iMac 2019 fans rather loud (and at 2850rpm) when only 30% CPU - is this normal? Situation: 
new iMac 
model retina 21.5-inch 2019, 3.2GhZ 6-core intel Core i7

I can not hear fans at all during normal non-cpu intensive operation (web browsing, etc.). They are at 1200 rpm. This is good.

However, when I load the CPU to an average of only 34% (by spinning up 4 yes commands, as described here), the fans already become rather loud and stabilise at 2850 rpm.

I consider 34% not very loaded. While the sound type of the fan doesn't sound abnormal (no ticking, or so) the sound level does sound unexpectedly high to me.  Loud being: an other person in the otherwise quiet living room asking 'what's going on?' when he hears it the first time, or almost approaching the sound level of a (generic brand) laptop with full fan speed, almost similar to a (soft) hair dryer. 

Question: Is this as expected for these iMacs?

Note: 


I already performed the reset-SMC-procedure as suggested at https://support.apple.com/en-us/HT202179
To be honest, it is the sort of sound level I sort of perhaps could expect when CPU would operate at 100% for some time, but not for anything lower than 75%.
I do have 20+ years experience with several non-Apple laptop brands, 1st gen macbook, and a mid-2008 iMac. By which I mean: computers and the related noise levels are not new to me.
reporting from macOS Activity Monitor seems to be consistent with what the menu bar app (iStat Menus) shows. See second screenshot. (cfr comment)
Without hearing the actual “noise” it’s pretty subjective as to what the actual level is.

However, I did notice that you’re running at least 4 instances of yes that you’ve probably detached from the user so what you’re seeing in that 3rd party “analyzer” is incorrect.  Activity monitor will give you a better view.  

That said, yes is designed to max out your CPU and, as stated, you have at least four instances running.  Your CPU fans are going to turn on especially when the CPU is running at 90 degrees (the thermal cut off is usually around 100 depending on the CPU) under load.

So, what you’re seeing is perfectly normal and resetting the SMC won’t “fix” it because nothing is broken.  Why?  The fans come on based on temperature not on load.  And, temperature is more than just CPU temperature.  There are several temp sensors, even ones that measure the ambient air that go into calculating when and at what speed the fans spin up.  So trying to evaluate a malfunction of the SMC or a problem with the fans based on one factor is incorrect methodology.  

So, is this expected for Mac computers?  

Yes.  It’s no secret Apple has poor cooling and uses undersized heat sinks/pipes, and even uses (IMO) sub-standard thermal paste between the conductive surfaces.  It’s a tradeoff between the super elegant and ultra thin design and efficient thermal performance.

Your Mac is running as expected.