During discharge of some types of bulk soids from silos vibratons occur. This phenomenon is clled "knocking". The vibrations are sometimes combined with loud honking sounds which might be a considerable nuisance for the neighborhood. Especially in industrial mixed areas this noise nuisance can even lead to a prohibition of the operation of the silo plant. These phenomena are known of bulk solids like maize and maize meal, coal or cement clinker, but also in case of hard plastic pellets. Recently even some cases with small-grained buk solids like PVC powder and TiO2 have become known.
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