Middle: Jumper is set in middle (middle speed)
Lowest: Jumper is set to lowest speed (opposite side of Full and lowest speed).
Vrms is directly proportional to the fan speed. When the fan is full / fastest the Vrms = 12V (max)
When it is set in the middle, it is 5.7V
When it is set at the lowest speed, it is 4.6V
There is a minimum voltage required to move the fan, so that is why the fan Vrms isn’t closer to 0V.
So, why would the slowest setting be noisier: ?
What happens is the fan is not fully engaged and isn’t running as smoothly as it could. It is litter jittering / shuddering as it moves. Because of the type of bearings this fan uses, there is a little slack / play in the fan axle; this starts vibrating more at lower speeds.
In summary, the slowest fan speed doesn't necessarily result in the quietest setting.
To achieve a quietest level you would need to set the fan in the middle (speed) setting. This is what was measured to have the lowest decibel reading.
Sound Level readings for the Thunderbolt ( front fan)
| PIN F | 36 db |
| PIN 1 | 29 db |
| PIN 2 | 22 db |
| PIN 3 | 28 db |
| PIN 4 | 29 db |
| Fan A Test | |
| No Jumper | No Spin |
| Pin F | 36 db |
| Pin 1 | 26 db |
| Pin 2 | 23 db |
| Pin 3 | 20 db |
| Pin 4 | 18 db |
| Fan B Test | |
| No Jumper | No Spin |
| Pin F | 42 db |
| Pin 1 | 30 db |
| Pin 2 | 25 db |
| Pin 3 | 21 db |
| Pin 4 | 21 db |