Alright, so I recorded using the Feickert Disc which has a 1 kHz tone which is actually at 0 dB. I adjusted the NPC gain such that it matched the measured input in Audacity at 0 dB, so I guess that’s calibrated correctly. I then recorded the tone at 1 kHz and in a silent groove on the disc. I also monitored the input gain with NPC on and the motor off and the NPC on and with the motor on but stylus up. Here’s what I got:
NPC on with motor off: -68 dB
NPC on with motor on with stylus up: -59 dB
NPC on with silent groove (using Nordost Sort Kones): -52 dB on the meter but on analyzing I get a peak of -60 at 3 Hz and -62 at 9 Hz. If I were to average the peak for 0-100 Hz (unweighted), I get -71.5 Hz by averaging measurements in 10 Hz increments. For 30-300 Hz (weighted), I’d guess it’s around -78 dB
NPC on with silent groove (using Fern and Roby sorbethane footers): -52 dB on the meter but on analyzing I get a peak of -55 at 4 Hz and -67 at 18 Hz. If I were to average the peak for 0-100 Hz (unweighted), I get -74.4 Hz by averaging measurements in 10 Hz increments. For 30-300 Hz (weighted), I’d guess it’s around -84 dB
NPC on with 1 kHz: 0 on the meter and 0 on peak at 1 kHz.
So what does all this say? I’m not sure since I don’t have a frame of reference! On Hoffman forum several guys say that a really, really good TT system can achieve in the -60 dB range using this technique. For this really to be done correctly it must be done with a Thorens Rumplemesskoppler which is a device which attaches to the spindle and has an arm with a flat spot for the stylus. Obviously I don’t have one of those, but it seems like my numbers are at least pretty good? I don’t know. I did find it interesting the difference between the measurements in the silent groove using the Nordost Sort Kones and the F&R sorbethane footers which allow the spikes in the TT to insert onto a plastic receptacle under which sorbethane is sandwiched between another layer of plastic. The sorbethane had a higher peak dB at very low frequency but significantly lower average dB lever from both 0-100 and 30-300 Hz, which also bears out in the graph. So is the sorbethane footer better? They both sound similar to me, honestly, but I haven’t done much critical listening to compare the two. The Sort Kones are designed to drain mechanical energy away from the component which the sorbethane is more of an isolating material to shield from outside vibrations. I doubt too many outside vibrations were going on since my room was quiet during these tests. Maybe the sorbethane footers are superior by this testing? More testing is needed!!