Technical Essays for Electronic Music Peter Elsea Fall 2002 Introduction
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Technical Essays For Electronic Music Peter Elsea Fall 2002 Introduction....................................................................................................................... 2 The Propagation Of Sound .............................................................................................. 3 Acoustics For Music...................................................................................................... 10 The Numbers (and Initials) of Acoustics ...................................................................... 16 Hearing And Perception................................................................................................. 23 Taking The Waveform Apart ........................................................................................ 29 Some Basic Electronics................................................................................................. 34 Decibels And Dynamic Range ................................................................................... 39 Analog Sound Processors ............................................................................................... 43 The Analog Synthesizer............................................................................................... 50 Sampled Sound Processors............................................................................................. 56 An Overview of Computer Music.................................................................................. 62 The Mathematics Of Electronic Music ......................................................................... 71 Simple Harmonic Motion............................................................................................ 81 A Look At Un-Electronic Musical Instruments ........................................................... 85 Acoustic Treatment For Home Studios......................................................................... 90 Making Connections .................................................................................................. 105 Converting Sound Into Numbers ............................................................................ 116 Another Way Of Looking At Audio............................................................................ 122 Making Waves From Numbers ................................................................................ 126 MIDI .............................................................................................................................. 131 Analog Recording Of Sound ........................................................................................ 139 A Primer On Microphones........................................................................................ 145 Notes on Fourier Transforms ...................................................................................... 159 Putting the FFT to work............................................................................................... 164 A Glossary Of Technical Terms Used In Electronic Music ...................................... 166 Some Good Books For Technical Background .......................................................... 179 Music 80c Technical Writings 2000 These essays cover basic information that every electronic musician should know. The toys are getting easier and easier to use these days, so practically anyone can get some sounds going and call themselves an electronic musician, but if you want to go beyond the factory presets and software demos, you need to know what's going on under the hood. To Quote Albert Einstein: "Everything should be as simple as possible, but no simpler." You can find slightly revised versions of these essays and some new ones on the electronic music web site http://arts.ucsc.edu/ems/music TE1:SOUND The Propagation Of Sound The notion of sound is rather remarkable. Something happens there, and we know it here, even if we are looking the other way, not paying attention, or even asleep. The fact that some sounds can produce complex physical and emotional effects beyond the mere signaling of events is just short of astounding. These notes will perhaps remove some of the mystery associated with sound and hearing, but probably none of the wonder. Sound, in the usual sense, is a disturbance of the atmosphere that human beings can hear. Such disturbances are produced by practically everything that moves, especially if it moves quickly or in a rapid and repetitive manner. For the purposes of discussion, I will limit possible sound generators to two: one that moves once to produce a sound IMPULSE (such as a cap gun), and one that moves the air repetitively to produce a TONE (such as a penny whistle). You should be aware that the air is made up of molecules just like everything else. Most of the characteristics we expect of air are a result of the fact that these particular molecules are very light and are in extremely rapid but disorganized motion. This motion spreads the molecules out evenly, so that any part of an enclosed space has just as many molecules as any other. If a little extra volume were to be suddenly added to the enclosed space (such as by moving a piston into a box), the molecules nearest the new volume would move into the recently created void, and all the others would move a little farther apart to keep the distribution even. Because the motion of the molecules is so disorganized, this filling of the void takes more time than you might think, and the redistribution of the rest of the air molecules in the room takes even longer. If the room were ten feet across, the whole process might take 1/100 of a second or so. If the piston were to move out suddenly, the volume of the room would be reduced and the reverse of the process would take place, again taking a hundredth of a second til everything was settled down. The interesting thing you would discover if you repeated this experiment is that no matter how far or how quickly the piston is moved, it always takes the same time for the molecules to even out. In a larger room (or outdoors for that matter), times measured until the piston's motion affects a particular molecule vary directly as the distance from that molecule to the piston, so you could say the disturbance caused by the piston moves at a constant rate through the air. If you could make 3 TE1:SOUND the disturbance visible somehow, you would see it spreading spherically from the piston, like an expanding balloon. Because the process is so similar to what happens when you drop an apple into a bucket, I call the disturbance line the WAVEFRONT. Fig.1 Propagation of pressure disturbance in air Under normal conditions a wavefront moves through air at 1130 feet/second. This is the SPEED OF SOUND. I mentioned that air gets its properties from the weight and speed of its molecules. Molecules of different weight (other gases or any material for that matter) or different velocity (that translates into temperature) will exhibit different speeds of sound. Motion of the sound source, the listener, or the air itself has no effect on the speed of sound, nor does the pressure of the air. A question comes to mind at this point: how far can a wavefront go? You can think of the piston as having imparted a certain-sized chunk of energy to the air. This energy is manifest as a difference in pressure between the affected and not-yet-affected areas of air, which is to say across the wavefront. The front will continue moving until the energy is used up. As the front expands spherically, the energy is spread over a larger and larger area, in a way suggested by the relationship between the radius and area of a sphere, A=4pR2. The total energy stays the same, the area expands, so energy per area unit decreases proportionally to the square of the distance from the source until there isn't enough to measure. The stronger the impulse, the farther the wavefront goes (the sound of the explosion of Krakatoa is said to have gone around the world three or four times). 4 TE1:SOUND Fig. 2 Reflection of sound REFLECTION What happens if the wavefront hits something before it dies out? If the something is soft the energy of the wavefront is dissipated in moving the object (or its surface) around. If the object is rigid (like a wall) two things happen. Part of the energy of the wavefront will set up a wavefront within the wall; just how big a part is transmitted this way depends on the material the wall is made of. The rest of the energy is reflected off the surface according to the same rules that apply to light on a mirror. (The most important of these is that "the angle of reflection equals the angle of incidence".) Figure 2 shows this happening in two kinds of situations, one in which the source of disturbance is relatively close to the wall, and one in which the source is far enough away that you don't notice the wavefront is curved. Several repeated wavefronts are shown in each example. At the risk of getting ahead of myself, I will point out two important effects of reflections. One is that a listener might hear something twice if the distances involved are great enough, the other is that the energy reflected back into a room is going to change our notions about how long the disturbance endures. 5 TE1:SOUND DIFFRACTION. Here's another question; suppose the wavefront hits a wall with a hole in it? A small portion of the wave energy leaks through the hole and begins propagating as if the hole were the source, that is, spherically around the hole. The amount