The Alarm spell states that the caster can choose to make it sound off in one of two ways: a mental ping or a localized bell sound.
Let’s say a dungeon room has two entrances. I want to Alarm them both, to tell if an intruder is coming or going. If I need to set both alarms for mental notification (since I’ll be gone deeper), can I still tell them apart?
In D&D 5e spells do only what they say in their description nothing more or less.
Nothing in the spell description for alarm says or implies that a magic user would understand the difference between the alarms they set. This means that a strict RAW reading would mean that a caster cannot do what OP wants.
Personally, and this is solely my opinion, I do not think this is the best way to adjudicate this as it seems needlessly legalistic and goes contrary to the ethos of enabling players to do fun things in general.
An arguably more lenient interpretation of RAW brought up by @TimGrant in their answer says that it is implicit that when the spell says "an alarm" and "a hand bell" that the spell is allowing room there for player choice in the matter. Examples of other spells might be using misty step to teleport to “an unoccupied space” and polymorph transforming a creature into "a new form". It may not be provable or definitive, but it makes sense and allows for creative and clever uses of the spell.
I don't see anything game-breaking or even close to it by allowing the spellcaster to know the difference or even to tweak the sound of the audible alarm. In fact, it makes a lot of sense intuitively that the spell would work this way in-universe even if the RAW doesn't support that.
This is a perfect example of a case where the DM plays an important part in adjudicating the rules in the game. In this case, they get to determine whether or not to allow a player to go slightly out of bound of the precise wording of the spell description. And this is perfectly good and encouraged by the designers.
In this case, I would rule this a clever use of a spell without any game-breaking potential and would not hesitate to allow it at my table. I can't think of a single reason that allowing it would be detrimental to any game.
If the DM is insistent upon following the strict RAW and disallowing the caster knowledge of which mental alarm goes to where, there may still be a clever way to get the desired effect. @BaconHero in their answer said that if you place several alarm spells close to each other in very clever ways you can use that to let the caster know which way they are coming from. For example, you could place one alarm spell on the door to passage A, another on the door to passage B, and another just inside of the door to passage B. Thus, the caster will hear 1 ping if a creature is coming from A and two if one is coming from B. If the caster is very careful and precise about placing the spells, they can get useful directional information.
We know that handbells can sound different than one another. If you were choosing physical handbells for alarms, you could easily choose ones with different pitches or tonal quality.
Since the spell simply says the alarm sounds like “a hand bell,” the caster gets to choose which bell.
Let’s turn this question around and ask: Can Alarm create two alarm bells that sound exactly alike? (I’m going to cleverly mislead my enemies with a false alarm.) The spell does not specify that you can do that, either.
If we want to remain consistent, we can’t say, “No, because the spell doesn’t say you can do that,” to both this question and the OP’s. We’ve got to fall back on context, on what actual bells are like.
(Aside: If we would be lenient, we would allow the caster to choose the bell’s ring, be it the same or different than other alarms, because it’s magic. But if we would be strict, we should at least be consistent and reasonable in our strictness.)
It doesn’t require any special effort to make (mundane) bells that sound different. Moreover, making two bells that sound the same requires tremendous craftmanship. Noted bell manufacturer Jan Felczyński writes:
Bell's acoustic is very complicated matter. The art of creating well-tuned bells is the secret of every bell founder. Other than the pure shape of the bell factors which determine the quality of bell sound are [material, casting, bell's artwork].
If you’ve watched Downton Abbey maybe you’ve noticed the alarm bells that ring for the servants to attend to the gentry all sound different. That’s not hard to accomplish; it is the default case.
If two regular hand bells sound different, magical bells should be able to have that property too. Magical effects should not be assumed to “work like mundane equivalents, only worse.”
I’d go a step farther and suggest the “mental ping” would work the same way. The differences between what the mental ping does and the bell does are spelled out, but it doesn't include any differences on whether different alarms can be distinguished.
Thus, if we accept that bells can sound different, we probably ought to accept that "mental pings" (whatever they are) can also be distinguished from one another.
There are limits to this, though. Two alarms could be distinct as two very different bells, but ten or fifty would be harder to keep track of.
But why does the caster get to choose the bell sound? Because other spells that allow effects within a category work that way, by default.
Silent Image (PH 296) can create the image of “a creature” (among other things). We assume which creature appears is the caster’s choice, and not some “generic” creature, even though the spell never says the caster can choose which creature.
More examples of spell effects which allow the caster to choose within set parameters include:
We understand the caster chooses which form, beast, image, sound, object, space, etc. — unless the spell description says otherwise. The same goes for which bell.
The restriction in the spell is that an audible alarm must sound like a bell, not a roaring tiger, etc. But within that restriction, the caster is free to choose the details.
From the Player's Handbook, page 211:
You set an alarm against unwanted intrusion. Choose a door, a window, or an area within range that is no larger than a 20 foot cube. Until the spell ends, an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area. When you cast the spell, you can designate creatures that won't set off the alarm. You also choose whether the alarm is mental or audible.
A mental alarm alerts you with a ping in your mind if you are within 1 mile of the warded area. This ping awakens you if you are sleeping.
An audible alarm produces the sound of a hand bell for 10 seconds within 60 feet.
Emphasis mine. It's necessary to point out that when the spell description says "the warded area" it is specifically referring to the area warded by that particular casting of the spell. You can cast the spell multiple times (as many times as you have spell slots) on separate areas. Each time you cast the spell, the area warded by that casting is entirely separate from the areas warded by other castings of the spell.
Assume you are in a room with two points of ingress/egress. We'll call these points A and B respectively. You cast Alarm twice and ward both point A and B separately.
Until the spell ends, an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area.
As per the spell, the caster must be alerted whenever a Tiny or larger creature touches or enters the area warded by that particular casting of the spell.
Regardless of whether the alarm is mental or physical, if one of the alarms goes off and you can't differentiate between them, then the spell has failed because it has not alerted the caster that a Tiny or larger creature has touched or entered the area warded by that particular casting of the spell. Instead, the spell has alerted the caster that a Tiny or larger creature has touched or entered either point A or point B. The resulting effect is similar to, but clearly not, what the spell actually says it does.
Therefore any interpretation that results in such ambiguity is by definition false because otherwise the spell would not function as described (hence the contradiction).
Any DM motivated to do so can come up with rationalizations and/or justifications for rendering any number of player options useless (at best) or counter-productive (at worst). D&D isn't supposed to be an adversarial activity where the players are actively opposed to the DM. Everyone at the table is on the same team, and that team's goal is supposed to be to have fun. If the DM is consistently twisting PC abilities to their disadvantage, rendering the game not fun for the players, then I respectfully assert that it is both fair and natural to question the DM's motives in doing so. In this case, the table has much more significant problems than arbitrating the details of a single spell.
And can be strongly interpreted either way by your DM.
But there is a way to achieve what you want no matter how Alarm is interpreted:
While the alarm description doesn't specify a difference in the ping you'll receive, you can still differentiate where the alarm came from by logical means even if they produce the same alert.
You can set one alarm on one entrance (let's call it A), and two alarms an inch apart since spells are not stackable, on the other entrance (let's call it B).
Thus, if you hear a single "ping" in an instance, you'll know that it came from entrance A. If you hear a "ping ping", you'll know that it came from B.
In fact if you follow this layout you can tell when a creature enters via entrance A, and when creatures enter or leave via entrance B: https://i.sstatic.net/rWn6l.jpg
In fact, with this layout you can accurately count how many creatures exited via entrance B. Knowing the precise number of creatures coming towards you will allow you to better prepare for the encounter!
You will also have achieved your goal of creating an alarm that tells you when creatures enter the room from which entrance.
You'll need to set your traps in such a way that their areas of effect do not overlap yet are as close to each other as possible. This way, when a creature crosses the threshold it will trigger all intended alarms at the same time.
In an instance:
Assuming creature movements are resolved on a grid and each takes turns according to initiative, a creature cannot accidentally trigger more or less than the intended alarms and creatures cannot set off alarms at both entrances at the same time.
I didn't notice until another question got flagged as a duplicate of this one that the update didn't take, and it was something none of the answers suggested, so I'll post it here.
The decision that our DM came to was to allow Alarm customization only if the spell was cast with a 2nd level slot or higher (which means regular ritual castings wouldn't be enough).
