The extensive literature on duration assessment generally uses tones with clear onsets and offsets. However, simplistic sounds can fail to evoke the same processes used when listening to sounds with time varying amplitude envelopes (Schutz & Gillard, 2020). To facilitate future research on the duration assessment of time varying tones, here we explore the ratio at which constant amplitude ‘flat’ and varying amplitude ‘percussive’ tones are perceived as the same duration. We will use an adaptive staircase procedure, in which flat and percussive tones are presented in pairs and participants state which tone sounded longer in duration. Each response changes the duration difference on subsequent trials and continues until responses converge around a specific point, with convergence defined as four consecutive reversals in response direction. One instance of these trials constitutes a single staircase; we will then calculate the millisecond point of subjective equality between flat and percussive tones by finding the average point of convergence between multiple, interleaved staircases. Beyond providing guidance on stimulus durations for studies comparing amplitude envelope, we aim to shed light on the process of duration assessment in sounds with time-varying amplitude envelopes.