|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
24.215.147.128
In Reply to: High compliance -- low mass posted by OLLY on December 18, 2005 at 16:51:31:
The reason that matching carts and arms is that like any mechanical system, they have resonant frequencies. If the fundamental resonant frequency is too close to audio frequencies (let alone actually audible), it will interfere with playing records (to put it mildly). Also, you want neither the turning of the turntable nor footfalls to interfere with it either.There is a formula to calculate the (approximate) resonance frequency of a tonearm-cartridge combination:
a/sqrt(CM)
a = 500/pi approximately 159
C is the cartridge's compliance. The units are called CU's which are equivalent to 10^-6cm/dyne or 10^-6m/10^-3N
M is the combined mass of the cartridge (including affixing bolts etc) and the effective mass of the arm in grams
(Reference: Van Den Hul FAQ Appendix 2.)This formula gives a frequency in Hz which ideally is between 9 and 11, but is OK between 8 and 12 and people have been known to live with those as low as 7 or as high as 13.
Now that you're sorry you asked, I can give you the good news that even if you're able to find out the specs for your equipment, they are not always measured in the same way (eg compliance can be static or dynamic and the latter measured at different frequencies; standards changed over the years).
I just go by the tracking force: under 1.6g is high compliance, between 1.7g and 2.3g is medium and 2.4g and above high compliance. YMMV.
Today, most cartridges are designed to be used with medium mass arms. If you have a low or high mass arm your cartridge selection will be more limited.
If you really want to know the resonance frequency of an arm/cartridge combination you can buy a test record with a frequency sweep and find out.
Follow Ups:
The resonant frequency in radians per second is given by ω = 1 / √C.m. if the compliance is stated in m/mN or cm/dyne and the mass is in grams. This is the basic law of simple harmonic motion.Since there are 2 pi radians per second in one Hz, the equivalent is 1/2pi√C.m. Since compliance is always stated in μm/mN there is a factor of 10 6 inside the square root sign, which is a factor of 10 3 (1000) outside, so the formula is 1000 / 2√C.m. which is equivalent to your formula.
BTW differing values of lateral and vertical compliance give differing resonant frequencies in the vertical and horizontal planes. Since resonance is a dynamic phenomenon only dynamic compliance matters.
You can measure effective mass directly using a simple torsion pendulum - a small ruler, a peg and a stopwatch will suffice.
The fact that the arm-cartridge system is being modelled as a simple harmonic motion (essentially a weight on a spring) shows why one should take its results with a grain of salt. It is a good estimate of what resonance to expect, but only an estimate.For example, temperature (which is an important factor in cartridge compliance) is not taken into account. Has anybody seen specs that give the temperature at which cartridge compliance was measured? The possibility of different resonances in different planes of motion is not addressed by specs from all manufacturers (I know of none besides Goldring).
The only way to know what the resonances are precisely is to measure them. Mark has suggested a way to measure effective mass directly. I imagine it would be quite risky for the life of a cart to measure cart compliance directly except to find the resonance frequency and estimate compliance with the formula given before.
Thanks Mark!
If you can measure the effective mass of your tonearm in both the vertical and lateral directions, then you can easily measure the compliance of your cartridge at your own room temperature using the HI-Fi News test record.
> > > You can measure effective mass directly using a simple torsion pendulum - a small ruler, a peg and a stopwatch will suffice.Hi Mark,
This intrigues me. Can you provide a more detailed explanation along with pictures if possible?
I'll have to set it up again to take phots, that may not be today.Once I have the phots I'll write up a description for my website and start a new thread with a link.
I'll ping you when it's done
I would like to measure the effective mass of my tonearm.
trying to understand this.. thereforemy goldring 1042 has static compliance of 24N vert& 16N Lat with cartride wweight of 6.3g. On a SME 3009 series 2 unimproved with 12.5g effective weight the resonance frequency is either
Dynamic compliance is the correct spec, but the question is at which frequency was it measured. Dynamic compliance is measured by moving the stylus, and this movement has a frequency. I don't know how much frequency affects the measured compliance. I certainly wouldn't try to use the formula I quoted to get more than 1 significant figure. Hence you are really asking, is it 8Hz or 9Hz, and the answer is your combination should be fine. If you want to know what the resonant frequency is exactly on your setup you have to use a test record with a frequency sweep.
The two arms I will be using are :- Audio Technica AT1501 and SME 3012. Not sure what version of the 3012 I have. The two cartridges I use the most are AT15XE and Shure M55E. I do have the Sumiko Oyster but I have not tried it with these arms.
OLLY
They are higher compliance and the arm is not too heavy.Here are some sources of information:
www.audiotools.com
www.cartridgedb.com
This post is made possible by the generous support of people like you and our sponsors: