels provided a potential sound barrier beneath the floor deck with the potential to perform as a double wall assembly with the floor deck to reduce sound transmission of MER noise radiating through the deck. We had concerns, however, that structure-borne vibration in both the floor deck and deck-supporting structural steel beams would transmit through the ceiling suspension wires and limit the effectiveness of the ceiling barrier. An added concern was that the structure-borne vibration also radiates as noise directly into the above-ceiling cavity, through the ceiling barrier, and ultimately, into the Quiet Room.

A detailed inspection of the actual building construction was made in the return air cavity above the Quiet Room ceiling. During the inspection, we noted that the angle iron and steel ceiling panels were suspended using steel spring and neoprene vibration isolators, presumably to interrupt the conduction of structure-borne noise from the floor deck and structural steel into the ceiling panels. These isolators were not shown on the design drawings and were apparently added during construction, presumably to interrupt the conduction of structure-borne noise from both the floor deck and structural steel into the ceiling panels. See Figure 3.

Figure 3. Photograph of the space above the ceiling of the Quiet Room showing the top side of metal ceiling panels that form the ceiling, the angle iron structural grid that supports these panels, and three of the spring and neoprene hangers that support the grid from the floor deck above. Also shown is the existing supply air distribution ductwork and the angled structural steel that supports a monorail beam in the Quiet Room below.

However, a closer inspection revealed that the spring hangers employed for this were the pre-compressed type. Pre-compressed spring hangers are available from hanger manufacturers specifically to limit movement in the isolated element (e.g., ceiling) as weight is successively added during construction. In the case of an isolated barrier ceiling, pre-compressed hangers maintain a constant elevation


of the ceiling as insulation and various layers of the ceiling are installed, rather than allowing a gradual and continual drop in ceiling elevation that would otherwise occur. Once the ceiling loading exceeds the pre-compression setting, the elements that create the pre-compression become disengaged and no longer are in contact. At that point, the ceiling load “breaks free” and the springs deflect slightly further to their final position.

Additional review of the isolating spring hangers that had been selected and installed to support this Quiet Room ceiling showed that these hanger springs were pre-compressed by the manufacturer to an equivalent load of 60 kg (133 lb) per hanger [3]. This was 60% of the maximum load rating of 101 kg (222 lb) per hanger. The combination of choosing hangers with load capacities that are too high for this ceiling under typical isolator spacing, and the use of an excessive number and concentration of hangers to support this ceiling resulted in an estimated average loading of less than 32 kg (70 lb) per isolator. It should be noted here that the installed spacing of the isolators was not in a simple rectangular pattern, but was instead somewhat irregular. This approach was chosen by the installation contractor in part to work around ductwork serving the Quiet Room (see Appendix for an illustration). The net result was that after construction, most spring isolators remained in a pre-compressed state such that the effectiveness of the springs for isolating vibration was compromised (short-circuited) by the nuts and washers installed to create the pre-compression. See Figure 4 for a view of one of the compromised hangers caused by the pre-compression nut and washer.

Figure 4. Photograph of the above-ceiling space with a close-up of the bottom of one of the isolation hangers. The red arrow indicates the pre-compression nut that forces the washer against the bottom of the housing causing a “short-circuit” of the spring and allows vibration to travel down to the ceiling panels.

In addition, inspection of the above-ceiling cavity revealed that there are three sets of steel framing elements that connect to the underside of floor-supporting steel, angle downward to penetrate the barrier ceiling plate, and support a horizontal beam to which a monorail beam

along the top and center of the Quiet Room is bolted. See Figure 5. These steel framing elements provide a direct path for structure-borne vibration from the underside of the MER floor deck to the monorail beam. A photo of one set of this framing (viewed from the above-ceiling cavity above the Quiet Room) is provided in Figure 6.

Figure 5. Photograph of the monorail beam for a hoist that traverses the centerline of the Quiet Room in a ceiling recess.

Figure 6. Photo of angled steel framing members that extend down from the floor deck to support the end of one of three horizontal beams that support the Quiet Room monorail beam that runs in a ceiling recess along the center of the Quiet Room. These steel members were also found to conduct vibration into the metal ceiling panels.

Acoustical MeasurementsMeasurements were made of the background noise level in the Quiet Room under various operating conditions of MER equipment. Refer to Figure 7. Measurements were made in octave bands and compared against the target noise limit of noise criterion (NC) 15 dB [4]. These measurement results show that operation of the MER refrigeration compressors is the most significant contributor to excessive Quiet


Figure 7. Third-octave band noise levels measured in the Quiet Room for various MER equipment operating conditions compared against the (octave band) target goal of NC-15.

Room noise. Noise produced by the electrical switchgear was also found to be a significant contributor to noise in the Quiet Room. One-third octave band sound pressure level (1/3 OB SPL) measurements were also made at key locations under specific MER equipment operating conditions to better understand the noise intrusion into the Quiet Room. Figure 8 shows the 1/3 OB SPL measured in the MER at positions ranging from close to and distant from the refrigeration compressors with the compressors in operation as well as the switchgear energized. Also shown is the spectrum of noise measured in the ceiling cavity above the Quiet Room and in the center of the Quiet Room. These results show that pronounced spectral peaks in the third-octave bands of 63, 125, 250, and 500 Hz exist in the MER, within the ceiling cavity, and in the Quiet Room.

Measurements were made of the airborne sound transmission loss (STL) performance of the floor-ceiling construction separating the Quiet Room from the mechanical equipment room above using the test procedure of ASTM E 336 [5]. Measurements were also made of the noise reduction (NR) versus third-octave band frequency between the MER and the above-ceiling cavity above the Quiet Room to approximate the STL of the floor deck alone. These are plotted in Figure 9 compared against laboratory measured STL data for the two respective floor and floor-ceiling constructions. It should be noted that because the actual laboratory STL performance for the overall floor-ceiling assembly was not readily available, the performance was estimated using empirical calculations based upon a combination of the STL of elements that were individually laboratory tested for STL performance (i.e., 6″ concrete and 18 ga.

sheet steel) [6]. Based upon this comparison, it is clear that the concrete floor deck performance is reasonably close to that expected based upon laboratory test data. However, the floor slab in conjunction with the suspended 18 ga. steel barrier produces considerably less than its potential as demonstrated by comparison against laboratory test data. This is attributed to the lack of resilient suspension of the barrier steel panel ceiling, which is the direct result of the compromised vibration isolation performance of the pre-compressed spring hangers.

Figure 8. Third-octave band noise levels measured in the Quiet Room before corrective measures, as well as above the Quiet Room ceiling, and at several positions in the MER for various MER equipment operating conditions compared against the (octave band) target goal of NC-15.

It should also be noted that the apparent drop in STL performance at 125 Hz is actually due to background noise in the measurements originating from the MER switchgear which could not be turned off for this investigation (see field test results provided in Figure 9).

Recommended Corrective MeasuresCorrective measures for reducing noise intrusion into the Quiet Room were identified to correct design and construction deficiencies discovered during this investigation. These included reworking the suspension system which supports the barrier ceiling plate above the Quiet Room, and related actions which reduce the airborne and structure-borne noise transmission into that ceiling barrier. In addition, a method for interrupting structure-borne noise through the MER floor deck was also identified. This consisted of first saw-cutting the floor deck to create a physical break in the otherwise continuous slab, and second, to interrupt the structure-borne path through structural steel beams that support that floor deck. The latter


would be achieved by breaking the contact between the ends of those beams and the main beams along column lines that support the beam ends. The “freed” ends of these floor-supporting beams would then be supported using spring isolators. These corrective measures were set up to be implemented in three separate phases. This approach allows for measurements of Quiet Room noise levels upon completion of each phase to determine the effectiveness of the work conducted in a given phase before proceeding with the next phase.

Figure 9. This graph shows a comparison of the third-octave band sound transmission loss performance of a 6″ concrete floor slab with and without an 18 ga. steel barrier suspended 60″ below the floor slab. Shown are laboratory versus field measured values.

Phase I: Above-Ceiling WorkSeveral correction measures were recommended for above the Quiet Room ceiling. The first was to reduce the number of isolators that support the ceiling from 100 to 72 by strategically removing isolators to make the spacing as even as possible. The second was to increase the loading across the ceiling plate by installing two layers of 1.6 cm (5/8″) drywall cut to fit the cavities formed by the angle iron framing, in addition to a viscoelastic damping adhesive. Third was to overlay the entire ceiling with 10 cm (4″) of mineral wool sound absorption batts. The insulation adds additional weight across the ceiling plate, and introduces sound absorption to the above-ceiling cavity. The added mass and absorption reduces the mass-air-mass resonance of the double wall system and thereby increases the STL performance of the floor-ceiling [See the Appendix] [7]. The added absorption also reduces sound passage through this cavity which serves as a return air plenum for ventilating the Quiet Room.

With the added weight of the drywall, damping, and insulation, plus the reduced numbers and reconfiguring of the spring hangers, the estimated total average loading per isolator is 72 kg (159 lb). This loading is both greater than the pre-compression setting of 60 kg (133 lb) and yet below the maximum hanger capacity of 101 kg (222 lb) [Refer to the Appendix].

The increased loading on each spring hanger was expected to overcome the pre-compression short-circuiting and allow the barrier ceiling to fully decouple from the MER floor above. The increased hanger loading was also expected to produce an average deflection of 1.8 cm (0.7″) in the isolator springs, which should produce a vibration isolation efficiency of 98% at 30 Hz, the lowest frequency of interest as determined from vibration measurements on the floor and structural steel. This degree of isolation efficiency tells us that the spring hangers will no longer compromise the double-wall STL performance of the floor/ceiling assembly [Refer to the Appendix] [8] [9].

Figure 10 shows the previously mentioned STL performance of the existing floor-ceiling construction between the MER and Quiet Room compared against the expected double wall STL for that assembly empirically determined using laboratory measured STL values for each individual element of the construction.

Also shown as a third curve is the estimated STL of the improved floor-ceiling construction after installation of the following recommended improvements:

1. The steel panels are overlaid with 2 layers of 1.6 cm (5/8″) drywall attached using a vibration damping adhesive.

2. The number and spacing of existing spring hangers are reduced and reconfigured to overcome the pre-compression condition.

3. The drywall-backed ceiling panels are overlaid with a 10 cm (4″) thick layer of mineral wool acoustical absorption material.

Another recommendation was to remove the rarely (if ever) used monorail and hoist from the Quiet Room (see Figure 5), including all of the steel elements that originate at the underside of the MER floor deck above (see Figure 6). This would remove a second major path of structure-borne noise that gets induced into the barrier ceiling plate as well as eliminate a source that radiates this structure-borne noise directly into the Quiet Room.

Phases II & III: Mezzanine Floor Deck IsolationAs of this writing, the Phase I: Above-Ceiling Work was completed. Subsequent work is planned and will be divided into phases. Phase II consists primarily of using a masonry saw to cut through the entire floor slab and deck parallel and close to the upper web of the main floor-supporting beam along the column line at two locations. This saw cutting is expected to create a 1.2 cm (0.5″) gap in the floor deck to interrupt structural-borne noise (vibration) induced in that floor slab by the low temperature chiller compressors and electrical switchgear which both rest directly atop the slab without any deliberate means of vibration isolation. This excitation travels through the floor slab and radiates as airborne sound into the ceiling cavity above the Quiet Room. Levels of compressor and switchgear noise measured in this cavity are higher than expected when


compared against the levels of airborne noise from the compressors and switchgear measured in the MER directly above the Quiet Room, adjusted for the noise reduction of the floor slab. The result is that these paths for structure-borne noise are significant contributors to the noise that intrudes into the Quiet Room and ultimately, create a limitation for the degree of improvement that the Phase I: Above-Ceiling Work can provide.

Figure 10. This graph shows a comparison of the third-octave band STL performance of a 6″ concrete floor slab with an 18 ga. steel barrier suspended 60″ below the floor slab, as well as the improvement in STL expected from the addition of drywall and cavity absorption.

Phase III is scheduled to be done should noise measurements made after Phase II (saw cutting the floor deck) show that structure-borne noise conduction through the structural beams supporting the MER floor deck continues to create excessive Quiet Room noise levels. The recommended method for interrupting these structural paths is by temporarily jack-supporting the ends of each of ten (10) W-27 beams that support both sides of the MER floor deck (See Figure 11), cutting away the structural connections of those beams from the W-36 main beams on column lines that support those beam ends, and adding structural steel seats that allow each beam end to be supported by a pair of steel spring vibration isolators (See Figure 12).

Residual FlankingThe vibration isolation of the MER floor deck described above under the Phase II and Phase III renovation work will be subject to some degree of limitation due to structure-borne noise flanking through paths that could not be interrupted. This includes the two main structural beams and structural concrete block walls along column lines 9 and 10. (See Figure 11)

Figure 11. Conceptual drawing of the 10 beam ends that would need to be cut free of the main beam support and then re-supported using two vibration isolating springs.

Post-Renovation MeasurementsMeasurements of the noise levels in the Quiet Room were made following completion of the Phase I: Above-Ceiling Work. Figure 13 shows the results for various MER equipment operation conditions. These results show that the work done above the Quiet Room ceiling was effective at reducing the noise intrusion of both the electrical switchgear and the refrigeration compressors on the background noise levels in the Quiet Room. These renovations reduced the Quiet Room noise levels from NC-35 with the compressors operating and NC-24 without (leaving only the switchgear noise) to NC-24 and NC-15 (respectively). It should be noted here that the above-ceiling renovation work also reduced the noise of the HVAC system serving the Quiet Room, but this was neither a requirement nor specified as a goal of this work since it can be readily turned off if needed for low-level acoustic measurements.

Follow-up measurements after implementation of the improvements being considered for Phases II & III (vibration isolation of the MER floor deck) are planned once that work is completed and will be reported in a subsequent paper.


Figure 12. These sections illustrate the re-support of each W-27 beam end from the main W-36 beams using a pair of spring isolators.

Figure 13. Quiet Room noise levels measured after completion of the above-ceiling work (Phase I Renovations). This shows that the NC 15 target background noise level is now met with the switchgear alone active, and that the room noise level with the compressors operating are significantly lower although still above the NC15 target.

Summary/ConclusionsConclusions of this study are that provisions originally intended to isolate the Quiet Room from electrical and mechanical equipment operating in the mezzanine floor deck were not sufficiently designed and were poorly installed. This combination resulted in excessive amounts of airborne noise being transmitted into the Quiet Room. Recommended corrective measures included removal of a monorail beam and hoist, and enhancement of the Quiet Room ceiling to add mass, recovery of the vibration isolation of that ceiling from the floor deck above, and improvement of the sound transmission loss performance of the combined floor/ceiling assembly. These improvements were found to be highly effective at reducing background noise levels in the Quiet Room.

Additional changes to further isolate structure-borne noise including interrupting both the mezzanine floor deck and structural steel that supports that deck have also been recommended for future phases of the Quiet Room noise reduction improvement project.

References1. International Standard, “Acoustics - Determination of sound

power levels of noise sources using sound pressure - Precision methods for anechoic and hemi-anechoic rooms,” ISO 3745, second edition, December 2003

2. Duda, John “Basic Design Considerations for Anechoic Chambers,” Noise Control Engineering, Vol. 9, No. 2, September 1977 doi:10.3397/1.2832070

3. Mason Industries, Hauppauge, NY

4. Noise criteria as defined by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) in the ASHRAE Handbook

5. American Society for Testing and Materials ASTM E 336-10 Standard Test Method for Measurement of Airborne Sound Insulation in Buildings

6. Wentzel, R. and Saha, P., “Empirically Predicting the Sound Transmission Loss of Double-Wall Sound Barrier Assemblies,” SAE Technical Paper 951268, 1995, doi:10.4271/951268.

7. Handbook of Acoustical Measurements and Noise Control, 3rd Edition, page 31.10 Cyril M. Harris, McGraw Hill ISBN 0-07-026868-1, 1991

8. Ibid, page 27.4

9. Noise Control in Buildings: A Practical Guide for Architects and Engineers, Chapter 9 Part 2 Cyril M. Harris, McGraw Hill ISBN 0-07-028887-8, 1994

Contact InformationRichard A. Kolano, P.E.INCE Board CertifiedPrincipal ConsultantKolano & Saha Engineers, Inc.3559 Sashabaw RoadWaterford, MI [email protected]


http://dx.doi.org/10.3397/1.2832070

http://www.sae.org/technical/papers/951268

http://dx.doi.org/10.4271/951268

AcknowledgmentsThe author gratefully acknowledges the efforts of Jeremy Bielecki, a Senior Project Engineer with Kolano & Saha Engineers, Inc., who conducted and analyzed the measurements in this project, skillfully plotted the results, drew the CAD drawings employed for figures, and gave this manuscript a critical reading. You are the best!

Definitions/Abbreviationsδ - Static deflection, in cm (inches)

fn - Natural resonant frequency of a deflected spring isolator, Hz

fmam - Mass - air - mass resonance frequency of a double wall assembly, Hz

NR - Noise Reduction, which is simply the difference in third-octave bands between the sound pressure level measured in the source room and that measured in the receiving room, in dB

STL - Sound Transmission Loss, in dB


APPENDIX

The Quiet Room ceiling was originally built using 100 spring isolators to support the weight of the ceiling from the underside of the Mechanical Equipment Room floor deck above. Figure A1 shows the original (“as-built”) layout of the isolators. Also shown is the supply air ductwork that serves the Quiet Room. The layout of this ductwork as well as the dimensions of the angle iron grid both influenced the original spacing and placement of the isolators.

Figure A1 includes a color coding of the existing, originally installed spring isolators to indicate which of those isolators should be removed and/or relocated to create a more equal distribution of the ceiling weight across the array of isolators without creating excessive spans for the angle iron grid which supports the ceiling panels and without under loading or overloading any given isolator.

Figure A1. Quiet Room Ceiling Isolators Color Code (Before):Green = Existing, to RemainBlack = Existing, to be RemovedRed = Pairs of Existing Isolators to be Removed and Replaced with a Single Isolator

Figure A2. Quiet Room Ceiling Isolators Color Code (After):Green = Final Layout after Excess Isolators are Removed or Repositioned

Figure A2 shows the revised isolator layout that was developed based upon maintaining the original angle iron grid, the added weight of the additional drywall and insulation, and the need to increase the average loading of the isolators to something greater than the pre-compression loading. This approach resulted in the use of 72 isolators to support the ceiling with the increased loading.

With 72 isolators and a total improved ceiling weight estimated to be 5179 kg (11,417 lb) (± 10%), each isolator will support 64 to 79 kg (142 lb to 174 lb) or about 72 kg (159 lb) on average and produce a static deflection of 1.8 cm (0.7″). Since the rated capacity of each existing isolator is 101 kg (222 lb), a 60% pre-compression of the isolators requires a load of at least 60kg (133 lb) to overcome the pre-compression.

The mass - air - mass resonance frequency (fmam) of a sealed double wall system establishes the frequency at which double-wall sound transmission loss performance begins to be achieved. The fmam of a double wall system can be determined using the following equation [7]:

Where fmam = mass-air-mass resonance frequency, Hz

m1 = surface mass of the first wall, kg/m2

m2 = surface mass of the second wall, kg/m2

d = separation between the two walls, mm

K = 60 for an empty cavity; 43 for a cavity filled with sound absorptive material


In this case, the fmam for the original floor-ceiling assembly is expected to be 10 Hz. With the improvements described here-in, the fmam of the floor-ceiling is reduced to 7 Hz. This assumes that air alone is the only element that acoustically couples the two masses, and that there is no absorber within the airspace. When the double wall system is actually vertically separated elements, such as the Quiet Room sound barrier ceiling suspended below the MER floor deck, the mass-air-mass frequency is not the only consideration. Also of concern is the stiffness of the elements that structurally support the barrier ceiling. This should also be considered, especially if the suspension system is stiffer than air. In such a case, the resonant frequency of the suspension system would be higher than the mass - air - mass resonance frequency of the floor and ceiling, thereby limiting the effectiveness of the double wall system for achieving high STL performance. In the experience of the author, practice has shown that the resonant frequency of the resilient suspension system for a massive ceiling hung below a structural floor deck may create a limiting condition if that resonant frequency is too high (i.e., above the mass - air - mass resonance frequency of the floor-ceiling system). In such a case, structure borne vibration measured in the floor may be transmitted through the elements that are supporting the ceiling and result in sound radiation from the barrier ceiling plane which is at a higher magnitude than would be expected simply from the sound transmission loss performance of the decoupled floor-ceiling and the source level/spectrum of noise measured in the space above that floor. As a result, the deflection and resonant frequency of the spring hangers that support the Quiet Room ceiling were initially a concern, and remained a concern following implementation of the recommended measures that allowed that ceiling to “break free” from the pre-compression settings of the isolation hangers. The concern was that even with the increased ceiling loading, and the average 1.8 cm (0.7 inches) deflection expected from the isolator springs, there may continue to be conduction of structure borne noise through the spring hangers at low frequencies which would reduce the double-wall STL of the floor/ceiling assembly at low frequencies. Based upon the following equation [8], the average deflection of the spring hangers is expected to produce a resonance frequency of 3.7 Hz.

Where fn is the natural resonant frequency of the deflected spring isolator, K is a constant of 15.8 (3.13 for I-P) and δ is the deflection of the spring isolator in millimeters (inches).

This average spring deflection is also expected to produce a vibration isolation efficiency of greater than 98% at frequencies above 30 Hz, which is the lowest frequency of interest as determined from vibration measurements on the MER floor and structural steel. This isolation efficiency is based upon the following equation [9]:

Vibration Isolation Efficiency

Where fd is the frequency of the disturbance which is to be isolated and fn.is the natural resonant frequency of the spring isolator.

With vibrational energy passage through the spring hangers reduced by 98% (or more), any reduction in the double wall STL due to structure borne noise transmitted through the spring hanger becomes negligible. This agrees with our standard practice of assuring that any resonant elements which potentially couple the opposing sides of a double wall assembly be selected so that the natural resonant frequency of those elements are below (less than) the fmam of the double wall assembly.

The Engineering Meetings Board has approved this paper for publication. It has successfully completed SAE’s peer review process under the supervision of the session organizer. The process requires a minimum of three (3) reviews by industry experts.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE International.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE International. The author is solely responsible for the content of the paper.

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