AnalogMagik Tutorial No. 7: How to measure Vibrations & Resonant Frequencies
Accurate signal retrieval during vinyl playback is highly dependent upon reducing vibrations which the turntable is subjected to. Vibration can come from many sources: foot stomping, motor rumbling, airborne sound waves, or improper turntable isolation, which will all translate into unwanted signals being picked up by the Cartridge. There are many ways to measure and quantify vibrations.
A good way to quantify vibrations is to measure the Intermodulation Distortion Level (IMD) between two frequency signals. Intermodulation Distortions are essentially signals not recorded the LP and are being picked up by the stylus, therefore it would be desirable to have the lowest IMD% number as possible. We have incorporated two low-frequency signals in this test. Note the test frequencies and algorithm in our formula employed in this test is different than the VTA IMD% analysis.
Play the "Vibration Measurement" track on the test LP and the Analog Magik software will display the amount of Intermodulation Distortion expressed in percentage on the computer screen. A good setup should yield a number lower than 2% to 3.0%. If the level of vibrations is too high, to begin with, it will mask the incremental changes which you are trying to measure on the different parameters and render them meaningless, therefore it is important to reduce vibrations to the minimal.
This measurement is designed to reflect immediate feedback which means the numbers should respond quickly to tapping or other external vibrations.
You can make changes to your turntable, such as going from direct drive to belt drive, adjusting the tension of the suspension, changing the isolation platform, or changing the location of the turntable. You can then repeat the measurement, a lower IMD% number would indicate an improvement has been made.
We wish to say that the test we have incorporated is only one of many which can be used to measure vibrations. In our product development engineer's laboratory, we have ultra-precise vibration measurement devices which are 100x more accurate than what we have provided, but the device cost more than a car. While there are far more sophisticated tools and test equipment available, we have to balance between cost, effectiveness and market demand. We believe the one we have provided will provide a good starting point to help users quantify vibrations.
When selecting cartridges, the total mass of the tonearm will interact with the compliance (elasticity) of the cartridge and produce a resonance frequency.
There are complex formulas which can be found online which will compute the theoretical resonance frequencies, however, we caution that such formulas are often overly generalized, and ignore factors such as temperature changes, barometric pressure, different resonant frequencies with different materials, etc. We have discovered that the computed numbers bear very little correlation to the actual measurement, so in reality, you really don't know what the numbers are unless if you actually measure it in a live setting.
It is generally agreed that the optimal resonance frequency should be between 8 to 12 Hz. A resonance frequency outside of this range will degrade playback performance.
The AnalogMagik Test LP has a test track which will work with the AnalogMagik software to determine the Lateral/Horizontal resonance frequency of your cartridge and tonearm combination. Play the Lateral test track in its entirety and if a Resonance Frequency is detected, it will will be displayed as a static number on the screen, under the 'Peak Frequency' window.
If the "Peak Frequency is not within the 8 to 12 Hz, you can attempt to change the frequency by changing the mass of the tonearm or the cartridge weight. Sometimes, the frequency is also affected by changing the tightness on the headshell screws or the counterweight. Temperature changes, as well as barometric pressure, will also affect the results so the numbers observed during hot and humid climate, may be different in cold and dry winters.
We have included a Vertical frequency track for reference against the lateral frequency, but the this track will show a static peak frequency only if significant resonance is detected. Otherwise the number will follow the frequency detected under on the test track, where the peak frequency number will not be static and correspond to the frequency detected on the test track.
Caveat emptor: In our lab, we tested a number of sound cards below $ 1000 dollars, and the majority of the them do not provide a linear response below 20 Hz. Also the LP format itself is incapable of delivering accurate signals below 10 Hz. The reliability of this test is subject to the limitation of the LP format, and equipment limitations.
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