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1 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Frequency response of of Infrasonic Noise Reducers Infrasonic Noise Reducers B. ALCOVERRO & A. LE PICHON

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2 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Infrasonic measurements need wind noise reduction. This noise reduction is performed by the mean of mechanic noise reducer before sensor measurement. The performance in term of noise reduction is dependant to to size of the used device. Large sizes are well designed for high wind speeds. But a large size introduce resonance in the upper frequency band that perturb the analysis. Impedance matching of acoustical circuit reduce them but introduce time lag. Poles & zeroes of noise reducers are calculated. They are used to estimate the effects of noise reducer on detected events. Introduction

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3 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Infrasonic measurement chain characterization Noise reducerSensor Digitizer Analysis Atmospheric pressure Filters coefficients Theoretical Poles and zeroes Spectrum analyzer Infrasound calibrator Spectrum analyzer Measured Frequency & Amplitude & Phase Measured Poles and zeroes ? actually Theoretical Poles and zeroes Mean of measurement Characteristic used Design Characteristics Use of noise spectra: Sufficient to validate theory

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4 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Examples of improved noise reducer design By PTS (D r Douglas Christies) Equal response of each inlets Number of inlets is large to improve the maximal noise reduction (96 or 144 inlets). Designs with large diameters improve noise reduction under high wind speeds.

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5 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Geometrical parameters: Number of inlets, Kind of geometry Construction parameters: Length of pipes, Diameter of pipes, Volumes of manifolds, Model of sensor used, Checking geometry Electroacoustical Model Frequency responses Poles & zeros calculation Wind speed: Wind Model Delayed Impulses responses FFT -1 Convolution Inlets position Stimulus Sum FFT Time Domain Frequency Domain Noise reduction Potential Design principle

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6 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon The calculation is easy: only pipes, cavities, and simple acoustic elements. Use an accurate electro acoustical model of the entire array including the sensor. Calculation of frequency responses: H i (j ) PiPi P sum Calculation of transfer function for each inlet between P sum and P i by the mean of a matrix method: With: [Z] the matrix impedance of the circuit, {P n } the pressure at each node, {Z i } the input impedance of each inlet, {P i } the stimulus pressure at each inlet,

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7 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Models used for acoustic elements : Cavity (Vb) Acoustic compliance : Short pipe (lp, rp) Acoustic mass : + Acoustic resistance : Long pipe (l,S) Dissipative Transmission Line : Zc and are the characteristic impedance of the medium and the constant of propagation in this medium, respectively. Benade (Benade, 1968) gives the expressions for Zc and in the case of fine tubes and wide tubes, as well as for a non- dimensional parameter rv. The approximation for the broad tubes is valid if: (m 3 /Pa) (kg/m 4 ) ( ) Zatl i 1 Zatl i 2 = Pa.s

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8 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Frequency response of large noise reducers Example of 144 ports PTS Noise Reducer. Verified on IS57 noise reducers ( Hedlin & Alcoverro 2002 JASA Publication) 70 m system has wide resonance around 2.6 Hz that amplify the signal & noise in the upper frequency band. Above 3 Hz, 180° phase differences between 18 m system & 70 m system. This may introduce differences in analysis of signals issued from various noise reducers. Introduce theoretical responses of large noise reducers during analysis. Reduce or shift these resonance. Example of frequency responses of 18 m & 70 m N.R. 18 m 70 m

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9 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Reducing the resonance by… Using shorter pipes by introducing multiple sensor in noise reducer. Sensors The longer pipe is less than the half length of « single sensor » noise reducer resonant frequency is shifted toward high frequencies. Requires stable & precise sensors for perfect electrical summing Adapting impedance with capillaries at the end of longer pipes. Longer pipe (Section S) Capillary (fine pipe of radius rp) Low impedance device. The longer pipe is terminated by a fine pipe that act as an acoustic resistance: the resistance must equal the characteristic impedance of the concerned pipe. lp Example: 15 mm pipe Zc = 2.37 M Capillary : Ø2 mm & 50 mm long These atractive solution must be validated over the time: Hedlin & Alcoverro 2002 JASA Publication is air viscosity coefficient

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10 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Capillaries use: effects on frequency response 70 m system with adapted impedance has a flat amplitude response over [0.02 – 4] Hz band. The amplitude response is similar to the 18 m system. The phase response increase slowly from low frequencies to reach –90° at 2.6 Hz. This phase behaviour could introduce problems in analysis. Example of frequency responses of 18 m & 70 m PTS noise reducers

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11 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Example of time lag of 18 m & 70 m PTS noise reducers SystemDASE PTS 18 m PTS 70 m PTS 70 m (Z adapted) t (s) < 0.002< 0.003< d (m) < 0.68< 1.1< Time lag introduced by noise reducers If two different noise reducers are used in a station, the relative position accuracy is > 1 m. (Specs CTBTO : < 1m). for example, if 18m & 70m adapted noise reducers are used simultaneously, an uncertainty of 33 m is added on the relative position accuracy. Solution is to introduce theoretical responses of adapted large noise reducers for analysis.

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12 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Example of poles and zeroes for noise reducers PTS 18 m 96 pPTS 70 m 144 pPTS 70 m A 144 pDASE 18 m 32 p Scale factor: poles: i i i i zeros: i i Scale factor: poles: i i i i i i zeros: i i i i Scale factor: poles: i i zeros: i i Scale factor: poles: i i i i zeros: i i Calculated from simulation frequency responses and regression analysis. Poles & zeroes values are mainly length pipes dependant.

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13 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Perturbations introduced by noise reducers on the detection Example of array used for simulations Synthetic signal generator Impulses responses Convolution PMCC Detector Azimuth Azimuth Horizontal trace velocity Horizontal trace velocity Azimuth Frequency Noise Poles & zeroes of Noise Reduc. Process used for simulations Analyze for various frequencies & various azimuths with a plane wave crossing the array at 345 m/s.

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14 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon III II I 18 m 70 m 18 m 70 m adapted 18 m 5.6° m/s 5.7° m/s 5.9° m/s Analyse at 2 Hz, 5° & 245°

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15 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Effect on detection results Configurations:I : 4 x 18m noise reducer II : 3 x 18m + 1 x 70m noise reducer II : 3 x 18m + 1 x 70m capillary noise reducer The use of large noise reducer adapted in impedance introduces errors in azimuth and wave speed detection over the entire bandwidth. The introduction of the frequency response during analysis will correct these effects.

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16 De Bilt Infrasound Workshop – 10/2002Noise reducers frequency response Date : 28/10/02DASE / Alcoverro & Le Pichon Summary The use of large noise reducers (Diameter > 70 m) is a necessity in windy conditions to maximize the signal to noise ratio. They introduce resonance around 2 Hz that could be reduced by using matched impedance systems. These noise reducers have non negligible response and particularly in phase response (tpg = 0.1 s). Detected events uncertainty increase (~5° in azimuth and ~20 m/s in wave speed). To prevent errors in detection, the filters responses could be introduced in the measurement chain characteristics as a part of sensor.

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