Investigation of sound propagation in a duct with a mean temperature gradient

This paper presents an analytical and numerical investigation of an impedance tube in the presence of a mean temperature gradient. Full Navier-Stokes simulation of the acoustic wave propagations through an impedance tube in the presence of a mean temperature gradient without mean air flow is investigated. Results from the simulations are compared with an analytical model of the behaviour of one-dimensional oscillations in an impedance tube with an axial temperature gradient in the absence of mean flow. Time and axial distance dependent acoustic pressure and velocity are visualised as 3D surface plots. The agreement between simulation and the analytical model is shown to be very good and represents a baseline validation of the numerical methodology for the simulation of impedance tubes in the presence of temperature gradients

Behaviour of acoustic waves in a duct with Helmholtz resonator in presence of a temperature gradient

Understanding the behaviour of one-dimensional acoustical wave propagation in ducts is very important for controlling combustion instabilities in propulsion, household burners, gas turbine combustors, and designing engineering mufflers. This paper is concerned with ducts in which temperature gradient exist. Computational Fluid Dynamics (CFD) simulation of the acoustic wave propagations through a duct with Helmholtz resonators in the presence of a mean temperature gradient without mean air flow has been investigated. Acoustic pressure and axial velocity amplitudes have been calculated as a function of time. Time and axial distance dependent acoustic pressure and velocity are visualised as 3D surface plots

THE INSERTION LOSS OF POROELASTIC PLATE SILENCERS IN A FLOW DUCT

Idealized silencers consisting of cavity-backed clamped porous elastic plates parallel to the direction of flow can result in low frequency attenuation of noise in the duct. The insertion loss of perforated and non-perforated poroelastic plate silencers have been measured with and without mean air flow. Peak IL when the plate is not perforated has a value of 4.1 dB at 1 kHz without air flow. With perforations the peak IL is 3.2 dB at 630 Hz. In the presence of mean air flow, without self-noise, peak IL have values of 17 dB at 250 Hz and 7 dB at 400 Hz for the air flow speeds of 37.65 m/s and 5.5 m/s respectively. Peak IL of perforated and non perforated porous plate has values of 11 dB and 11.5 dB at 160 Hz with noise and air flow at speed of 37.65 m/s. The displacements of a perforated and non-perforated porous elastic plates separated from duct wall by an air cavity in the duct have been measured and compared with those of the duct wall

Ultrasonic wave propagation through porous ceramics at different angles of propagation

The anisotropic pore structure and elasticity of cancellous bone cause wave speeds and attenuation in cancellous bone to vary with angle. Comparisons between predictions of a Biot-Allard model allowing for angle-dependent elasticity and angle-and-porosity dependent tortuosity and transmission data obtained on water-saturated replica bones at normal and oblique incidence are extended to water saturated porous rigid ceramic at different angles of propagation. It is found that predictions of the variation of transmitted waveforms with angle through porous ceramic are in reasonable agreement with data

Sound propagation through bone tissue

Effect of perforation on structure borne sound propagation through rigid porous materials has been investigated. Experimental works has been carried out on rigid porous materials with and without perforations. A low frequency vibration has been applied to excite the material structure by using a force transducer connected a shaker to detect the changes in resulting response. Applied vibration on sample surface causes structure borne sound wave to propagate through the material. The resulting response of this structural borne vibration is detected by using an accelerometer. The results with and without perforation of the sample have been compared. The results show that changing the structure of the material has an effect on the amplitude, shape and arrival time of the transmitted acoustic wave

Investigation of acoustic performance of compressed wool carpets

Sound absorbers including porous materials are used widely for noise control. The most widely exploited and acknowledged absorption mechanism in porous materials is viscous friction due to relative motion between solid and fluid. Acoustical performance of carpet made of wool by using a traditional compression technique has been investigated. The results are very interesting

Behaviour of ultrasonic waves in porous rigid materials: anisotropic Biot-Attenborough model

The anisotropic pore structure and elasticity of cancellous bone cause wave speeds and attenuation in cancellous bone to vary with angle. Anisotropy has been introduced into Biot theory by using an empirical expression for the angle-and porosity-dependence of tortuosity. Predictions of a modified anisotropic Biot–Attenboorugh theory are compared with measurements of pulses centred on 100 kHz and 1 MHz transmitted through water-saturated porous samples. The samples are 13 times larger than the original bone samples. Despite the expected effects of scattering, which is neglected in the theory, at 100 kHz the predicted and measured transmitted waveforms are similar

A review of the state of art in applying Biot theory to acoustic propagation through the bone

Understanding the propagation of acoustic waves through a liquid-perfused porous solid framework such as cancellous bone is an important pre-requisite to improve the diagnosis of osteoporosis by ultrasound. In order to elucidate the propagation dependence upon the material and structural properties of cancellous bone, several theoretical models have been considered to date, with Biot-based models demonstrating the greatest potential. This paper describes the fundamental basis of these models and reviews their performance

Sound propagation through bone

Effect of perforation on structure borne sound propagation through rigid porous materials has been investigated. Experimental works has been carried out on rigid porous materials with and without perforations. A low frequency vibration has been applied to excite the material structure by using a force transducer connected a shaker to detect the changes in resulting response. Applied vibration on sample surface causes structure borne sound wave to propagate through the material. The resulting response of this structural borne vibration is detected by using an accelerometer. The results with and without perforation of the sample have been compared. The results show that changing the structure of the material has an effect on the amplitude, shape and arrival time of the transmitted acoustic wave

Predictions of angle dependent tortuosity and elasticity effects on sound propagation in cancellous bone

The anisotropic pore structure and elasticity of cancellous bone cause wave speeds and attenuation in cancellous bone to vary with angle. Previously published predictions of the variation in wave speed with angle are reviewed. Predictions that allow tortuosity to be angle dependent but assume isotropic elasticity compare well with available data on wave speeds at large angles but less well for small angles near the normal to the trabeculae. Claims for predictions that only include angle-dependence in elasticity are found to be misleading. Audio-frequency data obtained at audio-frequencies in air-filled bone replicas are used to derive an empirical expression for the angle-and porosity-dependence of tortuosity. Predictions that allow for either angle dependent tortuosity or angle dependent elasticity or both are compared with existing data for all angles and porosities