silence followed by a gradual return of tinnitus to its original level. The effect lasts for 5 minutes or less in 90% of persons but in rare cases, an hour or more. The period of complete silence is usually seconds to about a minute. If the trigger sound is repeated at intervals, tinnitus can be suppressed as long as the repetition lasts. Residual inhibition The residual inhibition (RI) effect, first noted in 1903, is the capacity of a sound of the right frequency and intensity to briefly attenuate or suppress tinnitus. To distinguish the tailored sound used for inhibition from the broadband undifferentiated sound employed in tinnitus maskers, it is usually referred to as the 'trigger' or 'quieting' sound. The trigger sound is closely matched to the pitch and spectrum of the tinnitus, and as loud or somewhat louder than the perceived loudness of the tinnitus. The trigger sound is applied to the ear or ears with tinnitus and typically lasts a minute or two. Not everyone can experience RI; for those who do not, tinnitus masking is likely to be ineffective. Acoustic qualities Two qualities of a tinnitus masker need to be taken into account: how effective the masking is, and how intrusive or distracting the generated sound itself is. The masking sound may have various temporal, spectral and amplitudinal characteristics. In some cases, the sound is tuned to the pitch, loudness and other qualities of the tinnitus. However, frequencies widely divergent from the tinnitus frequency may also mask the tinnitus, if they are of sufficient loudness. The minimum energy (or loudness) mask is usually one closely matched to the central frequency and bandwidth of the tinnitus. But it has also been discovered that a tinnitus mask close to the frequency of the tinnitus can be ineffective, while a masker with a different frequency or bandwidth effectively masks the tinnitus. This indicates different origins of tinnitus. Tinnitus maskers may use music or natural sounds, wide band white or pink noise, narrow band white noise, a notched soundfield, frequency or amplitude modulated sound, intermittent pulsed sound, or other patterned sound. Temporally patterned sound may be more effective than white noise or background music in masking tinnitus. Studies have found that bandpass maskers are less annoying than pure tone maskers, that the annoyance rating of bandpass maskers is proportional to bandwidth, that intermittent maskers were more annoying than continuous ones, and