Acoustic characteristics

It is of interest to note that the lengthy and complex calculation Debye made was published in the same (first) edition of the Journal of Chemical Physics as an article by Bernal and Fowler, who first suggested several seminal concepts about the structure of water that are now commonly accepted in solution theory. The velocity amplitude is measured in cm s". It is the ratio of the pressure of the ultrasonic wave to the characteristic acoustic impedance of the media. 

Characteristic) acoustic impedance of quartz Resistance ratio... 

An amplification caused by a filter is called a resonance, and in speech these resonances are known as formants. The frequencies at which resonances occur are determined solely by the position of the vocal tract they are independent of the glottis. So no matter how the harmonics are spaced, for a certain vocal tract position the resonances will always occur at the same frequencies. Different mouth shapes give rise to different patterns of formants, and in this way, the production mechanisms of height and loudness give rise to different characteristic acoustic patterns. As each vowel has a different vocal tract shape, it will have different formant pattern, and it is these that the listener uses as the main cue to vowel identity. The relationship between mouth shapes and formant patterns is complicated, and is fully examined in Chapter 11. 

Temperature can propagate with the speed of sound. When the critical point is approached more closely than a crossover value given by asymptote analysis, the characteristic time of the piston effect does not monotonicaUy decrease to zero, but tends to reach a constant value, which is the characteristic acoustic time. For CO2 contained in a 10 mm long container set at 1 K above its critical temperature, the crossover value is some mK. At these conditions, the... 

The soimd radiated by the rotorcraft into its environment is what the critical observer perceives as noise pollution and what delivers a characteristic acoustic signature for aircraft detection and classification. The tjq)ical rotor-craft sound is composed of several components with destinctive directivity and intensity, depending on the flight conditions. In general, at a distance, the main rotor noise is dominant, the high frequency emissions of the tail rotor have some relevance, and the engine noise is secondary. 

Although the presented results prove the efficiency of radioscopy, this system have certain characteristics which justify to develop and employ further process integrated testing systems. One of this characteristics is that the integration of radioscopy in industrial applications is doubtful because of reasons of radiation protection. This means, that the results from radioscopy should rather be used to fit other systems (acoustic emission analysis or temperature analysis) for industrial applications. 

To analyse and evaluate the acoustic emission signals send out from the casting part and the child mold, which means to determine informative characteristic values a conception was realized which contains as well four partial steps (fig. 7, right). 

The last part, following the method to analyse radioscopy and acoustic emission values, will be to correlate the characteristic values of the radioscopic detection of casting defects with extracted characteristic values of the acoustic emission analysis. The correlation between the time based characteristic values of acoustic emission analysis and the defect characterizing radioscopy values did not come to very satisfactory results referring the low frequency measurements. The reason can be found in the... 

At these tests was parallel to vibrations observation discovered acoustic response of balls contact with sample surface. Characteristic courses of total number of AE signal counts (in all observed levels) dependence on time are shown on Fig.6. 

At contact fatigue tests of different steel and cast iron types was used the acoustic emission technique. Processed records from the AE analyser show importance of acoustic response of tested surface continuous sensing. In graphs are obvious characteristic types of summation curves, or may be from significant changes of AE signal course identified even phases of the wear process. 

A resonance in the layered stracture occurs when echoes between two boundaries travel back and forth due to differences in acoustic impedances at the boundaries. For multi-layer structures a number of resonances can be observed depending on their geometry and condition. For each particular defect-free structure and given transducer we obtain a characteristic resonance pattern, an ultrasonic signature, which can be used as a reference. 

The aim of the experiment was to study the transmission of a signal through the weld and to measure the frequency and phase dependencies of parameters of acoustic signals from the angle of incidence and beam path within the weld volume. One of the shift of the spectral characteristics the signal is shown in Figure 4(a,b). 

The Champ-Sons model is a most effieient tool allowing quantitative predictions of the field radiated by arbitrary transducers and possibly complex interfaces. It allows one to easily define the complete set of transducer characteristics (shape of the piezoelectric element, planar or focused lens, contact or immersion, single or multi-element), the excitation pulse (possibly an experimentally measured signal), to define the characteristics of the testing configuration (geometry of the piece, transducer position relatively to the piece, characteristics of both the coupling medium and the piece), and finally to define the calculation to run (field-points position, acoustical quantity considered). 

For conventional probes, acoustic verification aims at characterizing the beam pattern, beam crossing, beam angle, sensitivity, etc., which are key characteristics in the acoustic interaction between acoustic beam and defect. For array transducers, obviously, it is also a meaning to check the acoustic capabilities of the probe. That is to valid a domain (angle beam, focus, etc.) in which the probe can operate satisfactorily. 

However, if the probe is used as linear scanning system, the acoustic beam depends on the element characteristics which are liable to change from one element to an other. Therefore, the only two alternative proposals are to characterise the aeoustie behaviour of all active sub-set of elements or to proeeed to a statistical characterization. 

In the case of the Superphenix probes we were asked to provide a 100% characterization of the probes, that meant to verify all acoustics characteristics over the 160 groups of element multiplexed around the probe. This task has required the development of an automatic acquisition and analysis system which is described below. 

The accredited laboratory must dispose of data about the state of the block surfaces, their geometrical sizes and acoustical characteristics. These characteristics are defined in periodical testing of the blocks. 

Laborelec is the Belgian laboratory for the electricity industry. It deals with measuring and studying problems arising with the production, transport and distribution of electricity to industrial and private customers. It has developed very diverse domains of expertise, such as acoustics, material characteristics, technical audits to telecommunications, vibrations monitoring, etc... 

Cross Correlation. Considerable research has been devoted to correlation techniques where a tracer is not used. In these methods, some characteristic pattern in the flow, either natural or induced, is computer-identified at some point or plane in the flow. It is detected again at a measurable time later at a position slightly downstream. The correlation signal can be electrical, optical, or acoustical. This technique is used commercially to measure paper pulp flow and pneumatically conveyed soHds. 

CH2—CI2—) —(—CF2— CFH—) (39). Ceramic crystals have a higher piezoelectric efficiency. Their high acoustic impedance compared to body tissues necessitates impedance matching layers between the piezoelectric and the tissue. These layers are similar in function to the antireflective coatings on a lens. Polymer piezoelectric materials possess a more favorable impedance relative to body tissues but have poorer performance characteristics. Newer transducer materials are piezoelectric composites containing ceramic crystals embedded in a polymer matrix (see Composite materials, polymer-MATRIX Piezoelectrics). 

Ventilation noise originates primarily from fans and the air turbulence generated inside ducts and around supply air and exhaust air terminal devices. The appearance of the noise is, of course, affected by factors such as the speed of rotation and the power of the fan, and by how the fan is stabilized or in other ways acoustically insulated. The noise level and the frequency characteristics are also largely derermined by the velocity of the air inside ducts and around terminal devices, where factors such as the dimensions and appearance of the ducts and terminal devices may play a decisive role in the appearance of the noise. 

The disturbance caused by a noise depends on its intensity (equivalent pressure level L in dB(A)), its frequency spectrum (that is its energy distribution), and the acoustic characteristics of the medium in which the listener is kicated. 

---