the noise at bullet exit from the silencer was 112-113 db 5 meters to one side, but that was firing a subsonic bullet at a chamber pressure of onl y 20,000 psi. That compared with 137 db without the silencer under the same conditions with the same ammo.
The problem is that I can't equate that db figure to anything. It could be equal to a whisper, a hand clap or a nuclear blast.
Well, I asked because I was skeptical of the idea that that "silencer" would be more effective than modern suppressors (
"The term "suppressor" came to be used because modern "cans" are not silencers, but the Maxim was, at least as far as any device can quiet a fireram firing bullets at supersonic velocities").
Most modern dB tests are done in accordance with the mil-spec procedure with equipment that meets certain rise-time requirements. To paraphrase the former, muzzle one meter off the ground, microphone one meter to either side of the muzzle, and some mic orientation requirements depending on the type of mic.
Using this setup, a typical .22LR might be around 150-155 dB unsuppressed, but 110-120 dB suppressed. A typical full power .308 might be 160-165 dB unsuppressed, but 130-140 dB suppressed. The variance depends on specific suppressor design under test, barrel length, and ammunition.
To translate dB numbers when adding distance, the formula is 20*log(d2/d1), where the ratio of the distances is d2/d1. In this case, if we're 5x as far (5 meters instead of 1 meter), that would add approx 14 dB to your numbers. This would put them at 151 unsuppressed and 126 dB suppressed using the modern standard (1 meter). The net dB reduction does not change in either case, about 25 dB.
For comparison, many modern centerfire rifle suppressors have net reductions in the 25-30 dB range using full power supersonic ammunition, while .22LR suppressors are in the 30-40 dB range (net reduction). Based on the limited data for the Maxim, it does not seem to me like it would give modern suppressors a run for their money.