Quietest screw on silencer?

[rhyslightnin]

You are close but not quite there. As long as the speed of exiting air remains subsonic the difference in flow speed causes the dB peak to shift up or down in frequency. Higher speed (small exit hole) makes the peak a bit louder in dB scale but also moves it up in frequency which is why many consider it quieter.

Like a pan pipe or any wind instruments.... Shorter tube (smaller hole) is higher note???

[riku]

Like a pan pipe or any wind instruments.... Shorter tube (smaller hole) is higher note???

Good comparison. Smaller hole, faster airflow resonating at higher frequency. It doesn't matter inside the silencer but at the exit hole it makes an audible difference. Then there's the spacing of baffles to break up certain frequencies but that's then another story.