Suppressors - often called 'Silencers' - work somewhat like a car muffler to reduce the noise level by slowing the 'exhaust' gas and giving it room to expand and loose pressure before venting to atmosphere. Even sub-sonic cartridges have super-sonic gasses that loudly blast past the bullet as it leaves the muzzle.
One of the most successful silenced guns around is said to be the World War II 'De Lisle Carbine' These were built in England as modifications of the Lee-Enfield .303" bolt-action rifles.
Why not just 'silence' the .45" Tommie Gun? - Well any automatic or semi-auto action is 'blown-open' or operated by the cartridges high pressure gasses which then leak noisily from the open action - so the best suppression is achieved by a manually operated gun like a bolt-action.
The large De Lisle suppressor tube works with the sub-sonic .45" round to yield a remarkable low noise signature measured at 85.5 dB. - these guns are said to have been so quiet that the noise of the bolt being worked was louder than the shot being fired - so they then fitted rubber pads to soften that 'clack' as the bolt handle hit the action metal. (An unsilenced .308" Winchester rifle might typically record around 156 dB).
The velocity of these sub-sonic 45"acp rounds is further slowed by the cross-drilled barrels that reduce gas pressure in the bore of the De Lisle - making them quieter than otherwise.
An early WWII De Lisle prototype was supplied for testing in the 9mm Luger calibre but it was rejected - probably because sub-sonic 9x19mm ammunition was hard to find back then - complicating use of that calibre for covert operations. Currently most standard 147 grain 9mm ammunition is reliably sub-sonic and can be nicely silenced.
Many suppressors manage around 30 dB. sound reduction - while ear muffs can give between 18 to 32 dB. reduction at the ears. Note that small bore guns can be made quieter than larger guns firing at similar velocity. A normal unsilenced super-sonic rifle shot can typically be heard up to four miles away.
Noise measurement is not simple - as the perceived level of noise is also dependent on the duration of the peak noise level. - While many current suppressor manufacturers boast that their sub-140 dB. silencers are hearing safe - US OSHA rules a maximum exposure limit of 130 dB. for under a second.
- 0 dB is silence - above 140 dB may cause permanent hearing loss. - 220 dB can kill you (by percussive shock).
When taking the mickey out of Magnum shooters I have been known to say that nobody was ever killed by a loud noise - I was wrong.
Both of my 9x19mm silenced rifles - a De Lisle Carbine copy and a Just Right semi-auto Carbine are very comfortable to shoot when using my home brewed 158grain sub-sonic ammunition - but as I am already deaf in one ear I seriously try to have my ears protecteded all the time when on the range.
Gunworks (NZ) Suppressors.
There must be a considerable cost saving to be made in treating hearing loss by getting shooters to buy and use silencers and other hearing protection.
Some countries including New Zealand have no restrictions on the purchase and use of 'silencers' and have zero resulting crime problems. - Other more 'paranoid' administrations strictly forbid them - or impose strong regulations, long delays and expensive permits. Since the 1934 Gun Control Act in America there is a discouraging delay and fee to pay for silencers and "short rifles"
- Quote from 'Tactical Use Of Low-Signature weapons For Military and Police'(Mark White):
"The US Department of Veterans Affairs spends more than a Billion Dollars a year on treating veterans with hearing damage"
Pakistan punishes possession of a silencer with a lifetime prison sentence.
WARNING: I've just come across a warning about condensation inside silencer bodies - When blowing hot shooting gasses through cold metal bodied suppressors there can be moisture condensed onto the baffles that may cause rusting in your barrel & action - especially if you store the rifle in the usual way - standing upright with the barrel pointing skyward. Any moisture collected in the silencer body can then run down into the barrel and cause corrosion.
The suggested solution is to unscrew & remove the silencer for storage and run an oiled patch through the guns bore before putting it away. - Take any gun out of its padded rifle bag (or leather holster) for term storage.