This Bureau of Mines study examines the acoustic vibrational spectra of rock slabs in the range from 0 to 5,000 hz. The power spectra of so-called drummy rocks were found to be diagnostically enhanced in the range from 200 to 1,000 hz. Comparison of the power in this band with that in a band from 3,000 to 3,500 hz provides a quantitative measure of rock stability. A prototype field instrument was designed, constructed, and field tested. This battery-powered instrument computes the power contained in the two frequency bands mentioned, compares the magnitude of these two quantities, and displays a number that is related to the detachment of the block tested. Field tests show that this instrument is capable of giving reliable measurements of block stability under conditions of noise and dust that would seriously degrade the accuracy of normal sounding techniques.