Critical remarks on standard calculation technique of ventilation noise

When sound energy from fan propagates along airway in ventilation system, portion of the energy is reflected back from bends, branchings, duct area reductions, open ends etc. In standard calculation technique of ventilation noise is considered that the reflected energy is fully absorbed inside the ventilation system. However in fact the reflected energy is partly consumed on reverberation field formation in ducts. Some part of reverberation field energy then radiates from openings. Standard calculation technique of ventilation noise does not take into account this part of radiation from terminal devices. Radiation from duct open end, caused by both travelling waves from sound source and reverberation field in duct is considered in the paper. It is shown by the example that standard technique for calculation of sound power radiated from terminal devices may lead to underestimated results up to 10 dB and even more.