Measuring for rumble
How many times have you made a new plinth, or "upgraded" your turntable with parts and components that promise to improve the sonic qualities of your turntable? What if there is a simple method to determine by repeatable methods that can put a number of that performance?
I suspect that there is and that it may be very simple to do this in the relative comfort of your own listening environment.
In this case I am measuring for rumble on my Thorens TD124. SN 2729.
The TD124 has been refurbished by yours truly some years previously and listening sessions told me that I had greatly improved the player in comparison to what it had been when I initially bought it. Now, however it seems appropriate to put a number on that performance.
Rumble is an important figure because it largely determines how much detail the player will be able to reproduce from the record grooves without being masked by the mechanical noises of the player. The quieter it is mechanically, the more pure clean music you hear.
With this in mind I considered a simple way to come up with a valid measurement. Here below is what I think works.
Above photo: Tracing the "blank" between tracks of HFN001 test record.
It seems important to choose a "blank" space that does not have a lead-in groove that will guide the stylus between tracks; like on a regular record. That is why I chose this test record. by recording this blank I can be sure not to be measuring any rumble that may have been imparted onto the test record by the cutting lathe that mastered this record.
I used my 2nd hand Masterlink 9600 to record a 24/96 file of the "blank" space. And I had it burn a CDR of this file.
Taking the CDR over to my PC, I used Audacity to analyze this sound file.
And it was the Spectrum Plot that gave me the above images. Frequency and Amplitude of the noise. The number (in negative dB) shown is closely in agreement with published rumble figures for the TD124. Probably closer to what a TD124 mkII will do, thanks to its improved motor mounting scheme. the algorithm shown was Spectrum. Whatever that means.:D
I used a Minus-K platform under the Thorens for this test. But it is how I listen to the player all the time. It is quite possible that the Minus-K will improve the noise floor of any turntable standing on it. I suppose in order to measure the contribution of the Minus-K into this equation, I would need to repeat the test but with the Thorens standing on something else. I won't be doing that today. But I imagine it is a factor. How much is unknown. but my listening sessions tell me that it is a small but identifiable difference.
Any comments on this methodology? Is it really this simple? for the moment, I think that it is.
The first thought is gee, is the record blank truly flat? Just looking at the top photo I see that images in the blank regions of the disk do not reflect background as a truly flat surface would.
My first search for "turntable rumble measurement" brought me to this link, which on page four brings up very same topic of "subsonic surface irregularities".
They go on to mention that these are typically much greater signal source than motor and bearings, and that they excite resonances of tonearm system.
So, perhaps instead of record, optically flat surface is needed. Apparently when you are riding the grooveless blank region the anti skating mechanism is set such that stylus remains in smooth grooveless region.
Rumble in real recordings is dependent on smoothness of master cut by lathe, smoothness of lathe travel, and then further by smoothness achievable in pressed records.
In your frequency analysis picture "size 1024" refers to number of samples used for FFT used in transforming time domain into frequency domain. This only has marginal resolution for the frequencies of interest.
You should compare your result with using 1kHz test tone.
Otherwise, yes, your test methodology is apparently very similar to that used in the industry, at least back in the 70's.
Ohhh I remember the thread regarding that antivibration device.
1) it's based on a series of leverages and weights, 100% it wasn't designed
for audio purposes
2) If it was designed for that, it's also a really bad implementation (100 %)
It would need an anti-seismic base, and not a wooden stand.
3) Not to talk about the machinery placed on it : whazzat? Graphite ? and the Thorens is...plastic ? Viewing the whole thing with X-ray ( eyes ) it's just a bunch of moving metal parts soaked in some moisty fluid.
4) so the whole column is totally wrong; it's called 'miracles on vinyl' and I'm with you on that side. It's a very difficult matter and needs to be studied very
The minus-K... K is the symbol of elasticity coefficient or something.
Compare apples with apples
Hi, I have just been doing some similar tests to try to determine and reduce the resonate Frequency of an 18inch pick up arm I am working on. It seemed to me that we are using similar (but not the same) methodologies. So I have every thing set up so I measured the rumble output on my TemaadAudio turntable.
Two of the reasons I bought this T/T , even though they are not well know and it is made in China (heavens forbid), Is when I inquired about rumble I was told “Rumble in our range of turntables is of such a little value it is of no consequence, all our bearing / spindle assemblies are hand honed for at least 2 hours to ensure the absolute minimum rumble possible”. And secondly their motor units are battery driven, which is something I desired very much as all of my system (apart from my Pioneer DVD/CD player) are battery driven
Now I have measured the rumble in my system and am very impressed and now see that their statement was very much correct. To measure this I have used a commonly distributed program called Audacity and the HFN test disc that you used. Having managed a QA department for many years I quickly learned that any test you do must be repeatable and comparable with other results from other parties.
Audacity is primary used for recording on a computer but can have many other uses. Because it is so common place now it is easy to compare apples with apples.
Try downloading this program and run the test again and see what results you get. Run the rumble test at the end of side 2 and record and analyze the results. If you have a record clamp use it to help flatten the LP at these frequencies record warps can have an effect. Although as this is the last track it should only be minimul.
below is the graph I got from my TemaadAudio turntable:D
(1) What? It was designed for anything placed on top to reduce movement to the lowest frequencys possible. That includes lasers, optical, turntables, etc.
Herzan is another active device that attempts the same results rather than passive here, seems like a serious attempt IMO
Your recorder must have a flat response down to 6 or 7Hz before you can make accurate measurements.
Hi user510. Thanks for pushing the measurement side of things. All too often the proof of concept is "it sounded better". I do think the record and arm contribute to your total reading but for changes you make to your own setup you have found a way to view changes. very cool. Might have been nice to have this kind of view before you modded your table just to see what actually changed. Could Audacity (or something similar) not have been used to capture the data instead of the Masterlink 9600? Not too many people have one of those :)
Hi pass filtering of phono preamp and rest of recording chain are easily assessed. A reasonable correction curve is made, and noise gain is kept in mind.
In your spectrum picture is 50Hz signal part of test track or due to 50Hz mains voltage?
How is rumble signal referenced to signals in groove?
You can't measure turntable rumble just by playing a disc. You must have a device which can measure only the low frequency noise from the platter.
Mixing & matching of different forms-functions- materials leads to a random - more difficult & pain - result
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