Hej Mike do you mind (if you have) posting some pictures of your baffle system which gave you tonal sounds because of the enclosed air? My friend didn't experience such thing and thought it completely silent. He assumes you didn't use sound absorbing material in your tunnels. So now I 'm curious
Of course I used damping materials!
You have to keep in mind that the little hum I referred to was extremely low in level. Anyone who visited my home office would remark on the extraordinary quiet in that room w/ the computer on. The general opin was that it was silent, inaudible.
It's only after I worked for many hours daily, many days weekly, and many months in a row in close proximity to the system that this little hum became annoying.
I didn't have all the measurement tools then that I do now, but I'd venture to say that if the system were to be measured for SPL and frequency spectrum in my anechoic chamber today, this would be the result:
1. before enclosure, the SPL would probably be 15~18 dBA/1m, with a broadband signature and perhaps a single narrow band peak of a couple dB, somewhere in the 120~300Hz range.
2. after enclosure, the SPL would probably be 10-13 dBA, with a broadband signature and perhaps a single narrow band peak at maybe 15-16 dB, somewhere in the 120~300Hz range.
The key here is the average broadband level. After enclosure, it dropped below the background level of my working room
, which meant that the remaining peak was left standing by itself (subjectively speaking), and thus audible as a pure tone. Prior to enclosure, the broadband (white/pink) noise of air turbulence, which was audible above the background noise of the room, masked the peak enough so that it was not audible as a pure tone. Or perhaps the peak/tone developed as a result of the tunnel/enclosure resonances.
The issue with tunnels (any enclosed or semi-enclosed space) is that the air in them always has resonances. If you have any noise makers in them with output at or near those resonances, their noise will be exacerbated. So while the overall noise and measured SPL of a fan at 600rpm might drop by say 6 dB inside one of those tunnels, let's say it has a fundamental at 250 Hz which happens to coincide with a peak in the tunnel at that frequency. Then, while the damping material might bring the level of that fundamental down by a couple dB, the resonance might bring it back up by the same level. Now, with all the masking broadband noise no longer audible (because it is below your room ambient), that peak becomes a pure tone.
Ditto HDDs, btw, which have fundamentals at 90Hz (5400rpm) or 120Hz (7200rpm)... and secondary tones at multiples of those frequencies.