A hard disk drive with a voice coil motor coupled to a head. The head moves relative to a disk. The voice coil motor receives a biasing voltage and has a back emf voltage. A test circuit is coupled to the voice coil motor to determine the back emf voltage. The back emf voltage can be used to analyze low frequency vibration in the disk drive. The test circuit can be constructed so that the back emf voltage is proportional to a pair of test resistors and/or a pair of test inductors. The biasing voltage allows the back emf voltage to be measured and analyzed while the head is at any location from the ID to the OD of the disk.