Altec 416A sn. 391792  hammertone.   Restored surround / coating.     
Red curve, state when bought, 148 Ohm free air  impedance, resonance frequency 29,32 Hz.

Purple and blue curve, state during cleaning.
Green curve finished cleaning and thin coating.  328 Ohm free air impedance,  resonance frequency 22,92 Hz.

Hasling Audio Systems.     March 12, 2012.     Per Hasling.     Copyright.