Thanks Warrjon, the last plot doesn't show the area below 100Hz, how is it down there?
M
All of the plots are almost the same below 60Hz as damping inside the arm tube does little to help down there. What's interesting is how the different damping materials affect performance. I even tried cat hair inside the wand which was quite effective combined with the plugs.
The measurements were performed with a calibrated microphone and the wand hanging on a string. I performed this test to gain a better understanding of how much damping would be needed in the arm wand.
The measurements were performed with a calibrated microphone and the wand hanging on a string. I performed this test to gain a better understanding of how much damping would be needed in the arm wand.
Propagation of the vibration energy from the cartridge body (not from the stylus, since its task is to vibrate) along the arm to the arm base and further to the subchassis can be best described as a transmission line. If the termination resistance is different from the characteristic impedance of the TL, there is reflection. Consequently it doesn't seem to be a good practice to dampen the arm. Like you don't place resistors in the length of an antenna coaxial cable but terminate at its end. The arm base or the subchassis should be resistively terminated (damped) instead. I am sure it is possible to find the porper damping material that matches the wave impedance of the arm.
Also any transition between mechanical structures of different characteristic mechanical impedance (between cartridge body and headshell, between headshell and arm, ..., between arm base and plinth and perhaps further) causes energy reflection. It is better to set uniform impedance.
I think a silicone oil trough can work well as a termination resistance, and the resistivity could be set by the viscosity of the oil. Or maybe a bitumenous pad below the arm base could work, however it cannot form a rigid structure.
Also any transition between mechanical structures of different characteristic mechanical impedance (between cartridge body and headshell, between headshell and arm, ..., between arm base and plinth and perhaps further) causes energy reflection. It is better to set uniform impedance.
I think a silicone oil trough can work well as a termination resistance, and the resistivity could be set by the viscosity of the oil. Or maybe a bitumenous pad below the arm base could work, however it cannot form a rigid structure.
Hi Icsaszar, good to have your input thanks, is this your field of knowledge, it isn't mine, mine is intuition, plus experimental evidence, that tells me, no damping until the "sink" at the end. i think that is what you are saying as well? it seems that's what Niffy has as well?
Thats what i shall try next!
M
Thats what i shall try next!
M
I'm also in the "Sink vibration energy away from the cartridge" I'm in the process of building a new plinth for my SP10 that is not really a plinth it uses a Permalli (Like Panzerholtz) ground plane and the motor and tonearms are mounted on alumimium pods. This build is to facilitate mounting my LTA as well as a pivoting arm for testing.
The antenna analogy where adding a resistor in the coax is a little incorrect as this would be akin to cutting the tonearm wand in half and inserting a bit of wood. The impedance is pretty accurate if impedance in an RF connection is not matched then there will be a VSW (Voltage Standing Wave) on the cable this is the same as an Acoustic impedance mismatch between materials causing a reflection of the incident wave. What damping inside the arm wand is designed to do is reduce the Q and amplitude of the standing wave and harmonics. Damping will MINIMAL affect on the propagation of vibration energy through a structure unless the structure is cut. Damping could affect Rayleigh waves which travel on the surface of the structure.
Some information on wave propagation in materials. If the aZ (Acoustic Impedance) of 2 materials is the same there will be 100% energy transfer. If the wave is traveling from HiaZ to LoaZ the reflected wave will be in phase with the incident wave if the incident wave is from LoaZ to HiaZ then the reflected wave is 180deg out of phase with the incident wave. This information can be used to laminate different materials in order to spread and dissipate energy in a structure.
My current DIY 4 point has a brass CW stub connected to the aluminium arm wand with a Lead disk between them. The Lead improves incident wave transfer into the CW stub. The stub has a profile so when the wave bounces back to the AL wand it hits the Lead disk at greater than the critical angle bouncing most of the wave back to the stub. This is like light and a pool surface if you look into a pool at greater than the critical angle you can't see into the pool.
The antenna analogy where adding a resistor in the coax is a little incorrect as this would be akin to cutting the tonearm wand in half and inserting a bit of wood. The impedance is pretty accurate if impedance in an RF connection is not matched then there will be a VSW (Voltage Standing Wave) on the cable this is the same as an Acoustic impedance mismatch between materials causing a reflection of the incident wave. What damping inside the arm wand is designed to do is reduce the Q and amplitude of the standing wave and harmonics. Damping will MINIMAL affect on the propagation of vibration energy through a structure unless the structure is cut. Damping could affect Rayleigh waves which travel on the surface of the structure.
Some information on wave propagation in materials. If the aZ (Acoustic Impedance) of 2 materials is the same there will be 100% energy transfer. If the wave is traveling from HiaZ to LoaZ the reflected wave will be in phase with the incident wave if the incident wave is from LoaZ to HiaZ then the reflected wave is 180deg out of phase with the incident wave. This information can be used to laminate different materials in order to spread and dissipate energy in a structure.
My current DIY 4 point has a brass CW stub connected to the aluminium arm wand with a Lead disk between them. The Lead improves incident wave transfer into the CW stub. The stub has a profile so when the wave bounces back to the AL wand it hits the Lead disk at greater than the critical angle bouncing most of the wave back to the stub. This is like light and a pool surface if you look into a pool at greater than the critical angle you can't see into the pool.
I just thought if anyone is interested here is the formula to calculate incident wave reflection
Z1 is source and Z2 is sink.
((Z1-Z2)/(Z1+Z2)^2)*100
If the result of Z1-Z2 is negative the reflected wave will be inverted.
Z1 is source and Z2 is sink.
((Z1-Z2)/(Z1+Z2)^2)*100
If the result of Z1-Z2 is negative the reflected wave will be inverted.
OK, so my carriage on its rails makes quite a good railway noise through the stethoscope when pushed along quick, you can hear it without the scope when pushed quick as well. Obviously it makes no noise when stationary...........how much of a problem is this when playing a record. i can imagine that the hard materials for low friction don't help in this, and maybe the undamped surfaces, I can imagine that could be improved, but I would want to calibrate the measurement a bit, at the moment its my judgement of the stethoscope and how fast i push the carriage...........
I can also do this experiment with parts only, i.e. rails and balls etc. i know Carlo uses softer delrin balls and i use harder ceramic and subjectively the ceramic are noisier.
However, how much of a problem is this, the movements when playing a record are very small and slow..........
M
Hi Warren,
I totally agree that tracking error due to inertia and bearing friction (once you get below a certain level) are unimportant. I use induced tracking error as a way of quantifying the various forces acting on the stylus as it brings them into perspective. Saying that the stylus undergoes a force of 1mN doesn't draw a picture. Saying that the cantilever is deflected by 0.1° does. Deflections of less than half a degree are pretty much inaudible. To hear tracking error at that level you would need to have a steady state error then quickly remove it and listen again. A constantly varying tracking error of this magnitude is beyond my ability to discern.
Back in post #4832 I asserted that FM distortion due to scrubbing induced by the inertia of a linear arm tracking an eccentric record would be inaudible. Because of the low frequency of eccentricity the side bands would be very close to the fundamental. The variation in speed (wow) due to the scrubbing action is very small resulting in the side bands being very low amplitude. Very low amplitude side bands very close to the fundamental are just going to get lost in the flare.
I am not saying that audible FM distortion by stylus scrubbing does not occur, just that it's insignificant in this case.
Where scrubbing can cause an issue is actually in the vertical plane.
The arm will have a natural tendency to "bounce" at the resonant frequency of the cartridges compliance/effective mass. The arm will bounce in line with the major restorative force, gravity. It will want to bounce up and down rather than left to right. This will occur even with a totally flat we'll centred record due to stylus drag. The affect is similar to how a flag flaps in a steady breeze. Warps will make the situation worse.
As the bounce is at a much higher frequency (~10hz) than that of eccentricity (0.55hz) the side bands will be further from the fundamental making them much easier to hear.
Luckily damping the vertical motion of the arm is quite effective at reducing this bounce.
Niffy
Put the stethoscope on the rail as the cartridge tracks the runout groove. If you can't hear noise in this case then you should be safe when the carriage is moving dozens of times slower when playing music.
The noise is mainly caused by imperfections in the surface of the rail. If possible, polish your rail as smooth as you can. Even though I polished my rail and wheels down to a half micron with diamond polishing compound I can still hear the wheels on the rail if I move it quickly. Much quieter than before I polished. Polishing also reduced rolling resistance by a surprisingly large amount.
Niffy
I was referring to a pivoting arm scrubbing not a linear. Scrubbing in an LTA tracking eccentricity is going to be a factor of the arc of the CL and very small.
Oops. Must have misread your post. So much interesting stuff.
The acoustic impedance of any particular material can be calculated.
It is the density (kg/m^3) multiplied by the speed of sound through the material (m/s). The unit of impedance is kg/m^2.s
Most solids will have an impedance in the millions of kg/m^2.s range. Many lists of material properties online divide this by 1 million. For example acrylic has an impedance of 3.26x10^6kg/m^2.s but may be listed as just being 3.26.
I shall do some work on these aspects Niffy. Polishing fits my ethos for construction, I have no machine tools but polishing mops in the drill are just fine. And it fits the idea that there are no expensive materials in the diy bit. I am not sure what level mine are polished to, I go through the grades and finish with P175 yellow Menzerna compound, the rails are stainless steel and appear mirror finished? i have polished the mobile rail, which is anodised aluminium but am frightened of penetrating the surface, that's something I need to consider again. the compound says it is used by jewellers on plated articles so maybe i could try a bit more on the anodised..........
I did some resonant tests today.
I bought a pack of 5 contact mics from Amazon and placed them strategically on my turntable in different locations. Please see the photo below. The smaller number, the closer to the arm.
The number 1 location was on the end of the bearing shaft. I used a piece of metal and let it fall freely on the outside of the air-bearing. Then, I recorded the signal at position #1 under two conditions, with and without compressed air.
Here is the result without air.
Here is the result with the air.
From the results, I can conclude the following:
1. The resonant frequency of my air bearing is about 3.5 K.
2. It is inconclusive if the film of air will make any difference.
3. It seems to me that the bearing with air has less ringing.
I also run a sweep of 30 Hz to 30 kHz signal from Cardas Frequency Sweep And Burn-in Record. For comparisons, I held the contact mic without touching anything. Here is the result. The result shows the noise level without any signal inputs.
Here is the result when the contact mic was placed at position #1.
Here is the result when the contact mic was placed at position #2.
I didn't do the tests for positions #3, #4, and #5 since the signals will be so weak that it is lower than the noise. It is already very weak at position #2. From the result of the sweep, it confirms again, this particular air-bearing of mine has a 3.5k resonant frequency. Comparing the result with the non-contact mic, I would say that the damping of my air-bearing arm is very good. Even at position #1, the 3.5 kHz signal, resonant frequency, is at -70 dB. In other words, only the resonant frequency was transmitted.
In summary, the air-bearing arm has excellent damping. This may contribute to the quietness I have heard.
I bought a pack of 5 contact mics from Amazon and placed them strategically on my turntable in different locations. Please see the photo below. The smaller number, the closer to the arm.
The number 1 location was on the end of the bearing shaft. I used a piece of metal and let it fall freely on the outside of the air-bearing. Then, I recorded the signal at position #1 under two conditions, with and without compressed air.
Here is the result without air.
Here is the result with the air.
From the results, I can conclude the following:
1. The resonant frequency of my air bearing is about 3.5 K.
2. It is inconclusive if the film of air will make any difference.
3. It seems to me that the bearing with air has less ringing.
I also run a sweep of 30 Hz to 30 kHz signal from Cardas Frequency Sweep And Burn-in Record. For comparisons, I held the contact mic without touching anything. Here is the result. The result shows the noise level without any signal inputs.
Here is the result when the contact mic was placed at position #1.
Here is the result when the contact mic was placed at position #2.
I didn't do the tests for positions #3, #4, and #5 since the signals will be so weak that it is lower than the noise. It is already very weak at position #2. From the result of the sweep, it confirms again, this particular air-bearing of mine has a 3.5k resonant frequency. Comparing the result with the non-contact mic, I would say that the damping of my air-bearing arm is very good. Even at position #1, the 3.5 kHz signal, resonant frequency, is at -70 dB. In other words, only the resonant frequency was transmitted.
In summary, the air-bearing arm has excellent damping. This may contribute to the quietness I have heard.
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Very little audible through the stethoscope when traversing the run out groove Niffy, first its moving quite slowly, even though that would be its fastest traverse on a record, and also there is another contact point of the stylus on the record compared to when I push it when free of the record, which probably changes how it would react anyway. so, whilst I shall still attend to polishing again I don't think bearing noise is doing anything much in my case.
Thanks, M
Here made the corksrew2,
Thanks Carlo again!
This sounds better than LC prism even though the lateral movement is a bit less smooth, while vertical is much better.
Using 3.5mm ceramic balls, and lightly polished rails, carriage without cartridge is 40g (counter weight 20g, the others 20g).
Not sure what else to improve the cantilever deflection (5mm+ balls? anodized bar? so hard to cut weight)
Cheers, Wei
Thanks Carlo again!
This sounds better than LC prism even though the lateral movement is a bit less smooth, while vertical is much better.
Using 3.5mm ceramic balls, and lightly polished rails, carriage without cartridge is 40g (counter weight 20g, the others 20g).
Not sure what else to improve the cantilever deflection (5mm+ balls? anodized bar? so hard to cut weight)
Cheers, Wei
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Hi Jim,
It's a bit difficult to tell exactly what is happening in your tests. It looks like there is very little energy being transmitted to the rail/bearing shafts.
Have you used a stethoscope whilst playing music?
3.5khz is higher than I would have expected for a resonance of the air gap but it could be. It might be the resonant frequency of the bearing shaft.
It's a bit difficult to tell exactly what is happening in your tests. It looks like there is very little energy being transmitted to the rail/bearing shafts.
Have you used a stethoscope whilst playing music?
3.5khz is higher than I would have expected for a resonance of the air gap but it could be. It might be the resonant frequency of the bearing shaft.
Hi, impossible to see whats going on here, tell us a lot more about it, looks very interesting!
M