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A model for sound transmission through heat exchangers has been presented. The model is based on an assumed anisotropic equivalent fluid system. This type of model is valid as long as the characteristic length scale of the heat exchanger inner structure is much smaller than the wavelength. The data needed for the model can be obtained from the geometry of a given heat exchanger. To obtain the losses in the model either pressure drop data can be used or for the parallel plate type of heat exchangers, the losses can be estimated using the Kirchhoff model for sound in narrow tubes. The model gives a good agreement with the measured results A new type of micro-perforated parallel baffle type of silencer has also been proposed. The transmission loss and flow-generated sound have been investigated for seven prototypes of parallel baffle mufflers. It is found that the transmission loss can be enlarged by increasing the number of baffles, adding inner rigid walls inside the baffle and using inclined baffles. The inclined baffles mufflers with inner rigid walls have the largest transmission loss. The inclination increases the high frequency performance while the inner rigid walls give an increase for the mid frequency range. The flow-generated sound is affected by the number of baffles and the inclination angle. Increasing the number of baffles and increasing the inclination angle will increase the flow noise.