by Robert Stockton
As audiophile-grade hi-fi systems have become more revealing, and therefore less likely to contain subtle types of coloration and distortion, more and more seemingly minor factors have been found to relate to the audible characteristics of these same music systems. In the past few years, it has become clear that the effects of tonearm and cartridge mass resonance, friction,' and even arm tube and headshell rigidity can all color the sound of a high performance system.
A number of organizations studying the electrical and mechanical characteristics of record discs have found that the physical properties of the turntable mat upon which the disc rides can have consequences which are both audible and significant. These studies have focused on two phenomena which had previously been rather poorly understood: The nature of static formation, neutralization and discharge, and the occurrence of vibration and resonance in the record itself.
Conventional records, being com posed primarily of PVC, represent a highly insulating dielectric. The vinyl compounds are very easily charged, particularly in dry climatic conditions, and store extremely high static voltages. These charges can be easily formed, in many cases, just by removing the record from its inner sleeve. A recent paper on the subject, prepared by Shure Brothers,' notes that static charges of as high as 30,000 volts were commonly measured during their tests. Experiments performed by Dr. D. W. Swan of 3M Technical Laboratory ( England) found equally high static charges. Dr. Swan notes that since the breakdown voltage of air at normal humidity is about 30,000 volts, any "snap" which is heard as the record is removed from the jacket or handled indicates that a static charge of such magnitude is present. Most records, in addition to easily acquiring static volt ages, have substantial charges molded into the plastic during the pressing process. These voltages render them intrinsically charged, much like the diaphragm of an electret microphone.
Static Problems
Although static charges themselves are very rarely amplified by the system, and are therefore not usually directly audible, they do cause a plethora of other problems in various indirect ways. These problems include:
1) Attraction of Dust--Dust and larger particles commonly have charges of their own which tend, in the majority of cases, to attract them to record discs. According to the Shure studies, the charge on a record is generally negative; most dust particles, apparently, have positive charges, which factors generate a classic case of electrostatic attraction. In addition, Dr. Swan of 3M describes, in detail, the behavior of complex electrostatic forces which may generate attraction of even uncharged particles. It is generally accepted that the primary mechanism of dust collection on record surfaces is static related and not the product of particles simply falling onto the disc. Once attracted, dust becomes extremely difficult to remove without some neutralizing activity and virtually impossible to adequately pick up with simply dry record brushes.
2) Electrostatic Attraction of Cartridge--The phono cartridge itself is attracted to a statically charged disc for reasons similar to those responsible for dust attraction. A number of studies 2' indicate an addition to the measured tracking force of 0.375 to 0.500 grams may occur under reasonably typical conditions (a charge of about 4,000 volts with the record on the turntable). The cartridge or arm may also be attracted to the unplayed records in a changer stack and, in some cases, even to the closed dust cover of the turntable . These various attractions change not only the tracking force but also many other dynamic parameters. As a result, stereo imaging and tracking properties of the system may be affected.
3) Modulation of Cartridge Output--Since static charges are not uniformly distributed on the record surface, it is possible for tracking force, anti-skate settings, vertical tracking angle, and channel balance to be altered enormously on a momentary basis as the cartridge passes over heavily charged regions of the record surface (static "hot spots"). This may have the effect of modulating the out put of the cartridge in one or both channels.
4) Cueing "Snaps" or Transients--Occasionally, as a tonearm is cued to the disc, a static discharge will be coupled through the cartridge or input leads to the amplifier and reproduced by the audio system as a loud snap.
This is annoying, but more significantly, a transient of this type is of very high level and can damage both the amplifier and speakers.
The audio marketplace offers a wide variety of devices and methodology to reduce static charges. These range from fluid and film applications (with no generally used substance which performs this function without interfering with some other aspect of the playback process) to polonium-strip emitters (Staticmaster) which are essentially positive-only Alpha emitters.
Air-ionizing devices (Zerostat and others) can bring static charges to zero, but such neutralization lasts only about one hour. One other practical solution lies in creating a conductive path from the record surface to ground, and this can be done locally via a conductive fiber brush or generally via an electrically conductive turntable mat.
The conductive mats, such as the Discwasher D'Stat II, eliminate static charges in two separate and distinct ways: 1) by conduction of the charge to ground, where the charge passes through electrically conductive fibers woven into the mat which spread out local charge concentrations and act as a low resistance path to the turntable ground through the metal platter or spindle; and 2) by ionic neutralization, a phenomenon in which ions of opposite polarity to the charged record surface travel from ground (which may be viewed as an infinite "bank" of both positive and negative ions) onto the conductive fibers of the mat to the fiber points which store charges as "point electrodes." When sufficient ionic potential has gathered at the fiber points, the ions transfer to the disc and effectively neutralize the static charge present there. See Fig. 1. Thus, when the record is first placed on the turntable, an ionically active mat will require several seconds to eliminate the static field. Some mats accomplish the "conductive" function but few the ionic neutralization.
Both conduction and neutralization of charges take place on the "bottom" of the record, and, at first glance, this seems to have little beneficial effect upon the upper tracking surface. Interestingly enough, however, a high degree of conductivity in the mat will cause a migration of the charge be tween the upper and lower disc surfaces. This migration reduces, in large measure, the field on the upper surface. Alternatively, the record might be flipped to the "play" side after resting on the mat for several seconds, though this is tedious.
Studies on Mats
The National Swedish Authority for Testing, Inspection, and Metrology has performed a number of studies on conductive mats, comparing the effectiveness of different types. Their findings indicate that both the number of conductive fibers and the diameter of the fiber points (which determines the effectiveness of the fiber as an electrode) are of great importance. In a comparative test of three mats which had front-to-back resistances of 5 megohms, 100 megohms, and greater, respectively, it was demonstrated that the lower resistance type (e.g. D'Stat) was almost totally effective in eliminating charges of 4,600 volts and more by reducing the static charge on the record to about 500 volts, a relatively negligible figure. The higher resistance devices accomplished much less reduction of static charge, from 4600 volts down to 3900 and 4000. These tests establish that the electrical ion donor properties of conductive mats must fall within a fairly specific range in order to yield effective results.
Other recent studies have examined another rather interesting concept:
The turntable mat performs another crucial function in isolating or damping. A number of different types of vibration may generate resonances and motion within the record itself, and there are, apparently, several sources of vibration which may affect the disc.
The best documented of these sources is the turntable drive mechanism, primarily the drive motor and/or the main bearing on which the platter rotates. Depending upon the drive system and the suspension used to support these elements, rumble, primarily in the form of vertical motion of the record surface, may be transmitted to the stylus and through the system.
This transmission path is the predominant source of rumble in modern turntables; very little vibration is transmitted through the tonearm structure to the stylus. Contrary to popular belief, the frequency spectrum of rumble from conventional turntables is quite broad and includes a substantial amount of output above 50 Hz (in the audible range), as well as frequency components down to as low as a few Hertz. A paper by Bauer details a number of rumble spectra and comments on the origins of various frequency components. Platter-transmitted rumble, if reproduced by the system, has a number of ultimate sonic effects: 1) Modulation of the mid-bass information on the record by the higher frequency portions of the rumble; 2) a reduction in the usable power output of the amplifier and increased bass-driver distortion (this occurs because the low frequency portions of the rumble are amplified to very high levels, given the disc equalization curve of the preamp stage, and thus "use up" a substantial amount of both the power available from the amplifier and the usable excursion of the bass driver); and 3) excitation of the fundamental arm-cartridge resonance, with all of its well-documented impact upon tracking ability and deep bass performance. The Japanese firm, Denon, and their importer, American Audioport, have done tests on another source of resonance in record discs-vibration excited directly by the loudspeaker. Denon has carried out experiments which confirm that midrange frequencies at fairly typical listening levels can set the record in motion and, consequently, add midrange coloration to the music.
In their tests, a conventional phono graph record was photographed by laser holography, while a loudspeaker reproduced a wide range of frequencies at 103 dB SPL (measured adjacent to the record). A major resonance at 932 Hz and lesser resonances at other frequencies were observed (see Fig. 2).
The peak displacement of the record surface was about 0.02 mm, which translates to an amplitude of several dB when reproduced by a typical sys tem. In related experiments, engineers at American Audioport were able to induce ringing at even higher frequencies in records placed on radially ribbed or other edge-supportive platters. Records on such platters could be made to microphonically reproduce through an audio system the sound of a technician shouting at the record! Taken in perspective these findings do not seem surprising. Architectural acousticians regularly find much larger objects--chandeliers, framed paintings, etc.--which have pronounced resonant modes. Indeed, it would be more surprising if an unsupported vinyl disc did not vibrate in a room with music being played at concert hall levels.
Fig. 1--Illustration of cross section of the record, mat, and platter, showing
the static discharge process.
Fig. 2--Laser interferometry tracing of a record on turntable mats: A) resonance
pattern of record on standard ribbed turntable mat, and B) resonance pattern
of record on a specially designed butyl rubber mat (10 mm thick). The circular
islands and holes show resonance while the large spaces show stability of
the record disc.
Disc Vibration
A third source of disc excitation has been investigated privately by an English engineer, G. Holliman. In the magazine, Hi-Fi Answers, Holliman describes a phenomenon in which the record plays the stylus, as well as the reverse: "When the groove moves the stylus to reproduce a signal, the groove itself is slightly deformed and radiates waves on the record surface like ripples in a pond. These waves travel across the record surface, reflect from the curved edge, and return to the stylus, where they are corrected each time they pass, to further sound pulses (like an echo plate)." He goes on to give a detailed explanation of the audible effects of this wavelike vibration, describing the result as resembling the exaggerated reverberant effect of a large hall. Holliman also suggests an interesting experiment as one means of verifying its sonic effects. Suspend a record above the turntable platter on a stack of large washers such that only the labeled portion of the disc is supported, and compare the sound of the disc played in that position with the same disc played under normal circumstances.
Although Holliman's work is not backed by the same degree of experimental data as the studies of other disc vibration phenomena, it seems to be theoretically accurate. At least two cartridge manufacturers have noted similar occurrences under laboratory conditions, and audiophile groups have reported repeatedly on the sonic "flavor" it adds to certain records. 11, 12. Support and damping for the record disc, or lack of them, seem to be the primary factors in determining the degree of record susceptibility to vibration. The Denon tests involved two different types of mats- a conventional ribbed synthetic rubber mat and a heavy 10 mm thick butyl rubber unit.
The results indicate that mechanical damping was critical to the mat's effectiveness. Rubber and plastic units damp out vibration visco-elastically by dissipating the mechanical energy of motion in the process of "stretching" a lossy, elastic medium (the rubber compound). Felted or fibrous materials, by contrast, perform the same dissipation of vibrational energy in the process of separate fibers rubbing together, causing friction and energy conversion to heat. This fibrous energy conversion is somewhat more effective over a wide range of vibrational frequencies and has the additional ad vantage of being able to incorporate conductive elements. The findings of other tests support this contention. In the American Audioport experiments, the harmful vibrational effects of the edge-supportive and ribbed platters could not be duplicated when felted or heavy rubber mats were used. The Holliman article notes that the reverberant character of sound vanished when a heavy pad was substituted for the washer stack.
Important in the selection of mats is the thickness of the mat as it relates to cartridge tracking angle. A thin mat will change the cartridge angle less than 0.7° (much less than warp angle), whereas the 3/16-in. thickness of some "over mats" can change the tracking angle as much as 2.5°. Tonearms that can be vertically adjusted are essential to compensate for such increased pad thickness.
Replacing a conventional turntable mat with an anti-static, anti-resonant type will not radically alter the sound of an audio system, for the effect of such a change is to provide an awareness of musical nuance. Such subtle enhancement is true of any improvement which does not replace the major components of the system.
Nuance, however, is what separates the excellent from that which is merely satisfactory.
References
1. John J. Bubbers, "An Evaluation of the Forces Required to Move a Tonearm," Journal of the Audio Engineering Society, October, 1970.
2. C. Roger Anderson, et al., "Phonograph Reproduction--1978," Audio May, June, 1978.
3. Staff Report, "Audio Accessories--Supplement," Hi-fi News and Record Review, December, 1977.
4. M. P. Hide, et al., "Electrostatic Forces on Pickups" (Letters) Wireless World, May, June, August, September, and December, 1974.
5. National Swedish Institute for Testing, Inspection, and Metrology Test No. 77100-33, Spring, 1978.
6. K. Clunis & M. Kelly, "Overcoming Record Warps and Low Frequency Turntable Rumble in Phonographs," Journal of the Audio Engineering Society, July-August, 1975.
7. Benjamin B. Bauer, "On the Measurement of Rumble in Phonograph Reproduction," Journal of the Audio Engineering Society, April, 1967.
8. Gary T. Nakai, "Dynamic Damping of Stylus Compliance/Tonearm Resonance," Journal of the Audio Engineering Society, September, 1973.
9. Benjamin B. Bauer, "On the Damping of Phonograph Arms," Journal of the Audio Engineering Society, July, 1963.
10. G. Holliman, " Belt Drive Versus Direct Drive," Hi-fi Answers, November, 1976.
11. Arthur Pfeffer, "Further Thoughts--The Dust Bug," The Absolute Sound.
12. Art Scott, "Skinny Discs and Platter Pads," The Boston Audio Society Speaker, July, 1978.
(Adapted from: Audio magazine, Jun. 1979)
Also see:
The Fine Art Of Tweaking Your Turntable (Jun. 1979)
Understanding Phono Cartridges (March 1979)
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