that are high frequency. Spectral cue A spectral cue is a monaural (single ear) cue for locating incoming sounds based on the distribution of the incoming signal. The differences in distribution (or spectrum) of the sound waves are caused by interactions of the sounds with the head and the outer ear before entering the ear canal. Principles of auditory cue grouping The auditory system uses several heuristics to make sense of incoming cues, based on the properties of auditory stimuli that usually occur in the environment. Cue grouping refers to how humans naturally perceive incoming stimuli as organized patterns, based on certain rules. Onset time If two sounds start at different times, they are likely to have originated from different sources. Sounds that occur simultaneously likely originate from the same source. Location Cues originating at the same or slowly changing positions usually have the same source. When two sounds are separated in space, the cue of location (see: sound localization) helps an individual to separate them perceptually. If a sound is moving, it will move continuously. Erratically jumping sound is unlikely to come from the same source. Similarity of timbre Timbre is the tone quality or tone character of a sound, independent of pitch. This helps us distinguish between musical instruments playing the same notes. When hearing multiple sounds, the timbre of each sound will be unchanging (regardless of pitch), and thus we can differentiate between sounds from different sources over time. Similarity of pitch Pitch refers to the frequency of the sound wave reaching us. Although a single object could produce a variety of pitches over time, it is more likely that it would produce sounds in a similar range. Erratic changes in pitch are more likely to be perceived as originating from different sources. Auditory continuity Similar to the Gestalt principle of good continuation (see: principles of grouping), sounds that change smoothly or remain constant are often produced by the same source. Sound with the same frequency, even when interrupted by other noise, is perceived as continuous. Highly variable sound that is interrupted is perceived as separate. Factors affecting auditory cue perception The precedence effect When one sound is presented for a long interval before the introduction of a second one originating from a different location, individuals will hear them as two distinct sounds, each originating from the correct location. However, when the delay between the onset of