Standard Tapping Machine Research Articles

Development of a low-frequency impact noise metric

Low frequency footfall noise (“thudding”) is a common source of complaints in lightweight joist-framed multifamily projects. Previous work by the authors has indicated that the low frequency impact sound pressure levels (LFISPL) from a standard ISO tapping machine are highly correlated with occupant reaction [LoVerde and Dong, J. Acoust. Soc. Am. 112, 2201 (2002), LoVerde and Dong, INCE Inter‐Noise 2004 Proceedings, 167 (2004)]. In order to be useful, the raw LFISPL must be translated into a single number that maintains the high correlation with subjective reaction while providing adequate dynamic range to distinguish the performance of different assemblies. Candidate metrics are evaluated.

On the relevance of impact source impedance at low frequencies

Many researchers have posed the question of whether the standard tapping machine simulates the impedance of real sources well enough to properly judge the impact sound insulation performance of a floor. Proposed solutions such as the modified tapping machine, the bang machine, and ball were results of these investigations. Recent data collected on bare (wood and concrete) floors, suggest that in the low frequency range, the impedance of the source has no influence on the power injected into the floor. This is presumably due to the fact that the bare floors have much higher impedances than most common sources, meaning only the blocked force of the source influences the injected power. This furthermore suggests that modifying or redeveloping the source is not necessary, and that through use of an appropriate weighting curve a single number rating that correlates well with subjective measurements can be defined. Supporting objective and subjective results will be presented.

Acoustic contributions and characteristics of floor treatments for elementary school classroom background noise levels

A two part investigation involving the effect of floor treatments on classroom background noise levels will be presented. Phase 1 determined the effects of hard versus soft flooring on overall speech and activity noise levels using long-term calibrated sound recordings in elementary classrooms. Two similar-sized classrooms were used: one with vinyl composition tile (VCT) flooring, and one with short-pile commercial carpeting. After parsing the recordings into separate segments of (a) teacher/student speech (alone), and (b) classroom activity noise, including footfalls, chair scrapes, and impacts (no speech), a significant decrease in overall levels was found in the carpeted rooms. Phase 2 determined the acoustical properties of nine different flooring materials ranging from resilient athletic floors to VCT to commercial carpeting. Sound absorption was measured following ISO 10534-2, while ISO 3741 sound power measurements were made while either (a) using a standard tapping machine, or (b) scraping a classroom chair back/forth over the floor surface in a reciprocating manner. In general, both carpet samples resulted in the lowest sound levels and the highest absorption. Relative performances of each material will be presented along with additional classroom usability factors, such as maintenance, cost and durability. [Work supported by Paul S. Veneklasen Research Foundation.]

Rating the impact sound insulation of flooring from its airborne sound reduction index

With the increasing adoption of higher density forms of dwellings in urban environments we need reliable but economic means of verifying that they meet specified sound insulation requirements. This is particularly so now that territorial and building code authorities are realising that performance checking is an indispensible part of the quality assurance process. This presentation concerns a simplified way of confirming the impact noise insulation in buildings. This makes impact sound insulation rating possible without the need to make impact sound pressure level measurements. The technique estimates the impact sound level by using an accelerometer attached to a hammer (similar to those used in the standard tapping machine) to measure the reaction force from the floor and combines this with the floor's sound reduction index. The method simplifies the equipment required for impact sound insulation rating and allows measurements to be made in the presence of high background noise.

Acoustic characteristics of floor treatments for elementary school classrooms

Phase 1 of this study [J. Acoust. Soc. Am. 129(4), 2523 (2011)], determined the effects of hard versus soft flooring on overall speech and activity noise levels in elementary classrooms. A significant decrease in overall levels was found in carpeted rooms. This phase sought to investigate a range of floor materials and their pertinent properties. Nine different floor materials were mounted to 3 in. concrete slabs and evaluated using a battery of acoustic, impact, and chair scrape tests. Tested materials included vinyl composition tile, resilient rubber athletic flooring (virgin, blended/synthetic, and recycled), polyurethane, vinyl cushion tufted textile carpet, and rubber-backed commercial nylon carpet. Impedance tube measurements of sound absorption were made using International Organization for Standardization (ISO) 10534-2, while sound power measurements according to ISO 3741 were made while either (a) tapping on each mounted sample with a standard tapping machine or (b) while reciprocating an elementary classroom chair back and forth to produce repeatable scraping sounds. In general, the two carpet samples resulted in the lowest sound levels and the highest absorption. The relative performance of each material will be presented along with a discussion of additional usability factors, such as maintenance, cost, and durability. [Work supported by Paul S. Veneklasen Research Foundation.]

Modelling the Excitation Force of a Standard Tapping Machine on Lightweight Floor Structures

Up to now the research and development in the field of building acoustics is based mainly on measurements. In consequence the development and optimization of a new building component is a very tedious and expensive task. A considerable reduction of these costs could be achieved, if the optimization relying on measurements would be replaced – at least to some extent – by a computational prediction model. For these models it is necessary to represent not only the component and the adjacent rooms but also the excitation in a suitable way. This paper gives an overview of models for the excitation generated by a standard tapping machine taking into account the interaction between the impacting steel cylinders of the tapping machine and the vibrating surface of the floor.

Vibration reduction of thermal break balcony connections

In energy efficient buildings concrete balconies have to be thermally insulated from the concrete floor to reduce heat loss and to increase surface temperature. This thermal break is obtained by polystyrene in combination with high-strength concrete thrust-force bearings and an arrangement of stainless steel straight and bent bars to act as tension and shear reinforcement. The acoustic performance of such thermal break units was investigated under laboratory conditions. Therefore the vibration level difference at different junctions between balcony and the thermal isolated concrete floor as well as the normalized impact sound pressure level of the floor when the balcony is excited with the standard tapping machine are measured. The influence of the number of tension and shear steel bars and the type of thermal insulating material was studied on typical sized balconies connected to small test floors. The vibration isolation at low frequencies was determined using modal testing.

Uncertainties and repeatability of the Reception Plate Method

The Reception Plate Method (RPM) as proposed by CEN/TC126/WG7 in prEN 15657-1 estimates the structure-borne sound power injected from a (high mobility) vibrating source into a (low mobility) building structure. In the near future, a round-robin test is planned to investigate repeatability and reproducibility issues of the RPM. As a preparation of this round-robin, uncertainties of the RPM are studied using the standard ISO tapping machine, placed on a 4 feet table on a reception plate. The injected power into the reception plate is calculated using different methods. The influence of the source position, accelerometer positions and airborne excitation are investigated, together with repeatability tests. Also, differences in injected power between two different brands of the ISO tapping machine will be studied. Finally, in-situ installed structure-borne sound power level will be predicted using prEN 12354-5 and compared to measurements on a simple building structure.

Building vibration excited by step dancing—Case study

A new three-story building (steel frame, metal deck, concrete floor slabs) for the performing arts includes a dance studio on the upper level. An isolated concrete floor was installed within the dance studio, which is effective at attenuating impact sources (as modeled by a standard tapping machine), and normal foot fall (walking, light dancing, or similar). Hard-shoe, percussive dance (e.g., step dancing) by solo dancers and especially a chorus of 20 to 30 dancers, generates significant foot fall noise and vibration within the building, that is audible in the spaces one and two floors below the dance studio, and causes perceptible vibration of the building structure. Structural analysis relating natural frequency of building structure with natural frequency of isolated floor and forcing frequency of dancers was performed. Noise and vibration testing, with tapping machine and dancers, was also performed. Stiffening of slab with additional steel column to reduce effective span was effective in reducing magnitude of intrusive noise and vibration, as demonstrated by testing with dancer as source.

Acoustic characterization of in-room footfall noise

Existing literature concerning footfalls is primarily focused on its transmission between spaces, such as a floor/ceiling’s Impact Isolation Class. This study measured in-room sound power spectra produced by footfalls on eleven different floor surfaces using human subjects and a standard tapping machine. Within a reverberation room, 7 male and 7 female subjects walked on each floor while wearing 3 different types of footwear: Leather-soled with hard heels, rubber-soled with rubber heels, and sneakers. Sound power spectra were measured in 1/3 octaves using the procedures of ISO 3741. A tapping machine was also used on each floor profile using both standard drop weights and with cored samples of the same shoe soles attached to the bottom of each weight. The data for each floor profile produced averages by shoe type along with corresponding 95% confidence intervals. A total of 143 average sound power spectra were generated, based on eleven floors with thirteen averages per floor (3 male, 3 female shoes, 6 shoed tapping machines, and 1 bare tapping machine). Spectral results for each shoe/floor were studied and compared. Correlations between sound power spectra produced with human walkers versus tapping machine were investigated. [Work supported by The Paul S. Veneklasen Research Foundation.]

Impact noise and walking noise in multifamily housing

Field measurements of impact sound transmission using the ISO standard tapping machine are compared with field measurements of human walkers in both wood and concrete buildings. In wood frame construction, low-frequency noise dominates the spectrum with most floor coverings. In concrete buildings, high-frequency noise is the controlling factor in non-carpeted areas. The relative effectiveness of different noise mitigation measures will also be discussed.

Evaluation of mid‐ and high‐frequency impact insulation methods

Impact insulation is currently measured using impact insulation class (IIC) and a standard tapping machine described in ASTM E492 and E1007 (and corresponding ISO metrics and standards). The resultant IIC value does not correlate well with subjective reaction. In previous papers, the authors have focused on the low‐frequency thudding from footfalls in lightweight wood construction and shown that the impact sound pressure level (ISPL) in the 63‐Hz octave band correlates better with subjective reaction than IIC or other metrics [LoVerde and Dong, J. Acoust. Soc. Am. 112, 2201 (2002); INCE Inter‐Noise 2004 Proceedings, 167 (2004); J. Acoust. Soc. Am. 119, 3220 (2006)]. Further, the low‐frequency ISPL is controlled by different variables than the mid/high‐frequency ISPL. This suggests that a two‐metric system would be valuable where the first metric describes the low‐frequency performance and the second metric describes the mid/high‐frequency performance. Field testing experience is presented showing that IIC does not adequately differentiate the mid/high‐frequency performance of various resilient materials. Candidate mid/high‐frequency metrics are evaluated to determine if any result in improved methods for evaluation and rank‐ordering of floor/ceiling assemblies.

Investigation on the flanking transmission of impact sound insulation of floor

Impact sound pressure level of floors depends, as general rule, on direct sound radiated by the floor excited by a standard tapping machine and on flanking transmission of lateral walls. Depending on the kind of junction between the floor and walls of the receiving room, flanking paths can be more or less important. Requirements of laboratory test specimens are not well specified in EN ISO 140 standards, particularly for junctions of the test floor and lateral walls. In this paper measurements performed at DIENCA laboratory are presented in order to evaluate the influence of flanking transmission on impact sound pressure levels of a standard floor and on impact sound reduction level of standard floor with a resilient layer. Measurements are performed on a 14-cm-thick concrete floor in two different conditions: first connected on two sides of the receiving room and then on all four sides. Impact sound pressure levels and vibration levels are analyzed in order to evaluate the contribution of different transmission paths. As the measurements were carried on in the new facility for measurement of the contribution of the flanking transmission, this facility will be exhaustively presented in the paper.

Impact Sound Insulation: The Standard Tapping Machine Shall Learn to Walk!

Previous research on the standard tapping machine has shown that the impact on a floor depends strongly on the combined system of floor and tapping hammers. This means that in general, a particular measure to reduce the impact sound pressure level can be advantageous with one impact source and disadvantageous with another. Conversion from one impact source to another has to be made for each floor individually, provided there is a sufficient knowledge of the dynamic properties of the source and floor. So, if the standard tapping machine is to be used to characterise a floor with respect to walking noise, the only simple way out of the problem is to make it ‘behave like a walker’. That means it should have the same source impedance. Investigations at the Fraunhofer-Institut für Bauphysik Stuttgart showed that this could be achieved by small modifications to the standard tapping machine. Comparisons using a concrete floor and a wooden joist floor have proved that results using such a modified tapping machine are much closer to walking noise than the results using the standard tapping machine.

Impact Sound Insulation of Timber Floors: Interaction between Source, Floor Coverings and Load Bearing Floor

It is well known that prediction of impact sound of timber floors contains large uncertainties. They partly originate from the dynamic interaction of impact sources and floor elements which makes it difficult to characterise the performance of the elements of a floor in the right way. First results of a project recently started at the Fraunhofer-Institut für Bauphysik on this subject are presented. Calculations reveal strong influences of impact source and load bearing floor on the acoustical performance of the floor coverings. There are considerable deviations between concrete and timber floors. The standard ISO tapping machine and human walkers interact quite differently with floors and coverings, thus modifying the impact sound improvement “of the floor covering”. To clearly separate performance of the floors, floor coverings and impact sources detailed information about their dynamic properties is necessary. Non-linear effects are expected especially with carpets. The results of the project should be helpful to design a laboratory measuring method for the impact sound improvement on lightweight (timber joist) floors and to design floors with a better impact sound insulation.

The Influence of Concrete Elastic Characteristics on the Impact Noise Insulation of Concrete Floors

The noise insulation characterisation of a bare floor for structure-borne sound is usually based on specific tests made in accordance with the established international standard EN ISO 20140 and the structure-borne noise insulation index calculated according to standard EN ISO 20717. For bare concrete floors of the same type, which one might suppose would have identical acoustical performance, different impact sound insulation frequency spectra may be obtained if the concrete at the zone where the impact hammer strikes is of different type. This situation is particularly the case for bare floors which have levelling layers of floor screeds. This work presents an analytical study of the influence of the elastic characteristics of the concrete used in the area of the bare floor where the hammer of the standard tapping machine strikes (whether it be the structural concrete of the slab or a light concrete screed applied as a levelling layer), on the impact noise insulation spectra, and on the corresponding value of the structure-borne sound insulation index. The study firstly involved theoretically modelling the applied impulsive force spectrum of the hammer impact. Secondly, the modelled force spectrum was compared with the results of impact characteristics obtained using several shock tests on defined specimens, for which the effective impact force is determined. Finally, using these resulting force spectra, some simulations on a standard floor were performed to determine the changes in the response spectra, and the corresponding insulation index for structure-borne sound. Some additional considerations and conclusions are also presented.

An investigation of the characteristics of impact sound sources for impact sound insulation measurement

At the assessment of the impact sound insulation of a construction, a standard tapping machine is used as sound source. However, the current standard tapping machine has been criticised especially for measurement of wooden joist floors since the sounds generated by a tapping machine differ from those generated by actual footfall. This study has investigated the waveform and frequency spectra of human footfall (walking, running and jumping), of sand balls, sand bags and dropped tires, and of the standard tapping machine. It is shown that the frequency spectra differ between the standard tapping machine and actual footfall, especially at low frequencies. Results from other excitation sources such as a sand ball falling from a certain height showed concordance with actual footfall. This means that a sand ball drop as an impact source provides a better representation of actual human footfall than the standard tapping machine.

Assessment of low-frequency footfall noise in wood-frame multifamily construction

The impact insulation class (IIC) rating of floor/ceiling constructions fails to identify low-frequency footfall noise problems common in contemporary wood-frame multifamily construction. Because the IIC procedure ignores the impact noise spectrum below 100 Hz, the rating is not influenced by the low-frequency impact spectra produced by a typical live-walker, which usually peak in the region of 15 to 30 Hz and are associated with the ‘‘thuds’’ and ‘‘thumps’’ commonly observed. Data comparing the shape of the typical low-frequency impact noise spectrum produced by a live-walker and that of the standard ISO tapping machine indicate that the tapping machine might potentially be used as the basis for obtaining low-frequency impact noise ratings [W. E. Blazier, Jr. and R. B. DuPree, J. Acoust. Soc. Am. 96, 1521–1532 (1994)]. However, a comparison made between the two sources in the range above 100 Hz shows significant differences in the spectra produced by the tapping machine and a live-walker. This suggests that a more meaningful IIC rating of mid- to high-frequency impact noise might be obtained by cushioning the tips of the hammers in the standard ISO tapping machine, in order to better align the impact spectrum with that of a live-walker.

Floor research project at NRC Canada: An update on progress—Measurements with experimental impact devices

A major thrhust in this floor project is the collection of low-frequency impact information, below 100 Hz, using special impactors: a tire-dropping machine, two experimental rubber balls supplied by H. Tachibana, and a walker. The frequency range for the standard ISO tapping machine is also extended at 50 Hz. A preliminary analysis of the data collected so far in the project will be presented. Factors that were expected to change the shape of the impact spectrum drastically at low frequencies, so far, have had little or no effect. Changing joist depths from 185 to 285 mm had negligible effect on low-frequency transmission. Changing I-beam depth from 235 to 460 mm did have a significant effect. The best wood joist floor tested to date is still worse than a 150-mm concrete slab. So far in the project, the most effective remedy for excessive low-frequency impact sound is mass.

Floor research project at NRC Canada: An update on progress—Airborne sound and standard tapping machine measurements

The extensive research project on sound transmission through floors is still in progress. This paper will present information on standard measurements made according to ASTM E90, ASTM E492, and ISO 140 parts III and VI. Since the last report in 1995, the effects of changing joist length, joist spacing and depth, wood I-beam type and depth, and the effect of adding concrete as a topping on the floor have been studied. Different types of subfloors and different types of gypsum board have been used in several combinations. For a given basic structure, the mass of the panels has the most effect on the single number ratings, STC and IIC, which are determined mainly by the sound transmission loss values around 125 Hz. Many structural changes have little effect around 125 Hz so STC values do not change much, although there are significant changes at other frequencies.