Noise control in a small household appliance by means of experimental analysis of vibration paths

Automatic machines for the preparation of coffee from the bean to the cup have now a wide diffusion among coffee amateurs. Since these machines are usually installed at home and in offices, low noise emission is an important design requirement. These small appliances are complex dynamic systems from the point of view of noise and vibration control, because they include some electric motors, a transmission, a grinder, some mechanisms, a suspension system, cavities and radiating panels. In the last decades various methods for the experimental study of vibrations have been developed in the aerospace and automotive fields, like transfer path analysis and modal testing. The first section of this paper describes the potentialities and the limits of these methods, when a small appliance (maximum size 300 mm) has to be tested in an industrial environment. Then a specific method for the study of the flux of energy from the motor-grinder group to the acoustic field is presented. It is based on the physical variation in the stiffness of the resilient connections, which is carried out with the design of experiments approach. The noise spectra measured outside the machine and the vibration spectra measured in critical points inside the machine are analyzed with correlation techniques. Results show that the resilient mounts are effective, but one transmission path influences noise transmission more than the others, mainly owing to the asymmetrical construction of the motor-grinder group and of the appliance structure. Suggestions for improved noise control are given.