Originally Posted by
clunge
The difference in size is small, but the difference in sound is going to be proportional to the difference in the square of the hole diameter.
So if you use a 5.5mm hole for a .177 the square of that is 30.25.
If you use a 6.5mm hole for a .177 the square of that is 42.25.
If you use an 8mm hole for a .177 the square of that is 64.
That is a useful difference in garden plinking, where keeping neighbours happy is important. Worth a decibel or 4.
The other end of the range is also important. I recommend shooting small leaves sticking out of a big pile of soft sand, inside paving slabs, in front of your backstop.
Cardboard targets are too noisy once the neighbours' windows are open or they're out in the summer.
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.
Viking Mk2 .177/.22 bullpup, BSA Scorpion SE .177, BSA Scorpion .25 100M gun, BSA Scorpion .224 100fpe 100M gun,
Evanix Blizzard .257/.357 200M BR, Evanix Sniper X2 .45 at 270 fpe