On the acoustic analysis and optimization of ducted ventilation systems using a sub-structuring approach.

Structural acoustic silencers (sas) consist of a thin flexible plate that is introduced into a wall of an otherwise rigid-walled duct in which acoustic noise is propagating. Transmission loss (TL) in sas arises from reflection due to impedance mismatch, as well as partial absorption due to material damping in the plate. Reflection-based TL is less preferable than absorption-based TL, especially where back-pressure is undesirable. In this study, the TL arising out of absorption is increased by increasing the effective plate damping using the piezo-shunting method. Experiments are conducted on a custom-made sas with copper sheets of two different thicknesses. Piezo-shunting to increase the damping of the copper plate is done by connecting an electrical resistor to the external circuit of a thin piezoelectric bimorph attached to the copper plate. The effect of piezo-shunting on the sas TL is analyzed using a model. Piezo-shunt increases the fraction of power absorbed and decreases the reflected power fraction. The dependency of these fractions on the value of the electrical resistor and the impedance of the plate are discussed. This study demonstrates that piezo-shunting can be used to improve absorption-based TL in sas.

Expansion chamber mufflers are often used for duct noise control of building ventilation systems. In this paper, the periodic arrangement of expansion chamber muffler is proposed to improve the sound attenuation performance. The sound attenuation of periodic expansion chamber mufflers is investigated theoretically with Bloch wave theory and a case is calculated for the distance between mufflers and the length of the expansion chamber are the same. The results reveal that the transmission loss of the expansion muffler is enhanced by the periodic arrangement.

Local resonance – Helmholtz lattices with simultaneous solid-borne elastic waves and air-borne sound waves attenuation performance

Fresh air ventilation openings or windows are essential to most buildings but the openings allow considerable portions of noise to be transmitted from the outdoor into the indoor environment. This project studies relevant theoretical models through accurate numerical solutions, and seeks better designs to gain extra noise attenuation through the openings.\n\n\n\nThe spectral method of Chebyshev collocation and the corresponding technique of out-going wave boundary conditions, for both ducted and flanged opening, are developed as numerical tools and validated by analytical examples.\n\n\n\nAfter establishing the numerical tools, two prototype problems are studied. The first is one in which there is no special sound attenuation device. The configuration includes aperture of negligible thickness, aperture of finite thickness and aperture of semi-infinite length. The characteristics of the noise transmission through aperture of finite thickness are investigated with examples of sonic boom and traffic noise. The second prototype problem is the sound attenuation performance of side-branch design of the ducted opening. Side-branch configurations including porous material, single micro-perforated panel, double-layer micro-perforated panels, parallel micro-perforated panels and parallel micro-perforated panels with connected cavity are investigated. They are tested individually with parameters optimized for each of two examples of noise sources. The sound attenuation performances compared using appropriate subjective ratings of the noise sources. It is found that the parallel micro-perforated panels give the best performance for the sonic boom and the double-layer micro-perforated panels serve as the best substitute for the porous material with traffic noise source. Results from experiment are also presented to validate some numerical results.

Theoretical investigation of the sound attenuation of membrane-type acoustic metamaterials

The present paper deals with transmission loss studies of acoustical muffler and duct systems using the boundary element method (BEM). Results obtained by using this approach are presented for both axisymmetric and fully three-dimensional models. The transmission loss of a muffler element can be computed using the transfer matrix approach. This method is ideally suited for the BEM because only the values of the unknowns at the inlet and outlet boundaries are required. The BEM values are compared to theoretical predictions and other available data such as finite clement data, and good agreement is obtained within a broad frequency range. It can be concluded that the BEM should give reliable estimates of transmission loss in practical situations. [Work supported in part by ASHRAE.]

Numerical analyses of the sound transmission at low frequencies of a calibrated Insulating Glazing Unit

Guided wave ultrasound has been used to continuously monitor welded rail track by transmitting guided wave ultrasound between alternate transmit and receive stations along the rail. A section of rail was encountered where transmission was not reliably achieved. It was found that there was considerable flank wear on the head of the rail, which varied in a sinusoidal pattern over approximately 10 m. The scattering due to the wear is investigated in this paper using numerical modeling. A 3D finite element (FE) model of the section of rail containing the wear defect was coupled to two semi-analytical finite element models of the incoming and outgoing waveguides. In the case of a 10 m long defect, which is over 100 wavelengths long, the 3D FE matrices were excessively large, and the global matrices could not be assembled nor solved. An approach of successively assembling layers of the mesh and reducing out internal degrees-of-freedom in the dynamic stiffness matrix was successfully adopted. The influence of the length of wear on the transmission loss was computed. It was found that short wear lengths generally cause more transmission loss although the relationship is not monotonic. It was found that the long-wear seen in the field does not cause transmission loss. In this case, where the change in cross section is gradual the incoming mode converted to a single mode in the smallest cross section and this mode converted back to the incoming mode in the second half of the wear section.

Using data from the Yellow Sea, arrival times of the direct wave and surface/bottom reflections from explosive sources to a vertical hydrophone array are used to precisely determine each explosive source’s location, the source energy spectral density (SESD), and the water depth. Long-range propagation waveforms reveal modal dispersion: the ground wave, water wave, Airy phase, etc. There are two high-frequency (HF) groups of water waves. One propagates with the sound speed in the water below the thermocline, the other with a speed close to the sound speed in the water above the thermocline. The HF group arrival times offer a time reference for dispersion analyses, including the ground wave speed at the cutoff frequency and the group velocity at Airy frequency. Associated with a top layer oflow-velocity sediments (LVS), seafloor reflections have two pulses: one from the water-sediment interface, one from the sediment-basement interface; Long-range transmission loss (TL) exhibits abnormal peaks at selected frequencies. Above-mentioned physics characteristics and the SESD-normalized TL(r) and TL(f) in 50–5000 Hz range up to 27.6 km are used for observationally driven inferences of seafloor geo-acoustic parameters, such as the sound velocity and attenuation in the LVS layer, its thickness, the sound velocity in the basement, etc.

The Cremer concept for annular ducts for optimum sound attenuation

A review on acoustic performance analysis of reactive muffler

Acoustic metamaterials are increasingly being considered as a viable technology for sound insulation. Fractal patterns constitute a potentially groundbreaking architecture for acoustic metamaterials. We describe in this work the behaviour of the transmission loss of Hilbert fractal metamaterials used for sound control purposes. The transmission loss of 3D printed metamaterials with Hilbert fractal patterns related to configurations from the zeroth to the fourth order is investigated here using impedance tube tests and Finite Element models. We evaluate, in particular, the impact of the equivalent porosity and the relative size of the cavity of the fractal pattern versus the overall dimensions of the metamaterial unit. We also provide an analytical formulation that relates the acoustic cavity resonances in the fractal patterns and the frequencies associated with the maxima of the transmission losses, providing opportunities to tune the sound insulation properties through control of the fractal architecture.

<div class="section abstract"><div class="htmlview paragraph">The automotive Air Induction System (AIS) is an important part of the engine systems which delivers the air to the engine. A well-designed AIS should have low flow restriction and radiates a good quality sound at the snorkel. The GT-Power simulation tool has been widely utilized to evaluate the snorkel noise in industry. In Fiat Chrysler Automobiles, the simulation method enhanced with Design For Six Sigma (DFSS) approach has been developed and implemented in AIS development to meet the functional requirements. The development work included different types of DFSS projects such as identifying new concept, robust optimization and robust assessment etc. In this paper, the work of a robust optimization project is presented on developing an AIS parametric model to achieve optimized snorkel noise performance for a V8 engine.</div><div class="htmlview paragraph">First, the theory of AIS acoustic modeling using GT-power and DFSS robust optimization using Taguchi’s parameter design method are described. Secondly, the effects of several AIS design control factors on the AIS sound attenuation and snorkel performance are studied. Finally, the eight steps of Taguchi’s parameter design method are presented on developing a parametric AIS model for a V8 engine. Based on the results from the verification step, an optimized AIS parametric model to this specified V8 engine is suggested. The optimized model is more robust against temperature variation and has better snorkel noise performance. The lessons learned from this project and future work are discussed in the conclusion section.</div></div>

This study contrasts two modeling techniques proposed to accurately predict the influence of sintered fibrous metal (a non-woven structure of metallic fibers attached to one another by sintering processes), as a liner substitute, on sound attenuation performance and resulting noise emission for conventional aftermarket dissipative mufflers. Predicted values are compared to sound measurement data from stationary engine exhaust tests of a commercially available single-cylinder 450cc off-road motorcycle. The performance prediction techniques rely on the appropriate application and combination of pre-existing silencer design, engine exhaust and gas flow performance models as an economic alternative to more complex and expensive modeling programs that are typically beyond the reach of most small to medium-sized businesses in the motorcycle aftermarket industry. With respect to test results that showed approximate acoustical parity between mufflers containing the two different liner types, application limitations on the most suitable prediction technique are presented along with suggestions for further model refinement or additional physical testing. Further research is also invited to explore the impact of this liner substitution on muffler backpressure and its consequential impact on realized engine power.

Bottom acoustic parameters play an important role in sound field prediction. Acoustic parameters in deep water are not well understood. Bottom acoustic parameters are sensitive to the transmission-loss (TL) data in the shadow zone of deep water. We propose a multiple-step TL inversion method to invert sound speed, density and attenuation in deep water. Based on a uniform liquid half-space bottom model, sound speed of the bottom is inverted by using the long range TL at low frequency obtained in an acoustic propagation experiment conducted in the South China Sea (SCS) in summer 2014. Meanwhile, bottom density is estimated combining with the Hamilton sediment empirical relationship. Attenuation coefficients at different frequencies are then estimated from the TL data in the shadow zones by using the known sound speed and density as a constraint condition. The nonlinear relationship between attenuation coefficient and frequency is given in the end. The inverted bottom parameters can be used to forecast the transmission loss in the deep water area of SCS very well.