Tactical Sound Detection Rules

[swaghauler]

This recently came up in my campaign because my PCs have now acquired enough suppressed weapons that I needed better rules for sound detection.

What I'm currently using is a system where I determine the background noise in Decibels (using commonly available charts online) and I subtract this "background noise level" from whatever noise level my PCs are generating and apply it to this chart to determine the chance of hearing that activity with a PERCEPTION check (my Observation skill)...

MODIFIED DECIBELS CHART (after subtracting activity noise from ambient noise)

0 Dbl or less = IMPOSSIBLE TASK
10 Dbl = FORMIDABLE TASK
20 Dbl = DIFFICULT TASK
40 Dbl = AVERAGE TASK
80 Dbl = ROUTINE TASK
160 Dbl = EASY TASK
161+ Dbl = Automatic detection, no roll needed.

For Silencers, I go a little "Hollywood" because games should be fun. The chance to be detected becomes one level harder for EACH RANGE BAND the weapon is away from the person listening to it. So if a suppressed 9mm pistol has a rating of 130 Dbl (down from 160), It is an EASY task to hear it at Short range. A ROUTINE task at Medium range, an AVERAGE Task at Long range, and a FORMIDABLE task at Extreme range to detect that pistol.

I also add...

- 10 Dbl of Suppressor reduction for specialty ammo designed for suppressed guns.
- 10 Dbl of suppression for a locked action (where the gun doesn't cycle to release noise from the action/chamber).
- 10 Dbl of suppression for light fluid in the Can (suppressor) like water and 20 Dbl for heavy fluid in the Can (for wipes or grease).
- Earplugs are good for from 20 Dbl to 30 Dbl of sound reduction.

For those wondering, here are my background noise levels...

0 Dbl (true 0 not modified Dbl) = Beyond the range of human hearing.

10 Dbl = Normal human breathing at rest.

20 Dbl = Silent walking, whispered speech

30 Dbl = Light rain, walking in the woods on twigs.

40 Dbl = The typical noise in a family home or in a quiet social gathering.

50 Dbl = Steady medium rain with wind, typical noise in an apartment complex at rest.

60 Dbl = Normal Conversation, typical social setting (office, upscale restaurant, etc...)

70 Dbl = A hard rain or very windy moderate rain, typical social setting (busy office, restaurant, command post, etc...), light traffic in the background.

80 Dbl = Fresh gale, A chaotic social environment (a typical city fast food joint, warehouse, or very busy office), heavy traffic in the background.

90 Dbl = A SHOUTED Conversation, train station, heavy (diesel) commercial traffic, warehouse with heavy equipment in the background.

100 Dbl = Subway platform, light construction, port facility, stadium crowd, a fistfight in a bar. *At this level, prolonged exposure to the sound will cause hearing loss after 15 minutes of exposure.

110 Dbl = Power tools, Rock concert, turbo-prop aircraft, high-speed train passing, melee weapon combat.

120 Dbl = Jackhammer, Siren at 100ft, power hammer on steel, turbofan aircraft, muscle car headers,

130 Dbl = Heavy industrial machinery, military ducted jet engines running. At this level, even short exposure to this sound can cause hearing damage.

140 Dbl = Comercial jet taking off, small caliber or low-charge black powder firearms under 750 ft/sec.

150 Dbl = Black powder firearms over 750ft/sec, firearms under 1,000ft/sec.

160 Dbl = Most rifle rounds under .40 caliber and under 3,000ft/sec, singer shattering a glass.

170 Dbl = Most rifle rounds over 3,000ft/sec (5.56mm m193 ball is 165Dbl)

180 Dbl = Larger (.50 cal & elephant guns) rifle rounds, artillery firing, explosions, the LRAD's "warble" used to disperse protesters.

190 Dbl = Major explosions. Above 195 Dbl, sound becomes a shockwave that can cause damage to all physical objects.

Barriers CAN affect ambient (background) noise with paper-thin apartment walls absorbing 5 Dbl while typical doors and walls reduce it by 10 Dbl. Well-built structures, fire doors, and security doors could decrease sound by 20 to 30 Dbls.

So how do you handle suppressed weapons and sound perception in your games?

* over the course of months or years, not immediately.

[swaghauler]

My second "beta test" in today's game has resulted in an evolution of what I wrote here. The changes are:

Detection Chances (Modified):

0 Dbl or less = IMPOSSIBLE (X0.1) Task
1 to 10 Dbl = FORMIDABLE (X0.25) Task
11 to 20 Dbl = DIFFICULT (X0.5) Task
21 to 40 Dbl = FAIRLY DIFFICULT (X0.75) Task
41 to 60 Dbl = AVERAGE (Skill) Task
61 to 80 Dbl = ROUTINE (X1.5) Task
81 to 100 Dbl = EASY (X2) Task
101 to 120 DBL = VERY EASY (X3) Task
121 Dbl or greater = Automatic success.

Eavesdropping On A Conversation:

The following rules are for when you need to gather "Intelligence" by listening to others talk. It is the chance to hear AND UNDERSTAND what is being discussed. IF the intended targets are talking normally, you have an AVERAGE chance to overhear them IF you are within 5 meters. For every 5 meters [or fraction thereof] farther away you are, your chances are HALVED. Whispered conversations require you to just be 2 meters away. Windows and doors HALVE your chance to hear the conversation!

For those who are still confused about how the system works, here is the procedure...

1) Establish the Decibel Rating of the activity being performed.
2) Subtract the Ambient Noise Rating in Decibels (ie background noise) at your location from the noise being performed.
3) Roll to hear the noise of the activity being performed over the background noise.

That's all there is to it! I have now beta tested these rules and they work pretty well.

Swag

[chico20854]

Swag,

These look great! One suggestion, which may overly complicate things - distance! You lose 6 db of intensity with each doubling of the distance, so that a 150 db gunshot at 1m is down to 86.5 db at 1500m. There are a number of calculators out there, I used this one.

[pmulcahy11b]

One of the US-based National Labs was experimenting in the early 2000s with a sound horn that can develop 260-280 dB. It's directional, can be tuned to a narrow beam, and can bore holes in reinforced concrete up to a foot. (Don't get in front of it!) It's not dangerous outside of its beam.

(I found that in an old Guinness Book of World Records. There's probably much better technology now.) I'll have to look that up and find out more about that. Be back later...have to let my dogs out soon.

Be something like interesting in a Merc 2000 campaign that a bad guy has captured and the team is to sent to destroy.

[pmulcahy11b]

Didn't find anything on that yet, but I found this, which may be interesting:

https://housegrail.com/decibel-equiv...hats-how-loud/
https://soundproofingguide.com/decib...parison-chart/
https://soundproofingguide.com/decib...parison-chart/

And one just for the hell of it:
http://www.surefire.com/products/sup...uppress…

Mods, someone needs to teach me how to turn a URL into a few pertinent words instead of quoting the whole URL in the post. I don't know how you do it. Could someone get with me in PM or email and give me a tutorial?

[Homer]

How do you model noise discipline precautions like digging generator pits or conducting short count starts in assembly areas?

[swaghauler]

Originally Posted by chico20854

Swag,

These look great! One suggestion, which may overly complicate things - distance! You lose 6 db of intensity with each doubling of the distance, so that a 150 db gunshot at 1m is down to 86.5 db at 1500m. There are a number of calculators out there, I used this one.

This will be a really cool way to do audible detection range during play. Just select the Decibel rating of an encounter (a military encampment with vehicles would run around 130Dbl) and plug it in to see how far away the PCs can recognize what they are hearing actually is (note: the OPERATIVE word here is "recognize" as sound can carry pretty far).

[swaghauler]

Originally Posted by Homer

How do you model noise discipline precautions like digging generator pits or conducting short count starts in assembly areas?

Put a "deduction of Decibels" in place to model it. A ditch might provide 10 Dbl of reduction while a woodline COULD give you as much as 30 Dbl. NOTE: 5 Dbl is a typical sound reduction (essentially "halving" the sound) and 30 or more is a SEVERE REDUCTION in sound (most hearing protection is only 20 to 25 Dbl in reduction). Even camo netting will absorb some sound (about 5 Dbl).

[Homer]

Thanks. Although I’ll confess that anecdotally generator pits seem to be infrequently dug in practice. More use of terrain and woodline seems to be the reality.

It does seem that a single generator running in the woods supposedly something like a beacon for reconnaissance units looking for TOCs and Field Trains.