Straight Forward But Still Confused Me

In GURPS, subsonic ammunition and suppressors are rated with a negative number to indicate how much quieter they are from normal shooting.

For example, subsonic ammo is a, "-2 to hearing."

The explanation for how hearing stuff is on page 358 of the Basic Set and 158 of High Tech, with gun specific sounds in High Tech.

Things are expressed in range steps based on the kind of sound, like a "light pistol".  Every time the range is doubled, -1 to hear; every time the range is halved, +1 to hear. 

The example is a Walther PPK, a light pistol.  At 256 yards, you make a hearing roll to hear the shot at +0.  With subsonic ammo you'd roll at -2.

At 7 yards, though, you are five "steps" closer so a +5 to hear the gunshot, +3 to hear a subsonic bullet.

Since they also give the decibel value for the sounds, you can also figure out how good your suppressor is in GURPS terms from the dB reduction.

A rifle, like an M16A1, is shown as 150 dB.  If I put my HEL E4A, TL7 detachable baffle suppressor on it, that'd reduce it to 124 dB, which is a two step reduction in sound pressure, thus a Hearing -2 value!  SCIENCE!  A more modern suppressor would drop it to 110 dB and get a -4 to hearing.

The +0 range for a rifle is 512 yards, so +2 at 100 yards and +5 at 10 yards.

The average person has a hearing roll of 10, so they only notice a +0 hearing roll half the time.  At +2 they hear it 74.1% of the time.  At +5 they hear it 95.4% of the time.

Is suppressing a rifle worth it?

In the M16A1 and HEL E4A example, the -2 means the average person will hear the shot at 512 yards 25.9% or the time, half the time at 100 yards and 83.8% of the time at 10 yards.  It's not insignificant.

A better suppressor changes the percentages to 9.3% at 512 yards, 25.9% at 100 yards and 62.5% at 10 yards. 

Don't forget the list of modifiers on High Tech p.158!  They can matter a lot.