Sound waves measurement methods using

Acoustical properties of soybeans can be used to help distinguish between healthy and diseased soybeans. Misra et al. (1990) measured acoustic properties of soybeans by transmitting sound waves through soybeans using acoustic transmission and by an impact force method. In the impact force method, a seed is dropped on an acoustic transducer creating an impulse wave. The acoustic transmission method was slow but was able to predict the mass of individual soybeans. The impact force method showed that diseased soybeans had a narrower bandwidth than healthy soybeans. Soybeans with wrinkled surfaces and diseased and damaged soybeans were detected from healthy soybeans based on wide variations at low frequencies. 

Because of the above dependence on frequency, sound waves represent a mechanical probe for particular wave motions, namely, motion that can occur in the period of the sound wave. Viewed as one technique for making mechanical measurements on polymers, sound wave measurements using ultrasonic or Brillouin scattering probe motions of the polymer on short length scales while methods such as audio or low frequency DMA measurements probe large-scale motions. 

Ultrasonic absorption is used in the investigation of fast reactions in solution. If a system is at equilibrium and the equilibrium is disturbed in a very short time (of the order of 10"seconds) then it takes a finite time for the system to recover its equilibrium condition. This is called a relaxation process. When a system in solution is caused to relax using ultrasonics, the relaxation lime of the equilibrium can be related to the attenuation of the sound wave. Relaxation times of 10" to 10 seconds have been measured using this method and the rates of formation of many mono-, di-and tripositive metal complexes with a range of anions have been determined. 

When controlled explosives are used, the most common method is to place carefully measured charges in shallow shot holes a few inches in diameter drilled from a truck-mounted drill. These charges are then detonated to produce the sound waves needed. In less accessible areas, a portable drill may be used and, in certain environmentally sensitive areas, the charges may be mounted on stakes above the ground to minimize plant disturbance. 

Relaxation methods can be classified as either transient or stationary (Bernasconi, 1986). The former include pressure and temperature jump (p-jump and t-jump, respectively), and electric field pulse. With these methods, the equilibrium is perturbed and the relaxation time is monitored using some physical measurement such as conductivity. Examples of stationary relaxation methods are ultrasonic and certain electric field methods. Here, the reaction system is perturbed using a sound wave, which creates temperature and pressure changes or an oscillating electric field. Chemical relaxation can then be determined by analyzing absorbed energy (acous-... 

The imaginary part of the transfer function Sim has a frequency dependence shown in Fig. 10(b). It represents the amount of energy transferred to the system per period of oscillation owing to the relaxation, and the relaxation time corresponds to the frequency at which the greatest interaction occurs. Again, the effect on sound waves has been widely used. Methods making use ofS, , all involve the measurement of energy absorption they are called absorption methods. 

Because of the very small effects expected and, consequently, the immense experimental difficulties involved in measuring them, techniques are not yet well developed which make use of these properties. The bulk of the work has made use of the disturbing influence of high-frequency sound-waves on reacting solutions, and the remainder of this section will discuss briefly the application of acoustical methods to the study of chemical relaxation. 

Stationary relaxation methods include sound absorption und dlNpcrRlon and dielectric dispersion. A sound wave is used to perturb thc system (hat causes temperature and pressure alterations on an oscillating electric field. Then, chemical relaxation is measured by determining adsorbed energy (acoustical absorption or dielectric loss), or a phase lag that is dependent on the frequency of a forcing function (Bernasconi, 1986 Sparks, 1989). In this chapter, only transient relaxation methods will be discussed. 

The methods for determining the nucleation pressure were different but all of them are characterised by one common property - the samples has one volume and then the nucleation anywhere in the sample means equalizing pressure to L-V equilibrium with speed of sound. The measurements were done using Berthelot tubes, shock waves, acoustic cavitation with semi- spherical source, U-type tubes in centrifuges, etc. 

At lower ultrasonic frequencies (0.5-10 MHz), Raman-Nath diffraction of laser light by plane acoustic waves was used to measure the decay of acoustic energy inside an aerogel specimen [57], This method relies on the density variations produced by the sound wave that create a phase pattern inside the specimen. The intensity of the first order of the diffracted light beams is proportional to the sound intensity at the intercept of light and sound beam. In a log plot of sound intensity versus distance from the transducer, the slope is a measure of the attenuation. 

Besides the intrinsic value of speed of sound data, such measurements are of great importance if the interest is in equation-of-state (EOS) parameters such as bulk moduli, compressibilities, Griineisen parameters and specific-heat ratios [2]. All of these EOS parameters depend on techniques for the determination of the speed of sound, which have been developed and used in only a few laboratories. Different techniques can be foimd in literature, such as displacement interferometry [109], longitudinal run-time measurements of a sound wave fi om an impacting projectile [29], or measuring file run-time of a laser-generated stress wave across fee sample [27, 110, 111]. The latter method proved to be the most stable one and is still used to obtain speed of sound data [112]. 

The normal incidence sound absorption coefficient of a material can be determined using the method described in BS EN ISO 9614 [69] (BS 4196. Part 5 [70]). In this type of determination, a plane sound wave is made to be reflected by the sample mounted in a standing wave tube. The single-frequency wave hits the sample at normal incidence, and the amplitudes of the incident and reflected waves are measured over a series of chosen frequencies. Usually the frequencies of interest lie between 50 and. 00 Hz, although some automotive specifications will cover much wider ranges (sec Ford Engineering Specification WSK-M2D406-A). 

In Fig. 4 these experimental values based on X-ray measurement, are plotted against the cohesive energy density of the material. It might be expected that the more polar the polymer, the higher the transverse modulus. This hypothesis seems to be supported by the data. Two further values for polyethylene have not been included since they were obtained by other methods. Samuels, using the sound wave method obtained a... 

Ultrasonic methods There exist a variety of methods which are used widely in the aircraft industry. The technique measures changes in acoustic impedance caused by defects in a bonded assembly when an ultrasonic transducer is liquid-coupled to it. The high frequency sound waves are simply scattered by the presence of porosity or voids in the bondline. Interpretation of the data can be difficult, although substrate thickness is not generally a limitation. 

Doppler Ultrasound. Doppler ultrasound has been widely used to provide quaUtative measurements of the average flow velocity in large to medium-size vessels if the vessel diameter is known. These include the extracranial circulation and peripheral limb vessels. It is also used in an assessment of mapped occlusive disease of the lower extremities. The frequency used for Doppler ultrasound is typically between 1 and 15 MHz. The basis of this method is the Doppler shift, which is the observed difference in ftequency between sound waves that are transmitted from simple piezoelectric transducers and those that are received back when both transmitter and receiver are in relative motion. The average frequency shift of the Doppler spectrum is proportional to the average particulate velocity over the cross-sectional area of the sample. When us to measure blood flow, the transducers are stationary and motion is imparted by the flowing blood cells. In this event, red cell velocity V is described by the relationship... 

One of the drawbacks of the two-microphone transfer function method is that the absorption coefficient determined may not be a true representation of the material s characteristic. In the case of a porous material, such as silica aerogels, the reflected wave from the rigid wall could contribute to a rise in the absorbed energy by the material. To account for this uncertainty, the four-microphone impedance tube setup is usually used to determine the transmission loss (TL) and absorption coefficient (Feng 2013). In the absence of additional microphones downstream of the specimen, a sound meter could be used instead to measure the TL of the specimen under test. However, the sound meter picks up discrete transmitted signals at periodic interval, which could result in a mismatch with the generated signals from the source. 

Another method for measuring mechanical properties on the macroscopic scale uses the relation between mechanical properties and the propagation of acoustic waves [89]. The velocity of sound waves and also the damping thereof can be directly deduced from the elastic and viscous properties. For polymers, ultrasound can be used since the damping of the acoustic waves is decreased at high frequencies. However, this method seems not to have been applied to fuel cell-related membrane materials so far. 

ElGCtrokinetiC Phenomena. Electrokinetic motion occurs when the mobile part of the electric double layer is sheared away from the inner layer (charged surface). There are several types of electrokinetic measurements, electrophoresis, electroosmosis, streaming potential, sedimentation potential, and two electroa-coustical methods. The first four methods are described in References 35 and 62. Of these the first finds the most use in industrial practice. The electroacoustical methods involve detection of the sound waves generated when dispersed species are made to move by an imposed alternating electric field, or vice versa (63). In all of the electrokinetic measurements either the liquid is made to move across a... 

The transmission coefficient is the fraction of incident energy transmitted through a partition or a textile layer when a sound wave is incident upon it, and the transmission loss is the logarithm of the reciprocal of the transmission coefficient. Two types of transmission loss are commonly measured with a layer of textile material. The random incidence transmission loss is measured in a reverberation room using the reverberant room method, while the normal incidence sound transmission loss is measured in an impedance tube using the transfer matrix method. 

The theory of sonic-electronic level measurement is fundamentally based on a sound wave emission source from a transmitter, and a reflection of the sonic wave pulse to a receiver. Measurement of the transit time of this sound pulse and its correlation with electrical impulses provide a means for liquid level detection. Two basic designs operating on this principle use the vapor phase and the liquid phase methods. As most of the attention is currently devoted to the latter type system, this discussion deals exclusively with ultrasonic gaging over a liquid path. 

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