7 - 8/22/2005 7: SYNTHESIS OF COMPLEX SOUNDS A....

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8/22/2005 7: SYNTHESIS OF COMPLEX SOUNDS A. I NTRODUCTION Of the three most basic properties of a sound, loudness, pitch, and timbre, surely the most interesting is timbre. Timbre is what makes a cello sound like a cello, a howling wolf like a howling wolf, Roger Rabbit like Roger Rabbit. All of spoken communication is based on differences in timbre. As you have heard, different types of vibration sound different, and when they are displayed on the oscilloscope they different shapes . But it turns out that studying signal shape is not the best way to study timbre. It was discovered in the 19th century that there is a most basic type of oscillation, the sine oscillation, which you have heard many times coming from your function generator. All other vibrations, all other vibrations, can be produced simply by adding together sine vibrations. Since all sounds are just combinations of sine sounds, all sounds other that sine sounds are called complex . When you make a complex sound by adding together a bunch of sine signals, you are synthesizing the complex sound. “Synthesizing” simply means “putting together.” In this lab, you will synthesize several complex sounds using a synthesizer which has a harmonic series of nine oscillators each of which has separate amplitude and phase controls. Also on the synthesizer is a summing amplifier which allows you to hear and see the result of adding all these signals together. You may be surprised by how much you can do with just nine sine sounds. B. P HASE To construct a given complex oscillation from sine components, it is important to know the relative amplitudes and frequencies of the components, and also to know their PHASES relative to each other. Phase has to do with where the oscillation begins. Below are three oscillations which are otherwise identical but differ in phase: The function in diagram c is zero when t = 0 and is called the sine function. c d e t 0
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`P108 Lab 7: page 2 The function in d is identical except that is starts 1/4 cycle later. This particular phase relationship has a special name: cosine. The cosine is sometimes said to be “90 degrees out-of-phase” with the sine function. The curve in e is a sine function starting 1/2 cycle late and is said to be “180 degrees out of phase with c ” or “exactly out-of-phase.” If you look carefully at these graphs you will realize that e can be thought of either as c out of phase or as c with a negative amplitude. In the next lab out of phase oscillations will be referred to as having negative amplitude. C. L EARNING TO U SE THE S YNTHESIZER The synthesizer consists of 10 oscillators. The first two are both labeled “1” and are the same frequency. The reason that there are two of the same frequency so you can study the result of adding two signals which have the same frequency but different phases. The remaining eight oscillators form a harmonic series above the lowest frequency oscillators (1-Left and 1-Right). Using the descriptions listed below, try out each of the controls on oscillator 1-Left until
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This note was uploaded on 01/20/2012 for the course PHYSICS 108 taught by Professor Kesmodel during the Fall '08 term at Indiana.

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7 - 8/22/2005 7: SYNTHESIS OF COMPLEX SOUNDS A....

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