Q. Leclere

Sound field separation with sound pressure and particle velocity measurements

In conventional near-field acoustic holography (NAH) it is not possible to distinguish between sound from the two sides of the array, thus, it is a requirement that all the sources are confined to only one side and radiate into a free field. When this requirement cannot be fulfilled, sound field separation techniques make it possible to distinguish between outgoing and incoming waves from the two sides, and thus NAH can be applied. In this paper, a separation method based on the measurement of the particle velocity in two layers and another method based on the measurement of the pressure and the velocity in a single layer are proposed. The two methods use an equivalent source formulation with separate transfer matrices for the outgoing and incoming waves, so that the sound from the two sides of the array can be modeled independently. A weighting scheme is proposed to account for the distance between the equivalent sources and measurement surfaces and for the difference in magnitude between pressure and velocity. Experimental and numerical studies have been conducted to examine the methods. The double layer velocity method seems to be more robust to noise and flanking sound than the combined pressure-velocity method, although it requires an additional measurement surface. On the whole, the separation methods can be useful when the disturbance of the incoming field is significant. Otherwise the direct reconstruction is more accurate and straightforward.

Direct formulation of the supersonic acoustic intensity in space domain

This paper proposes and examines a direct formulation in space domain of the so-called supersonic acoustic intensity. This quantity differs from the usual (active) intensity by excluding the circulating energy in the near-field of the source, providing a map of the acoustic energy that is radiated into the far field. To date, its calculation has been formulated in the wave number domain, filtering out the evanescent waves outside the radiation circle and reconstructing the acoustic field with only the propagating waves. In this study, the supersonic intensity is calculated directly in space domain by means of a two-dimensional convolution between the acoustic field and a spatial filter mask that corresponds to the space domain representation of the radiation circle. Therefore, the acoustic field that propagates effectively to the far field is calculated via direct filtering in space domain. This paper presents the theory, as well as a numerical example to illustrate some fundamental principles. An experimental study on planar radiators was conducted to verify the validity of the technique. The experimental results are presented, and serve to illustrate the usefulness of the analysis, its strengths and limitations.

Some characteristics of the concert harp's acoustic radiation.

The way a musical instrument radiates plays an important part in determining the instruments sound quality. For the concert harp, the soundboard has to radiate the strings vibration over a range of 7 octaves. Despite the effort of instrument makers, this radiation is not uniform throughout this range. In a recent paper, Waltham and Kotlicki [J. Acoust. Soc. Am. 124, 1774-1780 (2008)] proposed an interesting approach for the study of the string-to-string variance based on the relationship between the string attachment position and the operating deflection shapes of the soundboard. Although the soundboard vibrational characteristics determine a large part of the instruments radiation, it is also important to study directly its radiation to conclude on the origins of the string-to-string variation in the sound production. This is done by computing the equivalent acoustical sources on the soundboard from the far field sound radiation measured around the harp, using the acoustic imaging technique inverse frequency response function. Results show that the radiated sound depends on the correlation between these sources, and the played strings frequency and location. These equivalent sources thus determine the magnitude and directivity of each strings partial in the far field, which have consequences on the spectral balance of the perceived sound for each string.

Time domain identification of loads on plate‐like structures using an array of acoustic velocity sensors

The FAT (Force Analysis Technique) method has been developped to identify loads on beams or plates from the knowledge of their flexural displacements. The method is based on a local discretisation of the differential operator of the studied structure : all derivatives of the displacement field are assessed at a given point from a finite difference scheme. The estimation of the operator gives as an output the value of the force distribution. Up to now, applications of the FAT method have been made in the frequency domain, scanning the studied structure with accelerometers or with a laser vibrometer, and using phase references to get the phase relation between different points. The aim of the present study is to show that the FAT method allows to identify loads in the time domain. This operation requires the simultaneous measurement of at least 13 points on the plate, that can be realized without contact using an array of acoustic velocity sensors in the very near field of the plate. The method has been app...

Nearfield acoustic holography using a laser vibrometer and a light membrane.

This paper deals with a measurement technique for planar nearfield acoustic holography (NAH) applications. The idea is to use a light tensionless membrane as a normal acoustic velocity sensor, whose response is measured by using a laser vibrometer. The main technical difficulty is that the used membrane must be optically reflective but acoustically transparent. The latter condition cannot be fully satisfied because of the membrane mass, which has to be minimized to reduce acoustic reflections. A mass correction operator is proposed in this work, based on a two-dimensional discrete Fourier transform of the membrane velocity field. An academic planar NAH experiment is finally reported, illustrating qualitatively and quantitatively the feasibility of the method and the pertinence of the mass correction operator.

Study of a concert harp's radiation using acoustic imaging methods

Recent studies have been conducted to understand the low frequency radiation of a concert harp. Experiments have been carried out in a semi‐anechoic room: the harps soundboard is excited with a shaker and the pressure is measured at more than 600 positions on a nearly hemispherical surface at a distance of about 2 m from the instrument. Simple source models with a set of monopoles have been optimized to fit the measured acoustic radiation. These models are very satisfactory in the low frequency domain but cannot properly reproduce the measured field above 350 Hz. The aim of this paper is to present the application of acoustic imaging tools to this academic case: volumetric velocity and acoustic power maps in the harp plane are computed with an optimized beamforming and with a regularized inverse FRF method up to 1 kHz. The optimization of the beamforming is realized using a variable windowing factor, and the regularization of the inverse FRF method with a Tikhonov approach. The presented results permit t...

Identification of Boundary Pressure Field Exciting a Plate Under Turbulent Flow

The characterisation of the aeroacoustic wall pressure field generated by turbulent flow is a difficult task that often requires instrumented panels and huge facilities like wind tunnels. In situ and non-intrusive experiments are rather not possible. In addition, the pressure field is dominated by the aerodynamic component and the experimental dynamics are not sufficient to measure correctly spectra in low wavenumbers by microphones. The present chapter deals with such a separation method by using the Force Analysis Technique (FAT). FAT is based on the use of the equation of motion of the structure (here a plate) and on the approximation of the fourth derivatives by a finite difference scheme. In the case of turbulent flow, the force auto-spectrum can be deduced at one point of the structure by measuring the velocity at 13 points synchronously. To this purpose, an array of 13 pU (acoustic pressure/particle velocity) probes has been made up. This array is moved in the near-field of the plate to identify map of the wall pressure level applied on the surface of the plate. In the present application, it is shown that FAT only identifies the component of the excitation with wavenumber lower than the natural flexural wavenumber of the plate, due to filtering effect of the plate and of the finite difference scheme. In most cases, the convective peak is then canceled and only the acoustic part of the turbulent flow is identified. This property can be of great interest for vehicle manufacturers to quantify the part of the wall pressure that is responsible of the radiated noise or to use FAT as a source separation technique.

Modal identification of a small-scale ducted fan

The subject of this paper is the experimental investigation of the noise radiated by a ducted rotating machine. A modal identification approach is used to decompose the radiated sound field into duct modes from acoustic pressure measured by wall-flush mounted microphones. Both azimuthal and radial decompositions are computed by means of an array with optimized microphone arrangement. The optimized array ensures a low condition number of the matrix relating modal coefficients to acoustic pressure over a wide frequency band, up to the second harmonic of the blade passing frequency. Above this frequency the number of cut-on modes is comparable to the number of microphones and the modal matrix suffers from ill-conditioning. A regularization procedure is then introduced to increase the high-frequency limit of the method. Results are presented for both tonal and broadband components of the radiated sound field. The difficulty in the broadband regime is that pressure fluctuations measured by in-duct microphones are strongly affected by hydrodynamic noise associated to the turbulent boundary layer (TBL). A technique to suppress the TBL related noise is thus applied prior to the modal identification, its interest is shown on experimental data from an academic test bench.

A Comparison of Microphone Phased Array MethodsApplied to the Study of Airframe Noise in WindTunnel Testing

In this werk, various microphone phased array data processing techniques are applied to two existing datasets from aeroacoustic wind tunnel tests. The first of these is from a !arge closed-wall facility, DLRs Kryo-Kanal Koln (DNW·KKK), and is a measurement cf the high-lift noise cf a semispan model. The second is from a small-scale open-jet facility, the NASA Langley Quiet Flow Facility (QFF), and is a measurement cf a clean airfoil selfnoise. The data had been made publicly available in 2015, and were analyzed by several research groups using multiple analysis techniques. This procedure allows the assessment of the variability of individual methods across various organizational implementations, as weil as the variability cf results produced by different array analysis methods. This paper summarizes the results presented at panel sessions held at AIAA conferences in 2015 and 2016. Results show that with appropriate handling of background noise, all advanced methods can identify dominant acoustic sources for a broad range cf frequencies. Lowerlevel sources may be masked er underpredicted. lntegrated levels are more robust and in closer agreement between methods than narrowband maps for individual frequencies. Overall there is no obvious best method, though multiple methods may be used to bound expected behavior.

Acoustic beamforming through a thin plate using vibration measurements

The aim of this paper is to propose a methodology to localize acoustic sources from the measurement of airborne induced vibrations of a thin structure. Targeted applications are the identification of acoustic sources through a thin wall, with a potential filtration of the incident field, which may be of practical interest, for instance, when identifying exterior acoustic sources from the inside of a moving vehicle. Two methods are coupled here to achieve this purpose: the Force Analysis Technique (FAT), used to identify the parietal pressure field exciting the thin structure from vibration measurements, and beamforming, used for the localization of acoustic sources from the (FAT-)identified parietal pressure. The coupling of the two methods is studied first from a theoretical point of view, and an experimental proof of concept is then presented, showing the feasibility and relevance of the proposed approach.