Calculation Of Noise Reduction Research Articles

A time-domain simulation method to predict insertion loss of a dissipative muffler with exhaust flow

Insertion loss (IL) is a more useful acoustic index than noise reduction and transmission loss for muffler design, but it is harder to predict because the prediction of IL requires knowing the characteristics of the muffler itself and the acoustic source. A muffler with porous material and complex meanflow makes the prediction even more challenging. Although calculation of noise reduction and transmission loss with the transient computational fluid dynamics (CFD) can naturally solve the flow–acoustic interaction problem, the published CFD methods have not been adopted to evaluate the IL of the dissipative muffler with exhaust flow. In this paper, a time-domain simulation method is developed to predict the IL of a complex engine muffler with porous material. The noise reduction and load impedance of the muffler were first calculated using a three-dimensional CFD model, in which the porous material was simulated by the governing equations modified with the material porosity and flow resistivity. The IL was then predicted from the noise reduction, the load impedance, and the engine source extracted from the measured exhaust noise. This developed method explores the application of the three-dimensional CFD method in calculating noise reduction, load impedance, and insertion loss of complex dissipative mufflers with exhaust flow.

A modified time domain approach for calculation of noise reduction and acoustic impedance of intake duct system

Calculation of noise reduction and acoustic impedance of the duct system is needed for the one-port source model, which is widely used in the field of duct acoustics for extracting the source characteristics and predicting the radiating noise at the open end. The flow field in the duct influences the acoustic characteristics when the flow velocity is relatively high. Recently, the time domain method has been adopted to study the flow effects on the acoustic response of the duct mufflers. However, the existing method only focuses on the calculation of transmission loss (TL) or transfer matrix. In this paper, a modified time domain approach is proposed for the first time to calculate the noise reduction and acoustic impedance of the intake duct system. In the proposed time domain model, the frequency dependent reflection at the intake port, rather the anechoic boundary used for TL calculation, is simulated by simply adding a correction pipe at the port. This approach has effectively simplified the implementation of the traditional time domain method in dealing with frequency dependent reflection at the open end. The porous sound-absorbing material commonly used in the intake system is modelled by modified equations with the material porosity and flow resistivity. The predicted time-domain sound pressure signals are then post-processed to obtain the noise reduction and the acoustic impedance in a different way from the traditional time domain TL calculation approach. The result of validation has shown that the noise reduction and acoustic impedance predicted with the modified time domain approach agree well with the measured results and the results calculated from frequency domain methods in the cases without and with meanflow.

Evaluation of wearing course mix designs on sound absorption improvement of porous asphalt pavement

This paper describes a laboratory procedure to study the sound absorption characteristics of porous asphalt pavement wearing course mix designs, and estimate their contributions to tire-pavement noise reduction. The effects of the following mixture properties were considered: percent porosity of mixture, degree of clogging occurred, and layer thickness. Four porosity levels of porous asphalt were studied: 12%, 16%, 20% and 25%. The mix design with 20% porosity was tested for the effects of clogging and specimen thickness respectively. A dense graded mix was included to serve as a reference for comparison. The effectiveness of the sound absorption and tire-pavement noise reducing capability of the mixtures were evaluated using the laboratory data and field measurements of trial sections. An approximate noise reduction calculation based on laboratory sound absorption measurements is proposed for comparison of different mix designs. The study found that changing the percent porosity from 25% to 12% caused some changes in the frequency characteristics of sound absorption, but made negligible changes in their contributions to tire-pavement noise reduction. On the other hand, clogging of the pores in the mixtures resulted in noticeable decreases in both the sound absorption coefficients and tire-pavement noise reductions. The analysis estimates that about 23–33% of the total tire-pavement noise reduction achieved by the porous asphalt mixtures can be attributed to their sound absorption capability. Finally, changing the thickness of the asphalt mixture tested from 63mm to 200mm had no significant effect on sound absorption and tire-pavement noise reduction.

A Verification by Practical Vehicle Driving Experiment on a Calculation Method of Noise Reduction of Stone Wall and Block Fence

In Kitakyushu, the site which exceeds from the environmental standard value for over 6db (A) in the arterial road at the night exists for about 30%. Authors notice the utilization of sound insulation ability such as stone wall and block fence of the roadside as the countermeasure. However, the noise reduction depends on positional relation among sound source, top of wall and observation point. It is better of the theoretical examination which can assume various situations than the experimental examination. Authors have already reported the calculation method of noise reduction by the wall and equivalent level noise from mad traffic. Purposes of this study are as follows. A) By practical vehicle driving experiment, it is verified that the calculation method which authors have already reported can also apply to the calculation of noise reduction of stone wall and block fence. B) The possibility of the noise reduction over 6db (A) is judged by the utilization of stone wall and block fence with the height seen in the roadside.

Keeping traffic noise outdoors

Proper design of housing near major traffic routes must include appropriate noise insulation provisions. A guideline has been developed which includes both a model for calculating the outdoor sound level (Leq) from road traffic and a building design procedure. The noise prediction model takes account of many acoustically significant parameters, and incorporates several novel features. The heights of relevant noise sources (tires, truck exhaust, etc.) are allowed for in evaluating the effective source height, which is of obvious importance in assessing the effectiveness of barriers. The barrier noise reduction calculation includes a simple correction procedure to allow for sound coming around the ends of a short barrier. Parameters in the ground attenuation calculation include source height, receiver height, horizontal source‐receiver distance, and the height of any intervening barrier. The building design procedure permits selection of appropriate components to provide the required noise reduction for eac...