Sound Absorption Spectra Research Articles

Laboratory Evaluation of Sound Absorption Characteristics of Pervious Concrete Pavement Materials

This paper describes a laboratory study on the sound absorption characteristics of clogged and unclogged pervious concrete (PC) pavement materials compared with those of porous asphalt (PA) mixtures and those of conventional dense-graded asphalt and portland cement concrete pavement materials. Examined in this study were the effects of the mixtures’ initial porosity on their sound absorption characteristics and how these characteristics were affected by subsequent clogging of the mixtures. Four porosity levels of PC and PA were studied: 10%, 15%, 20%, and 25%. The mixtures with 20% porosity were tested for the effects of clogging. The test results showed that the shapes of sound absorption spectra of PC and PA materials were similar displayed high sound absorption values within the frequency range of 250 to 1,000 Hz. However, for all four porosity levels studied, the PC materials produced about 0.1 or 20% higher sound absorption coefficient values throughout the entire measured frequency range from 100 to 2,500 Hz. The same order of magnitude of differences in the sound absorption values between PC and PA was also observed in their clogged states. The finding that PC exhibits a better sound absorption capability than PA is of practical significance. The results of this study also demonstrate that laboratory testing of sound absorption can be employed to provide a useful indicative assessment of the noise reduction properties of porous pavement materials without the need for full-scale field testing.

Calculating vibrational mode contributions to sound absorption in excitable gas mixtures by decomposing multi-relaxation absorption spectroscopy

Sound relaxational absorption spectroscopy of excitable gas mixtures is potentially applied for gas composition detection. The relaxation of vibrational modes of gas molecules determines the sound relaxational absorption. However, to our knowledge, the contribution of each vibrational mode available in gas mixtures to sound multi-relaxation absorption has not been calculated in existing literature. In this paper, based on the decoupled expression of the effective isochoric molar heat for a gas mixture, a sound multi-relaxation absorption spectrum is decomposed into the sum of single-relaxation spectra. From this decomposable characteristic, the contribution of each vibrational mode available in the gaseous medium to the multi-relaxation absorption is obtained at room temperature. For various gas compositions including carbon dioxide, methane, nitrogen etc., the calculated contributions of vibrational modes are verified by the comparison with experiment data. We prove the following views with quantifiable outcomes that the primary molecular relaxation process associated with the lowest mode plays the major role in acoustic relaxational absorption of gas mixtures; the mode with lower vibrational frequency provides higher contribution to the primary relaxation process. This work could provide a deeper insight into the relationship between the sound relaxational absorption spectroscopy and gas molecules.

Influence of the Microstructure in the Acoustical Performance of Consolidated Lightweight Granular Materials

This paper reports an investigation of the acoustical performance of consolidated lightweight granular materials, made of perlite, arlite and vermiculite mixed with polyurethane resin. These materials could help combat the noise control and replace some products that lack the sufficient structural strength and require expensive protection when exposed to the elements. The advantages of using these composites in acoustic applications, like noise barriers, compared with commercial materials, are their very light mass combined with a relatively high structural strength, high physical–chemical stability and low cost. Besides, these materials could be considered more acceptable, from the health point of view, and better suited to operate in an aggressive environment. Acoustical properties were assessed according to ISO 10534–2: 1998. The microstructure, through non-acoustical properties (porosity, tortuosity and flow resistivity), and the acoustical performance (sound absorption spectrum and normalized acoustic impedance) were studied, showing the influence of the non-acoustical properties in the acoustical performance.

A theory for monitoring combustion of natural gas based on the maximum point in sound absorption spectrum

Compositions of natural gases are different between each other because of different sources, resulting in the fact that natural gases have different energy contents and monetary value. This paper presents a theory to monitor the combustion properties of natural gas by using the acoustic relaxation phenomenon in which the maximum point of acoustic spectrum varies with gas composition. The theory is developed from the frequency-dependent sound absorption spectrum which can be reconstructed from its maximum point synthesized in the acoustic measurements at two frequencies. The theory uses the relation between the two values of the maximum point (i.e. the relaxation frequency and the maximum relaxational absorption) and gas composition to quantitatively monitor the gas. Moreover, the theory has the advantages of avoiding the detection of the gas density and the variation of the ambient pressure, which is necessary in the traditional way of measuring the maximum point of sound absorption spectrum.

Decoupling multimode vibrational relaxations in multi-component gas mixtures: Analysis of sound relaxational absorption spectra

Decoupling the complicated vibrational—vibrational (V—V) coupling of a multimode vibrational relaxation remains a challenge for analyzing the sound relaxational absorption in multi-component gas mixtures. In our previous work [Acta Phys. Sin. 61 174301 (2012)], an analytical model to predict the sound absorption from vibrational relaxation in a gas medium is proposed. In this paper, we develop the model to decouple the V—V coupled energy to each vibrational—translational deexcitation path, and analyze how the multimode relaxations form the peaks of sound absorption spectra in gas mixtures. We prove that a multimode relaxation is the sum of its decoupled single-relaxation processes, and only the decoupled process with a significant isochoric-molar-heat can be observed as an absorption peak. The decoupling model clarifies the essential processes behind the peaks in spectra arising from the multimode relaxations in multi-component gas mixtures. The simulation validates the proposed decoupling model.

A novel acoustic sensor approach to classify seeds based on sound absorption spectra.

A non-destructive and novel in situ acoustic sensor approach based on the sound absorption spectra was developed for identifying and classifying different seed types. The absorption coefficient spectra were determined by using the impedance tube measurement method. Subsequently, a multivariate statistical analysis, i.e., principal component analysis (PCA), was performed as a way to generate a classification of the seeds based on the soft independent modelling of class analogy (SIMCA) method. The results show that the sound absorption coefficient spectra of different seed types present characteristic patterns which are highly dependent on seed size and shape. In general, seed particle size and sphericity were inversely related with the absorption coefficient. PCA presented reliable grouping capabilities within the diverse seed types, since the 95% of the total spectral variance was described by the first two principal components. Furthermore, the SIMCA classification model based on the absorption spectra achieved optimal results as 100% of the evaluation samples were correctly classified. This study contains the initial structuring of an innovative method that will present new possibilities in agriculture and industry for classifying and determining physical properties of seeds and other materials.

Random incident absorber approximation

A method for measuring a first angular specific sound absorption spectrum of a first sound absorbing sample. The first sound absorbing sample has a predetermined random incidence sound absorption spectrum. The angular specific sound absorption spectrum is determined from a sound wave incident on a surface of the first sound absorbing sample with a predetermined angle of incidence that is greater than zero degrees from a plane parallel to the surface of the first surface. A first weighting factor that provides a fit of the angle specific sound absorption spectrum of the first sound absorbing sample to the predetermined random sound absorption spectrum is then determined. Next, a second angular specific sound absorption spectrum of a second sound absorbing sample at the predetermined angle of incidence is determined. The weighting factor is then used to determine the random specific sound absorption spectrum from the angle absorption spectrum of the second sound absorbing sample.

Bottom-up approach for microstructure optimization of sound absorbing materials

Results from a numerical study examining micro-/macrorelations linking local geometry parameters to sound absorption properties are presented. For a hexagonal structure of solid fibers, the porosity phi, the thermal characteristic length Lambda('), the static viscous permeability k(0), the tortuosity alpha(infinity), the viscous characteristic length Lambda, and the sound absorption coefficient are computed. Numerical solutions of the steady Stokes and electrical equations are employed to provide k(0), alpha(infinity), and Lambda. Hybrid estimates based on direct numerical evaluation of phi, Lambda('), k(0), alpha(infinity), Lambda, and the analytical model derived by Johnson, Allard, and Champoux are used to relate varying (i) throat size, (ii) pore size, and (iii) fibers' cross-section shapes to the sound absorption spectrum. The result of this paper tends to demonstrate the important effect of throat size in the sound absorption level, cell size in the sound absorption frequency selectivity, and fibers' cross-section shape in the porous material weight reduction. In a hexagonal porous structure with solid fibers, the sound absorption level will tend to be maximized with a 48+/-10 microm throat size corresponding to an intermediate resistivity, a 13+/-8 microm fiber radius associated with relatively small interfiber distances, and convex triangular cross-section shape fibers allowing weight reduction.

Influence of sound absorption on close proximity noise of porous pavement

The main objective of this research is to correlate sound absorption of a new bituminous porous pavement (air void content 21%) with the close proximity noise. Different core samples were taken from a porous road surface. The normal incidence sound absorption spectra were measured for these samples using the two-microphone impedance tube. Noise levels close to the tire/pavement contact patch were measured continuously in a geo-referenced way, when the test vehicle was rolling. The emission properties of the porous surface as a function of vehicles speed are analyzed in the acoustic frequency range showing different behaviors for the relationships between noise levels and speed. An abrupt variation in the relationship between the coefficient B and frequencies below 1 kHz was observed (Lcp=A+Ḃlog(V)). This feature could be highly influenced by sound absorption mechanism, particularly noticeable for a non-clogged porous surface with a high content of air voids.

Evaluating the bonding condition of NASA spray on foam insulation (SOFI) using audio frequency sound absorption measurements

The bonding condition of the spray on foam insulation (SOFI) used to insulate the external tank of the NASA space shuttle can be found by using the audio frequency sound absorption coefficient. The ASTM E1050 standard method for sound absorption measurements was used with an open-ended 1-in-diam cast acrylic impedance tube sealed to the SOFI with closed cell PVC foam. Two artificially disbonded locations, measuring 1.0 in. by 5.5 in. by 0.0625 in. and 2.0 in. by 8.0 in. by 0.0625 in., were detected by peaks in the sound absorption coefficient spectrum. The peaks in the sound absorption spectrum between 1000 and 4000 Hz were 25% to 50% higher over disbonded areas when compared to bonded locations. The maximum and minimum sound absorption levels for the foam ranged between approximately 0.1 and 0.3. The entire sample was scanned using the sound absorption peaks as indicators. Samples of 2-in.-thick polystyrene foam were used with different sized defects at different locations in the foam to relate defect size and location to peaks in absorption coeffi-cient spectrum. [Work supported by NASA under Award No. NAG102098.]

Propagation of zero sound in superfluid3He-B under magnetic field

We present a theory of zero sound propagation in super fluid3He-B with an order parameter strongly distorted by magnetic field. A general formula of the dispersion relation for arbitrary magnetic field in the collisionless regime is derived within the weak coupling theory and under the assumption of particle-hole symmetry. The Landau parameters are taken into account up toF 2 andF 0 . Numerical results for the sound velocity and absorption spectrum are presented. We show that the collective modeJ = 1,J z = 0 yields a sizable peak in the sound absorption spectrum under weak but finite magnetic field. The quasi-particle excitations under magnetic field also exhibit cusp-like fine structures in the absorption spectrum. We show that the anomalies discovered by Ling et al. and Sounders et al. near the pair breaking edge in theq ⊥H geometry consist of theJ = 1,J z = 0 collective mode and the pair breaking cusps in theJ z = 0, ±2 channels.

Evidence of critical concentration fluctuation contributions to the ultrasonic absorption spectrum of 2,6-dimethylpyridine/water

The sound absorption coefficient has been measured between 0.2 and 400 MHz as a function of frequency for pure 2,6-di-methylpyridine (2,6-DMP), for a dilute aqueous 2,6-DMP solution, and for a 2,6-DMP/water mixture of critical composition at various temperatures near the critical. The sound absorption spectra of pure 2,6-DMP and of the dilute solution do not show contributions in addition to the classical ones. The spectra of the critical mixture exhibit excess absorption over the whole frequency band. The measured data are consistent with the Bhattacharjee-Ferrell theory of critical concentration fluctuations.

Sound wave propagation in fluids with coupled chemical reactions

A hydroacoustic theory is presented which accounts for sound absorption and dispersion in a multicomponent mixture of reacting fluids. A set of first-order acoustic equations without diffusion is assumed for the case where several coupled reactions can occur simultaneously. General results are obtained in the form of a biquadratic characteristic equation (called the Kirchhoff–Langevin equation) for the complex propagation variable χ=−(α+iω/c) in which α is the attenuation coefficient, c is the phase speed of the progressive wave, and ω is the angular frequency. Computer simulations of sound absorption spectra have been made for three different chemical systems, each comprised of two-step chemical reactions using physicochemical data available in the literature. The chemical systems studied include (1) water–dioxane, (2) aqueous solutions of glycine, and (3) cobalt polyphosphate mixtures. Explicit comparisons are made between the exact biquadratic characteristic solution and the approximate equation (sometimes referred to as a Debye equation), previously applied to interpret the experimental data for the chemical reaction contribution to the absorption versus frequency. The relative chemical reaction and classical viscothermal contributions to the sound absorption are also presented. Several discrepancies are discussed that can arise when estimating thermodynamic data (chemical reaction heats or volume changes) for multistep chemical reaction systems when making dilute solution or constant density assumptions.

Hydroacoustic analysis of multiple relaxation kinetics

This investigation presents a hydroacoustic theory which accounts for sound absorption and dispersion in a multicomponent mixture of reacting fluids (assuming a set of first‐order acoustic equations without diffusion) such that several coupled reactions can occur simultaneously. General results are obtained in the form of a biquadratic characteristic equation (called the Kirchhoff‐ Langevin equation) for the complex propagation variable χ = (α + ωi/c) in which α is the attenuation coefficient, c is the phase speed of the progressive wave, and ω is the angular frequency. Computer simulations of sound absorption spectra have been made for three different chemical systems each comprised of two‐step chemical reactions using physico‐chemical parameter data available in the literature. The chemical systems studied include (1) water‐dioxane, (2) glycine‐water, and (3) cobalt polyphosphate water mixtures. Explicit comparisons are made between the biquadratic characteristic solution and the approximate equation (sometimes referred to as a Debye equation) previously applied to interpret the experimental data for the total chemical absorption versus frequency. The relative chemical reaction and classical viscothermal contributions to the sound absorption are also presented. Several discrepancies that can arise when estimating thermodynamic data (i.e., chemical reaction heats or volume changes) for multistep chemical reaction systems when making dilute solution or constant density assumptions are discussed.

Simulation of sound absorption spectra of seawater systems

Sound-absorption spectra of three chemical systems (I: CO2/H2O, II: Mg++/CO2/H2O, III: Mg++/SO4=/CO2/H2O) are simulated on a computer using available, and estimated kinetic and thermodynamic data. System III is used as a simplified model for seawater and is described by a nine-state mechanism. Using the chemical-relaxation theory described, six nonzero relaxation times are obtained for III. The relaxation frequency and absorption amplitude of two of the times agree reasonably well with oceanic data, which indicate excess sound absorption in the 1–5- and 150-kHz frequency regions.

Ultrasonic absorption by aqueous magnesium and calcium acetate solutions

Calcium and magnesium acetate complexes are formed in aqueous solution with identical free energies but with totally different heats and entropies of complexation. These differences have been rationalized in terms of different structural forms of the complexes. The sound absorption spectra have been measured over the frequency range 15–170 MHz and the data used to investigate the validity of this premise. Use has been made of the similarities of the systems to interpret the differences in the amplitudes of absorption in terms of the entropies of complex formation via the Maxwell relation (∂S/∂V)T=(∂P/∂T)V. The results indicate that both complexes are of the solvent-separated or outer ion-pair type and that the absolute magnitude of the entropy change is consequently not a useful criterion for distinguishing between inner- or outer-sphere as the predominant complex in solution.