This is quieter than my electric wall clock.
That's that cleared up, then.
Just for fun I thought I would explain a little more.
Just to illustrate I have attached a link to an official "scale" that compares sound pressure levels.
http://www.smgov.net/uploadedImages/Departments/OSE/Categories/Landscape/Sound_Level_Chart.jpgThe vast majority of audible noise in electronics is driven by cooling fans. I hate fan noise so generally replace fans in computers etc. A really good PWM PC fan with <30 db and reasonable flow rate will cost ~$15.00-$20.00 retail vs <$2.00 for a cheap industrial one .
Noise in fans comes from 3 main sources, bearings, turbulence from obstructions chopping the air flow (usually bracing) and turbulence coming from poor blade design, blade dirt or manufacture defects. Traditional computer and PSU fans generate 40 to 50 dba of noise at full RPM. Most teardowns I have watched on eevblog show these cheap industrial fans in the scopes.
Mitigation:
If you halve (50%) the sound pressure level (spl) you will drop about 3db, example: if I have 2 fans and remove one I will get a 3db drop. Now that means a regular industrial fan @ ~41 db fan is about 64X more energized than a 23 db fan. (this means it is perceived to be about 6X quieter)
Next, air flow rate which is proportional to fan RPM, is the dependent variable that creates noise levels and removes heat. It is generally a non linear function. So, lower fan speeds will be a lot quieter. This why "cheap" computer case makers have increased fan sizes. The volume of air flow is a function of (flow rate X Area). So doubling the diameter of a fan provides 4X the air flow (per rpm) which in theory (stretch) doubles the heat removal from the system or cuts RPM by 4X for the same dissipation.
It is much cheaper in a large PC case to throw a big, cheap, non-pwm fan in the case and then use marketing to sell the kids on just how cool this looks. (pun intended)
The other way to reduce noise is to adjust fan speeds downward to just match the air flow required to meet thermal transfer requirement (PWM fans) These are more expensive and require control circuitry. They are now the solution of choice in better quality computer equipment (PSU, Video Cards, CPU and case fans).
Now, if you want to engineer for low sound the first thing you do is minimize heat generation by using efficient design, quality components and well placed thermal zones on the PCB. Ideally achieving a purely passive solution. However, I am sure we all know, that this level of engineering is a complex game that optimizes cost, features, complexity and function.
Once the thermal load function from a device is known, the airflow required to cool it is "determined". I assume that thermal point loads in a "scope" would not need or require heat pipes or other more expensive esoteric cooling solutions. At least I have not seen it yet in a mid scope teardown.
Since heat is usually a function of "load" the quietest solution will generally have the following characteristic.
1) No fan stage - use of passive cooling only (Characteristically large cooling grate big holes and lots of them. Minimally obstructed air flow over cooling elements )
2) Fan Stage - Fan is under PWM control to vary the flow rate as needed and minimize fan RPM noise. Not necessary if you engineered for a low thermal load and maximized passive cooling options. (engineering in style) .. an inexpensive fan at low RPM.
It will be interesting to see Dave's teardown of the RTB2004, Based on the pictures .. Lots of Big holes .. unobstructed interior.. extremely low measured sound specification.
My guess.. RTB scopes are a properly engineered solution designed for the long term. Part of why I bought one. BTW the 25 db from my wall clock comes from the click of the second hand when your ear is about 75 cm away. a soft whisper like "tick" sadly there is no "tock"
SPL meters are so fun .. NOT!