title_body: what kinds of vocal sounds echo less? Please bear with me as I elaborate. 
  I'm looking to create a language for humanoids that live in a set of cave systems and tunnels. While their language may have been 'born' on the surface, they have moved under the ground ages ago. The Ecosystem under the ground is very hostile, and so they had to adapt their language to carry less when spoken. 
  For this purpose, I'm trying to find out which of the known vocal sounds would be more prevalent in such a language.
  (I am a true novice in the field, mind you, so I'm, reading several language construction guides to help in this endeavor)
  So, what I'm asking, essentially, is which of the vocal sounds (labial, dental, palatal etc etc) would echo more, and which would echo less. I've searched in vain for an answer, and I don't have a convenient cave to test that in.
  Thank you!
upvoted_answer: It would probably be a question of volume and frequency pattern. Loud sounds would travel further (and thus have more potential for echo). I would like to stress that I too have no cave to try this out, so there is a certain speculative aspect to this answer.
  
  So, avoid vowels (obviously), and voiced sounds (prefer /p/, /t/, /k/, /f/, /s/ over /b/, /d/, /g/, /v/, /z/).
  
  However, such a language would be pretty hard to use, so you'd need to compromise. Assuming low frequency sounds have less energy, the vowels /u/ and /o/ might be suitable, and perhaps the schwa.
  
  Plosives (/p/, /b/, /t/) feature a burst as the air is released, which might also not work well, so you're probably better off using fricatives.
  
  That leaves you with voiceless fricatives: /f/, /th/, /s/, /S/, /h/. Though voiced, nasals might also be suitable, as the mouth is closed during the articulation and thus it is possible to pronounce them without releasing too much acoustic energy.