Haha, sorry, but you sound like an Apple commercial: "Stuck trying to decide which fan to buy based on extremely subtle and likely imperceptible sound pressure level differences, but don't happen to have a studio-quality anechoic chamber handy? There's an app for that!"
I'm just messin, but I couldn't help but read that post with the apple commercial guy's voice in my head lol.
Anyway, I came back to repost and to clarify that I hope that what I said in the prior post didn't offend you, jonnL. I was not aiming that at you, and I don't want you to get the wrong impression of this place (or me; I'm friendly I promise), and I didn't realize until later that it sounds a bit harsh. Not my intention, so for what it's worth, sorry.
I do get frustrated sometimes by the sheer amount of people who (across the multiple forums I am actively involved in) ask people to find them ultra extreme performance fans but add a qualifier about how it must be absolutely silent, which is absolutely impossible. I realize that is not what you are doing, that's why I am back to apologize, but it is pretty common and always annoys me. Like I said in the last post, I do a ton of uncompressed audio recording editing which requires that I be able to hear the audio EXACTLY as the average listeners would, but also as people would with the most common audio device in the world (iPod earbuds... Barf) up to how the sound is reproduced via 10ft tall 8-cone "carved-from-a-single-piece-of-200yr-old-extremely-rare-wood-from-a-tree-located-at-the-heart-of-the-Congo-and-only-accessible-by-a-ten-day-walk-and-carried-back-by-hand" extremely expensive speakers that cost as much as a brand new Ferrari 599GTB HGTB with Carbon-Ceramic Brakes (and still have enough money left over from the cost of the car to have a full makeover courtesy of Novitec Rosso including their literally jaw-dropping twin-supercharger kit as well as full aerodynamic, weight-reduction via Carbon Fiber, customized interior, wheel & brake, and of course audio system packages). In fact, that's actually what "real" studio monitor speakers are designed to do, although many companies decided that calling their home theater or Blose-esque speakers "studio monitors" and headphone companies are even worse, especially since headphones can be more difficult to work with as a good set can deliver almost infinite types of sound but are very hard to calibrate as there aren't different drivers for different frequencies,just one big driver (yes, multi-driver headphones exist but they aren't even close to where single drivers are at yet). I have owned more headphones than you would imagine because of how headphones, just like speakers, have a break in period but it is far more noticeable with headphones, and the only pair I use consistently are my Grado RS1i Studio Reference cans.
Sorry, I went on rambling again...
My point is, I don't just know sound because I like to, I know it because I need to, and the environment I do my editing in is of good significance. That said, it has been about six months since I have been able to do my "side job"/hobby at any real capacity due to a number of reasons, but I don't think I've lost my ear. To me, the difference between 29dB and 33dB in terms of volume from a fan is truly imperceptible unless you are completely free of ambient noise (I can't get below around 12dB ambient without going from $2k in sound-absorbent/dampening materials to 10-20x that amount. If you live in a city, you probably have an ambient noise level well above that of these fans, and if you game with speakers or headphones on, the fans will not be perceptible. In fact, my RS1i headphones, which are open-air/dynamic (the space behind the driver is exposed, which allows for much better reproduction of realistic sound, but can also let more outside sound in), drown out every fan I have tested (at the equivalent of 60 percent volume on an iPod) with the sole exception being a 5400rpm Delta that I just had to play with. Now that is the kind of fan you don't want.
I just really urge you to base your purchase on something other than solely sound noise, because the factory specs are never accurate (yes, even Gentle Typhoon AP15s are specced wrong, albeit only by about 2dB which is still more accurate than 99 percent of fans), and when you do see the stated specs you should consider that the actual volume is between 4dB and 20dB higher than stated. I have heard many fans that are significantly quieter than others rated 15dB lower, in fact it's probably the least accurate spec given for fans regardless of make/model.
Not to mention that a louder fan with a lower-pitched but steady sound is vastly more comfortable than one that is actually quieter but is higher pitched and/or has a very irregular pitch.
Tl;dr version: don't buy a fan based on the given specs for sound, and realize that the type of sound is vastly more important than the level of sound when it comes to listening comfort. Also, always consider the sound specs given are off by 5-15dB.
And unless you are running some crazy fans, speakers or headphones will drown them out anyway.
The best thing to do? Fan Controller!