A researcher is interested in estimating the noise levels in decibels at area urban hospitals.?

A researcher is interested in estimating the noise levels in decibels at area urban hospitals. She wants to be 90% confident that her estimate is correct. If the standard deviation is 4.18, how large a sample is needed to get the desired information to be accurate within 0.64 decibels? Please show me the work so i can try to understand.