Audible Effects of MechanicalResonances in Turntables

Paper presented at the AES Convention in New York, 1977-233

Audible effects of mechanical resonances in turntables

by Poul Ladegaard. Briiel & Kjaer

IntroductionFor many years now, it has been

one of a Hi-Fi enthusiasts libertiesto choose and combine all the ele-ments in his sound system withonly price and persona! taste as li-miting factors. Therefore, integratedreceivers and factory tayiored Hi-Fisystems have only had little appealto the serious Hi-Fi consumer. Natu-rally, manufacturers of top-gradeequipment have sensed that trendand offer products in as small partsas reasonable, to aliow greater flexi-bility for the consumer when build-ing up his system.

Unfortunately, one- of the most im-portant factors in getting goodsound quality from a system is thatthe various elements interface wellwith each other. To ensure that, re-quires both insight and experiencethat most Hi-Fi buyers simply do nothave.

It is not uncommon, therefore, tosee Hi-Fi systems put together b ychoosing the most advanced andbest equipment in each group comeout with only an average soundquality.

From our experience, the con-struction of a good turntable froman individual motor, tonearm andcartridge is one of the most demand-ing tasks in this respect. This is ofcourse due to the many complex in-terface problems involved.

Moreover, most manufacturershave serious troubles with this, onereason being that only very few

The measuring set up

ditional way tell us virtually nothingabout what they are supposed to do— the motor. Likewise, the presettracking force with the tonearm instand-by condition has very little todo with the values found under ac-tual playing conditions.

Other factors not yet subject tostandard measurement techniquesseem to have a strong influence onthe total sound quality. This is theresult of spurious mechanical reson-ances in the tonearm itself, in thechassis structure and the platter,mat, record interface. This latterproblem will also be deait with.

manufacturers run a productionwhich incorporates all three impor-tant parts. Among them only a cou-ple have realized the interface prob-lems and tried to solve them. How-ever, the results from those firmswho have, are so impressive, thatthey really show where to gain im-provements.

In this paper we shall try to seehow these interface problems inreality govern both the objectivelymeasured and subjectively evalu-ated parameters. It is shown howspecifications like rumble and wowand flutter, when measured the tra-

The mechanical resonancesA turntable consists, in principle,

of a rotating platter and a fixture forthe cartridge. The purpose of this isto give the record groove a velocity,relative to the cartridge diamond,which is precisely 33 1/3, 45 or78 rpm. The support for the car-tridge has only the purpose of keep-ing the diamond in a constant andsteady contact with the groove.This, in such a way, that the forcebetween the needle and groove atall times has the same, pre-deter-mined, magnitude and direction.

This sounds quite simple in the-ory but in practice one has to dealwith mechanical parts which arenot perfectly stiff and non-flexingdue both to economical and techni-cal reasons. The flexing of the var-ious mechanical parts in the turn-table seriously disturb both of theabove mentioned requirements for aturntable. The actual places where

flexing can occur are indicated withsmall springs in Fig. 1.

If we take a look at these compli-ances, all of them, except one, canin theory, be avoided. The flexing inthe tonearm itself and its fixture tothe platter can be avoided by theproper choice of materials. It is notas easy to kill resonances in theturntable platter, its mat and the re-cord itself. However, improvementscan be achieved by proper design.

The only one which in principlecannot be removed is the compli-ance between the needle cantileverand the cartridge body, which unfor-tunately seems to be the most trou-blesome.

In the first part of the paper weshall only deal with this and seehow it affects the standard specifica-tions of a turntable and the audiblequality. Resonances in the cartridgeitself will not be treated in this text.

Fig.1. Sketch of the placement of elastic couplings between the various parts in a turntable.The compliance of these couplings builds up resonances which disturbs the relative posi-tion between the cartridge needle and the record groove. This results in deterioration ofthe sound quality

Parti

The fundamental tonearm resonanceRecent cartridges have been con-

structed so that they are able totrack modern recordings at a track-ing force in the range around10mN. To achieve this at low fre-quencies, it is required that the com-pliance in the cantilever suspensionbe relatively high, usually around35 — 50//m/mN.

This compliance together with theeffective mass of the tonearm car-tridge combination determines thefundamental tonearm resonance,see Fig.2. The calculation of the re-sonance frequency follows simplyfrom

This resonance of course, causesa boost in the frequency responsewhich has an amplitude dependenton the amount of damping in thesystem.

It is a relatively simple matter tomeasure this. The set-up is shownin Fig.3. It uses the B & K test re-cord QR 2010 which has a laterallycut frequency sweep from 5 —20 Hz.

Fig.2. The fundamental tonearm resonanceis determined by the compliance C inthe cartridge and the effective massof the cartridge m and the tonearmM. Both referred to the needletip

1f res =27rVC(m + M)

Here M and m are the effectivemasses of the tonearm and car-tridge respectively, both referred tothe stylus tip. C is the compliancein the cantilever supsension. Fig.3. Set-up for measuring the fundamental tonearm resonance of a turntable

reason for suspecting any of theseto cause trouble in the audiblerange. For instance, the frequencyresponse in the range 20 —20000 Hz is essentially unaffected.But as we will show in the follow-ing, the actual size and frequencyof this resonance strongly affectsparameters as rumble, wow andflutter and the tracking force.

As pointed out by e.g. PeterRother, (Ref,1) and Happ and Kar-low (Ref.2), the optimum tonearmresonant frequency must lie aroundor slightly above 10 Hz. To try togive a realistic evaluation of theside effects of this suggested opti-mum tonearm resonance, we havefor the following sections in thepaper only used cartridge nr. 3,which fulfills the mentioned criteriain each of the three arms. SeeFig.4.

However, it must be pointed outthat the cartridge nr. 3 has an unty-pically low compliance, and Arm nr.1 an unusual low effective mass (be-low 2,5 grams). This leaves combi-nations Arm 2 and 3 with cartridge1 and 2 as representative for the ac-tual situation, with arm resonancesaround 5 — 8 Hz. The influence innegative direction on both mea-sured and subjectively perceivedrumble, flutter tracking is muchmore aggrevated than the followingresults might indicate. See e.g. themeasurements on an "average"turntable Figs. 11 and 18.

Fig.4. At first sight these curves look rather irrelevant since they do not alter the frequency re-sponse in the audible range. The indirect consequences are on the other hand severe, giv-ing rise to measured rumble, wow and flutter and tracking problems

Three arms combined withthree different cartridges

As already mentioned, the funda-mental tonearm resonance dependssolely on the compliance of the car-tridge and the effective mass of thetonearm and cartridge, i.e. the ac-tual combination.

To see how this affects the var-

ious parameters we have chosenthree cartridges and combined themwith three arms of different con-struction and effective mass. Theywere mounted on the same turn-table while measurements were car-ried out.

As the examples shown in Fig.4indicate, there is, at first sight no

Rumblegures for turntables. If he uses theweighting filter B it is possible toreach values in the vicinity of65 dB. This is in fact so close to thebackground noise from the cartridgeand preamplifier that this measure-ment tells him very little about thequality of the turntable. If, on theother hand, he switches to filter Bthe figure may show up around30dB. Is this then rumble? The an-swer is no. If he changes to anothertonearm or cartridge it may nowdecrease to 50dB. How confusingresults one can get is shown inTable 1 where we have listed the Aand B rumble readings measuredon the same turntable and cartridgebut with three different arms.

Fig.5. Basic set-up for measuring rumble in a turntable

The standard way of measuring ,rumble is to use the set-up shownin Fig.5. The response test unitB & K 4416, is equipped with twoweighting networks A and B in ac-cordance to the standards. The filtercharacteristics are shown in Fig.6.

The rapidly increasing standard inthe quality of modern turntable mo-tors — better main bearings, directdrive and belt drive systems with ef-fectively decoupled motors — hasmade it very difficult for the manu-facturer to specify useful rumble fi-

To explain this, the results shownin Fig,4 come out as useful. In thearea around 5 — 20 Hz, the car-tridge response as shown is not atall linear so the total weighting func-tion is the combination of thecurves shown in Fig.4 and curve Ain Fig.6. But how can this give sucha great difference as reflected bythe figures in Table 1? The reasonis that records, also good test re-cords, have been through the samepressing process as normal ones.This means that they contain alarge amount of "rubbish" from themanufacturing process. Harpp andKarlpw (Ref.2) have shown thatthese sub-audible components havean amplitude distribution whichpeaks at around 3 — 4 Hz (Fig.7).

Fig.6. Standard weighting networks A and 8 for rumble measurements

Therefore, what we measure withrumble filter A also has very little todo with the rumble caused by irregu-larities in the motor. It is mostly sub-sonic surface irregularities in the re-cord boosted by the arm resonance.

Therefore the record chosen forthe rumble measurement has agreat influence on the results. InFig.8 we have on the Narrow BandAnalyzer B&K 2031 rumblespectra from three different test re-cords. The standardized DIN test re-cord, the B&K QR 2010 and a La-guer record. The results here reflectboth a different content of subsonicnoise and irregularities in the re-cord and how it excites the tonearmresonance.

If one really wants to know whatthe rumble is from, it is necessaryto make an analysis. With the B & KHeterodyne Analyzer 20,10 in theset-up shown in Fig.9 we have ob-tained the results shown in Fig. 10.

RumbleA

RumbleB

Arm 1 Arm 2 Arm 3

38 dB 28 dB

58 dB 58 dB

22 dB

56 dB

Table 1. Rumble figures measured withB&K Type 2010 test record (ref.lOcm/s at 1kHz left channel).Note the strong dependence of theactual arm used on the A-weightedresults

B&K QR 2010 100Hz

Fig.7. Velocity spectrum of phonograph re-cords

Lacquer record 100 Hz

100 HzFig.8, Unweighted rumble spectra from

three different test records shown onB&K 2031. Arm number 3 was used

Fig.9. Set-up for making narrow band analysis of rumble using Heterodyne Analyzer Type2010

Unweighted njmble — spectrum —mainly due to tonearm resonanceboosted surface irregularities fromtest record B & K QR 2011 . The ref-erence a 1 / 3 octave pink noise at1 kHz has a level of —22 dB dB ref.10 cm/sa t 1 kHz

Fig.10. Some typical results from the measurements made with the equipment shown in Fig.9

It is now easy for both the manu-facturer and user to see how to im-prove things. The motor rumblecomponents are a job for the manu-facturer to handle, while the boostaround the arm resonance can beremoved, or at least decreased by aproper choice of arm/cartridge com-bination. The hum componentaround 50 Hz can usually be solvedby the user by re-arranging groundconnections etc.

A much more elegant and fastway of getting the rumble spectrumis the use of parallel analyzers suchas B&K 2131 and B&K 2031. InFig.11 we have on the 1/3 OctaveReal Time Analyzer 2131 shownthe rumble spectrum of a standardturntable when playing the test re-cord QR 2011. On side 2 the re-cord contains 1/3 octave filteredpink noise intended for use in loud-speaker tests. Fig.11 now showsthe rumble level with the measur-ing signal, a band of 1 /3 octavenoise at 1 kHz as reference.

Since our ears do not detectsound much below 20 Hz, onecould ask: Why then bother aboutthis subsonic "rubbish"? The an-swer is simply that there are no di-rect audible effects of that, and ifthere were, it is a simple matter toinstall a high pass filter, with a cut-off around 20 Hz.

What then about the indirect ef-fects? Well, the physical meaning ofa resonance is increased ampli-tudes of the relative movements be-tween the record surface and car-tridge and, in addition, the lowerthe frequency the greater the ampli-tude. Therefore, all the time the car-tridge will oscillate at the tonearmresonance with large excursions —just the prescription for making in-termodulation distortion on tones inthe audible band. Remember, boththe rubber suspension in the car-tridge and the preamplifier are notiinear for large amplitudes. The firstthing done in an RIAA preamplifieris to boost the low frequencies by20 dB.

1 6 Hz Arm 1 1 25kHz

Arm 2 1,25 kHz1,6 Hz

Arm 3 1,25 kHz1,6 Hz

Fig. 12. Unweighted rumble spectra usingthree different arms in combinationwith one motor and cartridge. Testrecord B & K QR 2010 was used.110 (SB at the screen is the refer-ence level 10 cm/s at 1 kHzFig.13. Set-up for measurement of rumble using Parallel Analyzers B & K Type 2031 or 2131

Wow and FlutterWhen discussing turntable mo-

tors the prime parameter apart fromrumble, seems to be speed stability.This is normally described in differ-ent ways depending on the fre-quency at which the speedchanges. Deviations in the range 0— 0,5 Hz are called "drift" and ismeasured as an average over a cer-tain period. Here the IEC standard(Ref.3) requires the use of a 30 s av-eraging time. When the frequencymodulation is in the range 0,5 Hz to10 Hz it is called wow and from10 Hz to 100 Hz flutter. But sincethe ear does not change sensitivityto frequency modulation very shar-ply at 10 Hz there is in practice noreason for measuring them separ-ately.

A very simple set-up for quantify-ing drift and wow and flutter isshown in Fig. 14.

indication. In this case the greatestdeviation in either positive and nega-tive direction is reflected in themeasurement. This makes the useof RMS detection less relevantwhen searching for optimum corre-

lation between measurable and au-dible evaluation. H. Saki (Ref.4) hasfound that the ear is able to detectas little as ± 0,06% wow and flutteron a complex 5 kHz tone when themodulating frequency is 3 Hz.

Fig. 15. Three possible characteristics of the 5&K Type 6203 Flutter Meter. The two markedLin 315 and Lin 1000 are useful when making an analysis of the flutter spectrum (seeFig. 15). The standardized weighting curve with its maximum at 4 Hz rejects the ear'ssensitivity to wow and flutter as function of modulation frequency

Fig.14. Set-up for measurement of driftand wow and flutter using 6 & KType 6203 Automatic Flutter Meterand Test Record QR 2010. The sta-bility of the recorded 3,1 S Hz signalallows wow and flutter measure-ments down to ± 0,06% weighted

In order to ensure a good correla-tion between the measured resultsand the degradation of the soundquality as judged by the ear, theweighting curve shown in Fig.15should be used. Extensive listeningexperiments have shown that theear is most sensitive to wow andflutter when the modulating fre-quency is around 4 Hz. Since it isthe actual deviations from the origi-nal frequency, that is sensed by theear, it is reasonable to use a + peak

Fig. 16. Instrument arrangement for third octave analysis of flutter

Weightedwow and flutter

Arm 1

+ 0,04%

Arm2

i 0,12%

Arm 3

± 0,06% .-

Table 2. Measured wow and flutter usingthe set-up in Fig. 14. The same mo-tor and cartridge was used withthree different arms. The test re-cord was B & K Type QR 2010 Fig.17. A sophisticated version for flutter analysis using B & K Type 2131 or B & K Type 2031

A closer look at the spectrogramin Fig. 18 clearly reveals four maincauses of wow and flutter. Thepeaks at around 0,5 and 1,2 Hz aredue to imperfections in the test re-cord. The centring is a little off andthe groove is not perfectly circular.Another important peak is seen at afrequency corresponding to thetonearm resonance, and thereforedependent on the chosen arm/car-tridge combination. The last peak ofimportance, in this case at 25 Hz, isthe only component that is relatedto the quality of the turntable mo-tor.

This indicates that a single wowand flutter figure is very unreliableas a guide for the manufacturerwho wants to improve his motor. Itis also misleading to the consumerwho buys his persona! choice ofarm and cartridge, since theystrongly affect the amount of bothmeasured and subjectively per-ceived flutter.

The situation is even more aggra-vated when adding the weightingfunction from Fig.15 to the analysisin Fig. 18. It then becomes evidentthat the only really important par-ameter contributing to wow and flut-ter is the influence of the tonearmresonance. This once again puts thetonearm resonance in focus.

To see how the relative move-ments between the tonearm and re-cord due to the resonance affectwow and flutter, one could look atFig.20. Here we have shown howthe vertical tracking angle for a car-tridge, now standardized to 20°transforms vertical oscillations ofthe tonearm into needle movementsalong the groove, causing frequencymodulation. In the same way thetonearm offset angle (usuallyaround 20 — 25°) causes in-creased wow and flutter when thearm is oscillating in the horizontalplane. To illustrate this close link be-tween the arm resonance and themeasured wow and flutter we haveshown the wow and flutter spec-trum for the turntable fitted withthe three different cartridge/armcombinations. See Fig.21. Apartfrom showing that the dominatinglines in the flutter spectrum isclosely related to the arm reso-nance(s) also the faults in the testrecord is seen to be important forthe measured flutter value.

Fig. 18. Typical result of B flutter analysis made with the set-up in Fig. 16. A standard recordplayer was used

of the actual arm used. Has this an-ything to do with the tonearm reson-ance?

To answer this, one has to makea frequency analysis. A simple in-strument arrangement for doingthis is shown in Fig.16. A more so-phisticated set-up in Fig. 17 useseither B&K 1/3 Octave Analyzer2131 or B&K NarrowbandAnalyzer 2031. Typical results fromthese measurements are shown inFigs. 18 and 19.

Just like the measurement of rum-ble with A or B filters, as demon-strated in the last chapter, has verylittle to do with the quality of theturntable motor, one could ask:What does a wow and flutter figureread on the meter really tell aboutwhat it is supposed to — the motor.

The standardized measurement ofwow and flutter was made on aturntable using three differenttonearms. The results, listed inTable 2, show a great dependence

Fig. 19. Weighted flutter spectra shown on Parallel Analyzers B&K Type 2131 and 2031. Amedium quality turntable was used and the test record slightly worn

Fig.20. Left: Oscillations of the tonearm at the arm resonance in the vertical plane causeswow and flutter proportional to the vertical tracking angleRight: Oscillations in the horizontal plane increases wow and flutter proportional to theoffset angle. (Nose that parallel tracking arms have an offset angle of zero degree andgives therefore superior wow specification)

Arm 1 50 Hz Arm 1 50 Hz

0 Arm 2 50 Hz 0 Arm 2 50 Hz

Arm 3 50 Hz Arm 3 50 Hz

1 different test records were used B & K QR 2010a n d f l u t t e r spectra for a turntable fitted with three different arms

(right) and a Lacquer record [left). Dynamic range shown on screen is 40 dB

Especially arm number 2 indi-cates that a main part of the spec-trum is covered with closely spacediines. Shifting the 2031 analyzerfrequency range to 0 — 10 Hz gavethe result shown in Fig.22. Fromthis, it is evident that the spectrumbelow 10 HE totally is influenced bythe eccentricity and irregularities ofthe test record. The fundamental fre-quency is around 0,55 Hz, but notethe high level of 10 — 13th har-monic. Naturally it also appears asamplitude variations (rumble) butthis time the fundamental 0,55 Hzand lower harmonics are filteredout by the frequency response ofthe arm/cartridge combination (seeFig.4).

10 Hz Rumble spectrum 10 Hz0

Fig.22.

Flutter spectrum

A closer look at the flutter and rumble spectrum reveals that record irregularities showup as discrete harmonic components. The fundamental frequency at 33 1/3 rpm is ap-pro*. 0,55 Hz. Note the high level and the 10—13th harmonic and compare withFig.21. Test Record B&K QR 2011 and arm number 1 were used

Audihie sidebandsIn the two proceeding chapters

we have discussed how the actualtonearm resonance affects the mea-sured values of rurnbie and flutter.It is also pointed out that the audi-bie effects of rumble and flutter areintermodulation which appears assidebands on single tones. Onecould therefore try to look for a di-rect correlation between the arm re-sonance and the number and sizeof the sidebands. With the set up inFig.9 is made a narrow band analy-sis of the playback of a 3 kHz tone.Three examples are shown inFig.23 (note the arm/cartridge com-binations here are not identical withany of those mentioned in Fig.4).The first (A) is the result measuredwith an arm/cartridge resonance of7 Hz. In (B) the resonance is around9,5 Hz and in (C) it has been put at16 Hz and some damping applied.The lack of sidebands in (C) com-pared with (A) gives a clear improve-ment in sound quality in terms of in-creased stability and transparencyin the stereo picture.

From this it is clear to see that toimprove audible quality the mainproblem is to reduce the relativemovements between cartridge andrecord as much as possible. !nother words, one has to damp thearm resonance and move it up-wards in frequency.

Fig.23. Narrowband analysis of a 3 kHz tone from three-arm/cartridge combinations with (A)undamped resonance at 7 Hz, (B) undamped resonance at 9,5 Hz and (C) damped re-sonance at 16 Hz. The corresponding plots of the tonearm resonance are shown below

Variations in tracking force

From Fig. 12 it is evident thatthere are relative movements be-tween the cartridge and record.These are larger the lower the re-sonance frequency and the less thedamping. However, this implies tovariations in the tension in the rub-ber suspension of the cartridge can-tilever. This means variations intracking force. To see how the fre-quency and damping of the tonearmresonance affect this in practice,we have made a couple of untradi-tional measurements.

The first is actually a transienttest which makes use of a speciallyprepared record. With a hacksaw atiny cut along the radius was made,enabling the two separated parts tobe displaced about 0,2 mm. Thisieaves a step which sets the arminto oscillation when travellingacross the notch (see Fig.24).

With this record we have testedthe three arm/cartridge combina-tions also used in Figs.4 and 7. Theresults are shown in Fig.25. Thesecurves were recorded on a storageoscilloscope, but the B & K Narrow-band Analyzer 2031 is also suited.In addition to the time response itcan also give the frequency re-sponse. An example is shown inFig.26.

This clearly illustrated the audibledifferences that can be heard be-tween the various combinations.Here the sound quality, especiallyin the iow frequency range, isgreatly improved the faster the oscil-lations die out. The phenomenon isquite similar to what can be heardwith loudspeakers where bass re-sonance shows different degrees ofdamping. It is common practise to-day in quality loudspeakers to havethe bass resonance damped to a Qaround 0.5 to 1,5. Why should ton-earms not behave that weli?

As mentioned, at resonancethere are relative movements givingvariations in tracking force. To get amore realistic view of these varia-tions under practical playback condi-tions we fitted the three differentarms with a strain gauge cartridge.The voltage from such a cartridge isdirectly proportional to the tension

a record. The set-up is sketched inFig.27. Some typical results areshown in Fig.28 using two differentrecords. Number 1 having a me-dium sized warp at the beginning of

in the suspension and trackingforce. It was therefore possible di-rectly to record on a storage oscillo-scope the variations in trackingforce during, e.g. two revolutions of

Fig.25. Oscillograms of the transient testusing the record mentioned inFig. 24. Each picture shows the vol-tage from the cartridge reflectingthe oscillations initiated by the step.As seen in most of the examplesthe oscillations continue for morethan 0.5 s (1/4 revolution of the re-cord)

Fig.24. A small cut in a record enables thetwo parts to be displaced a bit. Awell-defined step is then obtainedfor transient tests of cartridge/armcombinations

Fig.26. Time and frequency response of the transient test Fig.24 shown on B&K Type 2031.Tonearm number 2 was used

10

the record. Number 2 had no visiblewarps and was played at a radius of8 cm.

The effect of this on the soundquality is evident. When looking aiittfe closer to the oscillograms inFig.28 it can be seen that in thecase of arm nr. 3, the tracking force20% of the time is below 5 mN (halfof the preset value). It follows thenthat the cartridge is not able totrack high frequencies without dis-tortion for a considerable part of thetotal playback time. In this connec-tion it could be mentioned that in acorresponding time interval the

Fig.27. Set-up for recording the tracking force variations during play-back of ordinary records

tracking force is far above what it is Fig.29. Here we have shown on theB&K Type 2131 1/3 OctaveAnalyzer, the distortion from theplayback of a 1/3 octave pink noiseat 20kHz (from test record B&KOP 2011).

supposed to be with possible accel-eration of record wear.

The actual increase in distortiondue to mistracking is illustrated in

Fig.28. Tracking force variations during playback of two average records. The pictures show a period of two revolutions of the record and the ver-tical scale is calibrated directly in mN. The initial set tracking force was adjusted to lOmN. In this test the most lightweight arm (number1! clearly outperforms the other two

25 Hz Arm 1 20 kHz 25 Hz Arm 3Arm 2 20 kHz 25 Hz 20 kHzFig.29. Increase in distortion due to mistracking using the same cartridge in three different arms. The signal is a 1/3 octave pink noise at 20 kHz

recorded at —22 dB ref. 10 cm/s at 1 kHz (from test record QR 2011)

11

BIM (Bass Intermodulation)

quency and the damping of thetonearm resonance that reallycount. For years it has been knownthat wow and flutter — low fre-quency modulation — folds up inthe audible range as sidebands tothe tones there. The result is simi-lar from the amplitude intermodula-tion due to rubber suspension andpreamplifier unlinearities. We havealso shown how a "scratch" (tran-sient) in the record sets the arminto oscillation at the tonearm reson-ance giving coloration to the sound.

In Fig.30 is shown another stri-king example of how closely ampli-tude and frequency variations arelinked together in the range below20 Hz giving distortion in the audi-ble band. Here is shown a rumbleand flutter analysis of a turntablewith a pronounced tonearm reson-ance at 7 Hz. In addition there arewow and flutter components at 20,40 and 80 Hz.

Lastly we demonstrated the influ-ence on tracking force giving distor-tion in the midrange during play-back of high frequencies.

As a parallel to the now widelyused term TIM (Transient Intermodu-lation Distortion) which indicatesthe distortion components fallinginto the audible band when high le-vel and high frequency (out of band)signals are fed to a feed-back ampli-fier — we could introduce the wordBIM (Ref.5). Bass Intermodulation— a result of a high level low fre-quency (out of band) signals from arecord boosted by an undampedtonearm resonance.

The last conclusion we can drawfrom these investigations is themeans of avoiding BIM. Since wehave to accept that practical records(Ref.2) contain a large amount of"rubbish" centred around 4 — 5 Hzincluding warps, the optimum solu-tion is clear. The tonearm/cartridgeresonance has to be placed at sucha high frequency 1 3 — 1 8 Hz that itmechanically filters out the sub-sonic signals. In addition somedamping should be applied to elimi-nate oscillations and influence onthe frequency response above20 Hz.

As shown in the text above, thedirect consequences of a turntableshowing a resonant frequency re-sponse below 20 Hz is rather unim-portant. It is normally only detectedin connection with vented loud-speaker enclosures as large low fre-quency excursions. To cure this, theswitching in of a rumble filter — ahigh pass filter with a steep cut-offbelow 25 — 40 Hz — is perfectlyadequate.

As regards the indirect conse-quences, the situation is muchworse. It seriously affects both themeasured and audible rumble andwow and flutter from turntables,making standard, one figure "state-ments of turntable quality" doubt-ful. At least it has very little to dowith the actual rumble and wowand flutter originating from the mo-tor. The strong influence on theseby the actual tonearm/cartridge res-onant frequency tells that unlessthe measurements are accompaniedwith specified arm and cartridgethey are of no value.

Furthermore, the results shownclearly indicate that it is the fre-

Fig.30. Subsonic amplitude and frequency intermodulation results in distortion in the audible band. Here in the form of sidebands added to a puresine at 3 kHz.

12

Part If

Resonances in the tonearm itself

From Part 1 it follows that a possi-ble way of improving the perfor-mance of a turntable is an increasein the resonant frequency of thearm/cartridge. This means thateither the compliance of the car-tridge, its weight or the effectivemass of the arm have to be de-creased. The compliance, however,cannot be lowered without requir-ing an increase in tracking force.The cartridge weight is closely re-lated to, and a function of its con-struction. This leaves a reduction ofthe effective mass of the arm as thepractical solution. However, it alsohas drawbacks. A more lightweightconstruction is more susceptible toflex. These flexings in the arm re-sult in peaks and dips in the fre-quency response (see Fig.31). Herethe results are listed from one car-tridge in combination with three dif-ferent arms. As the B & K 2011test record is used it has a siowsweep (lateral cut 20 — 1000 Hz,50s/decade). This allows all res-onances to build up to their fullsize. We have recorded results fromboth right and left channels. Whenthe flexings show up " in phase" itindicates bendings in the verticalpiane and "out of phase" showsbending in the horizontal plane.

Fig.31. A slow sweep from 20 — 1000Hz, using B&K Type 2011 test record, reveals flexingin the tonearm itself. The same cartridge and turntable was usad

13

Acoustical and mechanical feedback

As already indicated in Fig, 1, aturntable can be described as anumber of completely stiff mechani-cal parts linked together with com-pliances. This leaves a great num-ber of different resonant modes pos-sible. To excite these and cause rel-ative movements between therecord and cartridge body not onlythe vibrations due to the informa-tion in the record groove should beconsidered. When loudspeakers areused for playback both the airborneand structure borne vibrations fromthe speaker should be taken into ac-count. Long before the system goesinto oscillation audible coloration ofthe sound is unavoidable. To give aqualitative view of this we tried theset-up shown in Fig.32. As a refer-ence the spectrum (Fig.33) is theacoustical response from the loud-speaker with the microphone at thearm position. With equidistant posi-

Fig.32. Measurement of the resonance modes in a turntable, when exposed to mixed acoustic-mechanical excitement. The signal fed to the loudspeaker is broadband pink noise

tions for the three arms relative tothe loudspeaker we measured upthe following three spectra shownin Fig.34. Here the stylus was rest-ing on a non-rotating record andpink noise fed to the loudspeaker.As seen, the three arms exhibit amarked difference in sensitivity tothis mixed acoustical-mechanical ex-citation.

A more detailed study of these re-sonances was then done with theNarrowband Analyzer Type 2031.The spectra from arm 1 and 3 wereread out on a Level Recorder Type2307 (see Fig.35). This indicatesthat only the resonances around35 Hz seem to originate from theturntable itself. The others mustoriginate from resonances in thetwo arms. The relatively small ampli-

tudes of these resonances in theturntable and arm tube make it diffi-cult to make direct correlation withthe subjective listening results.However, one must realize thatthese resonances build up when hitby transients in the music, either di-rect from the groove or indirect viathe loudspeaker. When the tran-sient is gone the resonances delivertheir stored energy back to the car-tridge and is now converted to elec-trical signals at a time where thereshould be no signal. The pheno-menon is directly comparable towhat in connection with loudspeak-ers is called "Early reflections orbox sound" (Ref.6). The importanceof a reduction of panel vibrationsand its effect on soufid quality hasbeen known for years.

1,6 Hz 1,25 kHz

Fig.32a. Acoustical response from the loud-speaker measured at the cartridgeposition

1,6 Hz Arm 2 1,25 kHz 1,6 Hz Arm 3 1,25 kHzArm 1 1,25 kHz 1,6 Hz

Fig.34. Spectra from the three different arms with the cartridge resting on a non-rotating record when exposed to broadband pink noise. Set up:see Fig. 32

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As regards turntables very littlehas been done yet. However, our lis-tening test has shown there is aclear preference for arm number 1in this respect. In addition it is sup-ported by the quantitative measure-ments shown.

One possible way of establishinga measurement method that couldgive more quantitative results couldbe the use of recorded tone burstsignals. When measuring the sig-nal level between the bursts onecould get an idea of the size and fre-quency of these spurious reso-nances, (Ref.6 and 7). Fig.35. Frequency and level of resonant modes in a turntable with two different arms

Conclusion

in this paper we have pointed outthat traditional specifications likerumble, wow and flutter and re-quired tracking force are both unreli-able and inadequate. Furthermore,they are strongly influenced by theactual combination of motor, arm,cartridge and record, all of whichare often left to random decisionsby the Hi-Fi consumer. By the useof modern test equipment we havetried to throw a little light on thecauses and influence of the inter-face problems between the ele-ments in a turntable. Assisted by lis-tening tests one can conclude that

the fundamental problem creatingparameter is the frequency re-sponse of the turntable below20 Hz. Most modern turntablesieave much to be desired, typicallythey have resonance peaks of 5 —10dB at 5 — 7 Hz. The first thingto do is to raise the frequency to 1 5— 1 8 Hz and then ideally damp thesystem to a Q of 0,5, letting re-sponse roli off at preferably1 2 dB/oct.

In pursuit of this goal one shouldnot make trade offs with respect torigidity of the tonearm tube and fix-

ture. Flexing in the arm and otherspurious resonances could then bethe result and destroy the stabilityof the stereo image.

Finally in Part 2 we have focusedon a type of distortion that is mostclearly seen in the time domain:Early reflections. Our investigationtells us that here is an area which,at present, has rather poor correla-tion between the measurementmethods available and the impacton the sound quality.

References

4. H. Saki(1970)Perceptibility of Wow and FlutterJAES vol. 18 pp 290—298

5. Henning Moller"Multidimensional audio", pre-sented Nov. 1977 at the 58thConvention of the AudioEngineering Society, New YorkB & K Application Note 17—206

6. Henning Mefler3-Dimensional acoustic measure-ments — using gating tech-niquesAES paper New York 1977 andB & K Application Note 17—163

1. Peter Rother: 'The aspects oflow-inertia tonearm design"J. Audio Eng. Soc, Vol. 25 pp550—559, (Sept. 1977)

2. L. Happ and F. Karlov: "Recordwarps and System Playback per-formance", presented September10, 1973 at the 46th Conven-tion of the Audio Engineering So-ciety, New York

3. 1EC Publication 386 (1 972)Method of measurement ofspeed fluctuations in sound re-cording and reproducing equip-ment

7. Henning M0ller and CarstenThomsenElectro Acoustic free-field meas-urements in ordinary rooms —using gating techniquesAES paper New York 1975 orB & K Application Note 1 7—1 96

8. Henning MollerElectro Acoustic MeasurementsB&K 16—035

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