Several interesting effects can occur when two or more tones are heard simultaneously. Among these are the creation of primary and secondary beats, and combination tones of various types, especially difference tones of various orders. The next four demonstrations illustrate some of these aural effects, as well as some audible effects of , distortion external to the ears, and the effect of echoes that occur in rooms. Some related binaural phenomena, such as binaural beats, which require the use of headphones, will be demonstrated in the last section.
Beats are an important contributor to the sensation of dissonance in music, and form an invaluable perceptual tool for the tuning of musical instruments. Usually one tries to tune for minimum beats when playing several notes together, but sometimes instruments are purposely tuned to produce beats as a special effect, for instance the "Vox Celeste" organ stop or the Western "honky-tonk" piano. Combination tones. which are formed in our ears when two or more tones are presented, usually happen to coincide with partials that are already present. They can sometimes be perceived as separate tones, however. Distortion refers to any frequency component appearing in the output signal from some equipment (amplifier, loudspeaker, etc.) that was not a part of the input signal. Three types of distortion that are most significant in a high fidelity sound system are harmonic distortion, intermodulation distortion, and transient distortion. Small amounts of harmonic distortion often escape notice, since musical tones already include several harmonics. Intermodulation distortion results when tones of two or more different frequencies are reproduced at the same time, and a nonlinear characteristic of some electronic component results in the creation of sum and difference frequencies. These new frequencies are often not harmonically related to the desired tones, and thus are quite noticeable. Transient distortion occurs when some electronic component cannot respond quickly enough to a rapidly changing sound signal. Some sound equipment is designed to introduce various amounts of distortion, for instance the typical guitar amplifier (soft, intensity-dependent distortion) and the so-called "fuzz box" (hard clipping type distortion).