That’s when you manage to hide completely behind the lectern. Unless you are either George Bush or Hillary Clinton you won’t need to hide to avoid flying shoes. Acousmatic is a fancy jargon term you can use to amaze your friends. Wikipedia says that:
“Acousmatic sound is sound one hears without seeing an originating cause. The word acousmatic, from the French acousmatique, is derived from the Greek word akousmatikoi (ἀκουσματικοί), a term used to refer to probationary pupils of the philosopher Pythagoras who, so that they might better concentrate on his teachings, were required to sit in absolute silence while listening to their teacher deliver his lecture from behind a veil or screen.”
If you hide like that, then your audience can’t see your gestures and you have no eye contact with them. That will make your presentation relatively ineffective.
Last year Brian Kane published a book titled Sound Unseen: Acousmatic Sound in Theory and Practice. Langdon Morrison reviewed it at Music Theory Online. In points  and  he notes that the Pythagorean veil really may have just been an allegory. Kane states the history for the key myth in a paragraph on page 50 of his book:
“The term ‘acousmatic’ refers to the disciples of Pythagoras who heard the philosopher lecture from behind a screen, curtain, partition, or veil (M1, M2b, M3, M4, M5, M6, M7, M8, M9, M10). The reason they remained on the far side of the veil was to promote a form of concentrated listening (M2a, M8b) or to emphasize the master’s message (M1, M3, M6, M7, M9) undistracted by the visual aspects or physical presence of the speaker (M1, M3, M6, M7, M8, M9). In addition to keeping a vow of silence for five years (M1, M5, M6, M7, M10), this exoteric ritual formed part of an initiation into the Pythagorean school where pupils would then see the master (M1, M5, M6). From the experience of the acousmatics, we derive the adjectival sense of the term, meaning a sound that one hears without seeing or being able to identify the originating source (M2a, M2c, M3, M4, M6, M7, M8a, M8b, M10, M11a). The term was transmitted by Diderot in the Encyclopédie (M1, M5) and in the pages of Larousse (M6, M10). A related term, ‘acousmate’ (M1, M3, M11), was found in the Dictionnaire of the Académie française (M11b), as well as Larousse (M3). Apollinaire, a lover of rare words, used ‘acousmate’ as the title of two short poems (M1, M11b). These poems tell of voices heard in the air (M1, M11b). The writer Jérôme Peignot was the first to employ ‘acousmatic’ as a term for describing musique concrète (M2c, M3). Schaeffer learned about the term from Peignot (M1) and, by attaching it to the phenomenological epoché, developed a concept of acousmatics that formed a significant part of this theory in the Traité (M2d). Modern audio technology preserves the ancient acousmatic tradition of the Pythagorean veil (M10) or its mystical variants (M11). Acousmatic music continues the tradition of musique concrète Pythagoreanism by veiling sounds, through the use of the loudspeaker, of all causal and contextual associations (M2a, M4, M8b). “
The image was created by combining a face and a lectern found at Openclipart.