. How does a magnet manufacturer make a permanent magnet—how does it vary in strength?
The manufacturer assembles the magnet from hard magnetic materials. These materials are intrinsically magnetic (ferromagnetic) so that they have tiny magnetic domains inside. They are hard, meaning that these domains have great difficulty changing their magnetic orientations. As the final processing step, the finished magnets are exposed to an extremely strong magnetic field; so strong that it flips all of the domains into the desired direction. The domains become trapped in this new orientation and the magnet becomes permanently magnetized. Unless it is exposed to other very strong fields or excessive heat or shock, it will remain permanently magnetized indefinitely.
. How does magnetism play a part in tapes to create sound?
The tape recorder first represents sound (pressure fluctuations in the air) as electric current and it then represents that current as magnetization of a tape. It magnetizes the tape to various depths to represent the different amounts of current and it uses the direction of the magnetization to represent which way the current should flow. During playback, the tape recorder measures just how deeply and in what direction the tape has been magnetized and uses that information to recreate the current and the sound.
. I want to know what dB, BIAS, HX-Pro, Dolby A-B-C-S Noise Reduction, and 20-bit LAMBDA Super-Linear converter (from DENON) mean.
The term "dB" is a measure of power. It appears in many contexts, including sound power and audio signal power. Like the Richter scale used to measure the energy released by an earthquake, the dB scale is an exponential one—a sound that is 10 dB louder than another sound has 10 times as much sound power in it.
The term "BIAS" refers to a technique used to assist magnetic recording of weak audio signals. The magnetic particles on a tape's surface do not magnetize easily and need help when quiet sounds are being recorded. To provide that assistance, the tape recorder superimposes a strong, high-frequency "bias" signal on top of the weak audio signal. This inaudible bias signal allows the weak audio signal to influence the tape's magnetization. The characteristics of the bias signal must be adjusted to match the tape type.
The term "HX-Pro" probably refers to a variable bias techniques that prevents the bias signal from saturating the tape's magnetization and wasting some of the tape's dynamic range. (I'm less certain of this observation—please tell me if I am wrong and I'll fix this remark).
The term "Dolby Noise Reduction" refers to a collection of techniques for reducing high frequency noise on magnetic tapes. The higher the sound frequency, the smaller the patches of tape surface that are used to record each cycle of the sound. Since the recording occurs by magnetizing individual particles that are almost a micron long, the cycles of a high frequency sound do not use very many of the particles. A few miss-magnetized particles in each cycle can produce noticeable noise in the reproduced sound. To counter this noise, Dolby boost the volume of high frequency sounds during recording and then reduces their volume back to normal during playback. The noise caused by the particles is also reduced in volume and is less noticeable as a result. The different Dolby techniques refer to different filtering protocols, with C being an improvement over B, which was itself an improvement over A.
The term "20-bit LAMBDA Super-Linear converter" probably refers to a high performance Digital-to-Analog Converter (DAC). When a compact disc is played back, the audio signal must be converted from a stream of numbers into a smoothly varying electric current, which is then amplified and sent to a speaker. Turning each number into a current requires a DAC. The more carefully this DAC is built, the more perfectly the current passing through the speaker will represent the numbers on the disc and the recorded sound information. While most DACs work with only 16 bits, the one you mention provides 4 more bits of precision. However, the compact disc contains only 16 bits of sound information, so the 4 added bits must be created by some numerical analysis on the part of the compact disc player. This sort of signal processing may lead to reduction in noise during playback, but I wouldn't expect most people to be able to hear any difference.
. Is magnetic flux another name for magnetic field?
The two quantities are related but they're not the same. If you think of a large magnet as made up of many tiny magnets all turned in the same direction, you can think of magnetic flux as strings that connect each tiny north pole to each tiny south pole. The large magnet effectively has many of the strings extending outward from its north pole and wrapping around to its south pole. The magnetic field at each location in space around the magnet is related to how many of these strings of magnetic flux pass through a small surface at that location. Near the poles of the magnet, the density of magnetic flux lines is high and so is the magnetic field. Far from the magnet, the density of magnetic flux lines is low and the magnetic field is weak.
. Magnets can be demagnetized by heat. Is that true for permanent magnets or materials that have been magnetized?
It's true for both because permanent magnets are just a special material that has been magnetized. In fact, permanent magnets are often demagnetized more easily than other simpler materials. Anything that spoils the internal order of a material (heat or vibration) can demagnetize it.
. When magnetic tape is put on top of another piece of magnetic tape, the tape on the bottom is demagnetized — its memory is erased. How are we then able to rewind and forward tape, scrolling the tape together, on a cassette tape with out damaging the magnetism?
The process of winding tape up on reels does damage its magnetism slightly. The adjacent layers of tape do interact with one another and they do cause the sound on one layer to appear on the adjacent layers. Fortunately, the effect is very subtle and takes a long time to appear so that the tape must remain tightly wound up for ages before you can hear the damage. Tapes don't age perfectly anyway because thermal energy slowly erases the magnetization, particularly in a hot environment.
. How does magnetic recording work?
During the recording process, an electromagnet in the recording head magnetizes the surface of a specially coated tape. This tape is coated with a thin layer of plastic that's impregnated with tiny cigar-shaped magnetic particles. As the tape moves past the recording head, the head magnetizes these particles back and forth to a certain depth, according to the audio signal reaching the recorder from the microphone. The higher the pitch of the sound, the more frequently the direction of magnetization reverses. The louder the volume of the sound, the deeper the magnetization extends into the layer. During playback, this magnetized layer moves past the playback head and induces electric currents in it. These currents are then amplified and used to reproduce the sound. A much more detailed discussion of this process appears in my book.
. How were tape recorders invented?
Magnetic recording dates to 1898, when Danish engineer Valdemar Poulsen developed a method for recording sound on a steel wire. He stretched this wire across his laboratory and put the recording apparatus on a trolley that traveled along that wire. He would run along with the moving trolley, talking into its microphone to record sound on the wire. To play back this sound, he would roll a second trolley containing the playback equipment along the wire and it would reproduce the sound. Having proven the principle of magnetic recording, Poulsen and others began to develop wire recorders. In these devices, a wire rolling from one drum to another was used to record and play back sound. In 1927, American inventor J. A. O'Neill replaced the wire with a magnetically coated ribbon and since then magnetic tape recorders have dominated the recording industry.
. Why does my voice sound different to me when I listen to a recording of myself?
When you hear yourself speak directly, much of the sound that reaches your ears travels to them through the bones and tissues of your head. This type of sound conduction tends to emphasize the low frequencies in your voice so that your voice sounds lower to you than it does to other people. When you listen to a recording of your voice, you are hearing your voice as other people hear it, without the modifying effects of bone and tissue conduction. Everyone else listening to the tape thinks that your voice sounds normal but you think it sounds higher than normal.
. Is there any substance that can stop magnetic fields — K, Mendenhall, MS
Magnetic fields are related to what are call magnetic flux lines. These magnetic flux lines extend unbroken from north magnetic poles to south magnetic poles. Where the flux lines are close together, the magnetic field is strong. Thus to avoid magnetic fields, you need to keep magnetic flux lines away. Because magnetic flux lines can't be broken, they can't simply be made to disappear. To "stop" a magnetic field in a particular region of space, you have to either terminate the flux lines at a magnetic pole or you have to divert the flux lines away the region that you're interested in. The first strategy has a problem: no isolated magnetic poles (so-called "magnetic monopoles") have ever been found. That means that every north pole you find has a south pole attached to it. Thus you can't simply end the flux lines with magnetic poles because for each flux line you end with a south pole, you'll start a new one with the attached north pole. But the second strategy is reasonable. There are many materials that divert magnetic flux lines. One of the most important of these is a metal called "mu metal," an alloy that's made from nickel, iron, chromium, and copper. Mu metal attracts flux lines. It draws flux lines through itself so that if you were to wrap yourself in a layer of mu metal, any magnetic flux lines that would have gone through you (and thus exposed you to magnetic fields) will go through the mu metal instead. Mu metal and similar alloys are used routinely to shield objects that can't tolerate magnetic fields.