Part 1. Preparing pre-recorded material for distribution
Sound, no matter how complex, is just waves like the ripples in a pond. It can be considered as the displacement of molecules from their place of rest. A more technical definition would be: Sound is a series of compression and rarefaction waves in a substance, solid, liquid or generally in our experience, gas. Our aim in recording is to precisely reproduce these waves in another place and/or at a different time. As exact, absolutely precise reproduction can't be done even today, we must be satisfied with successive approximations and keep aiming to improve. A cheap, bad telephone will make something that sounds “sort of” like you. A mechanical phonograph will sound “better” but still not exactly the same as you. We can successively improve our approximations until, with modern speakers, decent amps and CD quality recording it would be very difficult to tell which was “real” you or the recording. But you might still be able to tell that there was a difference.
First exercise for students—take an inflated balloon, hold it in front of your face, and sing at it. You can feel the vibrations through your fingertips (and in good lighting, by singing loudly, watch them travel over the surface). As the sound hit's the balloon it wiggles, which is about how the ear drum works. If we were able to attach a needle to our balloon with glue or tape, we could have made it inscribe a wiggly line on a soot-covered piece of paper, which shows that we can make a record of the sound. This would demonstrate that enough energy is being transferred to do mechanical work.
How to turn Wiggles on a Disk into Sound
Air seems very easy to move, but trying to move it fast requires energy—as any manufacturer of sports cars could tell you. They call it air resistance, we call it acoustic impedance, but it comes down to the same thing. Despite what residents of Florida might believe, air prefers staying in the same place to chasing around. In the above drawing, our needle wiggling along the groove will move a tiny bit of air in contact with it, but that moving air doesn't have enough energy, enough “oomph,” to make sound that can be heard throughout the room. If we hook the needle up to a lever, and use the lever to wiggle a thin diaphragm (like a drum head) back and forth, then the diaphragm moving back and forth moves more air, which then has more energy, (more oomph) and can be heard more clearly. This uses more energy. We need to push the needle back and forth harder against the grooves. Pressing harder means that there is more friction. That causes more wear on the record, but we get a louder sound. This is a good thing. But we can't keep making the lever longer and longer, making the diaphragm bigger and bigger, because eventually, we would just snap the needle off. Needles aren't infinitely strong. There has to be another way to turn the small weak wiggles of the needle into a loud sound that can fill a room. To do this, we can use a bit of physics in the form of a horn.
Those big flower shaped horns on old crank up record players weren't just decorative. They were very important to hearing the music. Without the horn, you have to put your ear right up against the diaphragm as though you were listening to a telephone. The horn is critical. A horn works as an acoustic impedance converter. It converts high velocity, low-pressure waves at the wide end into low velocity high-pressure waves at the small end. Or vice-versa, it converts low velocity, high-pressure waves at the small end into high velocity, low pressure waves at the wide end. Look at the diagram. We have a little needle wiggling back and forth, getting some energy from the turning of the record player; (we're literally moving the needle back and forth with the energy from the turning of the record. That is what powers the wiggling.) So, our needle wiggles back and forth, and moves the lever, and the lever wiggles the diaphragm. So far, so good. But the diaphragm moves back and forth fast, literally hundreds to thousands of times a second. (The “A” above middle "C" on the piano requires the needle to wiggle 440 times a second.) So, the air right beside the diaphragm is nearly being torn apart. The pressure spikes up fast, and then, as the diaphragm moves back the other way spikes back down fast. So, you have very high pressures right next to the diaphragm. Now. If we put a properly shaped horn next to it, the horn can take this signal and “spread it out.”
If you're a physics geek, the horn acts as an impedance transformer. If you're a poet, the horn takes the thin, reedy sound at the diaphragm and makes it “bigger.” This is exactly the same sort of thing that happens in a trumpet or a saxophone. The trumpeter's lips going bzzzzzz make a thin, reedy sound that has no carrying power. The “trumpet shaped” trumpet takes those sound waves and transforms them into Louis Armstrong's powerful music. The sound horn on a record player does the exact same thing. The horn on the record player modifies the output of the diaphragm, making it more listenable.
It has long been known that a trumpet could annoy people a lot further away than a flute. The first record players, (phonographs) used horns that were like cheerleaders megaphones. They were small simple cones. They were adequate for speech, but didn't reproduce highs or lows well. The people building phonographs pretty quickly changed the horn shape. They settled on what turns out to be the theoretically perfect shape – that big-belled flower you've seen in pictures. It's called an “exponential horn.” The Grantville developers don't need to know why this shape is best, they can merely copy an existing design, which had been polished by previous trial and error.
We can use the same horn, diaphragm and needle to CUT records if we want to. Attaching a diaphragm across the narrow end of a horn concentrates the sound energy and allows the needle to cut the wiggly line into a piece of wax—or, as in the original, a piece of tin foil wound round a drum (Please look up Edison, Phonograph on the web). So, at this point in our development of a recorder, we're up to where Edison was in his patent. We have a line on a soft surface which exactly follows the movement of the diaphragm (and slightly less exactly the variations of air pressure at the diaphragm, and even less exactly the variations in the room, but it's a start. We can do intelligible speech already).
If we reposition the system and drag the needle back across this groove we'll get a sound at the mouth of the horn which sounds at least a little bit like the original. The problem is, as we do that, it destroys the recording we made. That's not exactly what we're looking for.
If we use a lighter needle/diaphragm assembly, we get a sound much more like the original, and we will be able to play our recording several times before our master wears out. Still, we would like to be able to play a recording many times. We would also like to make many copies of a recording. Even so, the principle of the recording system is simple. We take something soft moving at a constant speed past a needle which is vibrated by the energy from the air. If this something soft hardens with time (like cheese or play-doh) or with varying temperature, or can be treated to harden chemically, or can take a hard, regular surface coating, we have the basis for a permanent medium. Simply dragging a long candle past the diaphragm won't work, however. For two minutes of sound, linear motion requires about fifty meters of candle. Storing and carrying them would be inconvenient. The solution was to coil the recording up some way. Two techniques were tried.
First: Edison's solution: Spin a cylinder and wrap your groove around it like thread on a spool. If you build a drum that you can slide thin cylinders onto, you can “change the records." This has the distinct advantage that the speed of the groove past the needle is constant. It has the disadvantage that making the recording play longer takes more and more “tube.” An Edison tube “LP” would be six or more feet long.
Second: The Victrola solution. Use a flat disc (a ”˜record'). The groove coils from the outside in towards the center. Unfortunately, since the disk turns at a constant angular velocity, when the needle is in the groove near the outside edge, the speed of the groove past the needle is much higher than the speed near the center of the disk. More about that later.
In both cases the play head was moved by the groove itself. Disks became the de facto standard, in the OTL for two simple reasons. They stack better. Tubes have all that annoying space in the middle of them. And they are more copyable. (More on that later.)
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- The Grantville Gazette Staff