This is a follow-up on the latest development in this continuing saga to rid my turntable of low-frequency resonances. The latest results are nothing short of incredible. The turntable is still limited, but it has just taken a very large jump in value. I’m going to describe the original configuration and the changes made in detail just in case someone else can either learn from this situation, or better yet, provide insight as to the reason for the results.
Two sets of Nobsound springs arrived at my doorstep this week. Per @millercarbon’s suggestion, these were purchased as a possible way to push the -30dB resonance measured at 20 Hz further down the scale.
I removed the original metal, conical feet from the turntable’s levelers. The rubber bumpers to which the conical feet were attached are still present. In the original configuration the conical feet had been sitting on Vibrapod pads.
So, conical feet now removed, I arranged three Nobsound springs beneath each of the turntable’s levelers. I put the test record on and started measuring with a track playing a 0 dB reference tone at 1 kHz. And not believing what I was seeing, I started running through the settings on my software to ensure that I hadn’t changed something. I hadn’t. All the settings were exactly the same as when I measured the previous configuration.
This turntable is now emitting a resonance at 20 Hz down near the area of -70dB.
The resonance rises slightly towards 16 Hz (the lowest frequency I can measure), but even that isn’t much more than -65dB. By comparison, the second- and third-order harmonics from a 1 kHz sine wave and also random surface noises register higher than that as measured by my equipment.
This hasn’t been a purely scientific test; there are flaws in my process. And I’m using software (Logic Pro 10.5.1) that isn’t strictly meant for this purpose, not to mention the fact that I don’t know the tolerances of its measurements.
While I cannot draw the conclusion that the Nobsound springs are entirely responsible for the dramatic drop in resonance (the removal of the Vibrapods and the turntable’s metal coned feet are both included in a list of variables that may have contributed), they should almost certainly be given the lion’s share of the credit. Furthermore, I cannot state definitively that the Vibrapods are ineffective in every application, but they don’t seem to offer any reduction of resonances in this frequency range.
But why was I originally seeing such a high reading at 20 Hz?
It would appear that a rotation-related noise from the bearing is creating a resonance that was traveling through the plinth, through the feet, possibly through the Vibrapods and into the shelf, and then back up through the feet and plinth, into the tonearm and, finally, into the stylus. At some point along that path the resonance was being excited by a still-unknown factor (determining this is what I still need help with). If not for this, the resonance from the bearing would simply travel directly through the plinth and into the tonearm. Since I no longer have a resonance as nearly as high as previously measured, the excitation of the original resonance is no longer occurring (or at least not to the same degree). And since I have done no further work on the bearing, the original root cause of the resonance has not been eliminated. But the Nobsound springs seem to be extremely effective at nullifying that problem or at least not amplifying the problem as the previous configuration did.
Thanks again to @millercarbon for his advice. This has to be simultaneously the cheapest and most effective fix for an audio problem that I’ve yet come across.
Appendix
The turntable sits upon a shelf which “floats” above the audio rack’s superstructure (“floats” in that it sits on another set of Vibrapod pads). It’s a very heavy rack and has proved to be quite effective at reducing the impacts of footfalls. But I have yet to measure its sympathy to other types of vibrations.
After the application of Nobsound springs to the turntable, I installed the second set of Nobsound springs underneath the shelf upon which the turntable sits. These replaced the aforementioned Vibrapods. I did not observe any measurable changes to the noise floor of the turntable spinning the same track on the same test record. It may be possible that the noise floor may now be low enough that no further reductions should be expected with the associated equipment.
