Free‐Field Thresholds vs. Pressure Thresholds at Low Frequencies

1950 ◽  
Vol 22 (5) ◽  
pp. 674-674 ◽  
Author(s):  
Wayne Rudmose
Keyword(s):  
Free Field ◽  
Low Frequencies

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.

Vibration of Containing Structures by Sound in the Contained Fluid

1969 ◽  
Vol 91 (4) ◽  
pp. 939-950
Author(s):  
F. J. Fahy
Keyword(s):  
Mode Coupling ◽  
Free Field ◽  
Density Ratio ◽  
Radiation Efficiency ◽  
Low Frequencies ◽  
Simply Supported ◽  
A Value ◽  
Modal Density ◽  
Flexible Wall ◽  
Approximate Dependence

A system which consists of a rigid rectangular box with one simply supported flexible wall is analyzed by numerical and statistical methods for the internal acoustostructural mode coupling factors and the corresponding modal average radiation efficiency. It is found that for subcritical frequencies, but above a frequency which corresponds to the lower limit for maximum proximate mode coupling, the radiation efficiency is equal to that of a baffled panel radiating into a free field. At supercritical frequencies a value of radiation efficiency half that of a freely radiating panel is found. Below the limiting frequency an approximate dependence upon (f/fc)3 is found experimentally in all cases. The same type of behavior has been observed with an internally excited, partically closed cylinder. The radiation efficiency of a clamped panel is calculated to be about 3 db greater than that of the simply supported panel at low frequencies. This factor falls to zero at the critical frequency and above. Corrections to the normal statistical energy response and radiation equations are presented which take into account the fact that it may not be assumed, purely on the basis of modal density ratio considerations, that many acoustic modes couple with an individual structural mode.

scholarly journals Internally coupled middle ears enhance the range of interaural time differences heard by the chicken

2019 ◽  
Author(s):  
Christine Köppl
Keyword(s):  
Free Field ◽  
Sound Stimulation ◽  
Experimental Conditions ◽  
Low Frequencies ◽  
Interaural Time Differences ◽  
Acoustic Stimuli ◽  
Binaural Stimulation ◽  
Neurophysiological Data ◽  
Summary Statement ◽  
Millisecond Range

AbstractInteraural time differences (ITD) are one of several principle cues for localizing sounds. However, ITD are in the sub-millisecond range for most animals. Because the neural processing of such small ITDs pushes the limit of temporal resolution, the precise ITD-range for a given species and its usefulness - relative to other localization cues - was a powerful selective force in the evolution of the neural circuits involved. Birds and other non-mammals have internally coupled middle ears working as pressure-difference receivers that may significantly enhance ITD, depending on the precise properties of the interaural connection. Here, the extent of this internal coupling was investigated in chickens, specifically under the same experimental conditions as typically used in neurophysiology of ITD-coding circuits, i.e. with headphone stimulation. Cochlear microphonics (CM) were recorded simultaneously from both ears of anesthetized chickens under monaural and binaural stimulation, using pure tones from 0.1 to 3 kHz. Interaural transmission peaked at 1.5 kHz at a loss of only −5.5 dB; the mean interaural delay was 264 μs. CM amplitude strongly modulated as a function of ITD, confirming significant interaural coupling. The “ITD heard” derived from the CM phases in both ears showed enhancement, compared to the acoustic stimuli, by a factor of up to 1.8. However, the closed sound delivery systems impaired interaural transmission at low frequencies (< 1 kHz). We identify factors that need to be considered when interpreting neurophysiological data obtained under these conditions, and relating them to the natural free-field condition.Summary statementThe interaural time differences that chickens can use for sound localization are significantly greater than their small head size suggests. Closed-system sound stimulation can, however, produce complex artefacts.

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