I will do some comparisons later today and post the results.
And here it comes:
The contestants and some test equipment:
On the backthe Laing Delphi 12VDC pump
. On the front from left: PAPST 8412NGL 80mm fan
(rated 0.045A / 12dB(A) but definitely louder), Thermaltake A2277 120mm UV fan
(0.3A / 21dB(A)) and Silverstone FM121 120mm fan (0.4A / 17-39.5dB(A))
As it's now daytime, I didn't even try to measure absolute noise level, just make relative head-to-head comparisons. In order to eliminate vibration noise all contestants were placed on 50mm thick soft foam. Acoustics on the measurement room were quite challenging: walls are mostly bare and hard (concrete and similar).
Fully bleeding the small loop I made for testing the pump proved out to be quiet a challenge and there definitely was some additional noise due to miniscular airbubbles when I made listening tests.
But even on this level I had to go really close to the pump to hear any sound. Mostly the sound seemed to come from water turbulence, as the pump rushed the water throught the tight loop and fairly restrictive T piece used to bleed the system. In these ambient conditions (ambient sound level below 25dB(A), maybe as low as 20dB(A)) it was hard to hear the pump at all.
Pump against PAPST:
PAPST at 12V was clearly louder than the pump. It was by no means noisy, as it's one of the most silent fans I have ran into, but it had some level of low-frequency clicking motor noise and air turbulence noise easily heard from distances below 1 meter. It was by no means noisy, and it would be very hard to hear if it was inside a properly
I have soldered a trimmer pot to the PAPST to be able to fine-tune the voltage level, but as I wanted to use only known voltage levels I set it to zero resistance.
Next step was to do the 7V trick and at this level PAPST fell clearly below the pump in noise level. I had to strain my hearing to hear either of the devices, but the pump was more easily audible at this test. My guesstimate is that the pump equals in noise PAPST volted to 9V.
Thermaltake fan against the pump:
Thermaltake at 12V was so much louder than the pump that were really was no contest. At 7V is was a tad louder than the pump, but this was maybe the closest pair in this test - it's more up to your hearing, ambient noise and preferences which one you determine louder.
Next I managed to test that the Silverstone fan at 5V started cleanly and was more silent than TT@7V or pump@12V, but also didn't provide much airflow to talk about.
At this point the ancient PSU I used to run the equipment shut itself down and didn't start again. I have blocked it's fan from running in order to eliminate it as a noise source and I guess something overheated... Oh well.
Anyway, the conclusion is: the Laing Delphi pump I own can honestly be defined as quiet. It equaled noisewise one of the most silent 80mm fans I have at around 9V and the most silent 120mm fan I have at 7V.
In the Laing Delphi there really is not much noise to talk about than the sound that water causes when is circulates in the tubing. I noticed some noise from the impeller, some infrequent very quiet clicks every now and then, but they were too quiet to be heard from any distance greater than 30cm and I think they might be due to poor bleeding of the system at this point.