Ulf Kristiansen

Extending the image method to higher-order reflections

Abstract The paper concerns extrapolation of number of visible image sources and average image source distances to higher-order specular reflections from low-order calculated results. For closed rooms, the number of visible sources follows a second-order function while the distance curve approaches a straight line. Comparisons are made with results from a parallelepipedic room, for which it is possible to efficiently calculate image sources correctly up to high reflection orders. To establish the extrapolation functions it is desirable to calculate the positions of images for as many low-order reflections as possible. The paper therefore also discusses methods that might be used to accelerate such calculations.

Experimental study of the influence of low frequency flow modulation on the whistling behavior of a corrugated pipe

It is well known that airflow in a corrugated pipe can excite whistling at the frequencies of the pipes longitudinal acoustic modes. This short contribution reports on the results of experiments where a low frequency, oscillating flow with velocity magnitudes of the same order as the airflow has been added. Depending on the oscillation strength, it has been found that this flow may silence the pipe or move the whistling to higher harmonics. It is also shown that the low frequency oscillation itself may excite higher frequency whistling sounds in the pipe.

Finite Difference Method for the Acoustic Radiation of an Elastic Plate Excited by a Turbulent Boundary Layer: A Spectral Domain Solution

A finite difference method is developed to study, on a two-dimensional model, the acoustic pressure radiated when a thin elastic plate, clamped at its boundaries, is excited by a turbulent boundary layer.Consider a homogeneous thin elastic plate clamped at its boundaries and extended to infinity by a plane, perfectly rigid, baffle. This plate closes a rectangular cavity. Both the cavity and the outside domain contain a perfect fluid. The fluid in the cavity is at rest. The fluid in the outside domain moves in the direction parallel to the system plate/baffle with a constant speed. A turbulent boundary layer develops at the interface baffle/plate. The wall pressure fluctuations in this boundary layer generates a vibration of the plate and an acoustic radiation in the two fluid domains.Modeling the wall pressure fluctuations spectrum in a turbulent boundary layer developed over a vibrating surface is a very complex and unresolved task. Ducan and Sirkis [1] proposed a model for the two-way interactions between a membrane and a turbulent flow of fluid. The excitation of the membrane is modeled by a potential flow randomly perturbed. This potential flow is modified by the displacement of the membrane. Howe [2] proposed a model for the turbulent wall pressure fluctuations power spectrum over an elastomeric material. The model presented in this article is based on a hypothesis of one-way interaction between the flow and the structure: the flow generates wall pressure fluctuations which are at the origin of the vibration of the plate, but the vibration of the plate does not modify the characteristics of the flow.A finite difference scheme that incorporates the vibration of the plate and the acoustic pressure inside the fluid cavity has been developed and coupled with a boundary element method that ensures the outside domain coupling. In this paper, we focus on the resolution of the coupled vibration/interior acoustic problem. We compare the results obtained with three numerical methods: (a) a finite difference representation for both the plate displacement and the acoustic pressure inside the cavity; (b) a coupled method involving a finite difference representation for the displacement of the plate and a boundary element method for the interior acoustic pressure; (c) a boundary element method for both the vibration of the plate and the interior acoustic pressure.A comparison of the numerical results obtained with two models of turbulent wall pressure fluctuations spectrums - the Corcos model [3] and the Chase model [4] - is proposed. A difference of 20 dB is found in the vibro-acoustic response of the structure. In [3], this difference is explained by calculating a wavenumber transfer function of the plate. In [6], coupled beam-cavity modes for similar geometry are calculated by the finite difference method.

Reduction of sound in a low velocity flow duct by the use of Bragg reflections

Abstract This paper presents the results of some experiments on the use of structures of aerodynamically-shaped elements as narrow-band sound reflectors in a duct. The structures are tuned to give maximum attenuation for frequencies determined by the Bragg reflection condition, and are thought to be of special value where space considerations prohibit the use of conventional acoustic wave filters.

A different type of resonator for acoustics

Abstract An investigation is described of quarter-wave resonators where the volume is divided by a slanted wall, creating volumes of decreasing and increasing cross-sectional areas. It is shown that such a resonator has two fundamental frequencies, on either side of the original. Simple prediction formulae are given as well as results from measurements and a finite element investigation.

Controlling the Acoustic Resonance in a Corrugated Flow Pipe

A new experimental study, aimed at investigating the coupling between the flow in a corrugated pipe, the acoustically generated flow oscillations, and the emitted resulting noise is carried out. Hot-wire anemometry, Particle Image Velocimetry, and microphone measurements are associated to characterize the flow. The flow response to the corrugation is shown to fit to the sixth to ninth acoustic modes of the pipe according to the flow rate. When low frequency acoustically generated oscillations interfere with this, one checks that they either significantly reduce the noise level or modify the peak frequencies. In addition, theoretical/numerical works are also performed, in order to provide an analytical framework describing the acoustical properties of such corrugated pipe flows.

Sound generation by airflow in a pipe having a small internal cavity

Gas flow through corrugated pipes is known to excite strong acoustic resonances within the pipe. In an attempt to better understand the flow acoustic phenomena involved, we have investigated experimentally a short pipe (0.6m long and 0.04m diameter) having a single small (5mm long, 2.5mm deep) circumferential cavity. It was found that if placed close to the pipes inflow end, strong acoustic resonances were generated. The experimental results were compared to a model based on describing‐function theory. The model involves two transfer functions, one associated with the pipe resonator, and the other the shear layer above the cavity. These are combined to a feedback system. This model gives the frequencies generated and the acoustic pressure levels (to within a constant) for different flow velocities. Reasonable agreement was obtained between the experimental results and the model predictions.

Studies of scattering from differently shaped objects using the TLM technique

The transfer line method (TLM) can be regarded as a discretized version of Huygen’s principle. Wave energy propagates along a system of lines connecting regularly spaced node points. An energy packet reaching a node along a certain line will at the next time step be split up and reradiated into all the lines connected to that node. Wave propagation is therefore regarded as a discretized process in space and time with local updating rules. Special rules are applied for reflecting, absorbing, and partially absorbing surfaces. The method has been used for some time in electromagnetic wave propagation, but not so much in acoustic studies. The present study is a 2‐D investigation of the scattering of sound pulses from objects having different shapes: circular, triangular, and rectangular. Performing this in the time domain allows the following of the scattering process in detail.

A Helmholtz resonator with elongated orifice

For panel absorbers there is an increasing trend toward a design not utilizing fibrous components. For the distributed Helmholtz type of resonator absorbers, the challenge is to increase the natural losses, making the absorbers reasonably broadbanded. In this paper a new type of distributed resonators is investigated. Using a double‐plate construction, the resonator necks have been substantially elongated in the lateral direction, while their widths have been kept small. Thus the viscous losses in the resonator necks have been increased compared to traditional resonators. Measurements are compared with a theoretical model using analytic solutions for the slit impedances, the end corrections, and the resistance of the inner and outer surfaces. For the volume between the plates, a finite‐difference approach is used. The results show a high dependency on the plate separation, and to attain high absorption over a broad frequency range, this separation should be on the order of one boundary layer.

Toward a global in situ method for materials acoustic impedance assessment at very low frequencies using evolution strategy

As the current in situ absorption measurement methods are both local and based on time‐domain processing, the time‐frequency uncertainty relationship limits their ability to measure at low frequencies, leaving the building engineer with an uncovered frequency range. The complementary approach proposed here is to work in the frequency domain instead and to consider the measurement problem as an inverse and optimization one. A critical assumption for practicality is that very few measurement points should be necessary to define the pressure field without ambiguity. As the optimization problem is clearly not local, a global algorithm is necessary in combination with a forward model of the room giving the pressure field from the boundary conditions. A numerical study is given concerning a 2‐D rectangular room modeled by the finite‐difference method. Each wall has a specific impedance. An evolution strategy is used as a global optimization tool, both alone and in combination with a local search method, to find...