Traditionally, stimuli for experiments involving monaural phase perception consist of complex tones with two to thirty-one harmonics produced by tone generators. A number of these types of experiments are explained in the following paragraphs.

Chapin and Firestone (1934) demonstrated that listeners were able to hear differ-ences in the tone quality of stimuli containing a relatively low fundamental frequency and a single harmonic. In one experiment, the listeners were presented with a pure tone at 108 Hz with a SPL of 104 dB. They were asked to listen to the tone and then perform two actions: (i) to adjust the loudness of a second tone, which was at 216 Hz, until it could be detected in combination with the first tone, and (ii) to change the phase of the second tone, reporting any changes in tone quality or loudness. All twelve participants reported having heard changes in tone quality and loudness when adjusting the tone’s phase.

Mathes and Miller (1947) used stimuli that consisted of complex tones generated by frequency modulation (FM) and amplitude modulation (AM). The resulting tones con-tained three components, equally spaced in frequency. The center frequency of the tone generated by FM was 90° out of phase from the center frequency of the tone generated by
AM.

Licklider (1957) used complex tones containing sixteen harmonics. A complex tone was first presented to listeners with the harmonics at 90° phase. Then, another tone was presented in which the phase of one or more of the harmonics had been altered. Licklider found that listeners could detect changes in tone quality when the phase pattern was changed for the complex tones with relatively low fundamental frequencies, e.g., 250 Hz. The effect decreased with higher fundamental frequencies. Licklider also noted that changing low-numbered harmonics had less effect than changing the high-numbered.

Plomp and Steeneken (1962) used complex stimuli containing ten harmonics. The first set of experiments used stimuli with a fundamental frequency of 292.4 Hz and used harmonic amplitudes that decreased with a slope of -6 dB/octave. They used between eight and ten stimuli from three basic stimulus types: sine only, cosine only and a combi-nation of alternating sine and cosine terms. Based on these experiments they concluded that tones composed of alternating sine and cosine terms are distinguishable from sine only or cosine only terms and that this difference was the maximal possible effect of phase on timbre. The remaining experiments used two other fundamental frequencies, 146.2 Hz and 584.8 Hz in addition to 292.4 Hz, as well as four different slopes changing in incre-ments of 1 dB per octave from the original -6 dB per octave. Plomp and Steeneken further concluded that phase alterations on stimuli with lower fundamental frequencies affect the timbre more than phase alterations made on stimuli with higher fundamental frequencies.

In addition, they determined that phase and amplitude are independent factors in terms of timbre perception. Patterson (1987) used four types of stimuli, each consisting of thirty-one equal-amplitude harmonics, to experiment with monaural phase perception, including: cosine-phase (CPH), random-phase (RPH), alternating phase (APH) and monotonic-phase (MPH). CPH tones are generated by starting each component at the initial cosine phase.

RPH tones are generated by choosing a starting phase at random for each harmonic. APH tones are generated by shifting the starting phase of every other harmonic by a fixed amount (the harmonics not shifted starting at cosine-phase). And, MPH tones are gener-ated by starting the phase of each harmonic at a phase increment, which increased mono-tonically from the highest harmonic to the lowest (i.e., the lowest harmonic had the smallest increment and the highest the largest increment). Having listeners compare the RPH and CPH tones at various fundamental frequencies, Patterson (1987) confirmed the findings, mentioned above, that phase effects were directly related to the fundamental period of vibration, i.e., fundamental frequency. Specifically, phase effects can be heard when the fundamental frequency of the stimuli is below 400Hz. Otherwise, phase percep-tion corresponds to Ohm’s conjecture that the ear is phase deaf.

Patterson’s (1987) experiments are designed to reveal information about two types of phase effects brought about by stimuli with local and global phase changes. First, Patterson showed that the listener’s ability to differentiate APH and CPH tones – stimuli designed to test an individual’s sensitivity to local phase changes, i.e., within channel phase changes – was not only dependent upon the fundamental frequency but also on the intensity and bandwidth (number of harmonics) of the stimuli. And second, Patterson showed that a listener’s ability to differentiate MPH and CPH tones – stimuli designed to test individuals sensitivity to global phase changes, i.e., between channel phase changes – did not depend heavily upon the fundamental frequency and intensity. MPH and CPH tones could be differentiated when the difference between the phase offset of the minimum and maximum harmonic exceeded four to five milliseconds.

Moore and Glasberg (1989) used complex tones consisting of 20 equal-amplitude harmonics, in which the phase of a single harmonic was shifted by some amount while other harmonics started in cosine phase. Their results indicate that the phase difference limens (DLs), i.e. the threshold at which a difference cannot be noticed, increased as the fundamental increased. Changes were most audible for harmonic numbers eight to nine-teen. Moore and Glasberg also reported that listeners heard another pitch in addition to the fundamental. This pitch corresponded to the upper harmonic and could be lowered by shifting the phase 90°.