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I am currently play-testing an Awakened Mystic character in D&D 5e, using the beta rules released by WotC. As such, I have an ability that lets me choose between having Blindsight or Tremorsense.

BLINDSIGHT: ...[B]lindsight can perceive its surroundings without relying on sight, within a specific radius.... (Monster Manual p.8)

TREMORSENSE: ...[T]remorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the [observer] and the source of the vibrations are in contact with the same ground or substance. Tremorsense can't be used to detect flying or incorporeal creatures.... (Monster Manual p.9)

I have been wondering what the difference is between these two, aside from the fact that Tremorsense requires contact with the ground and cannot detect flying or incorporeal creatures. Specifically, I want to know whether Blindsight cannot detect creatures through solid matter such as walls, floors, and ceilings, whereas Tremorsense can? Can Blindsight detect creatures on the other side of a dungeon wall, or on another floor of said dungeon? Are there monsters in 5th Edition that possess both Blindsight and Tremorsense, and if so, when and why would they use one as opposed to the other?

I am aware of and appreciate this question regarding Blingsight's capability to detect invisible creatures, but it doesn't exactly address my issue of whether solid matter effectively "blocks" Blindsight.

I will gladly consider either rules-as-written or rules-as-interpreted answers. If there is a published game mechanic or a posted developer ruling I have overlooked, please include and cite them in your answer.

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    \$\begingroup\$ I'm finding that Blingsight typo more amusing than it should be \$\endgroup\$ – Adeptus Jul 12 '16 at 5:55
  • \$\begingroup\$ I've removed the "blind" tag, as that has to do with things like the "Blinded" condition or otherwise being blind, not simply a feature that has "blind" in its name. \$\endgroup\$ – V2Blast Aug 30 '18 at 19:08
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Blindsight (monster ability)

A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as oozes, and creatures with echolocation or heightened senses, such as bats and true dragons, have this sense.

This isn't super helpful mechanically, however it does mention 'creatures with echolocation like bats'. Echolocation in real life still requires a line of effect, and wouldn't work through solid walls. This isn't specifically spelled out in the text, but in the absence of a rule we have to take into account the words being used. In this case, a bat would be able to echolocate in complete darkness, but would not be able to echolocate a person on the other side of a 5 foot stone wall. Since the manual doesn't go on to describe echolocation in any greater detail, we have to assume it means in DnD what it means in real life.

It looks like a Purple Worm has both blindsight and tremorsense, which would seem to show that there is a reason to have both, which would again point to the above definition where blindsight does not work through solid objects but tremorsense does.

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The simplest answer is that, as far as I am aware, there are no hard and fast rules for when you may use each ability. However, looking at what the abilities are supposed to represent can shed some light and help us come closer to a "right" way to implement them.

First, let's look at tremorsense. Tremorsense is an additional sense, a way of perceiving things that is completely separate from normal human forms of perception. As stated in the description you provided, it allows you to pick up vibrations through materials you are in contact with and that are within the radius, meaning that it functions regardless of obstacles so long as the vibrations are being made. The result is that the thing being sensed must normally be moving. As I see it, this gives the ability its balance by providing one advantage and one drawback.

Now let's think about blindsight. Unlike, tremorsense, blindsight does not really add a separate sense. Instead, it replaces sight within a certain range. This again provides one advantage, which is that anything within a normal line of sight and within the range of the ability can be perceived (regardless of whether it is moving, for instance), as well as one drawback, in that things outside of a normal line of sight cannot be "seen" using the ability.

To reiterate, there are other ways to interpret the rules, but in terms of game balance, this way seems the most logical to me.

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    \$\begingroup\$ Jeremy weighs in: twitter.com/JeremyECrawford/status/753426004060680192 \$\endgroup\$ – nitsua60 Jul 14 '16 at 17:55
  • \$\begingroup\$ Thanks for sharing that. One thought: If you take that definition in its fullest extent, I do wonder what reason there would be to ever choose Tremorsense over Blindsight. It seems to me like anything that "cloaks your presence entirely" would also render Tremorsense ineffective; and in any other situation, Blindsight is the better option. Perhaps I'm missing something. Otherwise, this may be a case where unofficial definitions or house rules are in order. \$\endgroup\$ – Yes-And Roleplayer Jul 14 '16 at 23:06
  • \$\begingroup\$ However, if you take "hide from a creature" literally, as "remain out of sight" (merriam-webster.com/dictionary/hide), then invisibility would work against Blindsight, as it would be still strictly visual, but not Tremorsense and the first the definition I gave in my answer stands, as does the balance of the two actions. \$\endgroup\$ – Yes-And Roleplayer Jul 14 '16 at 23:06
  • \$\begingroup\$ FWIW, there is more to that discussion \$\endgroup\$ – KorvinStarmast Aug 30 '18 at 18:33
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No, Blindsight does not grant the ability to see through solid objects, such as walls, floors, and ceilings.

Blindsight states

A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as oozes, and creatures with echolocation or heightened senses, such as bats and true dragons, have this sense.

Seeing this, we then need to look at sensory details of the given examples, as well as for other cases.

Ooze

blindsight 60 ft. (blind beyond this radius)

Bat

blindsight 60 ft.
Echolocation. The bat can't use its blindsight while deafened.

As for other cases, underwater creatures such as Crabs and Sharks also feature Blindsight. As well as the Grimlock, with these traits

blindsight 30 ft. or 10 ft. while deafened (blind beyond this radius)
Blind Senses. The grimlock can't use its blindsight while deafened and unable to smell.

Looking at all these examples, there are some patterns:

  1. For some creatures, Blindsight is all about sound (Bats for example), and sound does not readily pass through air, a solid object, and then air again.

  2. For others, it also includes the ability to smell (Grimlocks), though scent does not pass through objects; it requires an air channel (entirely GM dependent).

  3. For still others, such as Crabs, Sharks, Oozes, and Dragons, it is down to just physical proximity, not dependent upon other senses, which cannot be detected through a solid object without an explicitly mentioned ability.

Since Blindsight does not state that it allows detection through solid objects, and no entry in the Monster Manual lists such an addition, we can be reasonably sure that Blindsight does not allow detection ('see') through solid objects such as walls, floors, and ceilings.

However, since Blindsight is based on sound, smell, and proximity, personally, I would rule that it can detect around solid objects, if doing so would be within the radius of detection, assuming such radius is treated the same as Movement distance.

Basically, you treat the detection radius as a speed, and wherever the creature could go, is how far it can detect.

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Here's how I would interpret it:

Blindsight would allow you to know that something is coming up, but not necessarily that it is around a corner. For example, you may hear something running, but due to the nature of how sound travels, it may sound like it's coming directly in front of you because the sound is bouncing off the cavern walls. This would be an approximation of a limitation on how echolocation works, which is mentioned in the manual specifically "creatures with echolocation or heightened senses, such as bats" - Monster Manual, P8.

Comparing this to Tremorsense you would feel the vibration coming, and possibly detect the specific direction. This, if it pointed straight at a wall, would tell you that there's a corner coming up.

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    \$\begingroup\$ These may be reasonable interpretations, but it would be good to show where you get those reasons from. (Especially as OP mentioned interest in designer statements/intent.) For instance, quoting the types of creatures called out in the "Blindsight" description could lead one to conclude it's a simple matter of hearing, sensing air movement, and even lateral lines, leading you to conclude that a stone wall would effectively block it. In other words, if that's the sort of thing you're thinking, spell it out. Otherwise it's just some NotADoctor's opinion on the internet. \$\endgroup\$ – nitsua60 Jul 11 '16 at 17:44
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    \$\begingroup\$ Jeremy weighs in: twitter.com/JeremyECrawford/status/753426004060680192 \$\endgroup\$ – nitsua60 Jul 14 '16 at 17:55
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I believe that the ability of a monster with blind-sight to be able to "see" through walls can not physically exist in the DND world. The question answers itself the way it is phrased. You have missed the key aspect of blind-sight though, ".[B]lind-sight can perceive its surroundings without relying on sight" Then it follows by giving different examples of monsters and how they achieve it. So Blind sight is determined by the Monster, not a sense. Intuition is also within the capability of a monster, A blind person can sense if something is amiss in their known environment, A blind dog can hear his food being prepared from the other side of the house, or know that something is in its environment that shouldn't be there. So depending on the Monster, Blind-sight may give it the ability to "Perceive" something on the other side of a door or wall. It should be linked to and rolled against perception/passive perception with appropriate modifiers. All this is also dependent that the target of the perception is withing the Monsters stated ability radius.

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You have to know the origin of your blindsight.

Bats use echolocation so they can't see through walls. Other creatures use sound or smell. If the origin of your blindsight would be magic, it could be arguable that it does work through walls.

The creatures in the Monster's Manual tend to have a short description on the way their blindsight works, but, since the mystic class doesn't specify anything it's up to the dm and the player to work it out together. Otherwise you could say your blindsight can detect absolutely everithing in the given range and that would be kinda broken < insert sarcastic mystic joke here. > .

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    \$\begingroup\$ Welcome to the stack! Please take our tour to learn more about we operate here. It would be helpful if you could add some support to your answer. And while the real-world blindsight examples are good for directional support, trying to equate game mechancis to real world physics often doesn't work. \$\endgroup\$ – NautArch Nov 4 '19 at 14:29
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    \$\begingroup\$ Also note that the bat has echolocation specifically and it differs from standard blindsight in that being deafened affects it. \$\endgroup\$ – NautArch Nov 4 '19 at 17:46
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Blindsight allows you to see everything in the radius. Tremorsense allows you to perceive everything in the radius that shares a medium of vibration.

Blindsight reads:

A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as oozes, and creatures with echolocation or heightened senses, such as bats and true dragons, have this sense.

The only stated limit to Blindsight is the radius.

Tremorsense reads:

A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance.

You can think of Tremorsense as a weaker version of Blindsight.

One less obvious point about Blindsight is that it is a kind of sight, where as Tremorsense is a sense. Effects that rely on sight do not include Tremorsense. For example, Unseen Attackers and Targets states:

When you attack a target that you can't see, you have disadvantage on the attack roll. ... When a creature can't see you, you have advantage on attack rolls against it.

A creature using Blindsight, Truesight, or Darkvision can see the perceived target. A creature using Tremorsense cannot.

You don't have to follow the rules 100%

The rules don't cover every single possibility. Even if you consider normal sight, humans have a great variety of capabilities ranging from total blindness to near-superhuman vision.

Blindsight is a catchall name for special senses that can detect anything within a radius. This include the "special sense" of a gray ooze, as well as echolocation of bats, and the keen senses of dragons. For some creatures, like bats, it may make sense to limit this to be blocked by walls, but in general you shouldn't default to that.

Your DM has the final say, and it would be reasonable of them to say that a dragon can sense you through a wall, floor, or ceiling, but perhaps a bat can't.

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