The sound pressure level of flow induced noise from a muffler may become high for particular muffler sizes and flow velocities. This phenomenon is mainly one of resonance as it is related to muffler natural frequencies. Usually, the sound pressure level is much higher in the case when predominance of tail pipe resonance rather than cavity resonance occurs. The predominance of tail pipe resonance intervenes for particular combinations of flow velocities and cavity lengths. The phenomena are considered to be strongly related to vortex ring generation at constant frequency inside the cavity by feedback mechanism from resonance noise to flow. The predominance of tail pipe resonance can be controlled by changing distributions of the flow velocity in the cavity through bellmouth shaped structure mounted at the entrance of the cavity. It can be also controlled through bellmouth shaped structure mounted at the exit of the cavity and/or by using absorbing material that is stuck to the cavity walls, reducing thus the sound pressure level of the tail pipe resonance.
In evaluating the acoustic power of the noise of a machine tool, the acoustic intensity of the measurement surface around the machine must be measured, In the case of large machine tools, the measurement surface must also be made large. Therefore, considerable space for measuring intensity is required, and the number of measurement points and measurement costs increase. If the acoustic power of the individual components of a machine tool were measured and added together, and if this total were to coincide with measurement of the acoustic power of the machine as a whole, the need to conduct single overall measurements of large objects such as machine tools would desappear. It would be sufficient to only measure the individual components of the machine and an estimate of the acoustic power of an machine entire system could be made immediately. This report will seek to validate this through the measurement of NC lathes.