fm tones,also known as warbled tones,are tones that vary in frequnecy over a range that is within few percent of the nominal frequency.This variation occurs several times per second.Under earphones,thresholds abtained with these narrowband stimuli are nearly identical to thresholds obtained with pure tones,with some exceptions in persons with steeply sloping hearing loss configurations.FM tones and narrow band noise are the preferred stimuli for sound-field thresholds measures.