Acoustic Transmissibility Research Articles

Active acoustic insulation system with quasi-zero stiffness supported nonlinear moving-coil diaphragm

Active acoustic insulation systems tend to suffer from narrow insulation frequency bandwidth and low-frequency deficiencies. In this paper, we employed a quasi-zero stiffness supported nonlinear moving-coil diaphragm to such a system to achieve the target energy transfer, and broaden the insulation frequency bandwidth. We established the electromechanical acoustic coupling model of the nonlinear active acoustic insulation system, and approximated the frequency response of the acoustic pressure transmissibility using the harmonic balance method. The analytical results were well supported by numerical methods. Further, we analyzed the effects of geometrical parameters and shunting circuit gains to explore the mechanical and electrical effects on insulation performance. An experimental design and a semi-physical simulation model were finally set up to validate theoretical analysis. It was revealed that a properly configured active acoustic insulation system could greatly reduce the sound levels and effectively expand the insulation frequency bandwidth.

On the reduction of the flow-induced noise using porous material plates with high acoustic transmissibility

This study investigates the efficacy of closed-structure porous materials for reducing flow-induced noise, given their high acoustic transmissibility and low fluid permeability. A multifaceted approach encompassing wind tunnel experiments, numerical simulations, and practical applications to heating, ventilation, and air-conditioning (HVAC) systems in commercial electric vehicles was adopted. Wind tunnel experiments demonstrated a significant reduction, over 20 dB, in the tonal noise generated from a rectangular cylinder and an increase in its frequency, attributable to changes in the vortex structure and velocity fluctuations in the wake of the cylinder. The numerical simulation of fluid permeation and pressure wave conditions on the surface of the porous material as a boundary condition aligned with the experimental outcomes, suggesting that such permeation conditions could lead to noise and drag reduction. Practical testing showed a 5 dB noise reduction at the driver's ear position when porous materials were attached to a portion of an electric vehicle's HVAC system. The noise reduction was associated with changes in velocity fluctuations in the flow field, aligning with the wind tunnel experimental findings. This study provides promising insights into the development of effective noise reduction strategies using porous materials.

Passive acoustic damage detection of structural cavities using flow-induced acoustic excitations

Cavities with different geometries represent the internal volumes of various engineering applications such as cabins of passenger cars, fuselages and wings of aircraft, and internal compartments of wind turbine blades. Transmissibility of acoustic excitation to and from these cavities is affected by material and cross-sectional properties of the structural cavity, as well as potential damage incurred. A new structural damage detection methodology that relies on the detectability of the changes in acoustic transmissibility across the boundaries of structural cavities is proposed. The methodology is described with a specific focus on the passive damage detection approach applied to cavity internal acoustic pressure responses under external flow-induced acoustic excitations. The approach is realized through a test plan that considers a wind turbine blade section subject to various damage types, severity levels, and locations, as well as wind speeds tested in a subsonic wind tunnel. A number of statistics-based metrics, including power spectral density estimates, band power differences from a known baseline, and the sum of absolute difference, were used to detect damage. The results obtained from the test campaign indicated that the passive acoustic damage detection approach was able to detect all considered hole-type damages as small as 0.32 cm in diameter and crack-type damages 1.27 cm in length. In general, the ability to distinguish damage from the baseline state improved as the damage increased in severity. Damage type, damage location, and flow speed influenced the ability to detect damage, but were not significant enough to prevent detection. This article serves as an overall proof of concept of the passive-based damage detection approach using flow-induced acoustic excitations on structural cavities of a wind turbine blade. The laboratory-scale results reveal that acoustic-based monitoring has great potential to be used as a new structural health monitoring technique for utility-scale wind turbine blades.

Active acoustic damage detection of structural cavities using internal acoustic excitations

A novel structural damage detection methodology that relies on the detectability of the changes in acoustic transmissibility across boundaries of structural cavities is investigated. The approach focuses on active damage detection by leveraging the acoustic pressure responses measured external to structural cavities while exposed to internal acoustic excitations. The active damage detection concept is first demonstrated on a 4 m wind turbine blade using acoustic beamforming techniques to confirm that the acoustic energy transmitted through a damaged surface increases local to the damage compared to an undamaged surface. The concept is further verified, only considering acoustic pressure responses measured from limited microphones positioned at various distances from a ~46 m wind turbine blade. A comprehensive testing campaign is developed and executed on the utility-scale blade considering various damage types, severity levels, and locations. The data are analyzed using a combination of spectral analysis and statistics-based metrics to detect and track the progression of damage as well as identify trends across the test variables. Overall, large increases in the power spectral density were observed from the pressure responses measured external to the structure in most cases. The spectral differences increased as the damage became more severe and damage as small as 5.1 cm in length was easily detected from multiple sensors up to 17.1 m from the damage location. Damage was easily detected when implemented before the mid-length of the blade using simple signal processing algorithms and preliminary test configurations. The data acquired in this work serve as a preliminary investigation into the capability of the approach on complex structures and paves the path for future research into the signal processing techniques and test configurations that will enhance the performance of the active acoustic damage detection approach.

Multimicrophone Measurement of Semi-Active Samples in an Acoustic Tube

The sample of a piezoelectric semi-active shunt damping material is analyzed inside an acoustic tube. The measurement of reflected and transmitted acoustic wave through the sample is realized using 4-microphone measurement method. Acoustic properties of the sample with the emphasis on acoustic transmissibility are analyzed. The composite piezoelectric sample comprising of a piezoelectric material connected to a linear shunting circuit is described, also the multimicrophone measurement methods are described in detail and the results are discussed in context of semi-active sound damping.

Adaptive support for aircraft panel testing: New method and its experimental verification on a beam structure

Acoustic transmissibility of aircraft panels is measured in full-scale test rigs. The panels are supported at their frames. These boundary conditions do not take into account the dynamic influence of the fuselage, which is significant in the frequency range below 300Hz. This paper introduces a new adaptive boundary system (ABS). It combines accelerometers and electrodynamic shakers with real-time signal processing. The ABS considers the dynamic effect of the fuselage on the panel. The frames are dominating the dynamic behaviour of a fuselage in the low-frequency range. Therefore, the new method is applied to a beam representing a frame of the aircraft structure. The experimental results are evaluated and the precision of the ABS is discussed. The theoretical apparent mass representing the cut-off part of a frame is calculated and compared with the apparent mass, as provided by the ABS. It is explained how the experimental set-up limits the precision of the ABS.

Structural engineering of three-dimensional phononic crystals

Artificially-structured materials are attracting the research interest of a growing community of scientists for the possibility to develop novel materials with advantageous properties that arise from the ability to tailor the propagation of elastic waves, and thus energy, through them. In this work, we propose a three-dimensional phononic crystal whose unit cell has been engineered to obtain a strong wave-attenuation band in the middle of the acoustic frequency range. The combination of its acoustic properties with the dimensions of the unit cell and its static mechanical properties makes it an interesting material for possibly several applications in civil and mechanical engineering, for instance as the core of an acoustically insulating sandwich panel. A sample of this crystal has been manufactured and experimentally tested with respect to its acoustic transmissibility. The performance of the phononic crystal core is remarkable both in terms of amplitude reduction in the transmissibility and width of the attenuation band. A parametric study has been finally conducted on selected geometrical parameters of the unit cell and on their effect on the macroscopic properties of the crystal. This work represents an application-oriented example of how the macroscopic properties of an artificially-structured material can be designed, according to specific needs, by a conventional engineering of its unit cell.

A study to determine chest wall vibratory attachment interface locations for a low frequency sonic vibrator in treatment of acute coronary thrombosis

Our institution is developing a non-invasive Diastolic Timed Vibrator (DTV) to enhance emergency clearance of acute coronary thrombosis. Sonic frequency diastolic vibro-percussion (i.e. 50Hz, 2mm amplitude) applied upon the rib-spaces of the left sternal border has shown to improve left ventricular performance and coronary flow in human volunteers. However, therapeutic acoustic penetrability cannot be assumed depending on varying chest size and lung position which attenuates acoustic transmissions. Furthermore, chest locations enabling a direct lung free pathway overlying the base of the heart (wherein the coronaries arise) should be promoted, while locations overlying the left ventricular apex (site of potential thrombus formation) should be avoided. We therefore set out to determine preferred chest wall placement positions for a vibratory interface suitable for treatment of ST Elevation Myocardial Infarction (STEMI). Inter-Costal Space (ICS) positions to the left or right of the sternum were interrogated in 90 adults following routine Echocardiography to ascertain whether the base of the heart could be imaged (hence inferring acoustic transmissibility), and to determine over what part of the heart each transducer position was overlying. The third and fourth ICS proximate the left sternal border provided acoustic transmissibility in 96 and 100% of cases respectively, with only one unwanted occurrence from the fourth ICS where the transducer overlaid the apical third of the left ventricle. Acoustic transmissibility from third and fourth ICS right sternal border was documented in 53 and 85% of cases respectively. A vibration interface in STEMI treatment should allow for contact overlying the left and right third and fourth ICS generally proximate the sternal borders. As vibration transmission to the cardiac apex and/or left atrium cannot be completely avoided, vibration therapy should be contra-indicated in late presenters for antero-septal apical STEMI, and in cases of new onset atrial fibrillation persisting greater than 48h which have not been adequately anti-coagulated.

An experimental rig for near-bit force measurement and drillstring acoustic transmission of BHA

An experimental rig for near-bit force measurement and drillstring acoustic transmission of bottom-hole assembly (BHA) is designed and implemented to investigate downhole dynamic behaviors of BHA. By using a developed dynamic force sensor device with a four-straight beam strain gauge, the rig can measure axial, lateral forces and torques exerted on simulated drillstring while rotating. For the upward transmission of downhole dynamic force data, drillstring acoustic telemetry method is further analyzed by transient numerical simulation. Considering acoustic signal attenuation and transmissibility, a narrow pulse on–off keyed modulated time-delay signal transmission method is developed. The near-bit force measurement experiments of BHA are performed under different weight on bit loads, deviation angles and drill pipe combinations. The obtained force data are transmitted via acoustic waves propagating along the simulated drillstring. The results show that acoustic transmission rate is approximated 77 bit/s along 6.4-m simulated drillstring under 1-W power excitation in the laboratory.

Acoustically transparent visor

A sun visor for use with a motor vehicle includes a visor body having sufficient acoustic transmissibility such that the visor body is sufficiently sound transparent. The visor body further being sufficiently opaque so as to inhibit passage of light through the visor body.

Naphthenic hydrocarbon oils transmissible for transverse waves

The acoustic shear transmissibility and rigidity of saturated naphthenic hydrocarbon oils (NHO) with bridged carbon rings were determined by ultrasonic transmission analysis. In contrast to an established theory, we found that the NHO has superior transmissibility for transverse waves. It arises from appearance of quasi-elastic shear response under ultrasonic vibration, based on interlocking of the sterically hindered molecules with bridged rings. The shear and bulk moduli are of the order of 0.8–1.3 and 2.3–5.7 GPa at room temperature, respectively, indicating shear rigidity like solid polymers.

A discrete method for the acoustic transmissibility of a pressure transducer-in-capsule arrangement

The theory presented in this paper was derived to determine analytically the transmissibility of the pressure transducer-in-capsule arrangement used in recent experiments to measure wall pressure spectra in a bundle of cylinders with application to the vibration of nuclear reactor fuel rods and heat exchanger tubes. Liquids such as water and silicon oil were considered as the main pressure transmitting fluids and attention was focused on the effects of trapped air on the method of measurement. Experiments were conducted to verify trends predicted by the theoretical model.

Acoustic transmission of the aortic component of the second heart sound within the ascending aorta of dogs

The spectral characteristics of the acoustic transmission of the aortic component of the second heart sound within the ascending aorta was studied using a Millar dual-micromanometer catheter. The tip micromanometer was located close to the aortic valve leaflets while the second micromanometer was located 3 cm above the aortic valve. The frequency response of the transmission properties (amplitude and phase) of the blood and the aortic wall was modelled by an equivalent acoustic transmission system. The signal recorded by the tip micromanometer located near the aortic valve was considered to be the input signal of the equivalent system and the signal recorded by the second micromanometer was used as the output signal. Results of the spectral analysis of the input and output signals show that the acoustic transmissibility of blood in the ascending aorta is high at 20 Hz (the attenuation is negligible). Between 20 and 60 Hz, the transmissibility decreases at a rate of -3 dB per octave while between 60 and 120 Hz it decreases at a rate of -14 dB per octave. Above 120 Hz the transmissibility is low and the resulting attenuation is greater than 20 dB. The phase of the transfer function is shifted by -60 degrees at 20 Hz and decreases at a mean rate of -2.0 degrees Hz-1 between 20 and 100 Hz and -0.75 degrees Hz-1 up to 400 Hz. The phase velocity of the sound transmission is relatively constant (5.5 ms-1) between 40 and 100 Hz and increases up to 9 ms-1 at 300 Hz.

Acoustic transmissibility of advanced turboprop aircraft windows

Advanced turboprop technology allows propeller aircraft to reach cruise speeds comparable to current jet aircraft yet with considerable fuel savings. Due to the higher blade loading and higher tip speed of these propellers, noise levels of up to 150 dB are expected on the outside of the fuselage in the propeller plane. In this study the transmissibility of triple pane windows, designed to provide 69 dB noise transmission loss at the blade propeller frequency of 164 Hz, was experimentally investigated using insertion loss and three-dimensional intensity techniques. A modal analysis on the outer window panes was conducted to determine pane modal frequencies. Coherence and phase relation of outer panes and window frame were established to obtain double/triple wall and lump mass resonance frequencies. Double/triple wall resonances were found to degrade the transmission loss of the two windows. It was shown that, at the blade passage frequency and the first two overtones, the combinations of window plus scratch shield provide less transmission loss than the average transmission loss of the treated fuselage. Strong disagreement was obtained between the experimental transmission loss of this investigation and the theoretical predictions from another study.