PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

INVESTIGATION OF REBUILD REPEATABILITY ISSUES IN
LABORATORY IMPACT TESTINGJohn LOVERDE1; Wayland DONG2; Erik HOLMGREEN3; Scott BERGQUIST4
1,2Veneklasen Associates, USA

3,4Maxxon Corporation, USAABSTRACT
Materials such as ﬂoor coverings, screeds, and resilient matting are commonly applied on top of a ﬂoorceiling assembly to improve the impact noise insulation. Since the test measures the insulation of the entire
assembly, it can be diﬃcult to isolate the eﬀect of these top-side components from the base structuralassembly. The authors previously reported on a laboratory test program to evaluate the repeatability in thelaboratory when rebuilding assemblies (1). As a continuation of that program, assemblies and products from
historical tests were retested, and the resulting repeatability was poor. Investigation determined that the causeof the discrepancies was variation in the base assemblies, not the top-side materials. The techniques andresults are discussed, and the eﬀect on the interpretation of laboratory testing is evaluated.
Keywords: Sound, Insulation, Transmission

1. INTRODUCTION
Acoustical testing characterizes the acoustical performance of materials and assembli es. Testing is
intended to evaluate assemblies with respect to building code or other requirements, to compare
materials or assemblies with each other, and to research and develop new materials or assemblies. In
order for acoustical testing to provide useful information to complete these goals, the characterizationmust be independent of environmental and other factors that are not directly related to the material,
installation or construction. The uncertainties of the test methods must be small compared to thedifferences being evaluated, and the uncertainties must be well known and stable. If these conditions
are not met, not only is the data unreliable, but the results can be misleading. For example, if twocompeting assemblies have similar actual performance, but the test results for the assemblies differ
by a significant amount for reasons unrelated to the assembly, an observer will be led to the incorrectconclusion that one product is superior to the other.
Unfortunately, it is our experience that such misleading results are common. The problem is notthe variation in the acoustical test results per se, but the fact that these uncertainties are large compared to the differences that are desired to be evaluated. Worse than the size of the uncertainties is thefact that the uncertainties are often not known or are inadequately measured and reported.
In this paper we report airborne and impact insulation testing of floor-ceiling assemblies that
demonstrate the undocumented and unreported uncertainties associated with re-installing and rebuilding assemblies, and how the uncertainty affects both manufacturers of acoustical products andthe acoustical engineers who depend on the data to make design decisions.

1

jloverde@veneklasen.com
wdong@veneklasen.com
3eholmgreen@maxxon.com
4sbergquist@maxxon.com
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2. STATE OF STATISTICAL CONTROLIn North America, most acoustical testing is performed according to standard methods publishedby ASTM International. ASTM E177 (2) defines the use of the terms “precision” and “bias” in othertest methods, and discusses the underlying statistical understanding. In most building acoustics testing,the “true value” is defined by the test method results, so there is no bias by definiti on. However, theprecision is a vital concept. Precision refers to the closeness of independent results performed understated conditions.
E177 states bluntly that, “The measurement process must be in a state of statistical control; else
the precision of the process has no meaning.” A process is in a state of statistical control if the variation
between independent measurements (under the stated conditions) can be attributed to “chance causes,”
sometimes called “common causes.” Chance causes refer to all of the unknown sources of variation,
presumably numerous but small in magnitude, that result in variation of a repeated measurement.
Chance causes represent a stable degree of variability and is generally normally distributed aroundthe mean.
Lack of statistical control is the situation when the variation exceeds that expected from the stable,
normal distribution of chance causes. E177 describes this condition as including “unassignable trends,cycles, abrupt changes, excess scatter, or other unpredictable variations ” as determined by statisticalmethods. The presence of outliers, data points outside of the normal distribution of chance causes,“may indicate a weakness in the test method or its documentation.” One possible cause of lack ofstatistical control is variation due to “special causes” or “assignable causes” that have not been
identified. Special causes result in a non-random distribution of results.

3. Testing program
3.1 Assembly description
The assemblies under consideration include gypsum concrete topping slab or screed over resilient
sound mat. The gypsum concrete screed and sound mat are considered floor coverings in the sensethat they are not an integral part of the structural design and can hence be installed on any structuralsystem, or “base assembly.” Here we consider two base assemblies, one with 300 mm deep wood Ijoists and one with dimensional timber joists (nominal 2 by 10 in., approximately 38 by 240 mm). For
both joist types, the base assembly included fiberglass insulation in the cavity, and a gypsum board
ceiling supported on resilient channels.
Since testing must be performed with numerous finish floors on numerous structur al assembly
types, the number of permutations is large. To facilitate testing, the laboratory constructs the baseassemblies in large frames which can be moved into and out of the test chamber relatively quickly.
The base assemblies are stored on site and the floor coverings are installed on the assemblies outsideof the chamber. A picture of the finish floor on the base assembly installed in the test chamber isshown in Figure 1.

Figure 1 – Assembly installed in chamber.

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3.2 Repeatability conditions“Repeatability” refers to the variation of independent test measurements performed under thestated repeatability conditions. Per ASTM E177, the preferred repeatability conditions are thoseinvolving the same laboratory, by the same operator using the same equipment in as short a time aspossible. E177 is aimed at more traditional measurements of properties of samples taken from a lot ofmaterials. The repeatability conditions must be modified for acoustical testing, where only a single
assembly is available to test in a short amount of time. Therefore, acoustical test methods define
repeatability as testing the same sample repeatedly by the same operator using the same equipment inas short a time as possible.
However, many other repeatability conditions are possible. In previous papers (1,3,4), the authors
evaluated additional repeatability conditions. Re-install repeatability is measured when the specimen
is removed from the chamber and later re-installed without modification. Re-build repeatability ismeasured when the specimen is completely or partially demolished and rebuilt.
ASTM E492 (5) section 14 reports repeatability standard deviations under several repeatabilityconditions. One is a re-build repeatability condition for a wood joist floor, and another is a re-install
repeatability condition for a concrete slab.
3.3 Previous Results
In the previously published study (1), the re-install and re-build repeatability were measured. Tosummarize, the re-install repeatability was consistent with the values reported in E492, althoughhigher at high frequencies, and consistent across finish floors and assemblies. The re-build
repeatability standard deviations were small in terms of single number ratings, but the third-octave
data showed significant discrepancies. One of the rebuilt assemblies showed only small changes
compared to the original, but the other assembly showed a systematic difference in both airborne andimpact isolation.
3.4 Current Testing
The gypsum concrete topping and sound mat were installed on the wood I-joist base assembly. The
result, in particular the impact insulation class (IIC) rating, was s ignificantly lower (roughly 4–5points) than expectations based on historical testing of the same products. Additional tests wereperformed to investigate the discrepancy.
The assemblies were re-installed and re-tested, which did not result in significant changes. Thiswas expected, as the re-install repeatability had been measured to be small. Next, the ceiling wasremoved and re-installed (partial re-build repeatability). Second, the base assembly was removed andre-installed 90 mm (3.5 in.) higher in the test opening.
In another series of tests, the gypsum concrete topping and sound mat were installed on thedimensional timber base assembly. Again the results were lower than expectations. This assembly wasalso raised in the test opening. Finally two modifications were made to the base assembly.
The single number ratings are shown in Table 1. The rationale for these changes and the results aredescribed in the following section.
Table 1: Summary of test results
Base AssemblyCondition
FinishIIC
STC
Bare
4359
First test
5.5 mm LVT
5260
Bare4460
I-joistCeiling rebuild
5.5 mm LVT
5360
Bare4958
Raised in opening
5.5 mm LVT
5658
First test
Bare
4257
Raised in opening
Bare
4454
Dimension timber
Frame reduced
Bare
4356
Lag screws removed
Bare
4655

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4. RESULTS AND DISCUSSION
4.1 Re-build repeatability
As described above, for the I-joist assemblies, the ceiling was removed and re-built, which resulted
in an improvement of 1 IIC point for both finish floors. Figure 2 shows the change in impact insulation(positive values indicate improvement, i.e., lower impact sound levels) in third octaves. There was aconsistent improvement, which might imply that the original assembly had errors or issues inconstruction.
In Figure 3, the absolute value of the differences is compared to the 95% repeatability limit basedon the re-build repeatability condition reported in E492. The change from the ceiling re-build istherefore smaller than previously documented changes, and well within the expected uncertainty forthis repeatability condition per the standard. The airborne transmission loss (not shown) did not havea large or systematic change. It was therefore concluded that errors (if any) or other variation in theceiling construction were not sufficient to explain the results.
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4.2 Position within opening
The opening between the top and bottom chambers in the test laboratory h as a depth that is set by
the overall construction of the chambers. The thickness of the base assemblies varies, so there is an214

ambiguity in the test method regarding the exact position of the specimen within the opening. The
specimens are generally positioned such that the floor of the specimen is flush with the floor of the
top chamber. However, this is not always the case, and the position is not documented in the test report.
Both the I-joist and dimensional timber assemblies were raised about 90 mm (3.5 in.) in the openingand re-tested. The resultant improvement in impact insulation is shown in Figure 4. The differencesat mid and high frequencies are large, and the single number ratings increased by 2-5 points. Forcomparison, Figure 5 compares the absolute value of the differences to the 95% repeatability limit
based on the re-install repeatability measured previously (1). The results are clearly outside theexpected variation.

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Figure 5 – Absolute value of differences due to re-installing the assembly at a different position in the test
opening, compared with the 95% limit for re-install repeatability previously established.
The position of the specimen within the opening has a large effect of the test result, yet thisparameter is not recorded or documented in the test report. Tests of otherwise identical assemblies
will exhibit abrupt and unexpected shifts in measured values depending on their position. An observer
who was not aware of the effect of the position might include such variations within re -install
repeatability. In fact, the position of the specimen within the opening appears to be a “special cause”
and failing to control or document this parameter means that the test process is not in a state ofstatistical control.

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4.3 Frame construction
For the dimensional timber system, investigation revealed that changes had been made to the baseassemblies. As mentioned, the base assemblies are stored at the laboratory and used to test a wide
variety of floor covering products. In an effort to stiffen the base assemblies to make them easier tomove, newly-constructed base assemblies were not identical to assemblies that had been used in thepast. The new base assemblies had been reinforced with additional pieces of lumber around the bottomof the frame, and large lag screws had been added to help to support the joists.
To determine if these changes to the base assembly affected the results, several tests wereperformed. First the new assembly with modifications was raised within the chamber opening as
described in section 4.2, resulting in an increase of 2 IIC points. Second, the additional lumber used
to stiffen the frame was removed from the dimensional lumber assembly, so that the floor level wasequal to the baseline test (low, flush with top chamber floor) but with the assembly supported on anon-rigid base. This resulted in a slight decrease in IIC rating. Finally, half of the lag screws wereremoved, one from the end of each joist. The change in impact and airborne isolation is shown inFigure 6. The individual changes are shown; that is, the curves should be summed to determine theoverall change.
Removing the lumber from the frame resulted in a large increase in airborne isolation, while theeffect on the impact isolation is smaller. Removing the lag screws reduced the transmission loss
slightly, but significantly increased the impact insulation.
Changes to the base structural assemblies are not included in existing definitions of re-rebuild orre-install repeatability conditions. It appears that changes to the base assembly are a special cause thatis causing a significant, non-random change in the test results, and that the test method is not in a stateof statistical control. The changes to the base assemblies were not documented or described to thelaboratory’s clients. It is not known how much testing the laboratory performed when constructing a
new base assembly to verify that the performance compared to the existing base assemblies.
It may be possible to reasonably define repeatability conditions that include the variation in baseassembly. This would require measurement and documentation of the variation among nominallyidentical base assemblies. To our knowledge this work has not been performed.
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Figure 6 – Improvement in impact isolation (left) and airborne isolation (right) for the changes to the baseassembly described in the text.

5. CONCLUSIONSThe nature of acoustical laboratory testing is such that numerous repeatability conditions occurthat may be uncommon in other types of testing. Some of these conditions have been documented
previously (1,3,4) and reported here. Measuring the variation under various repeatability conditions
is a critical task, and determines how well the test method can resolve differences between assembliesand products.

216More importantly, we have determined that some of the variations in laboratory test results cannot
be assumed to be statistical (common cause) variation. This work documents several special causesof variation within acoustical testing that are not reported and have not been suitably investigated by
the test laboratories. Since these special causes are not controlled as part of the repeatability
conditions, the test method is not in a state of statistical control, and the repeatability i s not defined.
Further, the size of the variations calls into question whether the test method can provide the desired
level of precision.
It is in the interest of all parties that the testing be performed in as fast and efficient a manne r as
possible. However, this study shows that the testing paradigm of building, storing, and installing baseassemblies has more parameters than then laboratories are currently tracking and reporting, or that
customers are specifying as part of their test programs. Additional testing is required to demonstrate
the reliability of the base assemblies and the statistical control of the test method.
Since the acoustical testing results are part of building codes and required by law, a top priority of
the community should be the research, analysis, and protocols required to bring the method into astate of statistical control and to properly document the uncertainty. Without this work, the market
will continue to be inundated with misleading and uninformative test data. The test method mustmatch the needs of the community.

ACKNOWLEDGEMENTS
The authors would like to acknowledge the assistance from Maxxon Corporation to support thistype of research. The authors also wish to thank Veneklasen Associates and the Paul S. Veneklasen
Research Foundation for their continued support.

REFERENCES
1. Dong W, LoVerde J, Holmgreen E, Bergquist S. Investigation of Extended Repeatability Conditions inLaboratories. In: Proc ICSV25. Hiroshima; 2018.
2. ASTM E177-14, Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods.
ASTM International, West Conshohocken, PA; 2014.
3. LoVerde JJ, Dong DW. Categories of repeatability and reproducibility in acoustical laboratories. J Acoust
Soc Am. 2015 Apr 1;137(4):2216–2216.
4. LoVerde J, Dong DW. Addressing the lack of statistical control in acoustical testing laboratories. J Acoust
Soc Am. 2017 May 1;141(5):3929–3929.
5. ASTM Standard E492, “Standard Test Method for Laboratory Measurement of Impact Sound
Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine.” ASTM International;
2009.

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