by EDWARD M. LONG
In my reports on turntables and tonearms for Audio, I have tried to measure various performance characteristics and correlate them with audible effects. In the main, I have focused on the tables and arms, almost ignoring turntable platter mats, spindle clamps, and vacuum disc hold-downs, and yet these devices can have large effects on the quality of sound reproduction in both the frequency and time domains. For example, if two identical transient signals of very short duration occur simultaneously, and one of them is 20 dB lower in level, the stronger signal will certainly mask the weaker. The total output is increased by less than 0.05 dB, which is not an audible change. If the two transients are not synchronous, but one is delayed with respect to the other, then it isn't the level change that is important, but the delay. If the delay between two transient signals is long enough, the result is what everyone would call an echo. For a 33 1/3-rpm record, the energy from a heavily modulated adjacent groove can produce a signal which is either 1.79 S before (pre-echo) or after (echo) the desired signal. This report isn't concerned with this phenomenon directly, although it can be a source of energy that reduces the clarity of reproduction in a less than obvious way. The problem of concern here is due to signal delays of much shorter duration. If a recorded signal is delayed slightly and then is added to the main signal, the level will be increased at each frequency where the polarities are the same, while at each frequency where the polarities are opposite, the level will be decreased. This causes the resulting spectrum to have peaks and dips. So not only is the spectral energy dispersed or smeared in time, by having one signal slightly delayed from the other, but it now has coloration due to the peaks and dips.
Delayed energy can be created in a record during playback, resulting in coloration and smearing in the reproduced sound. My attention was first drawn to the problem and its cause in an article by Peter Moncrieff published in the International Audio Review. Another equipment reviewer concerned about the problem is Martin Colloms, who writes for the British publication Hi-Fi News & Record Review.
Let's briefly review how a record playing system works. A record groove contains information which is stored in the form of very small changes in the average position of the spiral groove.
When a record is rotating, these rapid variations in the relatively slow-moving spiral groove impart mechanical energy to the stylus, causing it to move in relation to the cartridge, which remains relatively stationary. The force exerted upon the stylus is then converted by the cartridge from mechanical energy to electrical energy, and this is what produces the electrical signal which is fed to an amplifier. So far, I have only described something that most Audio readers probably already know. However, there is a problem which may not be as well known. As Newton's Third Law states, "Every force produces an equal and opposite force. While the undulating groove is imparting a force to the stylus, the stylus is, in turn, imparting an equal and opposite force back against the groove. What happens to this energy? I haven't seen a definitive study of this phenomenon in print, but it appears that the energy travels inside the record, bounces around, and is eventually dissipated, with some of it finding its way back to the stylus. This energy is delayed in time and therefore can be heard. And that's the problem. The remedy is to cause this energy to be absorbed and dissipated as quickly and completely as possible. The best way to do this appears to be by allowing the energy to be transferred from the undersurface of the disc into a record mat, where it is dissipated. What is attempted here is an initial quantitative study of the effectiveness in energy dissipation by a few classic or well-known mats, as well as of the effect of record clamps and vacuum record hold-downs on this process. (This is not an attempt at an exhaustive survey of any of these three categories, and we are aware that most of these mats are no longer on the market.)
The Test Procedure
The problem of inducing mechanical energy into a record, to simulate the effect of a reactive force of a stylus as it travels in a groove, is not a simple one. There are some sophisticated ways of doing this, and they are certainly worth investigating. For instance, the use of the techniques employed in time-delay reflectometry is a possibility, but the very short time delay between the induced signal and the reflected signals makes it difficult. Time Delay Spectrometry, using the modern instrumentation now available, would probably be a better method of obtaining data and would allow us to understand what is happening inside the record in a more precise way. The test signal could be injected into the record and recovered using the same transducer, but this is not as easy as it sounds; the transducer's characteristics would have to be carefully controlled to allow it to act as both sender and receiver.
Since I haven't come up with a transducer or the send/receive circuitry needed for such an elaborate test, I have opted for a different method. For this report, the test signal was applied as a mechanical impulse to the edge of the record. It was then picked up by a phono cartridge with its stylus resting in a stationary groove. Instrumentation consisted of a Nicolet Explorer III digital storage oscilloscope, which was used to obtain amplitude-versus-time waveforms, and a Nicolet 660A 2D digital Fast Fourier Transform analyzer, used to obtain the magnitude-versus frequency spectrum data. All individual mat tests were conducted twice, first without a record clamp and then with a clamp placed down against the record to force it into more intimate contact with the mat. The same record was used for all the tests to make certain that variations in the plastics used by different manufacturers did not affect the results.
The Mats and the Clamp
The mats tested in this report include one made of polyurethane foam, four made of what might be called synthetic rubber, and one made of acrylic. After testing these individually, I evaluated several combinations; finally, I tested the SOTA Star Sapphire turntable, which uses a vacuum system to pull the record against the mat. The polyurethane mat is very similar to a variety of mats which have been offered in the past and is not specifically designed for maximum energy absorption. It was included because mats of this type are widely used, and I thought it would be interesting to compare it to those designed specifically for energy absorption. The rubber mats vary from the relatively hard Oracle to the very soft Turomat. The acrylic mat from SOTA is a fairly recent offering and represents a slightly different approach to the delayed-energy problem.
Fig. 1--Output vs. time for mechanical impulse applied to edge of stationary
record resting on Scotch Dustguard mat, unclamped, with stylus in groove.
Fig. 2--Same as Fig. 1 but with clamp.
Fig. 3--Spectrum (averaged) of output caused by mechanical impulses
applied to edge of stationary record resting on Scotch Dustguard mat,
with stylus in groove, with and without clamp. (Reference signal level
for this and other output spectrum figures is 10 cm/S at 1 kHz.)
Fig. 4--Output vs. time using Platter Matter mat without clamp.
Fig. 5--Same as Fig. 4 but with clamp.
Fig. 6--Output spectrum using Platter Matter mat, with and without clamp.
Fig. 7--Output vs. time using Mission mat without clamp.
Fig. 8--Same as Fig. 7 but with clamp.
Fig. 9--Output spectrum using Mission mat, with and without clamp.
The clamp used for this report was the Planax clamp, which is distributed by Monster Cable. It is machined from nylon, is 2 5/16 inches in diameter and 1/2 inch thick, and weighs 45 grams. A slot is machined across most of the clamp's diameter; the slot is enlarged at the clamp's center to tit over the turntable spindle and is V-shaped at its open end. A small cylinder is held against the V by an elastic band. Pushing in on the little cylinder wedges the slot open, causing the hole at the center of the clamp to expand. The clamp can then be slid over the spindle and down against the record. This causes the record to be pressed into contact with the surface of the record mat.
Another clamp which could yield the same results is the SOTA Reflex. This black-anodized aluminum clamp weighs 215 grams and is 3 5/8 inches in diameter and 1 1/2 inches thick. It resembles the type of clamp used to hold 10 1/2-inch reels on a professional tape recorder. The Reflex uses a flip lock system. When the lock tab is flipped up, the clamp can be slid over the turntable spindle and pushed against the record. When the lock tab is flipped down, the stainless-steel center is locked to the spindle, and the outside part is pulled down against the record label. This forces the underside of the record into intimate contact with the record mat.
Also, Souther Engineering makes the Clever Clamp, which appears as if it would do a reasonable job of holding a record against a turntable mat.
Interpreting the Figures
The first two figures for each individual mat show the amplitude versus time of the cartridge output when a mechanical impulse was applied to the stationary record, without and with the clamp in place. The total time span of the window is 102.35 mS. In the third figure for each mat, I have shown the magnitude (averaged) versus frequency of the spectral components caused by a series of mechanical impulses applied to the edge of the record, again both with and without the clamp.
The magnitudes shown next to each of the cursors, which are at 200, 500. and 2,000 Hz, are in dB relative to a reference tone of 10 cm/S at 1 kHz. The top value is without the clamp and the bottom value is with the clamp. In a few of the figures, the with-clamp values climb up above those without the clamp. Ideally, this should not happen, but unfortunately it does. A record mat, or combination of mats, which produces a smooth spectrum seems to be preferable to one which actually provides more absorption at some frequencies but has a more uneven spectrum. This uneven distribution of absorption causes coloration in the reproduced sound.
Scotch Dustguard Mat
This mat is made of polyurethane foam which has been treated to give it anti-static properties. It is 11 3/4 inches in diameter by 1/4 inch thick and is completely flat, with no indentation to accept the thicker part of a record near its center. If the diameter were 11 1/2 inches, and if it had a slight recess at least 4 1/8 inches across at the center, it might have been more effective in removing energy from the record. This is because a record is thicker at both the edge and the center, causing the grooves to be held away from a flat record mat. As it was, even the clamp was not able to press the record into complete contact with the mat across its entire surface. This particular brand of mat is no longer being sold, and with the inclusion of the anti-static treatment, it represents a high point for the basic type. Mats like this, without the treatment, were quite prevalent in the past, and some manufacturers provided them with their turntables.
This mat had the lowest energy absorption of the group tested. It did neutralize the static charge buildup on records, but that was not the main concern of this series of tests. Figure 1 shows the result with no clamp. While the clamp did help to reduce the initial transient, as shown in Fig. 2, it also caused the magnitude of the spectrum (Fig. 3) to be more ragged, especially below 800 Hz, and this would tend to color the sound. Further, the clamp increased the output at a number of frequencies.
Listening verified that the clamp did actually increase the coloration of the reproduced sound. These results, both with and without the clamp, indicate that mats of this type probably should be avoided if sonic purity is the goal.
Platter Matter Mat
This blue mat was very effective in absorbing energy above 800 Hz. It is 11 3/8 inches in diameter on the bottom, 11 5/16 inches on top, and 3/16 inch thick. To accommodate the record label, it has a recess measuring 4 1/16 inches in diameter, but it also has raised lettering that would keep the label area from making intimate contact with the mat. Comparing Figs. 4 and 5, one can see that the clamp reduced the initial transient, and the spectrum shown in Fig. 6 indicates that the clamp improved the performance over virtually the whole range. The mat did have a greater absorption of energy at about 1 kHz (shifting down to about 800 Hz when the clamp was in place), and this single dip is a problem because it allows the energy around 1 kHz to stand out. This caused a slight coloration in the reproduced sound, probably because the ear is very sensitive in this range. Figure 5 shows that the output has a definite periodic characteristic corresponding to about 10 mS. This would be about 100 Hz, but since I don't know the actual velocity of sound in the record's particular plastic formulation, I can't be certain of the exact frequency. There is a peak at approximately 150 Hz in the spectrum shown in Fig. 6.
The outside diameter of the Platter Matter is less than that of a record, and this allows the underside of the record to make uniform contact with the mat.
When the clamp pushes the record down against the mat, much more energy is drawn from the record. The performance of the Platter Matter was definitely improved when the clamp caused intimate contact between the record and the mat. Not currently available, this mat was rated good because, though the improvement in clarity was only moderate, there was a relatively low level of coloration.
Mission Mat
The Mission mat has a slightly beveled edge and measures 11 9/16 inches in diameter on the top surface and 1 15/8 inches in diameter on the bottom.
This dark red mat is 1/8 inch thick and is relatively compliant. It is made of Sorbothane, a synthetic material originally developed to have the characteristics of human flesh and which is currently being used for the mats sold by Audio Quest, though these don't appear identical to the Mission mat. The 4 1/2 inch center section is recessed more than is necessary and has raised lettering, which means that the label area of the record does not make intimate contact with the mat.
The Mission has very good absorption above 200 Hz. Figures 7 and 8 indicate that, while the clamp is effective in reducing the initial transient, it actually increases the output at about 150 to 200 Hz. This is verified by the breakthrough area at the lower frequencies of Fig. 9, indicating that the magnitude is greater when the clamp is used to push the record down. The spectrum of Fig. 9 also indicates that the Mission mat's absorption characteristic varies with frequency. The peaks appear to have a certain harmonic relationship, which may be the main reason for the slight coloration that this mat adds to the reproduced sound. The clamp helps to increase the absorption above 200 Hz; even with the resulting unevenness in the absorption, I rate the mat as being good, because the slight coloration is easily counterbalanced by the increased clarity of reproduced transients.
Oracle Mat
No longer offered, this moderately stiff mat has a smooth surface which can be kept shiny by periodic cleaning; to this end, the manufacturer had supplied a bottle of cleaning fluid and a bottle of polish. The top and bottom diameters of the mat are 11 1/4 and 11 3/8 inches, respectively, which gives a slight taper to the mat's edge. This dark gray mat is about 3/16 inch thick and has a 37 5/16-inch recess for the record's raised label area. The recess is deeper than necessary and has raised lettering, which means that, as with the Platter Matter and Mission mats, the label area is not allowed to come into complete contact with the mat's surface.
A comparison of Figs. 10 and 11 shows that the initial transient is not reduced much with the clamp in place, but the lower frequency components of the secondary transient are reduced considerably. Figure 12 shows a very smooth absorption-versus-frequency characteristic, both without and with the clamp. Because of this smooth characteristic, the Oracle mat caused very little coloration. As is evident in Fig. 12, the clamp is very effective in increasing the absorption of the delayed energy from the record, and it does not cause any significant change in the coloration of the reproduced sound. I rate the Oracle as being very good. Even though some other mats have greater absorption, the smoothness of the Oracle's absorption characteristic is better, and it has less coloration.
Fig. 10--Output vs. time using Oracle mat without clamp
Fig. 11--Same as Fig. 10 but with clamp.
Fig. 12--Output spectrum using Oracle mat, with and without clamp.
Fig. 13--Output vs. time using Turomat without clamp.
Fig. 14--Same as Fig. 13 but with clamp.
Fig. 15--Output spectrum using Turomat, with and without clamp.
Turomat
This mat., the most limp of all those tested, is light tan with a dull finish. It has a very distinct bevel on its outer edge a diameter of 11 1/8 inches on top and 11 3/4 inches on the bottom. and a thickness of lust less than 1/4 inch. The upper diameter allows the record to overhang slightly so that its thicker edge does not keep it raised above the mat. This allows the underside of the record to make good contact with the top surface of the mat. The center indentation of the mat has a diameter of 4 1/4 inches, wide enough to allow for the record's thicker center part. However, it is deeper than it needs to be and also has raised lettering, which does not allow intimate contact between the mat and the record near the label area. The maker has turned largely to cleaning materials and apparently no longer offers the mat.
Comparing Figs. 13 and 14, it is easy to see that not only is the initial transient reduced with the clamp, but the secondary transient is reduced even more. The magnitude-versus-frequency data of Fig. 15 shows that, with the clamp in place, the Turomat has the greatest absorption of all the mats for frequencies above 200 Hz. Except for the little peak that stands out at 500 Hz. the spectrum also exhibits excellent smoothness. The peak is very narrow and low in level, so it probably doesn't cause coloration which can be heard easily. Figure 15 also clearly indicates that the use of a clamp with the Turomat is definitely recommended.
The clamp does increase the output slightly at a couple of spots below 200 Hz. but close inspection reveals that the spectrum is actually smoother when the clamp is used. The Turomat is the best of the group tested.
SOTA Supermat
This mat is very different from the others in that it is made of acrylic material and is very hard. The rationale behind the use of acrylic is the idea that energy transfer should be greater and more uniform across the audio spectrum if the mat material is similar to the record material. The mat is 3/16 inch thick and is translucent because its top surface is machined. The outside edge is rounded rather than straight, and the mat's diameter is 11 1/2 inches on top and 11 3/4 inches on the bottom. The smooth center recess is 4 1/4 inches across and just deep enough to accommodate the thicker part of a record near the label. The Supermat is designed to be used with another, more conventional, mat underneath it. Accordingly, the first tests were performed with the standard SOTA rubber mat under the Supermat.
Figure 16 shows the amplitude versus time for this combination with the clamp in place, and Fig. 17 shows the magnitude versus frequency, both with and without the clamp. Figure 17 indicates that there is a good amount of absorption but that there are peaks at 500 Hz and 1 kHz. The clamp causes an increase in output between 150 and 400 Hz and between 800 Hz and 3 kHz. This combination is good, but the next one is better.
SOTA Supermat and Turomat Combination
Comparing Figs. 16 and 18 shows that the initial transient is much reduced when the softer Turomat, rather than the standard SOTA mat, is used under the SOTA Supermat. There is still a considerable amount of output below 200 Hz, as Fig. 19 shows. The peaks at 500 Hz and 1 kHz are still present when the clamp is not forcing the record against the Supermat and the Supermat against the Turomat.
When the clamp is in place, the peaks are reduced considerably and would not be expected to cause any coloration in the reproduced sound.
Comparing Figs. 15 and 19 gives some insight into the reason why the Supermat, Turomat combination is preferable to the Turomat by itself When the clamp is in place, the more gentle slope of Fig. 19 compared to Fig. 15 is less likely to cause coloration. The clamp is very effective and does not increase the magnitude of the output except in a couple of places in the spectrum, and then only very slightly.
When the clamp is not used, the spectrum of Fig. 19 is certainly better than that shown in Fig. 15. This indicates that when the clamp is not securely in place, doing its job perfectly-which is likely to happen from time to time this combination will still do a very effective job in absorbing energy from the record. The spectrum will also have a gentle slope, which is good. This is the best combination of mats that I tested, with good absorption and very low coloration.
Fig. 16--Output vs. time using SOTA Supermat resting on SOTA regular mat, with
clamp.
Fig. 17--Output spectrum using SOTA Supermat resting on SOTA regular
mat, with and without clamp.
Fig. 18--Output vs. time using SOTA Supermat resting on Turomat, with clamp.
Fig. 19--Output spectrum using SOTA Supermat resting on Turomat, with
and without clamp.
SOTA Star Sapphire Vacuum Turntable with SOTA Supermat and Rubber Mat
Figure 20 shows the amplitude versus time for the Supermat used over the rubber mat on the SOTA Star Sapphire Turntable, with the turntable's record-holding vacuum system activated. The initial transient is very low, as is the secondary. The magnitude-versus frequency spectrum shown in Fig. 21 indicates an extremely high output at low frequencies when the vacuum is not applied. The record is actually held up away from the surface of the Super mat by the rubber lip which surrounds it at the periphery. When the vacuum is applied, the absorption of energy is very smooth across the whole spectrum. The absorption in the low frequency range is the best of any that I measured during this investigation.
The increase in output around 800 Hz when the vacuum is applied is very slight and does not stand out, and the depression between 700 Hz and 1.5 kHz which is present when the vacuum is not activated is now gone. The clarity of the reproduced sound is excellent, the best obtained during this series of tests. Another advantage of using the vacuum is that the entire record surface is pulled flat, thereby eliminating the vertical component of wow which occurs during play.
Fig. 20--Output vs. time using SOTA Supermat over rubber mat on SOTA Star Sapphire
turntable, with turntable's vacuum system on.
Fig. 21--Output spectrum using SOTA Supermat over rubber mat on SOTA Star Sapphire
turntable, with and without turntable's vacuum system on.
MEASURED DATA (table)
Conclusions
With the exception of the Dustguard, all of the mats and mat combinations discussed in this report improved the clarity of the sound, especially when the clamp was in place. The coloration of the sound due to the variation of absorption versus frequency was the main difference between the Platter Matter, Mission. Oracle, and Turomat: in this regard they rank in reverse order, with the Turomat having the least coloration. The combination of the Turomat and the SOTA Supermat was slightly better than the Turomat by itself, because it seemed to have slightly less coloration. The sound reproduction with the combination of the Super mat and the regular SOTA turntable mat was brighter and more colored than that of the Turomat by itself. The SOTA vacuum system yielded the best results with the greatest clarity and lowest coloration of anything I tested.
The lack of wow was also a definite plus. I think the vacuum provides such an improvement in the reproduced sound that other manufacturers should look into the possibility of offering similar systems, at least on their best turntables.
During the turntable tests I have conducted over the last few years, I have found a definite improvement in clarity and a lessening of coloration in analog record reproduction. I know much of this has been due to improvements in the way delayed energy has been dealt with by both turntable and accessory manufacturers. I strongly recommend that you investigate the current crop of turntable mats and clamps for your system. Then you will hear for yourself the improvement in the quality of reproduced sound that they can make.
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