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The title is a generalisation, but nonetheless...

Our group (consisting of three third-level PCs and me DMing) is using the Lost Mine of Phandelver adventure module.

We are in the Cragmaw Castle area, and are about to start combat against a bugbear, a wolf, and a doppelganger who the party thinks is a female drow, inside the castle (we finished our session here because I thought it was a good tension point). The PCs closed the door behind them, and the nearest enemies (hobgoblins) are through that door, a twenty-foot hallway, a curtain, and another ten feet (So, thirty feet, a fully closed door, and a curtain. The module states that they are actively looking out for people attempting to sneak up on the castle but says nothing about whether they are listening out for things inside the castle.

The adventure states also that a slightly further, with one curtain more, area is also where the goblins eat (and are eating at the time of combat). They make, as stated in the module, a clatter of crockery, which is probably not enough to overpower the sounds of combat... But combat might sound similar to goblins having a meal... They are goblins, after all.

So, my question is... if the party started combat, would you rule that pretty much every goblin in Castle Cragmaw come via the hobgoblins hearing and yelling out (the adventure states that they are smart, and loyal to King Grol, who is the bugbear in question). Or would you rule that unless one of the enemies manages to leave the room, or open the door somehow, and subsequently yells, the characters would remain undetected during the combat?

Or to phase it without spoilers: Assuming a setup that contains a door, then 20 feet of hallway, then a curtain and another 10 feet into a room with ample ambient noise, could combat be heard there?

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  • \$\begingroup\$ @doppelgreener I guess Fivesideddice was trying to make the text invisile to avoid spoilers. Fivesideddice, I've made no edit because I'm not sure about your real intentions. If my guess is right just use >! before each paragraph you wish to hide. \$\endgroup\$ – Ols Oct 22 '17 at 11:31
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    \$\begingroup\$ @Ols Aaah, that'd make sense. Multiline spoiler quote blocks are sorta broken, and have been for years. There's an obscure fix, which is to use HTML paragraph marks. \$\endgroup\$ – doppelgreener Oct 22 '17 at 11:58
  • \$\begingroup\$ @doppelgreener Ah, that's the point. I was trying to edit the question before you had made your edits and was wondering, what's wrong with Markdown. Thanks for clarification. \$\endgroup\$ – Ols Oct 22 '17 at 12:05
  • \$\begingroup\$ The headline and body copy here ask quite different questions. Which do you want answered? If it's "both" maybe we need two questions? \$\endgroup\$ – Tim Grant Oct 22 '17 at 13:02
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    \$\begingroup\$ I edited your head line question/title. Is that what you are asking? The text of your question seems to be that you are interested in how far sound of melee combat carries ... @TimGrant I hope that resolved your point. If not, we can hopefully clear it up further. \$\endgroup\$ – KorvinStarmast Oct 22 '17 at 15:42
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Use the guidance in the module

There are specific cases where noise in one area will draw the monsters from another area:

Any loud noises here attract the attention of the goblins in area 7.

(LMOP, p. 37)

However, the castle is not defended with particular discipline:

Because their goblin neighbors are always getting into fights, they don't pay attention to noise in areas 2 or 3

(p. 35)

So we know that noise in one part of the castle is often ignored in other parts.

There isn’t guidance specifically about whether noise in the king’s chamber will be audible elsewhere, or how the other monsters will react, however, we are told:

King Grol is a fierce old bugbear... He rules the Cragmaws through pure intimidation.

Because of this, I would suggest if there is noise in the king’s chamber, many of the goblins won’t be eager to get involved, out of fear, or antipathy towards the king. (The monsters in Area 6 even hope to dispose of the king, and might be quite willing to let some “accident” befall him.)

Moreover, since the king’s power over this subjects is based on fear, he may be reluctant to call on them for help in battle. His loyal guards in area 12 may be an exception.

But what can they hear through the king’s closed door? We do know:

Any character who listens at the door hears two voices in a heated discussion-a loud, growling voice…

This loud, angry voice is apparent if the party listens at the door, but not if they are merely in the adjacent room. So the door blocks a fair deal of noise. As the king has his temper up, it might be reasonable for even the alert guards in area 12 to assume muffled noises of combat were just part of the king’s negotiating tactics.

Thunderously-Loud Noises

Several spells that do thunder damage (e.g., Thunderwave and Thunderous Smite) are explicitly audible at considerable distances (defined in the spell description) and may also alert the entire castle. What any particular denizens will make of such noises is up to the DM, but braver creatures would be more more likely to investigate.

Dungeon Crawls

I would posit that the castle is meant to play a lot like a classic dungeon crawl, where the party has a good chance to deal with the monsters, one room (or encounter area) at a time.

If the party does anything particularly foolish, like blowing battle horns to alert the goblins of their intent to invade, then certainly the goblins will react to that reasonably. If and when the goblins are able to organize into a unified defense, it’s unlikely the party will be able to defeat them all by force.

If, on the other hand, the party is able to keep the goblins from specifically alerting others about an external invasion, it’s appropriate to let the party deal with the goblins one encounter area at a time.

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It's Physics time!

Ok, what is "loudness"? To say it easy, the amount of noise something produces is the amplitude of a compression wave of air. When it hits a surface, it transmits through that material and/or is reflected by it. The transmitting sound is distorted in the process before exiting on the other side.

Now, we measure sound pressure - which correlates with the ampliude of the compression wave - in decibel, or dB for short. dB is a logarithmic scale, where 3 dB means 2 times the energy and \$\sqrt 2\$ the amplitude of the wave. In other words: something that is 10 times as loud has 100 times the energy and is 20 dB louder.

It also has a neat chart that tells us how many dB something creates when it is a set distance away on wikipedia. Combat could from own experience (a sword clashing onto a helmet next to me) reach the noise of a vuvuzela horn at 1 meter distance (120 dB, 20 Pa) but usually will be a bit lower than what a Jackhammer does at the same distance (100 dB, 2 Pa).

Dish out the math already!

So, let's assume at 1 meter (\$r_1 = 1\text{m}\$) it's as loud as a noisy road in 10 meters distance, so \$p_1 = 0.632 \text{Pa}\text{ or }90\text{dB}\$.

What is that pressure at the door? \$p_2 = p_1\frac{r_1}{r_2}\$ tells us. let's assume the room has 2 meters to the door. So \$p_2= 0.632\times\frac{1\text{m}}{2\text{m}}=0.316 \text{Pa}\$. This is equivalent to a ca 84dB at ear.

Let's assume the door does not distort the sound or reduce the sound pressure in a meaningful way (it does, but one can't calculate that easily at all). So we have a 0.316 Pa noise emitted from the door to be heard in 20 feet (=6.096m). That's \$p_3=0.316\text{Pa}\frac{1}{6.096}=0.052\text{Pa}\$, so about the sound of a loud conversation or low key yelling (ca. 68dB).

A curtain (which again should dampen the sound but does not for the sake of simplicity), and another 10 feet (=3.048m) later, the sound pressure is down to \$p_4=0.017\text{Pa}\$, which is around 58.5dB. Remember, a normal conversation is 40 to 60dB, so this could be barely audible against the ambient noise of a lively room (of 70dB+).

But what if they are louder?

If we assume the people yell and scream at the top of their lungs and produce 120 dB or 20 Pa at the 1m Range, the sound travels still 11.144 meters, so the sound pressure drops - assuming no doors and curtains but one straight line without obstacles - \$p_2^*=20\text{Pa}\frac{1}{11.144}=1.8\text{Pa}\$ or ca. 100 dB, and taking each of the obstacles as a new source for the sound, just about 0.54 Pa, ca 88.5dB. This does not take into account the losses of the curtain and door, but if they would not be there, this yelling and screaming would be audible clarly.

That doesn't make sense to me!

Why does the formula seem to quirk out for iterative setup in comparison to cummulative? The cummulative assumes we have one corridor of sound. The iterative assumes we transmit the sound through some perfect, non dampening material between three areas of a set length. Think like a tube in which you put in thin layers of foil: even their mere presence does dampen the sound.

tl;dr:

At this distance the sounds could be drowned out by the noise of the lively mess hall if one takes into account the dampening properties of a door, the hallway and the curtain, but if for some reason they all would be silent for just a moment, they will hear the basic level of combat noise even over the ambient 60dB (like a TV) of a mess hall. In any case they should at least hear the spikes of the death screams or barbarians raging, fireballs detonating and other particular loud noises.

If the guards don't notice the basic combat sound for some reason it doesn't however stop a sloppy guard that is late for lunch break from passing the door that the players fight behind and hearing it through the door and then run to alert the rest.

In that case the closed door doesn't help the players at all, as their own combat noise would drown out these alerted guards advancing down the corridor and taking position on both sides of the door. Running in armor should be roughly 70dB at 1 meter. Assuming the heroes are 2 meters in the room and the goblins rub their bellies against the door in going into position, this is 63dB at the player's positions and that surely will drown out in their own combat noise of 90dB at least. But again, the door and air in the hallway dampens that sound before pressing through the door, so... if you rule that somebody manages to alert the garrison, and they manage to be quick enough to come while still fighting, the players should not hear them coming.

Disclaimer: The calculation makes a few assumptions and simplifications and takes a few shortcuts that result in the numbers being unrepresentative for real calculations, especially since the dampening of various materials (including air) is among other things frequency and pressure dependant. Also, the basic assumption does not include the channeling, dampening or directing properties of a stone corridor and the wall geometry and their decoration. But these could cancel out with the door and curtain properties. It does however allow an estimation as the results should be within the same order of magnitude.

To make it easier: a table of equivalent noise levels based on the one provided by Wikipedia, using this to convert the dB to pressure and vice versa and \$p_2=p_1\frac{1m}{distance}\$ between each point of reference. This ignores the unknown properties of the door, curtain and hallway - it basically assumes open space, so the result should be treated as an order of magnitude.

$$\begin{array}{r|lll} \text{noise level} & \text{Pressure in the room/} & \text{equivalent noise level} & \text{This is...} \\ \text{in the Room} & \text{at the door/at the curtain/} & \text{in the next room}\\ & \text{10 ft. in next room [Pa]} \\ \hline 120\text{ dB} & 20/10/1,64/0,538 & 88,6\text{ dB or a busy roadway} & \text{...clearly audible!} \\ 110\text{ dB} & 6,32/3,16/0,518/0,170 & 78,6\text{ dB or a loud car passing by} & \text{...clearly audible!} \\ 100 \text{ dB} & 2/1/0,164/0,0538 & 68,6\text{ dB or a loud TV} & \text{...audible over a normal conservation when the moment is not too noisy} \\ 90\text{ dB} & 0,63/0,315/0,0516/0,016 & 58,6 \text{ dB or a normal TV} & \text{ ...audible against a normal conservation when the moment is somewhat lively} \\ 80\text{ dB} & 0,2/0,1/0,0164/0,00538 & 48,6\text{ dB or a normal conversation} & \text{ ...maybe audible if the conversation is more quetly and the ambient sounds are dimmed} \\ 70\text{ dB} & 0,063/0,0315/0,00516/0,00169 & 38,6\text{ dB or a not too calm room} & \text{ ...barely audible against a normal conversation} \\ \end{array}$$

a few pointer: 70 dB ambient is like a loud TV yelling at you, or a lively mess hall at lunch time. 100 dB at 1 meter from the origin is what you hear when standing next to a guy using a jackhammer - or a dog barking in your face. 120 dB is a vuzuzela in your face - a wolf howls louder than that. A gun can go to 170 dB.

Now, ruling out the clearly audible ones, what would be the perception check difficulty for the 100 dB (wolf howls, metal on metal clashes) and lower levels?

  • In the next room, hearing 68 dB in a 70 db loud ambience (a lively mess hall, a loud TV) clearly should be medium test, over a 60 dB TV-ambience would be easy and thus automatic with passive perception.
  • Hearing 58 dB over a 60 dB ambient noise again is probably medium, and hard over a loud 70 dB room.
  • Hearing 48 dB in a 60 dB ambient should be hard, while in a 70 dB ambience it should be impossible. Against the only conversation in the room (50 dB) it would be medium.
  • Hearing 38 dB against a the 60 dB ambience is close to impossible (the room is 10 times as loud as the noise), against the only conversation (50 dB) hard, against whispered 40 dB conversations medium and against a quiet room of people staring at each other (20-30 dB) automatic.
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    \$\begingroup\$ I'm skeptical about this answer because the numbers used for sound pressure-at-distance assume an open space. The scenario in the question features a long (presumably stone) hallway, which is likely to direct and amplify the sound because the pressure isn't spreading evenly in all directions. Also, the combatants include a wolf who can hit 100dB pretty reliably. \$\endgroup\$ – BESW Oct 22 '17 at 12:20
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    \$\begingroup\$ Please add citations of these noise levels. Currently your work is based on assuming combat has the noise levels of a busy road -- back that up, and ensure you're factoring in very loud noises, like clangs of metal to metal. (Consider, car crashes and bangs can be heard at distances the road would be inaudible.) You could improve readability by also providing some kind of conclusion section which clearly lists (lists!) for comparison the range of noise levels expected in combat, the noise ranges inside the dining hall, and the noise ranges of the combat from the dining hall's distance. \$\endgroup\$ – doppelgreener Oct 22 '17 at 13:42
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    \$\begingroup\$ Also, do these calculations factor in the wolf, and the fact we're operating within enclosed tunnels rather than an open space, as BESW has highlighted? \$\endgroup\$ – doppelgreener Oct 22 '17 at 13:45
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    \$\begingroup\$ A good answer would show at least some involvement of the 5e rules in it. \$\endgroup\$ – mxyzplk Oct 22 '17 at 14:53
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    \$\begingroup\$ @BESW, for what it's worth, this castle is almost a ruin, so piles of rubble are everywhere. There are a number of tapestries too. Pretty complicated sonically, but definitely not your typical echo-filled stone hallway. \$\endgroup\$ – Tim Grant Oct 22 '17 at 16:53
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The DMG p105 talks a bit about sound in a dungeon, specifically talks about a creaking door echoing for hundreds of feet, and how a fight's noise might reverberate through the whole place; and some thunder spells (Thunderwave, Thunderous Smite) are specifically audible up to 300 feet away. It's not much to go on, but it's a start.

Since a fight is clearly much less loud than a crack of thunder, we could figure 100 feet as a base distance at which the sound of a fight is obvious under normal circumstances. Then double the distance if it's particularly quiet and if echoes are likely to make it more audible (double it for each); halve it if the environment is particularly loud and for each obstruction like a door, heavy curtain, or normal wall. Within that range, there's no check to hear it, it's just obvious; outside that range I'd make a check to see if it's audible.

At some point, like if you're directly next to a waterfall, the fight isn't audible at all, but everyone is also deafened by the noise.

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