An adaptive muffler with a flexible internal structure is con- sidered. Flexibility is achieved using controlled flow valves. The pro- posed adaptive muffler is able to adapt to changes in engine operating conditions. It consists of a Helmholtz resonator, expansion chamber, and quarter wavelength resonator. Different combinations of the control valvesʼ states at different operating conditions define the main working principle. To control the valveʼs position, an active noise control approach was used. With the proposed muffler, the transmission loss can be increased by more than 10dB in the selected frequency range.
COBISS.SI-ID: 14021147
Information about vibration modes is needed during the planning of noise control measures on different parts of machinery. A visualization of vibration modes is a starting point and different methods can be used to visualize vibration modes. Some methods which incorporate an inverse calculation of surface velocity from the sound pressure on some boundary have already been proposed, among others. A direct inverse method based on the discretized Rayleigh integral was used in our work, to demonstrate that microphones should be placed close to the vibrating structure to provide an acoustic transfer matrix with a low condition number. It is demonstrated, that there is practically no need for the calculation of the inverse matrix if microphones are placed in a very near field of the vibrating structure. A single microphone placed in a very near field together with a reference vibration sensor provides sufficient information for producing the images of vibration modes. Analytical results, numerical results, FEM simulations and measurement results were used to prove that properties of the sound pressure in a very near field permit a cost effective visualization of the vibration modes.
COBISS.SI-ID: 13042203
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 geom- etry 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 rever- beration 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 1500Hz). 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.
COBISS.SI-ID: 13908763
In some cases an impulsive noise source such as a gunshot can be a preferred alternative when investigating building acoustics, including sound insulation measurements, when compared to conventional steady state noise sources. A gun equipped with blank cartridges is an impulsive noise source that is lightweight and small enough to be easily transported. The differences in the noise characteristics between individual cartridges for the same gun are usually small, so the impulsive source can be replicated to a high degree. This paper is focused on the practical application of the sound exposure levels produced by a gunshot with a known sound energy level in the rooms under investigation. In this way, the equipment and methods required by the conventional method are simplified significantly. Furthermore, reverberation times need not be measured, since the equivalent absorption area can be directly obtained from the measured sound exposure levels. Using Greenʼs theorem, the roles of the sound source and measuring microphone were exchanged, which simplified the determination of sound insulation as it was easier to change the position of the gun than the microphone. The results obtained using the impulsive noise source were in good agreement with those obtained using the conventional method. Above 100 Hz, their difference in any frequency band of interest was less than 1 dB.
COBISS.SI-ID: 14389019
Background: A nationwide study was conducted to explore the short term association between daily individual meteorological parameters and the incidence of acute coronary syndrome (ACS) treated with coronary emergency catheter interventions in the Republic of Slovenia, a southcentral European country. Method: We linked meteorological data with daily ACS incidence for the entire population of Slovenia, for the population over 65 years of age and for the population under 65 years of age. Data were collected daily for a period of 4 years from 1 January 2008 to 31 December 2011. In line with existing studies, we used a main effect generalized linear model with a log-link-function and a Poisson distribution of ACS. Results and Conclusions: Three of the studied meteorological factors (daily average temperature, atmospheric pressure and relative humidity) all have relevant and significant influences on ACS incidences for the entire population. However, the ACS incidence for the population over 65 is only affected by daily average temperature, while the ACS incidence for the population under 65 is affected by daily average pressure and humidity. In terms of ambient temperature, the overall findings of our study are in line with the findings of the majority of contemporary European studies, which also note a negative correlation. The results regarding atmospheric pressure and humidity are less in line, due to considerable variations in results. Additionally, the number of available European studies on atmospheric pressure and humidity is relatively low. The fourth studied variable—season—does not influence ACS incidence in a statistically significant way.
COBISS.SI-ID: 1536032195