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The Preservation of Recorded
Sound Materials
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.
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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.
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The Recording, Retention and Playback of Sound
The Microphone
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.
The Speaker
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.
Discs
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.
Tapes
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 is to 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.
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To contact the Music Division:
Phone: 613-996-5115 or 1-866-578-7777 (toll free in Canada and the U.S.)
Fax: 819-934-7272
Electronic Mail: mus@lac-bac.gc.ca
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