# Unmasking Nasality to Assess Hypernasality
## 5 Discussion
### Phonetic Differences  
The results of experiment one show that, when considering four classes, the accuracy of the monophonic, mouth, and nose DNN models was almost identical. However, the models differed notably in the precise error types. Specifically, in the nose condition, nasality errors were clearly less frequent than vowel/consonant errors. In the other two conditions, the nasality errors were more frequent than the vowel/consonant errors. One explanation for the increased accuracy of the oral/nasal classification in the nose signal is that the classifier improves when the spectral distance between the classes is sufficiently clear. As shown above, the spectral distance for oral/nasal vowels is the highest in the nose signal, which would explain why accuracy increases in this case.  
Another relevant result is that the ratio of oral vowels wrongly classified as nasal was three times larger in the monophonic and mouth conditions as compared with the nose condition. This result is relevant because, in the clinical context, hypernasal patients are typically evaluated with oral sounds; this means that false negatives may have a relatively small impact in computing hypernasality, but false negatives may result in healthy speakers (and in patients who have been treated successfully) being wrongly classified as hypernasal. Unfortunately, we do not have an explanation for the observed bias.  
It is also relevant that in the three conditions, consonants were better classified than vowels. This result might be due to the criteria used to identify nasal sounds. [PERSON] et al. [30] classified 30% of the vowels in contact with nasal consonants as nasals. In this study, the vowel length was adapted slightly (by increasing that percentage in shorter vowels). However, phonetic research has shown that vowel nasalization varies as a function of multiple factors, including dialect, individual speaker, syllable stress, and word position [41; 42]. This means that our criterion may have led to misclassifying many vowel frames, which might explain the relatively poor results as regards vowel classification.  
Again, the monophonic signal seems to be more similar to the mouth signal than to the corresponding nose signal. This result is compatible with, and is explained by, the results of the MFFC distances: as the spectral distance is reduced for nasal/vowel sounds, nasality information transmission is less effectively transmitted. However, we are not saying that the information is not present in the monophonic signal, we merely consider that this information is better encoded in the nose signal, which implies that it is partially masked in the coupling process. This conclusion is compatible with evidence of the role of different acoustic cues in speech communication [10]. It is important to remember that the speech signal carries multiple types of phonetic information. These include, apart from the nasal/oral contrast, the following: consonant/vowel, manner of articulation, place of articulation, voicing, and many other intonational and rhythmic characteristics. It seems reasonable to assume that the speech signal must have evolved to give preference to developmentally or communicatively most relevant information. For instance, a fundamental step in speech development consists of differentiating vowel and consonant sounds; it helps children break the speech code by separating the signal into syllables, which are a keystone for phonological and language development [43]. Also, most of the acoustic cues are encoded in the mouth signal. To conclude, the oral/nasal contrast is relevant, but it certainly does not play such an important role as other cues, for which it seems more efficient for humans to have a dominant mouth signal. In other words, masking the nose signal might help ensure optimal speech communication.