*In canon, the functionality of the Montrals is not clearly defined. EDIT: A StarWars.com article (July 2017) restated the Legends material. Does that make it canon?
From the Legends Wookieepedia Article on Montrals (emphasis mine) :
Montral was a word used by the Togruta to refer to the hollow horns atop their heads. The Togruta possessed a form of passive echolocation by means of their hollow montrals which allowed them to sense the proximity and movement of physical objects around them. (1)
Echolocation, often referred to as "bio-sonar" uses properties of sound waves to detect the location of objects.
Echolocation--the active use of sonar (SOund Navigation And Ranging) along with special morphological (physical features) and physiological adaptations--allows bats to "see" with sound.(2)
From this, we can also infer that "Passive Echolocation" is, effectively, "Passive Sonar". Passive sonar is similar to listening for a sound.
From NOAA (emphasis mine)
Passive sonar systems are used primarily to detect noise from marine objects (such as submarines or ships) and marine animals like whales. Unlike active sonar, passive sonar does not emit its own signal, which is an advantage for military vessels that do not want to be found or for scientific missions that concentrate on quietly “listening” to the ocean. Rather, it only detects sound waves coming towards it. (3)
Notably, passive sonar does "detect sound waves coming towards it", in laymen's terms, it hears.
Going back to creatures who posses Bio-Sonar and echolocation, let's look at land mammals that are capable of echolocation (This is to eliminate some other factors that water presents). Both Bats and humans are capable of using echolocation sense their surroundings. The average bat is far more capable of echolocation; however, there have been some studies that say the human brain is actually suppressing the echoes that it hears as it relies more on sight (These studies are still being worked through, there is a study that also says humans using echolocation are using echolocation and stimulating visual processing sections of the brain) Echolocation Study . While what parts of the brain that are stimulated are in question, one thing remains clear with both species, their ears are the primary receptors of the sound waves.
As I write out the last part of this discussion and due to some comments, I realize there is a major caveat. This does not eliminate that Torgruta hear through other organs (ears) and operate a passive sonar ability through their montrals. I may delete this as this is a large gray area now that I reflect more on it.
- Motrals, Wookieepedia (Legends)
- Scientific American
- NOAA Sonar