Mirko Čudina

Use of noise and vibration signal for detection and monitoring of cavitation in kinetic pumps

Cavitation in kinetic pumps causes lowering performance, mechanical damage and increase of vibrations and noise. Therefore, it is important to detect inception and development of cavitation phenomenon in the pump. This paper deals with signals of vibrations and noise, which will be used for detection and monitoring of cavitation in kinetic pumps, and also to prevent the effect of cavitation in the pump and pumping system. When the cavitation is increasing, the flowing conditions are changing, which leads to an increase of vibrations of the pump and emitted noise in the surroundings. Because vibrations and noise are transferred from the pump through its casing, the signal is non‐uniformly distorted due to transfer losses and structure of the casing surfaces. Even so, it is possible to determine development of cavitation phenomena from the measured signal. Noise and vibrations are increasing steadily, but in some specific frequency ranges the signal is more pronounced than in other part of the spectrum. Exp...

Investigation on identifying structural modes by sound pressure signals

ZusammenfassungUnterschiedliche Methoden zur Visualisierung von Eigenschwingungsformen einer ebenen Platte werden verglichen: Ein Laser-Doppler-Vibrometer, eine Abtastung des Schalldrucks mittels Mikrofon im Nahfeld, Akustik-Holographie im Nahfeld, eine analytische Berechnung und die Berechnung mittels Finiten Elementen. Für diese Untersuchung wird eine ebene Platte von der Größe 380×382×2,9 mm verwendet. Für die Messungen mittels akustischer Holographie und die Schalldruckmessung mittels Mikrophon wurde die Platte durch einen elektrodynamischen Shaker angeregt, wobei als Anregungssignal weißes Rauschen im Frequenzbereich von 20 Hz bis 6 kHz verwendet wurde. Die unterschiedlichen Methoden ergeben ähnliche Ergebnisse. Trotzdem hat jede Methode ihre spezielle Eignung für verschiedene Frequenzbereiche und Anwendungen. Die Ergebnisse der akustischen Holographie und mittels Mikrofon abgetastetem Schalldruck im Nahfeld sind relative Größen und können ausschließlich für die Identifikation von Schwingungsformen verwendet werden, während das Laser-Doppler-Vibrometer absolute Ergebnisgrößen liefert.SummaryDifferent methods for a visualization of plate vibration modes are compared: a laser scanning vibrometer, a microphone scanning in a very near field, a near field acoustic holography, an analytical calculation, and a finite element method calculation. A plate with dimensions of 380×382×2.9 mm was used for the experiment. During measurements for acoustic holography and during the scanning with a pressure microphone the plate was excited with a shaker, using a white noise signal, in the frequency range from 20 Hz to 6 kHz. Different methods provide similar results. However, each method is intended for a different frequency range and for different applications. Results of the near field acoustic holography and of the scanning with the pressure microphone in a very near field are relative and can only be used for mode shape identifications, while the laser scanning Doppler vibrometer can produce traceable results.

Noise generation by vacuum cleaner suction units

Among all household appliances, the vacuum cleaner is the most powerful source of noise. The built-in suction unit, consisting of an electric motor and a centrifugal blower, represents the main source of noise; this is partially aerodynamic, mechanical and electromagnetic in origin. The contribution of the particular origin of noise generation to the total noise level depends on the geometry of the suction unit and operating conditions. The aerodynamically generated noise within the blower mostly prevails in the total emitted noise at design and off-design operations, and is especially distinct at partial flow rates when rotating stall and surge appear. The vaned diffuser built into the blower has an important effect on the emitted noise, so on the airborne as well as on the structure-borne noise and their spectra. Detailed analyses of measurement results have shown that the built-in vaned diffuser has numerous disadvantages, including: increase in the tonal and turbulent noise, increase in production costs and reduction of the flow rate, therefore, it could be omitted or replaced by a vaneless one. In this paper, an overview of the effects of the vaned diffuser on the aerodynamic, mechanical and electromagnetic noise origins at different operating conditions is given.

Identification of noise sources in centrifugal blower with acoustic camera

A centrifugal blower represents the main noise source in a vacuum cleaner. Noise emitted by the centrifugal blower working as a suction unit has aerodynamic, mechanical, and electromagnetic noise origins. Noise emitted by a suction unit consists of airborne and structure‐borne noise. The contribution of the individual noise source to the total noise level depends on the geometry of the suction unit, rotational speed and on the operating conditions. Operating conditions of a suction unit depend on the amount of dust particles in the dust bag and are constantly changing during operation until a zero flow rate is reached. In order to reduce noise of suction unit at broad range of operating conditions, an identification of noise sources needs to be performed. Identification of most important noise sources on the centrifugal blower was performed with an acoustic camera at the design and off‐design operation conditions. From the analyses we can conclude that the rotational noise usually prevails at the design p...

Algorithm for acoustic camera with post processing analysis

Acoustic camera is usually used for identifying the dominant noise source. Commercial version is easy to use and in many cases gives satisfactory results. However, high complexity of the sound sources, their movement, and their spatial distribution often present difficulties during the identification of the dominant noise source. Older versions of acoustic camera can not provide an acoustic image of the moving sources. The inability to record targets in motion renders old versions useless, when faced with a task of recording actual traffic or identifying a sound that is spreading from certain parts of the vehicle. To be able to analyze moving objects new acoustic camera algorithm was designed. During the design process few different algorithms have been investigated. Advantages and disadvantages of algorithms are discussed and the algorithm for sequential signal analysis which gives the ability to track moving targets is outlined.

Inter Laboratory Comparison of Environmental Noise Measurement

Results of an inter laboratory comparison for environmental noise measurements are presented in this article. 18 laboratories took part in this study. A purpose of individual measurement was to quantify environmental noise levels, calculate noise descriptors and compare them with community noise regulations. Measurements and calculations were performed according to standards ISO 1996-1:2005, ISO 1996-2:2007 and national legislation. A loudspeaker was used as a reference noise source. It was mounted at the top of the hayloft just below the roof. The loudspeaker emitted a pink noise, representing a constant broadband noise source. The noise source was operating from 8:00 in the morning to 16:00 at the afternoon, each day for 4 weeks. During this time, laboratories performed all required measurements. Each laboratory performed measurements independently from other laboratories and according to its own procedures. Measurement results of equivalent noise level L Aeq , percentile values L 1 , and calculation of noise descriptors L d and L den were compared. An estimation of measurement uncertainty for a given environmental conditions and measurement setup conditions is given.

The impact of paintings hung on lecture room walls on the speech intelligibility and perception of background noise

The most important parameter for rooms designed for speech (classroom or lecture room) is speech intelligibility. Speech intelligibility depends on the reverberation time, speech-to-noise ratio and geometry of the room, among others. At the Faculty of Mechanical Engineering, University of Ljubljana, the reverberation times in lecture rooms observed exceed the optimal value from 0.31 to 0.87 s and in some of them, speech intelligibility is below the threshold of good or even satisfactory. For reducing reverberation time and improving speech intelligibility, an alternative method was proposed, by hanging art paintings on the room walls. Experiments have shown that by doing so, reverberation time can be reduced by increasing the absorption coefficient by more than 30% in the most audible part of the frequency spectra (between 500 and 1500 Hz). The absorption coefficient can be increased by using different dimensions of paintings, different air spaces behind the canvas, by appropriate thickness of the paint layers and by adding absorption material behind the canvas. Subjective tests have also shown that paintings with proper colour combinations (pastel colours with prevailing green, blue and grey colours) are appropriate for soothing the undesirable effect of background sound (noise) by changing perception in the brain.

Analyses of adjustment of the impulse noise

Impulse noise is one of the most hazardous and annoying types of noise, which is present in the working and community environment. A-weighted equivalent sound pressure level, describing most types of noise, appears not to be the appropriate descriptor, especially not for high-impulsive and high-energy impulsive noise. However, this descriptor is often used as a basis for impulse noise evaluation, when combined with appropriate adjustment terms. But despite of objective character of such evaluation, care should be taken regarding certain facts, especially the source of impulse noise, its environment and time of measurement. In this article, the relationships between all these influential parameters have been investigated in detail. Today, on the other hand, a sophisticated sound level meter offers the possibility of simultaneous measurement of many acoustical descriptors. By combining some of these descriptors with some analytical investigations, as shown in this article, more useful information concerning impulse noise can be obtained. In this article some of them are used for more detailed analysis of impulse correction, according to some international standards.

Sound pressure around dipole source above porous surface

A technique for in situ measurements of acoustic properties of a fibrous porous material is proposed in this paper. Proposed technique exploits a directivity pattern of a dipole source in its very near field. Theoretical analysis for the proposed technique is based on the Rayleigh integral with a complex reflection included. Results are compared with results of FEM analysis and show that flow resistivity of a porous material placed in the very near field of the dipole source has significant influence on the sound pressure at its ring. Results provide an excellent starting point for the design of the sensor for sound absorption.

Identification of machinery sounds

Speaker recognition methods are well known and widely used in the ASR (Automatic Speech Recognition) systems. The use of these methods for the classification of machinery sounds in noisy environments is presented in this paper. Influence of background noise was reduced by using a highly directive sound recording, which can be understood as a spatial filter. A fusion of microphone antenna with beamforming algorithm forms such a filter, which improves SNR (Signal to Noise Ratio). Features of machinery sounds have been extracted using standard MFCC (Mel Frequency Cepstral Coefficients) parameterization method with Mel and linear frequency scaling. SVM (Support Vector Machine) classifier was used for the classification of sound features. A significant improvement of the classifier decision performance was achieved in noisy environment when 8 microphones were used together with beamforming algorithm. Results of using Mel and linear scale are also presented and show similar results in recognition of machinery sounds.