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The Recording, Retention And Playback Of Sound
The Degradation Mechanisms Of Sound Recording
Preservation Of Sound Recordings
Conclusion And Bibliography
Consultation And References
Sound recordings are machine readable artifacts; they are documents for which the integrity of the information they contain is directly related to the artifacts' physical well being. Since the majority of sound recordings are made of plastic, conservation must be treated as a plastics degradation problem, requiring a different approach than paper conservation. It is important to understand the basic chemical degenerative processes and the principles of the retention of sound by the various media in order to ensure that proper action is taken to slow the rate of degradation.
SOUND AND HEARING
Sound can be defined as the change in air pressure above and below an equilibrium (usually the barometric pressure). For example, when a bass drum is struck, the skin vibrates back and forth. As the skin travels outwards, away from the centre of the drum, the air pressure surrounding the drum rises above the barometric pressure; conversely as the drum skin travels inwards, the air pressure lowers. This to-and-fro action occurs numerous times per second creating waves of compression and decompression in surrounding air.
As air pressure increases by the outward motion of the bass drum skin, the eardrum is pushed towards the centre of the head; conversely, as pressure decreases, the eardrum travels away from the center of the head. Therefore, the eardrum physically moves in a parallel motion to the movement of the vibrating bass drum skin. The inner ear converts the change in air pressure into sound by translating the eardrum's mechanical motions into impulses that the brain will perceive as sound. The ear can detect changes in air pressure as slow as 20 cycles per second (a cycle being a complete to-and-fro motion) to as fast as 20,000 cycles per second. The higher the vibration speed, the higher the pitch; the larger the change in air pressure, the louder the sound.
THE RECORDING, RETENTION AND PLAYBACK OF SOUND
The interior of a microphone is comprised of a permanent magnet, a coil of wire and a diaphragm which, like the eardrum, vibrates to changes in air pressure. The vibration of the diaphragm in conjunction with the permanent magnet and the coil converts changes in air pressure into changes in electrical voltage.
As air pressure increases, the diaphragm within the microphone is pushed towards the back of the microphone, inducing a voltage; as pressure decreases, the diaphragm travels outwards inducing a voltage in the opposite direction. Like the eardrum, the diaphragm will move in a parallel motion to the movement of the example sound, the vibrating bass drum skin. The resulting voltage will be a continuous parallel voltage image of the movement of that bass drum skin.
If the bass drum were to be tuned at a higher pitch (the skin tightened) the skin would vibrate faster, causing the air pressure to compress and decompress faster, meaning that the diaphragm within the microphone will vibrate faster, consequently forcing the induced voltage to change direction more frequently. A higher pitch will thus be captured on the recording medium. If the drum were to be struck harder, producing a louder sound, the skin vibration would travel a greater distance, creating a higher compression of air, consequently forcing the microphone diaphragm to travel a greater distance thus inducing a larger voltage. The recording would thus be at a higher volume.
This chain of events occurs with the recording of any sound. If an orchestra were to be recorded, the collective air pressure change surrounding the orchestra (caused by the mixture of vibrating strings, reeds, etc.) would be captured by the microphone.
Once sound has been converted to an electrical voltage, the "voltage image" can be amplified and then used to drive speakers. Like the skin of the bass drum, the movement of the speaker compresses and decompresses air to produce sound. If the voltage is going upwards, the speaker will travel outwards; if the voltage is going downwards, the speaker will travel inwards. The resulting movement of the speaker will be parallel to the movement of the skin on the bass drum, to the movement of an eardrum, to the movement of the diaphragm within the microphone, and to the induced voltage.
All grooved discs physically retain information in the same fashion and are recorded in a similar manner. Just as a speaker converts a change of voltage to a parallel mechanical motion, so with discs a cutting stylus converts a voltage change to a mechanical motion. When the voltage applied to the cutting stylus goes up, the stylus will move in one direction; when the voltage goes down, the cutting stylus will move in the opposite direction. The movement of the cutting stylus determines the pattern of the groove which, of course, moves in a parallel motion to the movement of the bass drum. Again, the resulting groove shape will be a continuous, identical physical image of the movement of that bass drum skin. (Acoustic recordings, (made prior to the use of microphones, ca. 1925), recorded sound by capturing and channeling changes of air pressure through a horn to a diaphragm mounted with a cutting stylus. The diaphragm would transform the changes of air pressure into a parallel mechanical motion with the cutting stylus etching the groove.)
To retrieve information from a disc, a stylus is used to track the groove. The cartridge will convert the movement of the stylus to an electrical voltage (in the same fashion that a microphone converts mechanical motions to an electrical voltage) that can then be amplified and used to drive speakers. The movement of the speaker will be parallel to the movement of the stylus.
The binder layer of magnetic tape contains a finite number of ferromagnetic particles whose permanent alignment within the binder records voltage (current) levels.
To record onto tape, the tape must first pass an erase head whose task isto arrange the particles completely randomly. If a small voltage is applied to the record head, then a small percentage of particles will become unidirectionally aligned. If a larger voltage is applied to the record head, then a larger percentage of particles will become aligned. Saturation occurs when there are no more particles available to align. The particles will remain aligned until exposed to a magnetic force.
At playback, the aligned particles will induce a voltage in the playback head. The voltage level will be proportional to the number of aligned particles.
COMPACT DISCS (CDs)
Tapes and grooved discs are analog recordings --the term "analog" referring to the transformation of sound into "parallel", or analogous grooves or particle alignments. Compact Discs on the other hand, are "digital" recordings (as are Digital Audio Tapes (DAT), Digital Compact Cassettes (DCC), Mini Disc, etc.). Rather than being a continuous physical image of changes in electrical voltage, digital recordings are based on a series of discrete electrical voltage measurements.
For the CD, the electrical voltage (produced by the microphone) is measured 44,100 times per second. At a certain period in time the voltage might be (for argument's sake) .5 volts out of a maximum 1 volt. 1/44,100th of a second later the voltage might be .5005 volts, the following 1/44,100th of a second .5009 volts, etc. As the skin of the bass drum travels outwards, the resulting series of voltage readings get progressively larger; as the skin moves inwards, the resulting series get progressively smaller.
Just as 2:00 p.m. can be expressed as 14:00 hrs, so any value can be expressed using binary digits --1s and 0s. Also, 1/3 can be expressed to .3, or more accurately .33, or better yet .333 etc. The greater the number of decimal places, the more precise the expression of the translation; hence the larger the number of digital bits used in a number, the more accurate the translation. For the compact disc, the number of digital bits used to translate or "digitize" a voltage reading is 16. Thus the compact disc stores one 16 bit number (in addition to other required information) every 44,100th of a second (per audio channel).
The CD sound recording stores information using pits and flat areas wound in a spiral starting at the center of the disc. The edge of a pit (either the ascending edge or descending edge) indicates a one, a flat area either at the bottom of the pit or the land between the pits indicates 0. For example, a 5 bit number of 10001, using pits, would be an edge, a long flat area and another edge.
To play a CD, a laser beam is shone through the clear polycarbonate bottom to the aluminum (sometimes gold) layer. The light then reflects off the reflective surface to a pickup which differentiates between the top and bottom of a pit and interprets these as 1s or 0s. The electronics then build a continuous voltage from these series of stored binary numbers representing the original voltage readings.
THE DEGRADATION MECHANISMS OF SOUND RECORDING
The lifespan of a plastic is largely determined at the manufacturing stage. Variables such as basic resin, the materials added to the basic resin to alter its properties, the lamination of materials with dissimilar properties, and the manufacturing process itself, all directly affect the lifespan of the plastic. Post-manufacture environmental factors such as storage conditions, temperature, humidity, and handling also contribute to the long-term stability of the plastics.
Plastics can be divided into two main classes, thermoplastic and thermosetting. Thermoplastics soften and flow when heated and are normally shaped by heat and pressure. They will soften and flow again when re-heated. Vinyl, used in the manufacturing of LP's, is a thermoplastic.
Thermosetting plastics are moulded under heat and pressure. A chemical reaction occurs so that once molded they do not soften when re-heated and will normally char before melting. Most 78s are made of thermosetting plastics.
Prior to the advent of magnetic tape, instantaneous recordings were made chiefly on acetate discs. The chemical makeup of these discs, therefore, had to be a compromise between ease of engraving and the quality of the recording that resulted.
Since the 1930s, most blank acetate discs have been manufactured with a base, usually aluminum (although glass was used during the war years and cardboard for inexpensive home recordings), that was coated with nitrocellulose lacquer plasticized with castor oil. Because of the lacquer's inherent properties, acetate discs are the least stable type of sound recording. Continuous Shrinkage of the lacquer coating due to the loss of the castor oil plasticizer is the primary destructive force. The gradual loss of plasticizer causes progressive embrittlement and the irreversible loss of sound information. Because the coating is bonded to a core which cannot shrink, internal stresses result, which in turn cause cracking and peeling of the coating.
Nitrocellulose acetate decomposes continuously and over time reacts with water vapour or oxygen to produce acids that act as a catalyst for several other chemical reactions. As with most chemical reactions, these reactions are accelerated with elevated temperature and humidity levels.
VULCANITE (EARLY BERLINER RECORDS)
Vulcanite (hard rubber) was the first material used commercially by Berliner and provided the necessary basis for the exploitation of the flat disc.
In 1839 Hancock in England and Goodyear in the US independently discovered vulcanization. Vulcanization is the process of treating crude rubber with sulfur or sulfur compounds in varying proportions and at different temperatures. The result is an increase in the rubber's strength and elasticity, yielding either soft rubber or vulcanite. Vulcanite has been used to make combs, buttons, jewels, fountain pens, musical instruments, etc.
Vulcanite is stable in the dark and retains its appearance and properties very well. In response to light and/or heat the material loses sulfur then becomes brittle and loses its shine. Light induces oxidation of the rubber and forms oxides of sulfur and sulfuric acid in the presence of humidity. The acidity builds up to a level at which the degrading plastic is attacked and eventually decomposes. (see reference 1) The degradation can be demonstrated when playing an afflicted Berliner. The surface of the disc is shaved off by the pressure of the stylus against the groove wall.
Vulcanite also posed problems in the production of discs. The uneven shrinkage during cooling caused severe warping; entrapped gas would produce blisters; hard particles create loud pops and clicks; and the coarseness of the Vulcanite structure produced terrible background noise.
The first shellac discs date from the early 1900s. Shellac is a composite word -- it's a combination of shell and lac. The word is a Hindus name of an insect that infests certain types of trees. The lac draws sap from these trees, processes it through its digestive system and secretes it so that it becomes an attached protective shell around its body. Thus the shell is generally smaller than a grain of rice. Harvest involved scraping off the encrusted shells from twigs and branches.
After WWII resins such as Vinsol, Valite, Vynil chloride acetate and other commercial resins replaced organic shellac as the main binder. These plastics are slightly more stable than organic type discs. It is often difficult to distinguish between shellac and shellac type discs by inspection.
Determining the causes of shellac disc degradation is rather difficult because a very wide range of qualities of shellac and "fillers" have been used by manufacturers. Therefore one cannot expect consistent behavior of all stored shellac discs. The disc properties are as much a function of the filler as they are of the cementing agent. The fillers used run the gamut of natural cellulosic materials as well as of minerals.
For example two separate chemical analyses of "typical" shellac
discs showed the following:
EXAMPLE 1 (see reference 2)
EXAMPLE 2 (see reference 3)
Flake Shellac 15.63%
EXAMPLE 1 (see reference 2)
EXAMPLE 2 (see reference 3)
Flake Shellac 15.63%
The average shellac content in these "shellac" discs is approximately 15% shellac.
Also, record manufacturers would introduce scrap as filler into new mixtures. The manufacturers would recycle returned, unsold shellac discs. It was not uncommon for the scrap to included soft drink bottles litter, pieces of masonry or other unwanted material, all of which were ground up together and mixed in with the next batch of compound. (see reference 4)
COLUMBIA DISCS (LAMINATED DISC)
In 1906 Columbia introduced the Marconi Velvet Tone developed by Giulemino Marconi. The manufacturing technique involved using a craft paper core cut to approximate record size. After the core was carefully flattened and dried, it was covered with a powdered shellac compound of a thin uniform thickness. The dust-coated core was put in an oven and the dust fused to the core. For two- sided records, the operation was repeated for the other side. (see reference 5)
The advantage of this construction was that the amount of surface material needed to carry the music grooves could be kept very small. This economy allowed the use the best plastic available at that time. Edison was to use this idea in 1912/13, in the manufacturing of his Diamond Disc.
In 1922 Columbia returned to the laminated record, this time with a coarser compound for the powder core that was bonded between two discs of craft paper.
In general shellac discs are relatively stable. The curing process of shellac during disc manufacturing generates a chemical reaction where certain simple molecules such as water or ammonia molecules are eliminated. Curing causes shellac to shrink, increasing its density and its brittleness.
This condensation continues at a much slower rate after disc manufacturing. The speed at which condensation occurred is a function of storage temperature, storage humidity and completeness of cure. (The condensation reaction reduces the potential concentration of reacting elements.
A semi-quantitative measure of the cure of shellac is its solubility in alcohol. Raw shellac is totally soluble in alcohol and completely cured shellac is insoluble, and the extent to which condensation has proceeded determines the degree of solubility of a shellac.) (see reference 6) Thus the condensation becomes the primary degenerative force.
The internal reaction of the material and the rate at which the reaction occurs are related to storage temperature, storage humidity (moisture increases the condensation reaction rate) and completeness of the cured shellac.
Storage stability of these fillers vary widely. Organic materials in the aggregates are susceptible to fungus attack, while shellac itself is resistant to fungus attack.
In a proper storage environment, these discs suffer a slow, progressive embrittlement of the shellac. This embrittlement causes a fine powder to be shed from the disc after each playback. The behaviour of the other aggregate components is unpredictable, due to the wide combinations and variety of materials (and of material quality) that were used.
EDISON DIAMOND DISC
The Edison Diamond disc has the distinction of having been made of the first completely synthetic plastic, a material called phenol (phenol was also used in the manufacture of Bakelite). The Edison diamond disc is a laminated disc made up of a thick core and a thin varnish layers covering each of its sides.
The 1/4" core, which is also known as a powder blank, was manufactured by compressing the following ingredients in the following proportions: (see reference 7)
Wood flour 58%
The varnish, named "Edison condensite varnish" was made-up
The varnish would be applied to the blank by a brush as the blank rotated slowly. Four applications or coats were given each blank face with a drying period between. After the last coat the varnished blank would be placed in a steam-heated oven. This completed the drying and also effected a partial reaction of the varnish ingredients.
Prior to pressing, the blanks would be heated before applying pressure to soften. After the pressure was applied the heat was left on to complete the curing or reaction of the varnish. Then the moulds were cooled and the pressure released.
Prolonged contact with moisture or severe changes in humidity may cause damage to the surface through moisture absorption. In general Phenol is very stable and presents no serious degradation problems, neither is it prone to attack by bacteria, fungi or insects although, occasionally, under humid conditions moulds may grow and cause some surface attack on a nutrient filler such as wood or cotton, or be supported by a nutrient contaminant on the surface.
Thus far, vinyl has proven to be the most stable of the materials that have been used in the manufacture of sound recordings. (see reference 8) However, although stable, its life is not indefinite. Pickett and Lemcoe, in Preservation and Storage of Sound Recordings, states that "failure by chemical degradation of a vinyl disc in ordinary library environments should not occur in less than a century". (see reference 9)
Vinyl discs are made of polyvinyl chloride (PVC) and a small percentage (usually less than 25 percent) of "fillers", stabilizer, pigment, anti-static substances, etc. Internal plasticization, through a copolymerizing of vinyl acetate with vinyl chloride, is needed to achieve the required properties for the desired application.
Polyvinyl chloride degrades chemically when exposed to ultraviolet light or to heat. Phonograph discs are exposed to high temperatures during moulding and pressing. Unless stopped, this heat would be a catalyst for ongoing dehydrochlorination, which is the release of hydrochloric acid (HCl) from the PVC as a result of thermo-degradation. Stabilization is therefore achieved by adding a chemical to the resin during manufacture. This does not prevent the degradation but controls it, mainly by consuming the free HCl. Sufficient effective stabilizer remains in a plastic phonograph disc to protect it for several decades after pressing.
Magnetic tape first appeared in North America just after World
MAGNETIC TAPE BINDER
Manufacturers are very secretive about the specific chemical makeup of their products. Binder chemical composition, uniformity and smoothness of application all affect audio quality, noise level, tape-to-head contact, and friction. These factors also affect the tape's aging properties. The most common binder resin used today is polyester polyurethane. The most common ferromagnetic particle used is gamma ferric oxide (Fe3O2).
Numerous additives may be used during the various manufacturing stages, including: solvents, used to obtain a suitable viscosity of emulsion and to improve the mixing and bonding operations; wetting agents, used to break binder/particle mixing tension to produce a more even ferromagnetic particle dispersion within the binder; plasticizers, used to add suppleness to plastic; stabilizers, used mostly as anti-oxidants to avoid chemical degradation that could lead to physical breakdown; lubricants, used to reduce drag so that speed deviation problems such as "wow" and "flutter" are diminished, and to minimize wear damage to heads; fine mineral powders, used to make polymers harder and more resistant to abrasion; conduction discharge (material such as carbon black), used to discharge electrical charges; and fungicides.
The most common and serious magnetic tape degradation occurs through hydrolysis, the chemical reaction wherein an ester such as the binder resin "consumes" water drawn from humidity in the air to liberate carboxylic acid and alcohol. Hydrolysis in magnetic tape results in the binder shedding a gummy and tacky material which causes tape layers to stick together and inhibits playback when it is deposited onto the tape recorder heads. The added friction increases tape stress and can cause machines to stop. Hydrolysis also causes a weakening in the bond holding the binder to the backing, which results in shedding or possible detachment.
Chromium dioxide (CrO2) is used extensively as the ferromagnetic particles in cassette magnetic tape. It has been found that CrO2 particles interact with the polyester polyurethane to accelerate hydrolytic degradation. Additives are now added to retard this degradation.
Other problems associated with binder manufacturing and deterioration are: incomplete dispersion of the ferromagnetic particles, causing momentary loss of signal ("dropout"); a weak bond that causes the binder to separate from the backing; lubricants that evaporate to the point where tapes are unplayable; fine oxide powders that shed from tapes and deposit onto heads, inhibiting playback.
MAGNETIC TAPE BACKING
The backing, which is the structural support of the tape, must resist stresses imposed by playback and storage without becoming permanently deformed (e.g., stretching), or losing dimensional stability (e.g., expanding through absorption of moisture or heat). Most magnetic tape backing has been made of either cellulose acetate or polyester, materials that have dissimilar physical and aging properties.
Cellulose acetate-backed tapes were manufactured from about 1935 until the early 1960s. These tapes rely heavily on plasticizer additives for suppleness, and these plasticizers are liable, over time, to evaporate and crystallize. These tapes have extremely low tensile strength and are easily broken. Cellulose acetate tapes are very susceptible to linear expansion in humid and/or warm conditions. Because of the different properties of the binder and the base, the absorption of humidity and heat result in tape curling and edge fluttering. These distortions greatly affect the tape-to-head contact, which in turn directly affects audio quality. Repeated dimensional changes due to environmental fluctuations grossly affect winding tension and can promote binder fatigue, cracking, and finally, catastrophic failure (i.e., the irreversible loss of data).
A serious problem affecting acetate tape is that of the "Vinegar Syndrome". Vinegar Syndrome is exhibited by the release an acetic acid (vinegar) odour from the tape and is a by-product of acetate tape breaking-down. The process is accelerated by the presence of moisture and iron ferromagnetic particles in the tapes. (see reference 10). When the acetate is degrading -- giving off acetic acid-- it will start to take up more moisture. The process of self-destruction is auto catalytic, once it has started it will continue with ever increasing speed; no solution for its interruption has yet been found. Tapes afflicted with the vinegar syndrome will "infect" healthy tapes.
Polyester ("mylar") came into use in the early 1960s, and quickly replaced cellulose acetate for magnetic tape backing. Accelerated aging tests have found polyester to be a stable material which in fact undergoes hydrolysis degradation at a much slower rate than does the binder, polyester polyurethane, with which it is combined. However, polyester-based tape has a high tensile strength that can cause it to stretch irreparably (instead of breaking cleanly and reparably as does acetate-backed tape).
A third coating is now added to professional, modern tape on the opposite side of the binder. Made of carbon black particles held in a thin binder, it protects the backing from scratches, minimizes static electricity, and provides a more even wind.
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