Showing papers in "Noise Control Engineering Journal in 1997"

Development of an annoyance index for heavy-duty diesel engine noise using multivariate analysis

Abstract: Development of an annoyance index for heavy-duty diesel engine noise using multivariate analysis

Effect of protective layer on the performance of absorptive ducts

Abstract: Acoustically absorptive lining in ducts and parallel-baffle mufflers is invariably protected from the abrasive effect of grazing flow and dust by means of a protective layer in the form of highly porous thin metallic plate or cloth, or a very thin impervious layer of synthetic material like Mylar. An investigation of effects of protective layers on the acoustic performance of lined circular ducts and parallel-baffle mufflers (modeled as two-dimensional rectangular ducts) is the subject of this paper. Analytical models are presented for both cases with bulk reacting as well as locally reacting absorptive linings protected by a perforated or un-perforated layer, These simple models based upon a stationary inviscid medium are then used in a parametric study to provide noise control engineers with necessary design information, The effect of grazing flow on the impedance of a perforated protective plate is duly accounted for in the model.

An active control mechanism to enhance and parametrically study a multi-fuel, Rijke-type pulse combustor

Abstract: An active control mechanism has been developed to study the effect of an actively controlled acoustic field on the characteristic parameters of a practical combustion system. This mechanism has been used to attenuate and enhance the combustion instabilities in a multi-fuel, Rijke-tube combustor. The main parameters for this control are discussed in both theory and application on the combustion process. Performance characterization as a function of important reactor and control system parameters for the control schemes developed, using either external loudspeakers or inline fuel modulation, are presented. The effect of the control mechanisms on the combustion process is studied by investigation of the changes in flame structure and heat transfer to the reactor walls. The results show that the burner position inside the Rijke-tube combustor is an important parameter for development and implementation of an effective control mechanism. Implementation of the control mechanism on a two-phase combustion system showed that the heat transfer to the wall of the combustor increased as much as 26% for oscillating conditions with a sound-pressure level of 150 dB „re: 20 mPa... over the non-oscillating value, along with an increase of temperature and mixing. The results from the implementation of the control mechanism on gaseous and liquid fuel showed a changed flame structure. The effects of controllable pressure modes on the flame structure are significant. In general the flame height decreased with oscillation and increased sound-pressure level indicating a more compact and efficient combustion process. © 1997 Institute of Noise Control Engineering. †S0736-2501„97...00404-9‡

Acoustic analysis and parametric studies of lagged pipes

Abstract: Pipelines of noisy air or steam flow systems are often lagged on the outside with an acoustically absorptive, highly porous material which in turn is covered with a thin impervious (metallic) jacket. Prediction of transverse insertion loss (reduction in break-out noise) due to this composite lagging is the subject of the present paper. A transfer matrix for the porous absorptive layer is derived for cylindrical waves in the radial direction. Making use of an impedance model, radial velocities of the bare pipe and the outer jacket for the lagged pipe are calculated. These are then combined with respective radiation impedances to calculate insertion loss due to the composite lagging as well as the bare (unconstrained) lagging. Predicted values of insertion loss are compared with the measured values from literature. Finally results of parametric studies are presented.

Noise reduction of diesel engines with internal stiffeners

Abstract: In recent years there has been an increased interest from truck manufacturers in reducing the noise from internal combustion engines through stiffening solutions like bearing beams, ladder frames, and bedplates. This paper presents a short literature review of experiences published on the topic. Results of an experimental study of engine modifications with a bearing beam, a ladder frame, and an isolated oil sump are presented as well. The beam and the frame were designed so that they can be installed together without vibration propagating connections and to give high stiffness in the areas where they are most effective. The study was performed on a relatively light 9-liter six-cylinder diesel engine with an engine block of deep skirt type. The influence of the engine modifications was investigated through measurements of sound intensity, sound pressure, running modes, internal and external vibrations, and mobility. The results show that a ladder frame effectively reduces the noise from a deep skirt engine and that the reductions are substantially increased by isolation of the oil sump. The results also show where the components of the ladder frame are effective and that a bearing beam can be an unsuitable solution for deep skirt engines as resonances are shifted to higher frequencies that can be more effectively transferred and radiated.

Acoustic analysis and design of compliant cable-hose systems

Abstract: Starting with basic equations of mass continuity and momentum balance, incorporating viscous damping, the finite radial impedances of the cable as well as the hose, and the convective effect of mean flow, coupled wave equations have been derived for waves inside the cable and also in the annulus of a cable-hose system. These have been solved together as an eigenvalue problem, and thence a 4 × 4 transfer matrix has been derived. Applying appropriate boundary conditions, the desired 2 × 2 transfer matrices have then been derived for the extended inlet and extended outlet type of elements. Then, axial transmission loss has been calculated for a typical cable-hose configuration and has been compared with that computed by the classical equivalent sound-speed model in vogue, which has been shown here (as well as earlier for hoses) to be a special case where the tube wall is assumed to be elastic but massless. Finally, the effect of breakout noise on the design of cable-hose systems is discussed.