that control over the center frequency and bandwidth was important in minimizing the annoyance of maskers. Limitations In addition to the spectrum of masking sound, the amplitude determines the degree of masking achieved. In some cases, the required masking amplitude is so high as to cause discomfort to the user of the sound generator, and the tinnitus is therefore considered unmaskable. The frequency and other characteristics of tinnitus can vary over time, and the shifts can be temporary or permanent. They may also be cyclical. Therefore, the optimal tinnitus mask may need to be adjusted periodically, within days, hours, or even minutes. So a fixed recording (i.e. CD, DVD or audio file) of a tinnitus mask is not a very efficacious application. There are also safety issues associated with use of tinnitus maskers: the "safe" level of continuous sound depends on the duration of the sound. For maskers worn continuously, or most of the day, a safe level is no more than 70 dB, in accordance with EPA maximum safe exposure level for 24-hour environmental sound. Maskers louder than 45 dB or so (the ambient background level of an average home living area) may start to interfere with understanding conversation (about 60 dB). Masking of high frequency tinnitus (>6,000 Hz) is difficult because of the resonance characteristics of the human ear as well as high frequency limitations of in-ear devices. The ear has a resonance frequency (highest frequency response) at 3,000 Hz and rolls off at a very steep 26 dB/octave on the high frequency side. Unfortunately, most tinnitus is associated with hearing loss, and most hearing loss and tinnitus is high frequency. Most maskers are limited to about 6,000 Hz of effective output. Higher frequency output results in increasing distortion, and may cause rejection of the device. Minimum masking level The minimum masking level (MML) is the minimum amplitude of narrowband noise centered on the tinnitus pitch (or frequency range) necessary to completely mask the tinnitus. Devices Tinnitus maskers are commonly used by tinnitus sufferers when trying to sleep or relax, as it is within these quiet environments that the tinnitus is at its most noticeable. They commonly take the form of CD or MP3 recordings, or bedside noise generators. When used in conjunction with a sound pillow (which contains small embedded speakers) they can mask a person's tinnitus sounds without disturbing his/her partner. These masker devices use soothing