Measurement of acoustic impedance of sound absorbing materials in a free field at low frequencies

1987 ◽  
Vol 82 (S1) ◽  
pp. S10-S10
Author(s):  
Y. Champoux ◽  
J. F. Allard
Keyword(s):  
Acoustic Impedance ◽  
Free Field ◽  
Low Frequencies ◽  
Absorbing Materials

Measurement of acoustic impedance in a free field at low frequencies

1988 ◽  
Vol 125 (2) ◽  
pp. 313-323 ◽  
Author(s):  
Y. Champoux ◽  
J. Nicolas ◽  
J.F. Allard
Keyword(s):  
Acoustic Impedance ◽  
Free Field ◽  
Low Frequencies

Acoustic Neuromas and the Acoustic Impedance of the Ear

1973 ◽  
Vol 38 (3) ◽  
pp. 345-353 ◽  
Author(s):  
J. H. Macrae
Keyword(s):  
Tympanic Membrane ◽  
Acoustic Neuroma ◽  
Acoustic Impedance ◽  
Theoretical Calculations ◽  
Low Frequencies ◽  
Cochlear Duct ◽  
Acoustic Neuromas ◽  
Affected Side ◽  
Contralateral Ear ◽  
Conductive System

The acoustic impedance at the tympanic membrane was measured at frequencies in the range 100–1000 Hz and found to be abnormal on the affected side in four patients with acoustic neuroma. In all four the resistance was abnormally high at low frequencies on the affected side, and in three the reactance of the affected ear was raised relative to that of the contralateral ear, particularly at low frequencies. The abnormality is attributed to an increase in the input acoustic impedance of the cochlea produced by the increase in protein content of the cochlear fluids and dilatation of the cochlear duct known to occur in acoustic neuroma. This explanation is supported by theoretical calculations carried out on an electric analogue of the conductive system, and it is suggested that similar abnormalities in the acoustic impedance at the tympanic membrane might occur in other pathologies which produce abnormal mechanical conditions in the cochlea.

Acoustic Barriers Based on Sonic Crystals

2007 ◽  
Author(s):  
V. Romero-Garci´a ◽  
E. Fuster-Garcia ◽  
L. M. Garci´a-Raffi ◽  
J. V. Sa´nchez-Pe´rez
Keyword(s):  
Geometric Distribution ◽  
Free Field ◽  
Low Frequency ◽  
Optimization Methods ◽  
Band Gaps ◽  
High Symmetry ◽  
Low Frequencies ◽  
Periodic Arrays ◽  
Sonic Crystals ◽  
High Sound

Environmental noise problems become an standard topic across the years. Acoustic barriers have been purposed as a possible solution because they can act creating an acoustic attenuation zone which depends on the sound frequency, reducing the sound transmission through it. It was demonstrated that at high sound frequencies the effect of the barriers is more pronounced than at low frequencies, due to the diffraction in their edges. Sonic Crystals (SCs) are periodic arrays of scatterers embedded in a host material with strong modulation of its physical properties, that produces band gaps attenuation in frequencies related with their geometry. These frequencies are explained by the well known Bragg’s diffraction inside the crystal. SCs present different high symmetry directions, where the Bragg’s peaks appears in different frequencies ranges due to the variation of the geometry in each direction. Recently, some authors have studied the possibility to use SCs to reduce noise in free-field condition. Also, it was showed that SCs built by trees are acoustic systems that present acoustic band gaps in low frequency range due to the geometric distribution of the trees. These results led us think that these structures are a suitable device to reduce noise, this means SCs could be use as acoustic barriers. Nevertheless the technological application of these devices for controlling the noise present some problems. First, the angular dependence of the frequencies attenuated when the sound impinges over the SC. Second, the fact that the necessary space to put the SC is bigger than in the case of the traditional acoustic barriers. Finally, the necessity of some robust and long-lasting materials to use them outdoors. In this paper we show the possibility to use different materials (rigid, mixed or soft) to make scatterers, explaining their advantages or disadvantages. These materials in conjunction with some optimization methods will allow us find some solutions to the problems mentioned above. We will relate both acoustic systems, acoustic barriers and SCs, making a comparison of the main properties of each one and then, we will present the technological possibilities to design acoustic barriers based on SCs.

The Use of Optimally Shaped Piezo-electric Film Sensors in the Active Control of Free Field Structural Radiation, Part 2: Feedback Control

1996 ◽  
Vol 118 (1) ◽  
pp. 112-121 ◽  
Author(s):  
S. D. Snyder ◽  
N. Tanaka ◽  
Y. Kikushima
Keyword(s):  
Feedback Control ◽  
Control Strategies ◽  
Free Field ◽  
Low Frequency ◽  
Acoustic Power ◽  
State Equations ◽  
Subsequent Increase ◽  
Low Frequencies ◽  
Control Approach ◽  
Iir Filters

Feedback control of free field structural radiation is considered. State equations are formulated with a transformation which decouples the acoustic power error criterion. Using the resultant equations, expressed in terms of “transformed mode” states, the order of the state equations can be significantly reduced at low frequencies. Two experimental implementations of feedback control strategies using shaped piezoelectric polymer film sensors to measure the transformed system states are described. The first of these is a simple analog implementation. The second implementation is in discrete time, where an adaptive algorithm for optimizing the weights of IIR filters for practical use is described. It is shown that by using the outlined control approach significant levels of low frequency acoustic power attenuation can be obtained with no control spillover and subsequent increase in higher frequency acoustic power output.

The Use of Optimally Shaped Piezo-electric Film Sensors in the Active Control of Free Field Structural Radiation, Part 1: Feedforward Control

1995 ◽  
Vol 117 (3A) ◽  
pp. 311-322 ◽  
Author(s):  
S. D. Snyder ◽  
N. Tanaka ◽  
Y. Kikushima
Keyword(s):  
Vibration Control ◽  
Polymer Film ◽  
Active Control ◽  
Computer Simulations ◽  
Basis Function ◽  
Feedforward Control ◽  
Free Field ◽  
Acoustic Power ◽  
Low Frequencies ◽  
Piezo Electric

Feedforward active control of free field structural radiation using vibration control sources and piezo-electric polymer film error sensors is considered. The problem of what should be measured by the sensors is first examined, where it is shown that orthonormal decomposition of the equation governing the acoustic power output of the structure will define the optimal quantities, which are described using the in vacuo structural modes as a basis function. Computer simulations show that by using only a few of these quantities as error signals, practically the maximum levels of acoustic power attenuation can be obtained at low frequencies. Tonal and broadband experimental results are presented using the shaped piezo-electric polymer film sensors which demonstrate the effectiveness of the described approach.

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