cooling fan attenuation

[steveu]

I suggest you try this. A N.O. thermostat switch and a ceiling fan capacitor (parallel) in series with the AC motor. You may need to tweek the cap size to get a useful speed.
https://www.walmart.com/ip/Temperature-Control-Switch-Thermostat-45-C-10A-Normally-Open-N-O-2pcs/152365217?wmlspartner=wlpa&selectedSellerId=571&adid=22222222227276032463&wl0=&wl1=g&wl2=c&wl3=331801447846&wl4=aud-566049426705😛la-635406407182&wl5=9029025&wl6=&wl7=&wl8=&wl9=pla&wl10=111838760&wl11=online&wl12=152365217&veh=sem&gclid=EAIaIQobChMIiMrv-eT85AIViLzACh3cMAMCEAQYAyABEgJIlPD_BwE


https://www.lowes.com/pd/Harbor-Breeze-1-Setting-Black-Ceiling-Fan-Switch/3638340

 

[PRR]

....perhaps a different blade shape would minimize blade noise while maintaining airflow?...

Shhhhhh!! That's a military secret, for obvious reasons. Aircraft, and the similar problem of submarine propellers. There was a scandal a decade+ back about sub prop secrets leaking to another side.

Yes, there's "cheap" and there's "less loud". Fan designers are very aware of this. Plain muffins are pretty good (being molded plastic, not bent tin, they can have complex curves). There is a line of extra-quiet fans with more complicated blade shapes. But aside from the worst fans (factory blowers, not our muffins) the difference is marginal.

The KEY way to reduce noise for airflow is SIZE. (And darn the efficiency.) A 10-inch fan at 100CFM will be much quieter than a 5-inch at the same airflow.

Duct mufflers can work. (They work on cars, which are just airpumps.) Taking out low frequency noise needs BIG mufflers, the size of a room (no matter the airflow, the muffler has to be a large part of a wavelength.) A fuzzy duct can take a lot of midrange out.

The peak dissipation of a cold-bias class B amplifier (most non-A amps) is at 50% max power (70% voltage). NO unclipped speech/music signal can get here long enough to hot-up. Average power with audible clipping is typically 10% maybe 20% of max. In the past many amplifiers were cooled only this much. FTC Rules demanded a significant run at 1/3rd power before testing. This is pretty-near the worst possible case. Makers complained but could not turn-over the stupid rule. Actually makers of industrial "audio" amplifiers were not bothered, because their servo/shaker motor customers were already holding part-power steady for days, and had the cooling for that. The music-only amps adapted in various ways. One down-rated the claims so the torture-test level fell to a tolerable zone. Another had a speaker-powered fan: on test it ran part-speed steady, but in real music use it only blipped on a few peaks, never really spinning. (Fun to watch.)