Archives of Acoustics

In the paper the author has described the visualization methods in acoustic flow fields and show how these methods may assist scientists to gain understanding of complex acoustic energy flow in real-life field. A graphical method will be presented to determine the real acoustic wave distribution in the flow field. Visualization of research results, which is unavailable by conventional acoustics metrology, may be shown in the form of intensity streamlines in space, as a shape of floating acoustic wave and intensity isosurface in three-dimensional space. In traditional acoustic metrology, the analysis of acoustic fields concerns only the distribution of pressure levels (scalar variable), however in a real acoustic field both the scalar (acoustic pressure) and vector (the acoustic particle velocity) effects are closely related. Only when the acoustic field is described by both the potential and kinetic energies, we may understand the mechanisms of propagation, diffraction and scattering of acoustic waves on obstacles, as a form of energy image. This attribute of intensity method can also validate the results of CFD/CAA numerical modeling which is very important in any industry acoustic investigations.

The development of digital signal processors and the increase in their computing capabilities bring opportunities to employ algorithms with multiple variable parameters in active noise control systems. Of particular interest are the algorithms based on artificial neural networks. This paper presents an active noise control algorithm based on a neural network and a nonlinear input-output system identification model. The purpose of the algorithm is an active noise control system with a nonlinear primary path. The algorithm uses the NARMAX system identification model. The neural network employed in the proposed algorithm is a multilayer perceptron. The error backpropagation rule with adaptive learning rate is employed to update the weight of the neural network. The performance of the proposed algorithm has been tested by numerical simulations. Results for narrow-band input signals and nonlinear primary path are presented below.

There are many industrial environments which are exposed to a high-level noise, sometimes much higher than the level of speech. Verbal communication is then practically unfeasible. In order to increase the speech intelligibility, appropriate speech enhancement algorithms can be used. It is impossible to filter off the noise completely from the acquired signal by using a conventional filter, because of two reasons. First, the speech and the noise frequency contents are overlapping. Second, the noise properties are subject to change. The adaptive realisation of the Wiener-based approach can be, however, applied. Two structures are possible. One is the line enhancer, where the predictive realisation of the Wiener approach is used. The benefit of using this structure it that it does not require additional apparatus. The second structure takes advantage of the high level of noise. Under such condition, placing another microphone, even close to the primary one, can provide a reference signal well correlated with the noise disturbing the speech and lacking the information about the speech. Then, the classical Wiener filter can be used, to produce an estimate of the noise based on the reference signal. That noise estimate can be then subtracted from the disturbed speech. Both algorithms are verified, based on the data obtained from the real industrial environment. For laboratory experiments the G. R. A. S. artificial head and two microphones, one at back side of an earplug and another at the mouth are used.

The aim of this project was to create a ranking of the nursery schools in Wrocław with regard to the quality of the acoustic environment on their premises, using a specially developed evaluation methodology. Each nursery school was rated according to an adopted grading scale on the basis of the noise level distribution on the playground and on the nursery school building facades. Using the grading scale one can classify nursery school premises into twelve categories characterized by different acoustic environment quality, from exceptionally good (< 45 dB) to exceptionally bad (> 70 dB).

The appropriately rescaled data from the acoustic map of Wrocław and the authors' own measurements and simulation analyses were used. The developed methodology was verified by comparing the ratings yielded by it with those determined on the basis of field measurements and simulation studies, carried out for several selected nursery schools. The paper presents the results of an acoustic environment quality assessment carried out, using the developed investigative methodology, for 118 nursery schools located in Wrocław.

Main energy conversion machinery used and to be used in cogeneration systems are schematically described. Some assets of the distributed generation are pointed out and small-scale cogeneration systems designed for energy units of distributed cogeneration are described.

In the small scale, turbines and bearings are a source of specific problems connected with securing stable rotor operation. Accepted has been two kinds of high speed micro-turbines of electric power about 3 KW with multistage axial and radial rotors supported on foil bearings. A concept which becomes more and more attractive takes into account a low-boiling agent, which is normally used in the thermal cycle of the micro-turbine, as the lubricating liquid in the bearings (so-called ORC based systems). Of some importance is the operation of these machines at a low noise emission level, sine being parts of the household equipment they could disturb the calm of the residents. The scope of the present article is limited to the discussion of dynamic characteristics of the selected design. The properties of the rotor combined with slide bearings (foil bearings in this particular case) were taken under investigation. A combination of this type is a certain novelty since a typical modal analysis of such objects refers to a rotor itself. Analysing the dynamic state of the "home" power plants requires qualitatively novel research tools.

There are many industrial environments which are exposed to a high-level noise. It is necessary to protect people from the noise. Most of the time, the consumer requires a miniature version of a noise canceller to satisfy the internal working place requirements. Very important thing is to select the most appropriate personal hearing protection device, for example an earplug. It should guarantee high passive noise attenuation and allow for secondary sound generation in case of active control. In many cases the noise is nonstationary. For instance, some of the noisy devices are switched on and off, speed of some rotors or fans changes, etc. To avoid any severe transient acoustic effects due to potential convergence problems of adaptive systems, a fixed-parameter approach to control is appreciated. If the noise were stationary, it would be possible to design an optimal control filter minimising variance of the signal being the effect of the acoustic noise and the secondary sound interference. Because of noise nonstationarity for most applications, the idea of generalised disturbance defined by a frequency window of different types has been developed by the authors and announced in previous publications. The aim of this paper is to apply such an approach to different earplugs and verify its noise reduction properties. Simulation experiments are conducted based on real world measurements performed using the G. R. A. S. artificial head equipped with an artificial mechanical ear, and the noise recorded in a power plant.

In the article a short historical outline of noise control conferences organized in Poland is given. Those conferences with the participation of Polish specialists have been organized since 1964; since 1976 they have been evolved into International Noise Control Conferences. Silhouettes of four Polish scientists, which have made a large contribution to the noise and vibration control in Poland, are presented. Also the current state of threats by noise and vibrations have been briefly mentioned. The significance of such conferences has been emphasized.

For the use of acoustic assessment of machinery, a global index of acoustic quality has been developed. Acoustic quality index is considered as a product of the following partial indices: sound power index, index of distance between the workstation and the machine, radiation directivity index, impulse and impact noise index and noise spectrum index. Each partial index always assumes positive value. If the value of global index does not exceed 1, the noise of the assessed machine will not exceed the admissible value of A-weighted sound pressure level at the workstation.

Experimental tests were carried out in order to determine the values of global indices for a group of engine-generators, with the use of inversion method allowing for the determination of sound power level. The correctness of the determined values of indices was confirmed by the results of A-weighted sound pressure level measurements, at the hypothetically assumed workstations in simulated in situ conditions.

An important aspect in assessing noise in urban agglomerations is the subjective one, which takes into account the sensitivity and specific reactions of residents to the noise in their living environment. This paper presents results of a sociological study initiated to determine the population awareness, regarding the urban acoustic environment and estimation of effects and disturbance. The survey was conducted in a Romanian city, to complement the information provided by the strategic noise map of the area. This approach allows the estimation of specific local patterns of reaction and response to urban noise of the exposed population and provides the information, needed to develop action plans and to set proper solutions for urban area planning.

In the last years the number of new forms of workplaces, such as call centers, increases. It is defined as a workstation where the basic tasks of a worker are carried out with the use of a phone and a computer. According to statistics, about 1.3-4% of workers are employed in call centers in the European countries. The noise is one of the harmful and annoying hazards of call center workstations. The paper presents the noise sources in call center rooms, assessment criteria of noise and results of noise measurements in call center workstations. The results of measurements show that the noise at call center workstations (during the use of handset receiver phone by operators) can be harmful (causing the risk of hearing loss) and annoying, as it makes it difficult to carry out the basic work activities and causes additionally auditory disadvantageous changes in health.

The reduction of structural vibrations on the example of two pedestrian bridges (in Poznań and Wrocław) with using of tuned mass dampers (TMD) has been presented in the paper. The results of theoretical and experimental studies of pedestrian bridge vibrations has been described and discussed. Basing on the results of calculations and measurements, tuned mass dampers (TMD) has been designed and mounted in the structure of the bridges. The measurements after the assembly of TMD show a high efficiency of vibration damping.

The physical phenomena occurring in sound-absorbing and insulating enclosures are subject of the present paper. These phenomena are: absorption in air and by the sound-absorbing material covering the walls and the coincidence effect. The absorption in the air can be neglected in small size enclosures for low ultrasonic frequencies (20-30 kHz). The coincidence plays a role in decrease of the sound insulation, however the main role play the leaks. The boards made of ceramic fibers have been chosen as the optimal sound-absorbing material. They are dense and have deeply porous structures. The enclosure for insulation of 20-kHz noise produced by a welding machine has been designed and manufactured, and reductions of 25 dB of peak and Leq levels have been achieved.

This paper presents the interior acoustical characterization of the 9,000-seat church of the Holy Trinity in the Sanctuary of Fátima, Portugal, inaugurated in 2007. In situ measurements were held regarding interior sound pressure levels (with and without the HVAC equipment working), NC curves, RASTI (with and without the installed sound system) and reverberation time. The results are presented and commented according to the design values. A comparison is made with other churches in the world, also with a very large volume (for instance the Basilica Mariacka in Gdańsk). The measured data are also used to calculate a global index of this church acoustic quality using Engel's and Kosała's Index Method.

A dynamic economy contributes to the increase in the number of workers exposed to mechanical vibration caused by machines and transport equipment. As the means of transport are insufficiently recognised sources of mechanical vibrations, this article presents the results of whole-body and hand-arm vibration tests of 30 most common means of in-house transport. An analysis of vibration signals recorded at each workstation according to PN-EN 14253 and PN-EN ISO 5349 made it possible to determine the weighted values of components of directional vibration acceleration and the values of daily vibration exposure A(8).

In order to assess exposure to whole-body and hand-arm vibration at the tested workstations of in-house transport, indices of vibration hazard related to admissible values, the total evaluation index (developed in a previous study at CIOP-PIB) and a three-degrees scale for assessing exposure to vibrations were used. The assessment showed that the workstations were a major hazard. Vibration hazards at all those workstations were classified as either medium or high.

Noise Control is the most important international acoustics conference regularly organized in Poland. The XV International Conference Noise Control 2010 is taking place between June 6 and 9, 2010, in Ksi?? in Wa?brzych. It is organized by the Central Institute for Labour Protection - National Research Institute, the Committee on Acoustics of the Polish Academy of Sciences, the Polish Acoustical Society, with the cooperation of the Department of Mechanics and Vibroacoustics of the AGH University of Science and Technology, the Department of Acoustics of the Building Research Institute and the Chair of Acoustics of the Wroc?aw University of Technology.