Incident Acoustic Power Research Articles

Prediction of random incidence sound absorption coefficients of porous materials

Measuring the random incidence sound absorption coefficient (SAC) of porous materials is costly and requires a full-size reverberation room. This paper shows that the predictions from impedance tube measurement and the empirical method have reasonable accuracy for the determination of the random incidence SAC above 400 Hz. It also shows that the measurement of the random incidence SAC in a small self-made reverberation room is consistent with the full-size reverberation room results in the frequency range above 800 Hz. The prediction of the random incidence SAC was made using the finite transfer matrix method (FTMM) with material data obtained using the two-cavity method, the two-thickness method, the four-microphone transfer function matrix method, the empirical equations, and calculation by applying the inverse method to the Johnson-Champoux-Allard (JCA) model. In general, the different methods show a similar trend and reasonable agreement in the middle and the high-frequency ranges when predicting the acoustic properties of fibrous materials. The characteristics of each method are compared and discussed, and recommendations for the use of each method are made. The ‘usual SAC’ method, which uses the geometric incident acoustic power, agrees with the full-size reverberation room results better, while the ‘true SAC’ method, which uses the power absorbed by an absorber whose impedance is equal to the radiation impedance of an infinite size absorber as the incident acoustic power, is more suitable for predicting the results obtained in the small-size reverberation room.

Investigation of vibro-acoustic characteristics of FRP plates with porous foam core

Both theoretical and experimental investigations are performed on vibro-acoustic characteristics of composite plate comprising fiber-reinforced polymer (FRP) laminates with a porous foam core (PFC) subjected to planar acoustic wave (PAW) excitation in present work. Firstly, a theoretical model of the PFC-FRP plates is established through a combination of the advantages of analytical and finite element methods. The equivalent Young's and shear moduli and density of porous foam core with different porosity distributions is considered. The free and forced vibration equations subjected to PAW load are derived to solve natural frequencies, modal shapes and vibration responses of the PFC-FRP plates on the basis of the first-order shear deformation theory and the four-node quadrilateral isoperimetric finite element method. To solve the acoustic radiation powers (ARP), the Rayleigh integral approach is used to determine the quantitative relationships between the vibration velocity responses and acoustic radiation pressures. The sound transmission loss is further determined with the pre-defined proportional expression involving the ARP and the incident acoustic power (IRP). After the theoretical model is validated coarsely based on the separated literature results, a TC300 carbon/ GEL2 epoxy resin plate specimen is fabricated and an integrated vibro-acoustic experiment system is set up to give further validation of the proposed model, of which vibration and acoustic results are measured simultaneously without changing boundary conditions of the specimen. Finally, a detailed parametric study is conducted to obtain the optimum suppression performance of vibration and noise of this kind of composite plate.

Laser textured GFRP superhydrophobic surface as an underwater acoustic absorption metasurface

Underwater acoustic absorption metasurfaces (UAAMs), featured with artificial periodic unit cells as acoustic micro-absorbers, are crucial for the enhancement of the underwater stealth, energy transition, sonar detection, etc. How to implement the acoustic metasurfaces for practical engineering applications requires the support of efficient manufacturing techniques. Here we present a simple way of laser texturing for fabrication of the patterned superhydrophobic surface on glass fiber reinforced plastic (GFRP). Unlike the traditional acoustic absorbing material whose thickness should match the corresponding wavelength, the laser textured superhydrophobic micro-grooves have thickness less than the absorption sound wavelength by 2–3 orders of magnitude but promisingly demonstrate over 88% of the underwater incident acoustic power absorption from 50 kHz to 250 kHz. The physical mechanism of the UAAM ascribes to the multiple interface scattering effect induced by the air layer between water and superhydrophobic micro-nano structures.

Estimating the sound transmission loss of a single partition using vibration measurements

The sound transmission loss of a homogeneous, isotropic, thin panel under a diffuse acoustic field excitation is derived from a measurement of its airborne induced vibration field. Using this single dataset, the virtual fields method allows identifying the wall pressure field exciting the panel and estimating the corresponding incident acoustic power, provided that the differential operator governing the plate dynamics and its material properties are known a priori. Using the same dataset, the radiated acoustic power is calculated using the radiation resistance matrix method. For an aluminium plate, a comparison of transmission loss values obtained using this approach and a standardized measurement shows good agreement off resonance but large discrepancies on resonance and close to resonance, due to ill conditioning of the virtual fields method. A simple correction is proposed on resonance.

Metasurface for Water-to-Air Sound Transmission.

Effective transmission of sound from water to air is crucial for the enhancement of the detection sensitivity of underwater sound. However, only 0.1% of the acoustic energy is naturally transmitted at such a boundary. At audio frequencies, quarter-wave plates or multilayered antireflection coatings are too bulky for practical use for such enhancement. Here we present an acoustic metasurface of a thickness of only ∼λ/100, where λ is the wavelength in air, consisting of an array of meta-atoms that each contain a set of membranes and an air-filled cavity. We experimentally demonstrate that such a meta-atom increases the transmission of sound at ∼700 Hz by 2 orders of magnitude, allowing about 30% of the incident acoustic power from water to be transmitted into air. Applications include underwater sonic sensing and communication.

Research on Sound Insulation Performance of Poroelastic Material in Complex Structure

A finite element model of the double-wall acoustic insulation structure with a air layer and an acoustic absorbent layer made of the poroelastic materials is set up, the responses of this acoustic-vibration system are calculated by using of the direct finite element method when having a diffuse incident acoustic field acting on the incident surface, the radiant acoustic power from the another surface are achieved, then the Transmission Loss(TL) are formulated using the incident acoustic power and the radiant acoustic power. The effects of the thicknesses, elastic modulus, flow resistivity and viscous lengths of the poroelastic materials on TL are analyzed. The results show that the thicknesses and elastic modulus have a significant effects on TL, TL are enhanced with the thicknesses increasing of the poroelastic materials layers, a 4.9dB addition of TL is achieved when thickness is added from 2cm to 3cm; TL are enhanced with the reduction of the elastic modulus in considered frequency range, and TL are reduced with the declining of viscous lengths and with the addition of the flow resistivity when the frequencies are higher than 600Hz.

Measuring Sound Absorption: Considerations on the Measurement of the Active Acoustic Power

Using a local plane wave assumption, one can determine the normal incidence sound absorption coefficient of a surface by measuring the acoustic pressure and the particle velocity normal to that surface. As the measurement surface lies in front of the material surface, the measured active and incident acoustic power will generally deviate from those at the material surface, leading to a possibly inaccurate sound absorption coefficient. This phenomenon is particularly pronounced for poorly absorbing surfaces if sound is not normally incident over the whole material surface. Based on an analytical model, it is shown that the accuracy can be improved by extending the measurement surface upon which the active acoustic power is measured. Experimental results demonstrate the usefulness of this approach, in particular for poorly absorbing surfaces.

고속철도차량의 터널 소음을 위한 차음 전략

터널 내에서 고속철도 차량의 실내소음은 개활지 대비 5dB~7dB정도 증가한다. 그 원인은 터널 내 차체 외부의 음향 인텐시티가 급격히 증가하기 때문인데, 특히, 측면재의 외부에서는 개활지에 비해서 음향 인텐시티의 증가가 가장 크다. 따라서 터널 내에서는 차체의 측면을 통하여 음향파워의 투과 전달이 커질 가능성이 상당히 높다. 본 논문에서는 현재 개발중인 차세대 고속 철도차량의 터널 내 실내소음을 저감시키기 위한 종합적인 차음 전략을 제시하고자 한다. 이를 위하여 차체의 주요 차음재인 바닥 적층재, 측면 적층재 및 복층 유리창의 시편을 제작하여 ASTM E2249-02에 근거하여 투과손실 측정한다. 측정 데이터에 근거하여 차음 성능상의 문제점을 진단하고, 차음성능 향상을 위한 층별 개선 방안과 적층 구조의 개선 방안을 제시한다. In a tunnel, interior noise of a high speed train increases by 5dB~7dB. The reason is that the sound intensity of the acoustic field in the tunnel significantly increases by the reflected waves occurred in the closed space. Especially, the incident acoustic power largely increases on the outside of the compartment side panel and large transmission of noise is available through the side panel and the glass window. In this paper, the sound insulation strategy in the tunnel is proposed for the next generation high speed train under development. Specimens of the aluminum extruded panels, layered panels and double glazed window are manufactured and intensity transmission loss is measured according to ASTM E2249-02. Based on the measured data, problems in the sound insulation performance are diagnosed and the sound insulation strategy is reviewed on each panel and layered structures.

Investigating the Efficacy of Subharmonic Aided Pressure Estimation for Portal Vein Pressures and Portal Hypertension Monitoring

The efficacy of using subharmonic emissions from Sonazoid microbubbles (GE Healthcare, Oslo, Norway) to track portal vein pressures and pressure changes was investigated in 14 canines using either slow- or high-flow models of portal hypertension (PH). A modified Logiq 9 scanner (GE Healthcare, Milwaukee, WI, USA) operating in subharmonic mode (ftransmit: 2.5 MHz, freceive: 1.25 MHz) was used to collect radiofrequency data at 10–40% incident acoustic power levels with 2–4 transmit cycles (in triplicate) before and after inducing PH. A pressure catheter (Millar Instruments, Inc., Houston, TX, USA) provided reference portal vein pressures. At optimum insonification, subharmonic signal amplitude changes correlated with portal vein pressure changes; r ranged from −0.82 to −0.94 and from −0.70 to −0.73 for PH models considered separately or together, respectively. The subharmonic signal amplitudes correlated with absolute portal vein pressures (r: −0.71 to −0.79). Statistically significant differences between subharmonic amplitudes, before and after inducing PH, were noted (p ≤ 0.01). Portal vein pressures estimated using subharmonic aided pressure estimation did not reveal significant differences (p > 0.05) with respect to the pressures obtained using the Millar pressure catheter. Subharmonic-aided pressure estimation may be useful clinically for portal vein pressure monitoring.

The Use of Helmholtz Resonators in a Practical Combustor

The drive to reduce emissions has led to the development of lean premixed combustors. However, lean premixed combustion is often associated with combustion oscillations which can be so severe that they can cause structural damage to the engine. Since the associated frequencies are typically of the order of hundreds of Hertz, there is a need for a compact device to absorb the noise which drives the oscillation. Helmholtz resonators are commonly used as absorbers of incident acoustic power. In addition they are relatively compact. However, their use in combustors creates practical issues, such as placement within the chamber, neck length, and cooling, which need to be addressed. In this paper we consider these practical problems and describe how to overcome them in a real combustor.

Calibration of contact transducers by means of thermoacoustic sensors

The utilization of the thermoacoustic effect for measuring the absolute value of acoustic power by means of thermoacoustic sensors is discussed. The sensor’s response to the incident acoustic power is measured as the temperature increase T. This temperature increase is computed by the differential equation describing the sensor, which contains only the sensor’s material constants: density ρ, specific heat capacity cw, and conductivity Λw, ∂T/∂t=(Λw/ρcw)∇2T+(1/ρcw)⋅W. It is shown how the distribution of heat sources W (caused by the absorbed ultrasonic field) can be optimized for the realization of thermoacoustic sensors adapted to experimental situations. A discussion of different types of sensors will show that the complex transfer functions are restricted by the material constants. It is shown how the solutions presented can be used to improve the dynamic performance. Experimental results obtained by the use of different sensors and contact transducers (calibrated by the reciprocity method) are compared with respective solutions of the parabolic heat flow equation.

On the passive power absorption potential of the eardrum

Experimental evidence reported by Khanna [Int. Cong. Acoust. B6‐4, Toronto (1986)] and Allen [Peripheral Auditory Mechanisms (Springer, New York, 1985), pp. 46–51] suggests that a very large fraction of the incident acoustic power in the cat ear canal is absorbed at the eardrum. In some animals, the high efficiency extends over a very wide frequency range from 300 Hz–30 kHz. These findings are sensitive to the condition of the animal and are subject to relatively large interanimal variations, particularly at high frequencies. Theoretical models describing the function of the middle ear differ significantly in predicting the power absorption and in predicting the fraction of the energy flux dissipated within the eardrum itself. Some insight into the behavior of the eardrum can be gained by studying the power absorption properties when the ossicular chain is immobilized or removed. In doing so, the dynamics of the ossicular chain, and to a large extent the dynamics of the cochlea, is removed from the system...

Stripping one‐dimensional acoustic pressure response into propagating‐ and standing‐wave components

Most realistic boundary conditions are partially absorptive, with the rest of the incident acoustic power being reflected. This results in a response that can be viewed as a summation of propagating‐ and standing‐wave effects. A one‐dimensional, measurement‐based, two‐microphone spectral analysis technique is developed which s_eparates the t_otal acoustic r_esponse i_nto p_ropagating and s_tanding (STRIPS) wave components. STRIPS uses measurements that contain implicit information about the boundary condition to extract the propagating‐ and standing‐wave components. These sum to be the total system response without requiring explicit knowledge of the particular boundary condition. An exact decomposition example for a one‐dimensional acoustic pressure model with a known mixed boundary condition is used to validate the STRIPS method. The utility of the STRIPS method is demonstrated using experimental measurements from a tube as input to STRIPS to obtain the propagating‐ and standing‐wave pressure components. Changes in experimental boundary absorptivity are seen in the STRIPS results when a flared end is attached to the tube.

Stripping the one-dimensional acoustic pressure response into propagating and standing wave components

Most realistic boundary conditions are partially absorptive, with the rest of the incident acoustic power being reflected. This results in a response that can be viewed as a summation of propagating and standing wave effects. A one-dimensional, measurement-based, two-microphone spectral analysis technique is developed which Separates the Total acoustic Response Into Propagating and Standing (STRIPS) wave components. STRIPS uses measurements that contain implicit information about the boundary condition to extract the propagating and standing wave components. These sum to be the total system response without requiring explicit knowledge of the particular boundary condition. An exact decomposition example for a one-dimensional acoustic pressure model with a known mixed boundary condition is used to validate the STRIPS method. The utility of the STRIPS method is demonstrated using experimental measurements from a tube as input to STRIPS to obtain the propagating and standing wave pressure components. Changes in experimental boundary absorptivity are seen in the STRIPS results when a flared end is attached to the tube.

Reflection of Gigacycle-per-Second Ultrasonic Waves from an Optical-Contact Bond

The use of optical contact as a low-loss acoustic bond at gigacycle-per-second frequencies has been demonstrated. The reflection of 3- and 9 Gc sec longitudinal waves from an optical contact bond between two X cut quartz rods has been measured to be of the order of 1% of the incident acoustic power at 1.5°K. Thin X-cut quartz disks optically contacted to Z-cut quartz and ruby rods have been used at 1.5°K and found to be about as efficient aa an X-cut rod.