Enhanced absorption in structural acoustic silencers using piezo-shunting.

This paper presents a general sub-structuring approach to predict the acoustic performance of ducted ventilation systems. The modeling principle is to determine the subsystem characteristics by calculating the transfer functions at their coupling interfaces, and the assembly is enabled by using a patch-based interface matching technique. For a particular example of a bended ventilation duct connecting an inlet and an outlet acoustic domain, a numerical model is developed to predict its sound attenuation performance. The prediction accuracy is thoroughly validated against finite element models. Through numerical examples, the rigid-walled duct is shown to provide relatively weak transmission loss (TL) across the frequency range of interest, and exhibit only the reactive behavior for sound reflection. By integrating sound absorbing treatment such as micro-perforated absorbers into the system, the TL can be significantly improved, and the system is seen to exhibit hybrid mechanisms for sound attenuation. The dissipative effect dominates at frequencies where the absorber is designed to be effective, and the reactive effect provides compensations at the absorption valleys attributed to the resonant behavior of the absorber. This ultimately maintains the system TL at a relatively high level across the entire frequency of interest. The TL of the system can be tuned or optimized in a very efficient way using the proposed approach due to its modular nature. It is shown that a balance of the hybrid mechanism is important to achieve an overall broadband attenuation performance in the design frequency range.

In the situations where conventional NDT ultrasonic techniques using immersion of the part under inspection or its contact with the transducers cannot be used, in-air investigation presents an alternative. The huge impedance mismatch between the part material and air (transmission loss in the order of 80 dB for a thin metallic plate) induces having to deal very small signals and unfavorable signal to noise ratios. The approach adopted here is the use of the crack of a spark generated by an induction coil as a sound source and an electrostatic polyethylene membrane microphone as a receiver [1]. The advantage of this source is that the spark power is high (several kilowatts) and its power is directly coupled to air during the energy release. In some difficult situations, an elliptical mirror is used to concentrate the sound beam power on the surface of the part [2,3]. Stability and reproducibility of the sound generated by the spark, which are a necessity in order to perform quantitative evaluations, are achieved in our experiment. This permits also an increase of the signal to noise ratio by signal accumulation. The sound pulse duration of few microseconds allows operating in pulse echo in some circumstances. The bandwidth of the source is large, of several hundred of kilohertz, and that of the microphone above 100 kHz allow the flexibility to address different kinds of materials. The technique allows an easy, in-air, non contact, inspection of structural composite parts, with pulse waves, with an excellent signal to noise ratio. An X-Y ultrasonic scanning ultrasonic system for material inspection using this technique has been realized. Results obtained in transmission and reflection are presented. Defects in carbon composite plates and in honeycomb are imaged in transmission Echographic measurements show that defect detection can be performed in thin plates using Lamb waves propagation when only one sided inspection of the part is possible.

Metal oxide thin films are widely used for energy saving glass coating. This coating has the ability of blocking the infrared signal while being transparent to other visible part of the spectrum. However, there is one critical disadvantage of this metal oxide coating which it attenuates useful radio frequency and microwave signal such as GSM mobile signal, personal communication, GPS signal through them. These important microwave signals are fall within the range of 800 MHz to 2200 MHz. Frequency selective structure has been applied to solve the attenuation of microwave signal. With the adding of frequency selective structure, it can bring huge improvement of the transmission loss through it. Computer simulation using CST software is used to investigate the transmission loss through the metal oxide coated glass. The frequency selective structure will be etched out from the metallic oxide coated on the glass. Results showed that different shape of the structure will have different peak transmission loss through the glass. When cross dipole and circle shape been simulated using CST software, it can clearly see that the transmission lost and peak frequency had changed drastically. Then, triangle and pentagon shape also have different transmission through it. In addition, conductivity and electrical properties of coated metal oxide thin film is also very important. The transmission through the different ohmic sheet resistance of metal oxide thin film was also investigated. The sheet resistance value was obtained from the reported experimental results. Simulated results showed that full width half maximum, maximum transmission loss and peak frequency loss was very much dependent on the metal oxide sheet resistance. Therefore, the control of the thickness and oxygen content in metal oxide thin film are very much important to optimize the transmission loss through it for energy saving glass applications.

Noise barrier with acoustic metamaterial has application prospect in power transformer noise reduction due to its low frequency noise reduction advantages. The acoustic diffraction along the edges of the noise barrier decreases the overall noise reduction obviously. An active noise control (ANC) method combined with noise barrier is proposed to counteract the diffracting sound waves near the edges of the noise barrier. The noise barrier is composed of the Hilbert fractal acoustic metamaterial (HFAMM) units with good sound insulation characteristics at low frequencies. The transmission loss (TL) of the HFAMM unit is higher than 20dB at the main frequencies of the transformer noise theoretically. The experiment system including the acoustic metamaterial noise barrier and the ANC system is established. The sound pressure level with ANC on the specified points in the target area is more than 10dB lower than that without ANC in the experiment. The results prove that the new method is effective for improving noise reduction performance of the noise barriers.

Satellite launch vehicle during lift-off generates intense dynamic environment mainly the acoustic noise, which in turn acts on the nearby launch pad systems. Reliable operation of these launch pad ground systems needs to be ensured by safeguarding against these dynamic loads with adequate protection measures. For future launch vehicle programs where the launch turnaround time is a crucial aspect, sensitive systems like ground check-out electronic packages are planned to be placed adjacent to the launch vehicle mounting pedestal on the launch pad to ensure minimal time for preparation. The enclosures housing these electronic packages needs to be configured to minimize the transmission of acoustic levels being imposed from the lift-off induced noise. In the present work, an experimental investigation is carried out on acoustic transmission characteristics of metallic enclosures of different material and thickness during the static firing tests of solid rocket motors. Acoustics generated from the supersonic jet noise has a wide frequency band from 100 to 10,000 Hz with sound power peaking near 1,000 Hz. The electronic components and packages, which are generally vulnerable to the high frequency noise, need to be protected from this lift-off induced acoustics. The enclosures are placed in the far field of this jet noise and the transmission loss measurements are obtained. Variation of transmission loss with material and thickness of the enclosures are studied with specific attention to the internal acoustic modes and the coincidence phenomena. Subsequently, the enclosures treated with acoustic blankets of different thickness are investigated to verify its influence on internal acoustic modes and also the additional reduction in transmission of sound in the dominant frequency band of supersonic jet noise.

<div class="htmlview paragraph">In this paper, sound transmission characteristics of typical stamped steel automotive body panels are investigated. In particular, focus was placed on the dash panel since it separates the interior space from the engine. Three dash panels were considered with the same geometry but different damping characteristics. These dash panels included a bare steel dash, a bare steel dash with a Patch Constrained Layer (PCL) and a laminated steel dash. Sound transmission loss was measured in a transmission loss suite. It was observed that the measured transmission loss of the laminated steel dash was significantly higher than the other dash panels throughout the mid and high frequency regions. A hybrid approach using both Finite Element Analysis and Statistical Energy Analysis (SEA) was used to predict and correlate the sound transmission loss for these panels. The geometry effects of the dash panels were considered while calculating SEA parameters. Coupling loss factors between the dash panel and acoustic cavity were predicted considering the effects of increased stiffness and the impedance mismatches due to discontinuities along the surface of the dash. The power contributions to the receiving cavity by resonant paths and non-resonant paths were investigated. It was observed that the resonant power contribution was significantly lower for the laminated steel dash as compared to the other dash panels. This resulted in higher transmission loss for laminated steel dash compared to the transmission loss for other two panels.</div>

In the process of power supply to final consumers, an important amount of energy is lost in the transmission and distribution systems due to technological and non-technological losses. Technical losses represent an economic loss with environmental effects at national level, and its optimization should be performed from an overall national's perspective, regardless of the institutional organization of the sector and ownership of operating electricity utilities. The volume of energy losses in the power transmission and distribution systems is important, as it is generally up to 3 - 13%. The volume of power losses in Romanian power transmission and distribution systems is important, the level being of about double to the average in Europe. The current paper presents the research results of the authors regarding the real time monitoring and analysis systems for power losses, in electrical transmission and distribution systems.Within the research project, it was developed a dedicated system, successfully tested within a 5 years partnership with CNTEE “Transelectrica” - SA, on a transformer of 250/250/80MVA 400/121/20 kV from 400/110 kV Pelicanu Substation and respectively on the 400 kV overhead power line (118 km) Bucuresti Sud - Pelicanu.The tests results showed the importance of evaluation by real-time measurements, compared to the results of evaluations by calculations, of active power losses at power transformers and overhead power lines, respectively. Values several times higher were found for the values of active power losses measured online, in real time, than those determined by calculations.Online measurements have shown that there may be cases when large industrial consumers produce current and voltage harmonics well above the maximum allowable limits specified in the standards. The harmonics are transmitted practically undamped by the windings of the power transformers and on the transmission power lines (in the case of tests, even more than 100 km away from the place of their production).The online measurement of active power losses on the 400 kV overhead power line, 119 km, showed that corona losses are insignificant especially during periods of atmospheric humidity below 85%, and that active losses on power lines are practically resistive losses. The resistive losses on the measured high voltage overhead power lines are much higher than the calculated ones. The difference is determined by the fact that the calculation of active power losses on power lines includes the value of ohmic resistances of conductors evaluated based on line conductor resistivity or possibly based on ohmic resistance measured at a certain moment, and not the real value of the ohmic resistance of the line in real time (which changes over time due to the change in the state of the multitude of joints of the OHL conductor sections).

Membrane- and plate-type acoustic metamaterials with elastic unit cell edges

Silica nanoparticle-impregnated Kevlar (SNK) fabric has better specific ballistic performance in comparison to its neat counterparts. For multifunctional structural applications using lightweight composites, combining this improved ballistic functionality with an acoustic functionality is desirable. In this study, acoustic characterization of neat and SNK samples is conducted using the normal-incidence impedance tube method. Both the absorption coefficient and transmission loss (TL) are measured in the 60–6000 Hz frequency range. The influence of parameters such as number of layers of neat or treated fabric, percentage by weight of nanoparticle addition, spacing between fabric layers, and residual porosity is examined. It is found that while absorption decreases with an increase in nanoparticle addition for frequencies above about 2500 Hz, increasing the number of layers shifts peak absorption to lower frequencies. By introducing an air-gap behind the fabric layer, dominant low-frequency (1000–3000 Hz) absorption peaks are obtained that correlate well with natural modes of mass-equivalent thin plates. Examining the influence of residual porosity by laminating the SNK samples reveals that it contributes to about 30–50% of the total absorption. Above about 1500 Hz, 3–5 dB of TL increase is obtained for SNK samples vis-à-vis the neat samples. TL is found to increase beyond that of the neat sample above a threshold frequency when an air-gap is introduced between two SNK layers. With an increase in the weight of nanoparticle addition, measured TL tends to be closer to mass law predictions. This study demonstrates that SNK fabric could provide improved acoustic performance in addition to its ballistic capabilities, making it suitable for multifunctional applications and could form the basis for the development of simplified models to predict the structural acoustic response of such nanoparticle–fabric composites.

Experimental study to measure the transmission loss of double panel natural fibers

The turbulent boundary layer excitation is one of the main sources of aircraft interior noise over a large frequency range while the diffuse sound field excitation is often considered as an approximation for noise analysis and noise control evaluation. This paper presents the study of add-on damping patches efficiency under both excitations. The structure of interest here is a simply supported thin plate partially covered with a damping treatment. Numerical results are carried out using an original methodology based on the Minimization of Error on a Sample of Observation Points (MESOP). The transmission loss index is assessed in the frequency range from 50 Hz to 7 kHz. This investigation points out different behaviors: a damping effect is observed only in the critical frequency region under diffuse sound field excitation whereas the transmission loss index is increased by damping over the whole frequency band under turbulent boundary layer excitation. The excitation field appears to be a key factor. The excitation effect on the damping treatment characterization is then observed for two patch sizes. The transmission loss under aerodynamic excitation with varying patch sizes is finally investigated in a context of damping treatments optimization. (C) 2010 Institute of Noise Control Engineering.

ON THE APPLICATION OF SHUNTED PIEZOCERAMICS FOR INCREASING ACOUSTIC TRANSMISSION LOSS IN STRUCTURES

The application of electrically shunted piezoceramic materials (PZTs) for increasing transmission loss is studied. A test setup in an SAE J1400 facility is used to analyze the sound transmission loss due to a test plate with shunted PZTs, and to compare the results with an undamped plate and a plate with constrained-layer damping materials. The test results indicate that shunted PZTs can increase sound transmission loss by approximately 6 dB for a pure tune input at 162 Hz, and 7 dB for a broad band input of 10 - 10000 Hz. The test results further show that, compared to the constrained layer damping materials that are commonly used in the automotive industry, PZTs provide a substantially higher sound transmission loss-to-weight ratio.

Large number of substations have been upgraded to higher voltage level by loop in loop out of existing transmission lines to meet the increasing load demand. Therefore; large number of loops of different voltage levels has been created between substations. Transformers of different MVA ratings; percentage impedance and tap ratios have been installed at the substations. Presently there is no coordination between adjoining substation operators for tap setting of transformers. Therefore; due to mismatch of transformers tap ratio and percentage impedance; circulating MVARs are flowing on transmission lines resulting in higher transmission losses; increase loading of transformers & lines and poor voltage profile. In this paper impact of circulating MVAR flow and its control through coordinated tap setting of transformers has been studied. Rajasthan power system has been considered to carry out the studies and has been modeled in Mi-Power software. Rajasthan Power System have total 750 buses comprising 2 nos. 765 KV; 35 nos. 400 KV; 147 nos. 220 KV; 504 nos. 132 KV and 62 nos. generator buses with load of 10000 MW. Effect of coordinated Tap setting of transformers on circulating MVAR flow; transmission losses; network voltage profile and lines & transformers loading have been analyzed. Proposed methodology has been successfully tested on Rajasthan power system to remove the circulating MVAR flows on 220 kV and 132 kV network.

Sound transmission and mode coupling at junctions of thin plates, part II: Parametric survey