The effect of ambient gradients on sound propagation in air filled narrow tube was investigated. The narrow tubes were taken to be nominally straight with very small pores in the walls. The solution includes the effect of a static pressure, temperature and density gradients in the presence of mean flow, which is assumed to have a uniform velocity profile. A dispersion equation is derived by assuming the spatial variations of the ambient variables can be lumped by using their average values. The complex wave number, density, speed of sound and the characteristic impedance of such media were evaluated. An application to fulfil narrow tubes with rectangular cross section and permeable walls such as; heat exchangers is developed and presented. An accurate acoustic model based on two-port matrix to calculate the transmission losses in the heat exchanger (HE) taking the ambient gradients effects into account are developed and used to study and improve the acoustic performance of HE. The developed model is validated with the measured results using normal incident and diffuse field at room temperature and a good agreement is achieved. Based on the results presented in this paper, the acoustic performance of the existing heat exchanger is bad especially at low frequencies, the operating conditions have some positive effects on its performance and its acoustic performance can be improved through channel wall impedance. Extra improvements are still needed to use it as a passive noise control element to damp the fan noise.

sound transmission, narrow permeable pipes, normal incident and diffuse field measurement, wall impedance optimization and heat exchanger