Mechanical sound generators

Figure 13.5 Principles of sound generation in mechanical sound generators (a) Galton whistle, (b) Hartman whistle, (c) wedge resonator, (d) dynamic siren, (e) modified Hartman whistle, and (f) Branson sound generator.

The sound generation mechanism involves the transmission of acoustic energy in space. Therefore, the power of a sound source is the energy emitted in time units and is measured in W. 

What then makes one voice sound more natural than another Again, this is a question that we are not in a position to answer fully, but we can provide a sketch of the factors involved. Firstly, any system that produces obviously non-human artefacts in the speech will readily be judged as unnatural. It is all too easy to generate speech with pops, clicks, buzzes and an endless variety of other mechanical sounds. Even speech that is devoid of any of these error types of sounds can readily sound unnatural. While speech exhibits considerable variability in the realisation of a particular phoneme, this is controlled by... 

Acoustic emission is the sound waves produced when a material undergoes stress (internal change), as a result of an external force. Acoustic emission is a phenomenon occurring in, for instance, mechanical loading generating sources of elastic waves. 

The turbulence mechanism for the KorotkofF sound has been excluded as a valid theory of KorotkofF by Drzewiecki et al. (1989), on the basis of the wide variety of observations available for the KorotkofF sounds. While a turbulence theory does not fit the KorotkofF sounds it maybe useful in explaining some sounds of vascular disease. Sound generation by turbulence maybe a popular theory for vascular sound because it is a very efficient sound-generating mechanism. For example, turbulence provides the sound source for the Woodwind type of musical instruments. 

Hence, it is not conclusive with regard to the specific mechanism of sound generation for the diseased artery. Moreover, beyond detection, the acoustics features of the stenotic sound can further be employed to classify the age and physical condition of the stenosis. In this chapter we will examine some of the theories for the generation of vascular sound in the stenotic artery. 

The flow dynamics of flexible vascular stenosis differ greatly from the rigid concentric stenosis as the normal comphant free wall section of artery can respond to local blood pressure (Siebes and D Argennio, 1989). This permits blood pressure to affect the lumen area, resulting in a pressure-dependent flow resistance, so that the free wall may lead to blood vessel constriction for low lumen pressure conditions. The condition where a compliant vessel free wall reduces the lumen area to increase flow resistance has been studied extensively in collapsible vessels such as the veins and results in a phenomenon referred to as flow limitation. Flow limitation is also known as the Starhng resistance effect (Drzewiecki et al., 1997). It is proposed here that flow limitation of a diseased artery with a flexible free wall leads to vascular sound generation as in the case of the Korotkoff sounds (Drzewiecki et al., 1989). The free wall mechanism of vascular sound is distinctly different from the turbulence theory and results in a characteristically different sound. 

This type of sound-generation mechanism was described earliest by Drzewiecki et al (1989) where the Korotkoff sounds of blood pressure determination were modeled by a computational fluid dynamic model. In this model, the brachial artery was represented by a similar free wall artery structure that was found to be an efficient sound-generating process. 

Clinical usefulness ofvascular sound information for disease diagnosis has been reviewed. Theories and experiments in the analysis of the production and origin of vascular sounds in a blood vessel are several and it is found that the plaque dome structure associated vibrating mechanism favors such sound generation in diseased arteries. Coupled with the previously established Korotkoff sound theory, a new approach to a more accurate diagnosis of carotid artery disease via auscultation is plausible. 

To quiet a noise-generating mechanism, the first impulse is often to enclose it. Sometimes an enclosure is in fact the best solution, but not always. If it can be determined what is causing the noise, appropriate action can be taken to be more specific and provide a cost-effective fix. In some cases the problem is caused by a component such as a stepper motor or gear set that does not produce objectionable noise by itself. The trouble typically develops because a small noise is transmitted to a metal frame or cabinet that then serves to amplify the sound using a plastic cabinet can isolate the noise problem. 

The harmonic oscillator is an important system in the study of physical phenomena in both classical and quantum mechanics. Classically, the harmonic oscillator describes the mechanical behavior of a spring and, by analogy, other phenomena such as the oscillations of charge flow in an electric circuit, the vibrations of sound-wave and light-wave generators, and oscillatory chemical reactions. The quantum-mechanical treatment of the harmonic oscillator may be applied to the vibrations of molecular bonds and has many other applications in quantum physics and held theory. 

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