Room modes cause audible artifacts in listening environments. Modal control approaches have emerged in scientific literature over the years and, often, their performance is measured by criteria that may be perceptually unfounded. Previous research has shown modal decay as a key perceptual factor in detecting modal effects. In this work, perceptual thresholds for the effects of modes as a function of modal decay have been measured in the region between 32 and 250 Hz. A test methodology has been developed to include modal interaction and temporal masking from musical events, which are important aspects in recreating an ecologically valid test regime. This method has been deployed in addition to artificial test stimuli traditionally used in psychometric studies, which provide unmasked, absolute thresholds. For artificial stimuli, thresholds decrease monotonically from 0.9 s at 32 Hz to 0.17 s at 200 Hz, with a knee at 63 Hz. For music stimuli, thresholds decrease monotonically from 0.51 s at 63 Hz to 0.12 s at 250 Hz. Perceptual thresholds are shown to be dependent on frequency and to a much lesser extent on level. The results presented here define absolute and practical thresholds, which are useful as perceptually relevant optimization targets for modal control methods.