Finding resonant frequency when you don't know the mass of your tonearm

doctor fuse

Super Member
Is there a way to figure out the resonant frequency, when you don't know the mass of your tonearm?

I've been enjoying my bedroom system for a few weeks now, after adding plasticine to a plasticky old, lightweight JVC L-A21 turntable.

It's got a Grado Green1 with 8MZ stylus on it, and I would like to figure out the resonant frequency. Just because.

Compliance of a Grado green is 20 whatever-they-are-called units. But the vinylengine owner's manual does not give the weight of the tonearm.

Is there any sort of trick to figure out the resonant frequency mathematically, if this weight is unknown?
 
A test record will reveal the RF for you and it's not the weight of the tonearm you're after, it's the effective mass. A tonearm can weigh at 400 grams but still have 13g of eff. mass. No math magic in this case. Too many variables that you don't have.
 
A test record is definitely the way to go here. I have two and both have resonance tests. One is the Shure Era IV Audio Obstacle Course. This one is pretty common, so finding a decent used copy at a fair price shouldn't be too hard. The other one I have is the Hi-FiNews Analogue test LP - The Producer's Cut. This one is still available new, but runs about 50 bucks. It also includes resonance tests. There are other new test records available from Clearaudio and Ortofon. I haven't used either of these, but they are also in the 45 - 50 buck range.
 
The cheapest option is to buy a used Shure test record - either the Era IV (V15 Type IV - test record number TTR-115) or Era V (V15 Type V - test record number TTR-117), as they both have resonance test frequencies as the last test on the second side. Because anyone who bought those V15 cartridges could got the records if they wished, there are plenty available.

Some of the Hi-fi News test records were found to have the resonance test frequencies slightly off the announced frequency, when someone tested them through a digital recording program, although they aren't that far out that you can't get a reasonably good idea of whether the tonearm and cartridge are a good match. The Shure records are spot on the correct frequencies.
 
The tonearm fundamental resonance will be the largest FR peak no matter what record you use. If you can hook the deck up to a computer and record a silent groove portion...a few seconds is all it takes...then you can read the resonant frequency by looking at the response graph.
 
The tonearm fundamental resonance will be the largest FR peak no matter what record you use. If you can hook the deck up to a computer and record a silent groove portion...a few seconds is all it takes...then you can read the resonant frequency by looking at the response graph.

Cool idea! Will give this method a go. Thanks!
 
The Ortofon test record is a good one as well. I see it must have been re-issued recently as it became available widely from being unobtainium some 10 years ago. This is what I use for resonance frequency and tracking ability to fine tune VTF as well.
 
Using the "Plot Spectrum" tool in Audacity, I can't figure out how to zoom in on the low frequencies where the resonant frequency will lie (5-12Hz?). Any recommendations for a free spectrum analyzer tool for Mac OS X?
 
Screen Shot 2018-04-16 at 1.11.44 PM.png

It looks to me like the resonant frequency is bang on, at 10 Hz. Then the next peak is around 20 Hz. Am I interpreting this spectrograph correctly?

Turntable is a JVC L-A21 with heavily damped platter, plinth and base (with plasticine), and a Grado Green1 with 8MZ stylus.
 
Or is the 10Hz peak the beginning of the cartridge's response (10Hz-45kHz)? Is the peak at around 7.5 Hz the resonant frequency then?

Screen Shot 2018-04-16 at 1.25.51 PM.png
 
Difficult to say from the above graphs. Usually there is a dip between the music signal and the tonearm resonance:

Sk%C3%A4rmavbild%202018-04-16%20kl.%2023.17.59.png
 
If I use this formula (from http://www.theanalogdept.com/cartridge___arm_matching.htm):

acnvbul1.gif
effective mass rating: x grams
acnvbul1.gif
cartridge weight: 5.5 grams (Grado Green)
acnvbul1.gif
fastener weight: 0.5 grams
acnvbul1.gif
compliance: 15


(x + 5.5 + 0.5) * 15 = y

sqrt y = z

159 / z = 14 hz calculated

Going backwards:

z = 11.36

y = 11.36 X 11.36 = 129

129 = (x + 5.5 + 0.5) * 15
8.6 = (x + 5.5 + 0.5)
x = 8.6 - 6
x = 2.6

Is it possible for a cheap tonearm to have an effective mass of 2.6? Have I done something wrong here? Is this formula no good?
 
Weird, not much difference going through the phonostage of a NAD 3020.
Yes, that is weird.

Not using Audacity, but this is what I see: The first image shows the resonance measured using a CBS STR-100 test record. The following image shows the spectrum of the noise following the test track. They relate quite well, if not exactly.

Res test DV 23r.jpg Res test noise DV 23r.jpg Res test DV 23r.jpg Res test noise DV 23r.jpg
 
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