Determination Of Acoustic Impedance Research Articles

Density, ultrasound velocity, acoustic impedance, reflection and absorption coefficient determination of liquids via multiple reflection method

The accuracy of density, reflection coefficient, and acoustic impedance determination via multiple reflection method was validated experimentally. The ternary system water–maltose–ethanol was used to execute a systematic, temperature dependent study over a wide range of densities and viscosities aiming an application as inline sensor in beverage industries.The validation results of the presented method and setup show root mean square errors of: 1.201E−3gcm−3 (±0.12%) density, 0.515E−3 (0.15%) reflection coefficient and 1.851E+3kgs−1m−2 (0.12%) specific acoustic impedance. The results of the diffraction corrected absorption showed an average standard deviation of only 0.12%. It was found that the absorption change shows a good correlation to concentration variations and may be useful for laboratory analysis of sufficiently pure liquids.The main part of the observed errors can be explained by the observed noise, temperature variation and the low signal resolution of 50MHz. In particular, the poor signal-to-noise ratio of the second reflector echo was found to be a main accuracy limitation. Concerning the investigation of liquids the unstable properties of the reference material PMMA, due to hygroscopicity, were identified to be an additional, unpredictable source of uncertainty. While dimensional changes can be considered by adequate methodology, the impact of the time and temperature dependent water absorption on relevant reference properties like the buffer’s sound velocity and density could not be considered and may explain part of the observed deviations.

Determination of Acoustic Impedance of Walls Based on Acoustic Field Parameter Values Measured in the Room

Acoustic eld in enclosed rooms in the low frequency range can be described by the wave model, based on solution of the wave equation. Solution to the wave equation for acoustic eld in the room can be obtained using numerical procedures, e.g. the boundary elements method. Determination of acoustic impedance of the room walls surface material, based on the knowledge of the distribution of acoustic pressure amplitudes in the enclosed space, requires application of the inverse boundary elements method and gathering a proper set of input data. The paper presents the possibilities of analysis of acoustic properties for industrial-type rooms, by using inverse methods in the low frequency range.

A direct determination of acoustical impedance of a thin layer on a thick elastic substrate by the ultrasound

A direct determination method of acoustical impedance of a linear-viscoelastic thin layer on a thick elastic substrate is proposed without the prior knowledge of other layer parameters. With the rectangular windows that start at the time zero and are rightward expanded, the successive windowing operations on the total ultrasonic reflected signals are performed. Using the transfer functions of the windowed signals and the analytical formula, the distribution characteristic of the calculated results of acoustical impedance with the different frequency intervals and the behavior of the corresponding estimated values are numerically investigated as the lengths of the sampling windows change. Acoustical impedance of the thin layer can be directly determined from the distribution characteristic of the calculated results and the behavior of the estimated values. The proposed direct determination method is independent of the parameters of the next substrates.

An ultrasonic method for determination of acoustic impedance, time of flight, and attenuation of a thin linear-viscoelastic layer

A method for the determination of acoustic impedance, time of flight, and attenuation of an immersed thin linear-viscoelastic layer on a substrate of infinite extent is proposed as the transfer functions of the normally incident ultrasonic reflected signals are given. Utilizing the derived analytical expressions related to the measured data, three parameters can be determined without considering the acoustic impedance of the substrate and giving the initial guesses and the corresponding convergence zones. The measurement relations between the layer properties and the three parameters are theoretically derived. The method stability against the experimental noises is numerically investigated.

Determination of Acoustical Impedance of Absorbing Surfaces by Two-Microphone Transfer Function Techniques: Measurement In-Situ

In the fourth and final of a series of papers on the measurement of acoustical impedance of absorbing surfaces using the two-microphone transfer function technique, the application of the technique to the measurement of a suspended ceiling and to soft grassland is described. Real materials often exhibit extended reaction, and detailed experimental data is currently difficult to obtain for such samples. But for precise sound field predictions, accurate acoustical data is required. The suspended ceiling tiles investigated are extended reaction panels and measurements are performed in-situ. Consequently, no restrictions on the sample area, edge conditions, or homogeneity are applied. Comparisons with theoretical models are presented. These investigations indicate that the panel absorber can be described by its surface impedance, and that the two-microphone transfer function technique can be used to successfully measure this characteristic in-situ. The surface impedance of soft grassland is also successfully measured using both normal and oblique incident sound. These measurements are compared with theoretical models, which are then used to obtain information concerning the structure of the ground, without invasive sampling.

Determination of Acoustical Impedance of Absorbing Surfaces by Two-Microphone Transfer Function Techniques: Effect of Absorption Mechanism

In the third of a series of papers on the measurement of acoustical impedance of absorbing surfaces using the two-microphone transfer function technique, the influence of surface absorption mechanism upon the measured impedance is described. The results from measurements of the impedance of bulk reactors are compared with values obtained from theoretical models. Materials investigated are an inhomogeneous polyurethane foam, a distributed resonance absorber, and a twin layer foam. This paper also investigates how the measurement technique behaves with samples which are bulk reacting and have surface roughness. A rough surfaced polyurethane foam sample is used. The results indicate that at frequencies for which the surface irregularities are small in comparison to the wavelength, the material can be accurately characterised by the acoustical impedance acting at an effective plane. For higher frequencies it is thought that the measuring technique becomes inaccurate due to scattering of sound by the surface roughness, and the consequent breakdown of the sound field prediction methods.

A novel method to determine the acoustic impedance of membrane material

A new method is proposed for the determination of acoustic impedance of a material present in the form of a membrane. It has been successfully tried on two different types of polymer membranes, viz. PMMA membrane of thickness 0.20 mm and PET membrane of thickness 0.02 mm. Significantly, the method does not require the knowledge of either ultrasonic velocity or thickness or density of the membrane. It is based on the measurement of magnitudes at two different frequencies in the frequency spectra of double through-transmission signals with and without the membrane.

Determination of Acoustical Impedance of Absorbing Surfaces by Two-Microphone Transfer Function Techniques: Geometrical and Measurement Errors

In the second of a series of papers on the measurement of acoustical impedance of absorbing surfaces in-situ, geometrical and measurement errors for the two microphone transfer function technique are detailed. The effects of finite sample area on the impedance measured are considered, and experimental results show that for edge effects to be negligible, the minimum dimension of a sample should be two wavelengths. The method also becomes inaccurate when the transfer function tends to unity. This is shown to occur at low frequencies, but more importantly, at higher frequencies when an integral number of half wavelengths exists between the path length differences to the microphones. The results of the error analysis show that great care should be taken in determining the precise locations of the microphones. In particular, care should be taken in the alignment of the microphones with respect to the normal to the sample, since errors due to the alignment of the probe combine features of errors not only in the microphone positions, but also in the source location and the angle of sound incidence. In comparison with typical errors in the experimental geometry, errors due to the precision of typical transfer function estimates are very small.

Determination of Acoustical Impedance of Absorbing Surfaces by Two-Microphone Transfer Function Techniques: Theoretical Models

In the first of a series of papers on the measurement of acoustical impedance of absorbing surfaces in-situ, the development of the theoretical methods for the determination of acoustic impedance by the two-microphone transfer function technique is outlined. Each method is based upon a model for propagation of sound above a plane. The impedance calculation models include an expression based on the Weyl-van der Pol equation for propagation over an impedance plane, an exact method based on specular reflection, and an approximate equation based upon specular reflection. The algorithm proposed by Vigran is also derived, and a plane wave model developed for comparison. Conditions under which the results from the different models are equivalent are discussed, along with the practical considerations in programming each model.

An Improved Method to Determine Complex Acoustic Impedance Using a Piezoelectric Resonator

On the basis of a propagating wave analysis, a simple method of acoustic impedance determination of material deposited on one face of a resonator is presented. In addition, an algorithm to determine the resonant parameters of the resonator from the measured frequency spectrum of the electrical impedance is provided. Using this method with the algorithm, the complex acoustic impedance is given by the electrical impedance measurement. Experimental determination of the complex viscosity of some liquids is presented as an example of the method, and it is found that the minimum detectable viscosity change is, at most, 1%.

Mounting and selection of microphones for two-microphone impedance measurements

The determination of acoustic impedance as specified in ASTM E-1050 requires the measurement of sound pressure at two positions along a standing wave tube. The choice of microphones and their mounting can have a significant influence on the measurement results. A variety of microphone types (i.e., phase-matched condensers, probe tube types, condenser microphones fitted with phase correctors, etc.) will be considered and the results from these various types will be compared. The mounting of the microphones (i.e., flush with the tube walls, near the middle of the tube, etc.) will also be considered.

Further studies on acoustic impedance of major bovine blood vessel walls.

Data on ultrasound velocity and specific impedance of major bovine blood vessel walls are compared to structural protein content measured by hydroxypoline assay as well as total solid tissue content of these tissues. Results indicate that acoustic velocity in these tissues is determined largely by the collagen and elastin contents of the tissues. However, acoustic impedance, which is the product of density and velocity, shows a stronger dependence on the total tissue content, suggesting that other solid tissues, in particular, muscle in blood vessel walls, may also play a significant role in the determination of acoustic impedance.

Acoustic impedance measurement using sine sweep excitation and known volume velocity technique

The direct determination of acoustic impedance requires the measurements of both pressure and volume velocity, but the latter in general cannot be measured directly. Thus, other means must be adopted to bypass this measurement problem. This paper presents two methods which utilize excitation of a system with known volume velocity. This, along with measured pressure response, provides both magnitude and phase of impedance as continuous functions of frequency. The first method uses a convertible acoustic horn driver for excitation. Its volume velocity is determined by measuring calibrated pressure in a fixed cavity at the back of the driver. The second method uses for excitation an electrodynamic shaker driven piston which carriers an accelerometer. This signal is integrated to yield input acoustic velocity information. The two-channel analog signal processing setup includes the following instruments: sine sweep oscillator, tracking filters, log converters, phase meter, etc. In order to establish the validity of the proposed experimental techniques, several example cases have been evaluated. Excellent agreement between theory and measurements are obtained for driving point and transfer impedances over the plane wave frequency regime. The advantages of the methods proposed here over some other established techniques are as follows: (1) our methods are easier to use, (2) continuous impedance spectra are obtained quickly, (3) excellent dynamic range is achieved, and (4) complications of digital systems are not involved.

The abc's of impedance audiometry

AbstractImpedance audiometry plays an important part in the otologic evaluation. To appreciate fully when it can be of help, the otologist must have a basic understanding of the impedance bridge and acoustic equivalent volume.The test battery consists of a tympanogram, the determination of acoustic impedance and eliciting the acoustic reflex.Impedance audiometry is of greatest value in young children because a voluntary response is not required. The otologist may differentiate conductive from sensori‐neural impairments. In the adult it has been of most use as part of the neuro‐otologic evaluation, to help differentiate a cochlear from a retrocochlear impairment.