Spectrum Analysis Of Ring Test Of 50% And 80% 1967 Coins

I've read a lot about the ring test and that people say there is a distinguishable difference in the tone of a known 80% quarter or dime and a 50% quarter or dime. The usual comments are that the 80% will ring higher (not completely correct) and and have more sustain (also partially correct).

Here's the shake down. I did a sound frequency spectrum analysis of the drops of some pre 67 and then '68 - meaning there was no question of the content. Briefly, for those that are not familiar, a spectrum analysis plots the amplitude over the frequency range. What I found was that the higher silver content coins had resonant peaks that were from 1.5 to 1.8 kHz lower than the 50% coins. If the numbers were removed, the plots for each type of coin would look very similar, because they are the same shape object (a disc). Some peaks would be higher than others, but they'd mostly all be there. However, the difference in numbers between the two types of coin was stark. This makes a lot of sense since the speed of sound in copper is a lot higher than that in silver, meaning, the more copper in the coin, the higher resonance frequencies it would be able to support.

Looking at the plots I also noticed, that with the 50% silver/copper, the sharpest peaks (the ones we perceive the most clearly - the ones with highest signal/noise ratio) was on the higher side of the frequency spectrum, and it was at the 13.3 to 14.2 kHz range (mostly around 13.6 kHz) for quarters and even higher for dimes (17-18 kHz), a range that a large population will lose as they get older. That is why the 80% silver one seems to ring louder with more sustain - its sharpest and most distinct peak is at about the 11.5 to 12.4 kHz range (mostly around 12 kHz) and is more in the middle of the sound spectrum that humans hear.
Most people throughout their life (again, MOST people) will not experience as great a loss of hearing damage closer to the middle of the human hearing range. So that's why the silver ones sound sweeter - because we can hear it better! The frequencies that resonate in the 50% coins that are more in the range that we can hear the best are the lower ones that do not have as high of a signal to noise ratio, therefore they sound deader. I think that's because they get muddled up in the sound of the coin clashing into whatever you've dropped it on.

Anyway, I'm really excited about this. I'm including an example of each type of coin. Picture1 is of a 50% 1967 quarter and picture2 is of a 80% 1967 quarter. Even though each quarters frequency plot was unique in its own way and even varied about 7% for each type, it was not at all difficult to tell the difference, there was never one where I was wondering, is this one or the other? I did not do a formal scientific analysis of the data, these are just rough observations so far. But the differences between the two types of coins is different enough that an "eyeball" test can even make sense of the results.