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Chapter 4
Music Performed at Home


      Monaural, stereo, and surround sound reproduction has left us with an artificial experience. When you come home and hear a quartette playing music in the next room, you might ask "Who has come to visit?" When you hear the stereo playing, you could have no such question. The auditory processing of the brain is very sophisticated. For all of the many decades of trying, the human ear is cannot be fooled by any known means of reproduction.

      The size and shape of your living room does not produce the acoustics of an auditorium, or even that of another room. Besides the acoustics, there are other glaring differences between production and reproduction. When more than one instrument is conveyed through a loudspeaker, the instruments interact. Intermodulation and frequency modulation between the instruments within the speaker produces artificial sound components that have never been completely eliminated by the technology of reproduction. These sound components are extraneous, artificial and detectable.

      There is another type of distortion that is produced by speakers. When a single tone is produced by an instrument, it produces the consciously heard pitch together with many others that each have a frequency that is an integer multiple of the pitch that we consciously hear. These are called partials or overtones. The relative loudness of these overtones accounts for the tone quality difference between different natural sounds. A speaker that is not perfectly unbiased in its conveyance of these overtones will produce a natural sound, but one that has a tone that is not identical to the source sound sent to the speaker. Fortunately, speakers do this in a consistent way. and therefore can be corrected. If a specific speaker is used to play the sound of a specific instrument, the series of overtones can be adjusted at the source by computations made beforehand by a computer that creates the data that drives the speaker.

      Frequency modulation distortion occurs when a low frequency and a high frequency is being sounded out of the same speaker or other vibrating surface. This is due to the Doppler effect. consider a train whistle having a constant pitch. When the train is approaching those who hear the whistle, the frequency of the whistle is measured as higher than when the train is moving away from witnesses or a measuring device (a frequency counter). When a speaker surface is moving toward the listener, any much faster vibration of the surface that is simultaneously being sounded has its frequency inflated by the slower motion. The higher frequency sound is diminished in frequency when it is moving away from the listener. When this higher frequency sound is not an integer multiple of the lower frequency, its frequency modulation by the lower pitch does not come out even at the end of each cycle of the lower pitch being sounded by the speaker. This is an audible distortion.

      Intermodulation distortion occurs when one sound modulates the loudness of another sound in a vibrating surface. The loudness of one sound varies at the frequency of the other sound. This is caused by a non-linear response in the vibrating speaker of surface. This produces an audible distortion if the two frequncies are not related (one being an overtone of the other.

      This means that any single musical line can be produced flawlessly in a place of your choosing by a computer driving one or more speakers, resonators or diaphragms. So long as each transducer produces only one musical line, it is a live production and not a reproduction. The acoustics are those of the room where the music is being performed. There is no intermodulation distortion or frequency modulation distortion because the partials are consistent. They are not unrelated frequencies coming from more than one instrument.

      One might wonder why the interactions between overtones does not produce distortions that cannot be corrected. It is because whenever the overtones are exact integer multiples of the lowest pitch (this is the one we hear), the movement of the vibrating body (the speaker in this case) repeats a cycle of movement over and over again exactly. Any such movement is mathematically identical to some sound that has integer multiple overtones. Because the relative amplitude of overtones can be adjusted, the distortion can be eliminated.

      The system is especially valuable to composers. Music that is either very difficult or impossible to play manually can nonetheless be composed, performed, and enjoyed.