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Chapter 4 Measuring Sound Absorption Area using the Radiation Box Gear
Introduction
The objective of this chapter is to introduce the other modified equipment method for impact sound measurement resulting in a simplified test procedure, as is similar to the purpose of the research in Chapter 3. Chapter 4 describes the modification of the airborne sound source in sound absorption area measurement (conventionally obtained RT for calculating sound absorption area according to ISO 354) that is one of essential components of impact sound insulation. The modified equipment establishes an economical measurement system for conducting complete impact isolation measurement.
The pilot equipment, Radiation Box Gear (RBG), is suggested as the replacement for the loudspeaker and power amplifier in RT measurement/for ISO reference sound source for sound absorption test, which is regarded as being an essential part of the measuring system used in impact sound insulation test system (see Introduction). If the method succeeds in the laboratory and the field, and is also accepted by researchers and consultants, the RBG could be used to directly measure the total sound absorption of a receiving room in accordance with the ISO 3741 series, whilst avoiding the disadvantages of measuring RT using a loudspeaker and amplifier in compliance with ISO 354. In regard to its design and process development, The Radiation Box has a normal four-stage cycle:
The designed prototype is to be built with suitable materials following the optimisation of its theoretical model; (2) Prototype is to gain its technical capability in controlled conditions; (3) Final prototype is to be released after product has been maximally optimised in the laboratory; (4) Final prototype is to be verified in field conditions. It is well known that the whole cycle is required to circulate several times in order to optimise the product to achieve the best design. In the case of the RBG in this thesis; this was finished in stage 3 when the RBG had been verified for its effectiveness in laboratory conditions. The measurements for verification in controlled conditions include the sound power level and its repeatability measurements; these results are compared with ISO equipment, the reference sound source, which is also applied to sound absorption measurement accord ing to the ISO Standard 3741(Standard ISO 3741 1999). In comparison with the traditional RT measurement (level method), in accordance with ISO Standard 354 (Standard ISO 354 2003), the sound absorption method (absorption method) has the advantage of allowing for fast measurement of sound absorption using the airborne sound source in the receiving room, for the assessment of impact sound insulation of floor/ceiling structures in buildings. In order to develop a comparatively new airborne sound such as the ISO reference sound source, the tests conducted during stage 4 are to conduct comparative testing on the same sample of building elements in both laboratory and field conditions; this involves the making of a comparative repeatability test for measuring a sound absorption area by using the RBG on a floor/ceiling structure to be built both on site (in an actual building) and in a laboratory. Unfortunately, there is a lack of funding and facilities (for the same construction built and tested in the same time period) to verify the concept of the RBG method during stage 4 (in the field) of the process.
The advantage of this modified equipment method is to be able to either replace RT measurement using the loudspeaker equipment as the airborne sound source, or to substitute the sound absorption measurement with the ISO reference sound source in dwellings. The author believes that it would be better to directly measure the sound absorption area (using the sound absorption method) rather than to derive the sound absorption area from RT (using sound level method in accordance with ISO 354) of the receiving room in buildings. The benefits of the RBG are as follows: First, avoidance of the necessity for the transporting of the heavy loudspeaker and power amplifier that are required to measure RT down to low frequencies (e.g. below 100 Hz). The total weight of the loudspeaker and amplifier used in the ARC is around 33 kg for making RT tests in buildings in accordance with ISO 354; whereas, at this stage of development, the weight of the prototype RB is around 10 kg, for the measuring of sound absorption in accordance with ISO 3741. Secondly, the results of total sound absorption can be directly applied to calculate the impact sound insulation according to the Formula (I-1); therefore, the method is both simple and effective. Thirdly, the sound absorption method requires only two, 30-second, tests to be undertaken in the receiving room; this saves more time and effort in comparison with the traditional sound level method. This means that during the same measurement period as required in the RT test, the use of the absorption method enables more tests to be carried out if necessary in order to further improve the measurement accuracy on site. According to the ISO 354 standard, in the case of the level method, the measurement at 2 loudspeaker positions with 3 microphone positions is required. For each of the loudspeaker positions, 2 noise decays are recorded for each of the 3 microphone positions. In short, one RT requires 12 measurements. In contrast, the absorption method requires a measurement of 30 seconds at 2 reference source positions with one Leq measurement, and only requires 2 measurements to be taken. Consequently, the absorption method is more efficient than the level method. Last, but not least, if accepted, the newly proposed equipment would be less costly than, either, the ISO reference sound source, or, the required loudspeaker systems. In conclusion, compared to the use of conventional RT measurement using loudspeakers as the airborne source according to ISO standard 354 in field conditions, the modified equipment method is more efficient and cost effective for the completion of the impact isolation test.
Although the subject of how to obtain accurate sound absorption measurements in buildings has been debated over the last 20 years, most field measurements are derived from RT results by using the sound level method (with loudspeaker and amplifier as the airborne sound source), while few field measurements directly measure the sound absorption area in buildings (using the ISO reference sound source). For most consultant companies, the reason for rarely using the absorption method is possibly due to the extra expense of the ISO reference source required. If these companies already have loudspeaker systems which enable a full ISO RT test to be conducted, it seems less necessary for them to purchase extra similarly functioning equipment such as the ISO reference sound source. However, from the author‟s point of view, in some instances, the latter method appears promising in order to avoid the difficulties of measuring RT; thus, the RGB method (with known SWL of sound equipment) is proposed and is being studied to simplify the measurement system. The scope of applying the RGB is as follows. As the sound absorption method is more suitable for exciting an evenly distributed sound field in bedroom-size volumes in residential buildings, the method may not be suitable for areas of greater volume in commercial buildings with high absorption, such as concert halls or movie theatres. Therefore, the RBG method may not be suitable for use in measuring sound absorption in commercial buildings with relatively greater volumes. The other limitation is that the RBG technique might be carried out in low ambient noise environments so as not to disturb neighbours; this is the same limitation as applies to the loudspeaker method. The ultimate purpose of the RBG is to use it as the new airborne sound excitation to undertake level difference and room absorption tests by replacing the loudspeaker systems, the ISO reference sound source in the sound insulation measurement in buildings. In comparison with the 33 kg weight of the loudspeaker system and its market price, the Radiation Box is lighter in weight at 10 kg and this will significantly lessen the cost of equipment. However, to date, tests have not carried out on the performance of the current prototype Radiation Box in the field, due to the fact that the laboratory stage of RB product design is still in progress. The RB also needs to be able to produce greater power to accurately measure the sound level difference between test rooms. In order to validate the RBG as a new airborne sound source in the field, a comparison repeatability test for the RT (level difference in the near future) between the field and the laboratory is required. Fortunately, for the RB product, even at its current stage of development, another application can be extended to make level differences tests on wall systems which have relatively poor insulation. The application involves the determination of faulty wall systems by screening the adjacent rooms for a short period time. In conclusion, the RBG technique is most suitable for use by acoustic consultants who screen the impact sound insulation of building elements (monitor building performance before a buildings is fully finished), or detect faulty wall systems during the construction stages for high-rise buildings; the requirement for such consultants is only to measure the impact sound insulation of floor in finished buildings (this conclusion was drawn after the author had interviewed consultants at over 4 consulting companies in New Zealand).
The RBG method has been examined for use in controlled laboratory conditions, and further research would focus on applying the Radiation Box Gear to the implementation of sound insulation measurement in finished buildings in the field. Although some problems arise; such as, the presence of non-diffuse sound fields or, high absorption levels in some rooms (if applied in certain buildings); these may influence the measurement accuracy of sound absorption, these are similar to the problems encountered in measuring RT using a loudspeaker source as an airborne sound source in the field . Further research should be carried out on the investigation of the comparison of sound absorption and sound level methods to determine whether or not the RBG method can successfully used in field conditions.
Invention of the Radiation Box
As discussed in Chapters 1 and 2, a certain amount of research work, on the standardization of sound insulation measurements as a feasible quick field check in buildings, has previously been undertaken by international and national standard organizations. In one case, a quasi-simplified method for field measurement of airborne and impact sound insulation, using an airborne sound source consisting of the ISO standard tapping machine and the sound Radiation Box (Liang et al 1992) was suggested. The method evolved as the GBJ standard 75-84 (Standard China GBJ 75-84 1985) in the sound insulation of building elements in residential buildings in China. In the GBJ standard 75-84, field measurement of sound insulation is significantly reduced to 5 octave band sound levels and the single number rating for sound insulation is derived from the ISO reference curve by the conventional ISO method. Extensive field measurements for inspecting sound insulation were carried out using the simplified method, in accordance with GBJ 75-84, in apartments and high-rise buildings in six cities in China. The evaluation of sound insulation was given as single number R‘w for airborne, and L‘nT,w ,for impact, sound as required by the ISO rating standard 717 series, although the simplified test procedures (GBJ 75-84) were quite different from the ISO measuring standards in the 140 series. The measured results of R‘ for airborne, or L‘nT, for impact, sound are inferred from the A-weighted sound levels examined in the source and receiving room; the rating numbers of R‘w and L‘nT,w, are achieved from a regression formula by the substitution of A-weighted sound pressure level difference or measured sound pressure level, for airborne or impact sound insulation, respectively. Due to the nature of the simplified test procedure, that has only 5 octave band frequencies in octave band, as described in the reported study, it is almost impossible to follow the direction of the research. However, to the contrary, the Radiation Box Gear as an airborne sound substitute for the loudspeaker system or the ISO reference sound source is so unique that it is worthy of continued application in simplifying impact sound insulation measurement.
The author has been inspired by the idea of the Radiation Box Gear to replace airborne sound source and thus has combined it into the research on simplifying the impact sound insulation. As the impact sound test is the primary site measurement required (by NZ acoustical consulting companies) thus, standard tapping machine is traditionally indispensable and require being transported into buildings. In many instances, in the floor areas in lounges and kitchens, the impact sound insulation is one of the weakest sound isolation areas and gives rise to noise complaints; thus it is primarily these areas that require sound inspection. According to Equation (I-3) for accessing the impact sound insulation of floor systems, apart from the sound level test, either reverberation time or sound absorption tests are necessary in order to infer the normalized impact sound pressure level. Therefore, any modified equipment will have a great advantage in replacing the current airborne sound source if it can be combined well with the tapping machine to act as an airborne sound source. This means that if modified equipment system, consisting of the Radiation Box (10 kg) with ISO tapping machine, is powerful enough to measure the sound absorption area in an impact sound test, the loudspeaker and power amplifier (33 kg) will not needed to be carried into the field area that is to be tested.
Abstract
Acknowledgments
List of Principal Symbols
Lists of Abbreviations
Glossary
Introduction
1. Setting the Scene: Noise Complaints.
2. The Motivation for the Thesis
3. Outline of the Thesis
Chapter 1 Literature Review
1.1 Introduction.
1.2 Normative References Sound Insulation Standards from ISO and Individual Countries
1.3 Literature Review of RT Test
1.4 Reviews on Impact Sound Insulation
1.5 Review of Sound Insulation Measurement
1.6 Rating Systems of Impact Sound Insulation in Buildings
1.7 Review of the Uncertainty of Measurement
1.8 Review of Simplifying Sound Insulation Measurements in Buildings
Chapter 2 Rating Standards, National Building Codes and Regulations
2.1 Introduction
2.2 Overview of the Reference Curves for Sound Insulation
2.3 Rating Standards in ISO and Building Codes in Europe
2.4 Building Codes in New Zealand and Australia.
2.5 Building Codes in the USA
2.6 Proposal for a Universal Concept for Sound Insulation Classification
2.7 Conclusion
Chapter 3 Using Correction Factors Applied to Airborne Insulation for Predicting Impact Insulation
3.1 Introduction
3.2 Relationship between R and Ln
3.3 Scope of Application of Correction Factors
3.4 Verification for the Hypothesis by Tests on a Bare Concrete Floor in the Laboratory
3.5 CF determined by the ARC Laboratory
3.6 CF determined by NRC Laboratory
3.7 CF Determined by Computer Simulation .
3.8 CF Determined from Field Measurements
3.9 Reliability of CF
3.10 Conclusion
3.11 Further Work and Prospects
Chapter 4 Measuring Sound Absorption Area using the Radiation Box Gear
4.1 Introduction
4.2 Invention of the Radiation Box
4.3 SWL of the Radiation Box Gear
4.4 Comparison of SWL between the RBG and ISO Reference Source
4.5 Application of the Noise Source to Test Absorption Coefficient
4.6 Further Suggestions for the Radiation Box
Chapter 5 Measurement Uncertainty and Repeatability in Field Conditions
5.1 Introduction
5.2 Testing Facility & Experimental Setup
5.3 Experimental Technique
5.4 Comparisons for Laboratory & Field Testing on Floors
5.5 Comparison of a Single Number Quantity for 10 Floors under Two Categories
5.6 Discussion of Errors and Uncertainties in Field Tests.
5.7 Discussion and Conclusion on the Results
5.8 Repeatability of ISPL
5.9 Discussion on rs and rw in the Field
5.10 Suggested Values for r of ISPL in the Field
5.11 Repeatability for CF in the Laboratory
5.12 Repeatability for CF in the Field
5.13 Repeatability for SWL of the RB in the Laboratory
5.14 Uncertainty Budget for Impact Sound in the Field.
5.15 Future Research and Prospects
Conclusion
Appendix
Bibliography
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SIMPLIFIED MEASUREMENT METHODS FOR IMPACT SOUND INSULATION IN BUILDINGS
