...The idea for a reference sound is to enable you to adjust your speaker volume to the point where when you hear a sound file say from MikeC.....you are hearing that file at the exact volume he is hearing it.
...I don't think a reference sound should be of an object (a specific fan for instance) that is not available to everyone.
You've got it exactly.
However, there's no avoiding the use of fans for reference files. It's the most common noise source in a PC and most of us end up with Panaflo 80Ls(and some others) sooner or later. Those who want the maximum usefulness from these sound files will get a reference fan sooner than later.
I've come to the conclusion that there is no such thing as a ubiquitous and perfectly consistent noise source in the home or office. Everything makes a different sound at a different level. Your marble idea is on the right path but a huge problem with it is that it is essentially percussive in nature while all the sounds I am recording are pretty much steady state. Very difficult even for highly trained listeners to compare loudness levels between such different types of sounds.
So here are some reference files that I will be making available:
in free air at 12V and on the Alpha 8045 HS -- at 12V (25cfm), 10V (20cfm), 8V (15cfm) and 6V (10cfm).
Nexus 92mm fan
in free air at 12V and on Thermalright XP-90 HS -- at 12V (25cfm), 10.7V (20cfm), 9V (15cfm) and 7V (10cfm)
Nexus 120mm fan
in free air at 12V and on Thermalright XP-120 HS -- at 12V (42cfm), 8.6V (30cfm), 6.1V (20cfm) and 5V (15cfm)
-- bare on kitchen counter, 3" distance, idle / seek; completely decoupled on foam on same kitchen counter, 3" distance, idle / seek.
SPL at 1m will be included with each file.
Yes, the Nexus fans will all be adopted as reference fans for SPCR reviews, along with the Panaflo 80L. All the Nexus fans are truly quiet & smooth, and show very good sample-to-sample consistency. They are basically very low-speed fans that don't push much air, which is generally true of quiet fans, but also have very low bearing noise and appear to have been carefully cherry-picked. As most extreme silencers can vouch, consistency is uncommon among fans. In my worst purchase of Panaflo 80Ls, I cherry-picked 5-6 units I felt good about using from a box of 20.
The Nexus 80 would be in the list above, but I managed to break a couple in a nasty lab accident that took out several fans before I had a chance to measure them completely. There should be a couple more in the lab but I seem to have hid them from myself for safekeeping.
(It is quieter than the Panaflo 80L but its airflow is lower than the Panaflo 80L.)
I still need at least one noisy reference. Any suggestion for a noisy fan that's cheap & ubiquitous?