1. Field of the Invention
This invention relates to record player turntables and particularly relates to devices for dampening, dissipating, and blocking vibrations and resonances that interfere with faithful sound reproduction. It especially relates to devices for centering a record on the platter and then isolating the record from the turntable drive mechanism while the record is being played in order to block such vibrations travelling through the drive mechanism.
2. Review of the Prior Art
Playback distortion from the turntable frequently occurs in even the finest equipment. It may be an obvious distortion which makes listening very unpleasant, an objectionable resonant coloration, a blurring of clear, distinct sound into an unrecognizable mass of sound, a subtly annoying but not totally unpleasant effect, or even an unidentifiable source of fatigue. Mechanical vibration in a turntable may originate in, or be transmitted by, the drive system, the loader assembly, the platter design, or the chassis design.
Feedback is a major source of mechanical vibration which may be either mechanical or acoustical. Mechanical feedback is energy transmitted through the floorboards and to the wall beams and the like so that the loudspeaker is mechanically coupled to the turntable. Acoustical feedback is created by acoustical energy emanating from the loudspeaker or other sources when it moves or pumps energy into the room in the form of low-to-high level pressures at multiple frequencies and in complex patterns and with changing forces. A mechanical force is thus created when the pressure patterns in the air are absorbed by solid objects.
The lower the frequency, the more obvious the mechanical force becomes until it reaches a frequency too low to be heard. But even at such low frequencies, sufficient energy can be absorbed to rattle windows and shake walls as well as to create mechanical energy in the turntable platter, it main board, its base, and its supporting structure. Each of these parts vibrates with its own characteristic resonances in accordance with varying amounts of acoustical energy in the room.
These mechanical and acoustical vibrations travel through the equipment and coincide from all directions at certain key pathways to the tone arm. The result of such combinations seems to be a compounded increase in the feedback to the tone arm at many key points which might be called "collision course vibrations". These collision course vibrations are also generated within the mechanism itself, by and between the motor and the main bearing and the chassis and the subchassis, and are transmitted to and picked up by each end of the tone arm.
Such vibrations are commonly measured in the laboratory as rumble. Rumble is a low-pitched vibration or frequency that is caused by a mechanical vibration acting on the turntable and tone arm when the vibration occurs at the rotation frequency of the motor, the idler, the bearing, or the platter, or at some multiple of any of these frequencies. The platter bearing is indeed the main source of rumble in turntables that are now available on the market. Rumble may be reported as weighted or unweighted. Weighted rumble measurements discriminate against subsonic frequency components which cannot be reproduced by loudspeakers or heard by the human ear, but such frequencies can overdrive an amplifier or speaker and thereby impair the reproduction of higher frequencies. Thus, an unweighted measurement can also provide useful information because both sonic and subsonic frequencies--from one to 100,000 cps--contribute undesirable side effects.
Flutter is a rapid pitch fluctuation in reproduced music which is caused by pulsations or changes of the turntable speed, i.e., a rapid variation from constant rotational speed. When flutter occurs at a low rate, it is called "wow", suggesting the characteristic sound it imparts to steady musical tones. When it occurs at higher rates, the effect is of a "gargling" or roughness. Wow and flutter are usually reported as a combined flutter measurement which is weighted to emphasize the most objectionable flutter rates at around 5-10 Hz. This combined flutter measurement is usually specified in hundredths of a percent of perfect accuracy with 0.03% being a typically good figure.
Flutter robs a musical instrument of its character by blurring the musical image. Flutter can be characterized as forward and backward movement. The composite of all of these vibrations creates a situation that has much the same distortional effect, with respect to playback, as flutter itself but with more severe characteristics because these vibrations react in all planes and in 360.degree..
Even though such laboratory measurements report excellent values, such as an average peak wow and flutter of not more than .+-.0.03% and a rumble low enough to produce an ARLL-weighted measurement of -73 db or even -80 db, collision course vibrations can produce annoying disturbances to the trained ear. Neither is consequently acceptable for quality equipment.
Numerous devices have been designed and built for decoupling the turntable from mechanical vibrations. However, the frequency at which the energy decoupler resonates must be above the rotational speed of the turntable, which at 331/3 rpm is approximately one-half cycle per second, and, at the same time, must be lower than the resonant frequency of the tone arm mass and cartridge compliance, which is preferably 8 or 9 cycles per second. Thus, the best frequency for decoupling mechanical energy to the turntable is two or three cyles per second. Such decouplers include the use of a dense, thick, massive support board, upon which the turntable is placed, and the use of a number of coil springs between the support board and the platform therebeneath. Adding additional mass to the frame of the turntable also changes its frequency of vibration and reduces distortion that may range from frizzy highs to muddy lows, e.g., that is, music "out of focus".
About twenty-four years ago, a turntable having excellent acoustical qualities is believed to have been advertised. This turntable featured a centering pin that expanded for precisely centering a record and remained in place during play.
U.S. Pat. No. 1,821,916 describes a resilient center for phonograph records which is a laminated structure comprising the record body and a relatively thin rubber layer with a pin opening therein which is substantially coincident with the central axis of the record.
U.S. Pat. No. 3,801,476 relates to accurately centering a centering core clamped around a lacquer foil original recording for the manufacture of record discs containing sound or video recordings. It provides a centering sleeve which closely fits both the spindle and the outer edge of the centering core, whereby any possible misalignment of the foil with reference to the spindle is avoided.
However, these arrangements do not effectively decouple the mechanism of the turntable from the center hole of a record. There is accordingly a need for a simple, generally applicable and efficient decoupler for collision course vibrations travelling from the motor or other parts of the mechanism and up the spindle towards the center hole of the record and then to the needle carried by the tonearm.