[Auditorium revenue bonds; mortgage or lien to secure

payment.] Whenever it shall be declared necessary by the governing body, by ordinance duly adopted, as hereinafter provided, municipalities are hereby authorized to make and issue revenue bonds, payable solely out of the net income to be derived from the operation of an auditorium, and to pledge irrevocably, such income to the payment thereof, and to execute a mortgage or other lien upon the auditorium premises to secure the payment of said bonds.