Page 48 - Summer 2018

Speech: Not So Acoustic
Figure 3. An utterance like “the dady slept in the trid” is non- sense, but if the same utterance is heard with sufficient back- ground noise, the brain replaces the unusual words with simi- lar-sounding alternates that create a well-formed sentence like “the baby slept in the crib.” See also Multimedia File 1, parts a through d, at acousticstoday.org/speech-not-acoustic.
Figure 2. Making the bed “with cream cheese” doesn’t make sense... it must have been “with clean sheets.” The amplitude envelopes of the sounds are nearly identical (top), which might explain why a seemingly drastic series of mistakes could be made. Within each interpretation, the words make sense in re- lation to each other; the listener would probably not think of “bagels” and “clean sheets” as belonging in the same sentence.
sonants articulated at the wrong place in the mouth (e.g., “His plan meant taking a big risk” transformed into “His tlan neant tating a did rist” (see Multimedia File 1, parts a through d, at acousticstoday.org/speech-not-acoustic).
They called the mispronounced utterances “elliptical speech.” Although these sentences were very challenging to hear in qui- et, a little bit of background noise invoked the strategy of us- ing “top-down” processing to replace each misarticulated work with a more reasonable counterpart. The conversion of a non- sensical utterance into a meaningful one is shown in Figure 3.
Even the Acoustic Parts Are Not
as Clear One Thinks
It seems intuitive to think of speech as sentences constructed from smaller units like words, which are themselves con-
Figure 4. Articulatory motions for speech sounds overlap in time, producing utterances that would be confusing if heard out of context. Within the context of this sentence, “hish” can be in- terpreted as “his” + the onset of “shoes” (see Multimedia File 2, parts a and b, at acousticstoday.org/speech-not-acoustic).
structed from smaller units called phonemes. This building block framework not only fails to account for the influence of the aforementioned “top-down” factors, it also assumes that the smaller building blocks (individual phonemes and words) are themselves intelligible. However, this is frequent- ly not the case. If the words from a sentences are stripped out and played in isolation, they are sometimes entirely unintelligible, again highlighting the importance of the surrounding context when identifying what is spoken. The intelligibility of short utterances excerpted from a conversa- tion was analyzed by Pickett and Pollack (1963), who found that performance was only above 50% when substantial sur- rounding context to a word was provided. Figure 4 shows how this is true for the sentence “His shoes were untied.” The first word in isolation sounds like “hish,” not like any recognizable English word (see Multimedia File 2, parts a and b, at acousticstoday.org/speech-not-acoustic). But when followed by the rest of the sentence, the original meaning
46 | Acoustics Today | Summer 2018