Vibration transmission in building structures
The evaluation of the sound transmission between two adjoining rooms is traditionally carried out through the analysis of their separating structure. Such an analysis takes into account exclusively the sound reduction index of the separating wall, as resulting from laboratory measurements, but other factors are of primary importance, namely:
- the airborne sound transmission through leakages and small buildings elements, like electrical ducts;
- the structural transmission through the lateral structures of the two rooms (the so called flanking transmission).
While the problem of leakages can be enclosed in the traditional analysis, the problem of flanking transmission requires a completely new approach.
In principle, the analysis of the flanking transmission between two rooms implies the study of the energetic balance between all the different subsystems involved (walls, ground and air volumes) with some simplifying statistical hypothesis, like the uniform distribution of the vibrational energy in the structures. The present limit of this method is the difficult evaluation of some parameters, like the radiation factor and coupling loss factor, for actual building structures.
An alternative method has been proposed, which requires input data of simpler evaluation, like the reduction index Ri of all the structures and the vibration reduction index Kij of all the junctions between the structures. At the moment, this method is under study by CEN/TC 126/WG 2 which has proposed a draft standard for the calculation of the apparent sound reduction index between adjoining rooms.
The Group of Applied Acoustics of the University of Bologna has a specific research project for the measurements of the vibration reduction index in building structures, both in laboratory and in situ. The main aims of the research are:
- to assess the best method for the measurement of the vibration transmission between building structures (transient or pseudo-stationary excitation);
- to measure the vibration reduction index of junctions between typical massive building structures (homogeneous and multi-layered).
Regarding the first point, the influence of the excitation type (stationary random excitation, using an electromagnetic shaker; pseudo-stationary, with different hammer impacts; transient, single hammer impact) and of the number and position of the measurements points is being evaluated both in laboratory and in situ.
At the moment, laboratory measurements are carried out on scale models, but it has already been planned the construction of four rooms for flanking transmission measurements, to be completed in 1998.
Figure 1. Different excitation methods of the structures under test.
Regarding the second point, it is planned:
- to study from an experimental point of view the vibration transmission in real building structures, in order to have a database of the vibration reduction index of junctions between typical building structures (homogeneous and multi-layered);
- to study the influence of elastic layers between the structures;
- to study the influence of the structural frame (concrete beams and pillars) in the flanking transmission.
Figure 2. A typical junction between a floor and a separating wall on a concrete beam.
- Two-channel real-time analyzer Nortronic RTA 830;
- Two-channel FFT analyzer Ono Sokky CF 5220;
- Accelerometers and charge amplifiers Bruel & Kjaer;
- Microphones, sound intensity probe and loudspeaker Bruel & Kjaer;
- Acquisition board and post-processing software MLSSA;
- Internally developed software;
- PR EN 12354 Building acoustics; estimation of acoustic performance of buildings from the performance of products, part 1, N° 137: airborne sound insulation between rooms, CEN, 1996.
- PR EN 12354, Building acoustics; estimation of acoustic performance of buildings from the performance of products, part. 2, N° 138: Impact sound insulation between rooms, CEN, 1996.
- CEN/TC126/WG6 N 30, Laboratory measurement of the flanking transmission of airborne and impact noise between adjoining rooms, second draft, May 1996.
- GERRETSEN E., "Calculation of airborne and impact sound insulation between dwellings", in Applied Acoustics n. 19, 1986, 245-264.
- NORTON M. P., Fundamentals of noise and vibration analysis for engineers, ch. 4, "Noise and vibration measurement and control" and ch. 6, "Statistical energy analysis of noise and vibration", Cambridge University Press, 1989.
- GERRETSEN E., "Calculation of the sound transmission between dwellings by partitions and flanking structures", in Applied Acoustics n. 12, 1979, 413-433.
- CREMER L., HECKL M., UNGAR E., Structure borne sound, Springer Verlag, Berlin, Heidelberg, New York, 1973.
- EWINS D. J., Modal testing: theory and practice, Taunton, Somerset, England, Research Studies Press, 1984.
- GERRETSEN E., Vibration reduction index Kij, a new quantity for sound transmission at junctions of building elements, Proceedings of Internoise 96, Liverpool, July 1996, 1475-1480.
- OSIPOV A., VERMEIER G., "Sound transmission in buildings with elastic layers at joints", in Applied Acoustics, vol. 49, n. 2, 141-162.
An overview publication (in Italian)
- M.Garai, S.Secchi Metodi di previsione delle prestazioni acustiche degli edifici: isolamento all'interno degli edifici, Atti del Convegno Nazionale Edilizia e Ambiente, Trento, 18-20 Febbraio 1998.