Abstract
There are several different types of nasal emission that can occur during speech due to either velopharyngeal dysfunction or abnormal articulation in the pharynx. Nasal emission can be inaudible or very loud and distracting, depending on the size of the velopharyngeal opening and the physics of the flow. Nasal emission can be obligatory and/or compensatory (due to abnormal structure) or it can be caused by a misarticulation that results in a substitution of a pharyngeal sound for an oral sound, despite normal velopharyngeal structure. Nasal emission can occur on all pressure-sensitive phonemes or it can be phoneme-specific. Although it is generally recognized that the loud and distracting form of nasal emission (called nasal turbulence or nasal rustle) is due to a small velopharyngeal opening, the causality of the distracted sound is debated. This article provides a brief review of the types of nasal emission, the terms used to describe it, and the potential causes. This article also stresses the need for further research to clarify the causality of the sound generated by a small velopharyngeal opening.
Keywords: nasal emission, nasal turbulence, nasal rustle, velar flutter, posterior nasal fricative, velopharyngeal insufficiency
Normal Velopharyngeal Function and Velopharyngeal Dysfunction
During normal speech production, the velopharyngeal (VP) valve directs both sound and airflow into the appropriate cavity for each speech sound. Nasal consonant sounds (/m/, /n/, and /ŋ/) are produced with the VP valve open, which results in an acoustic coupling between the oral and nasal cavities. On the other hand, oral consonants require complete closure of the VP valve so that all sound and airflow are directed into the oral cavity.
If sound escapes through the VP valve during production of oral sounds, there will be hypernasality. In hypernasality, speech distortion is created by an acoustic mechanism where undesirable sound propagates (resonates) in the nasal cavity. This particularly occurs with a relatively large VP opening and is perceived on vowels and voiced consonants. If airflow escapes through the VP valve, there will be nasal emission of the airstream.
Audibility of Nasal Emission Based on the Size of the VP Opening
Nasal emission is often characterized by its audibility, which provides clues as to the size of the VP opening (Kummer et al., 2003). Nasal emission is totally inaudible when there is a large VP opening because there is relatively little impedance to the flow to cause audible friction. In addition, its sound is masked by the sound of hypernasality, which is the predominant characteristic of a large VP opening. Although inaudible, the effects of nasal emission with a large opening include weak or omitted consonants, the need to take more frequent breaths to sustain phonation for connected speech, and the potential for development of compensatory productions to increase intelligibility. In comparison to a large opening, a midsized opening usually causes audible nasal emission because there is more impedance to the flow, causing friction, and less hypernasality. Nasal emission is most audible with a small VP opening. In this case, the nasal emission is not only audible—it can be very loud and distorting, making speech unpleasant to hear and often unclear (Kummer et al., 1992; Kummer et al., 2003; Peterson-Falzone et al., 2006; Zajac and Preisser, 2016). Overall, when there is a large VP opening, hypernasality is the predominant characteristic of speech, whereas nasal emission is inaudible. When the opening is small, nasal emission is the predominant characteristic of speech, whereas resonance is normal (Kummer et al., 2003).
Nasal Emission as an Obligatory Distortion Versus a Compensatory Error
An obligatory distortion occurs when articulation production is normal, but abnormal structure alone results in distortion of the speech sound. In contrast, a compensatory production (or compensatory error) occurs when articulation production is altered in response to abnormal structure in an attempt to improve overall intelligibility. When there is velopharyngeal insufficiency (VPI), nasal emission occurs as an obligatory distortion of speech. As such, physical management is required for correction. Nasal emission may persist after correction of the VPI; however, if the patient developed compensatory productions (eg, pharyngeal fricatives or posterior nasal fricatives) for sibilant sounds (s, z, sh, ch, j) to compensate for the lack of adequate oral airflow. Because the nasal emission after correction occurs only on the misarticulated sounds, these compensatory productions would cause phoneme-specific nasal emission (PSNE), which is nasal emission that occurs only on certain sounds due to incorrect placement.
Nasal Emission Due to Abnormal Speech Sound Production (Misarticulation)
Although nasal emission is most commonly caused by VPI or by compensatory productions secondary to VPI, nasal emission can also occur due to velopharyngeal mislearning in the absence of VPI. Although the cause is unclear, some children learn to produce a pharyngeal sound (ie, pharyngeal fricative or posterior nasal fricative) for an oral sound. This substitution causes PSNE because the airflow necessarily is released through the VP port and then nasal cavity.
Terms for Audible Nasal Emission and Presumed Causes
Several terms have been used to describe the severe form of nasal emission that occurs due to a small VP opening. These terms include nasal turbulence, nasal rustle, and nasal snort. In addition, several causes have been attributed to this distracting sound, including turbulent airflow (McWilliams, 1982), bubbling of secretions (Kummer et al., 1992; Kummer et al., 2003), and velar flutter (Trost, 1981). It should be noted that this same type of audible nasal emission can also be heard when the cause is due to misarticulations in the pharynx with production of a posterior nasal fricative or even a pharyngeal fricative. These terms and their presumed causes are summarized in Table 1.
Table 1.
Terms and Presumed Causes of Loud and Distorting Nasal Emission.
| Term | Description | Type | Due to… | Occurrence | Presumed Sound Source |
|---|---|---|---|---|---|
| Nasal turbulence | Although it was briefly mentioned by (Mason and Grandstaff, 1971), the term “nasal turbulence” is largely attribute to McWilliams (1982) who suggested that severe audible nasal emission is caused by “marked resistance to the stream air,” where nasal resistance was defined as “when airstream is greater than the space through which it must circulate.” According to McWilliams’ theory, the small VP opening accelerates the flow and the fast-moving flow in the nasal cavity then produces the turbulent noise. It should be noted that the term “severe” is a perceptual measure and there are known issues with rater agreement. Peterson-Falzone et al. (2010) indicated that nasal turbulence “may be caused by soft tissue vibrations or by displacement of mucous pooling in the area of the constriction.” It is not clear how these mechanisms would be related to the method of generating turbulent noise as was suggested by McWilliams. It might be that these authors suggest all these mechanisms can generate turbulent noise, but if such is the case, then one would expect for the turbulent noise to be (slightly) different for each mechanism. In other words, turbulent sound from tissue vibration should be perceived differently than turbulent sound generated by secretion bubbling |
Obligatory distortion | VPI only | Inconsistent on all pressure sounds | Turbulent airflow |
| Nasal rustle | Term was first used by Mason and Grandstaff (1971) and further defined as an obligatory distortion by Kummer and colleagues who suggested that the audible nasal emission was due to an interaction between the nasal airflow and the secretions in the nasopharynx (Kummer et al., 1992; Kummer et al., 2003). Using nasopharyngoscopy, these studies observed bubbling of secretions on the superior aspect of the VP port, specifically with small openings, and attributed the distorted sound to the secretion bubbling. However, these studies did not explain how this process can actually generate the sound/noise that is perceived as the distortion (ie, they showed correlation, but not causation) | Obligatory distortion | VPI only | Inconsistent on all pressure sounds | Bubbling of secretions |
| Nasal snort | Although used less often nowadays, the term used to be common by clinicians to describe audible nasal emission (Mason and Grandstaff, 1971). Morley (1970) described nasal snort as a possible mechanism for defect in cleft palate speech and explained the snorting as sound generated by the passage of air through a small sphincter that is not tightly closed. Mason and Grandstaff (1971) similarly described this term as “airstream being expelled through a small aperture in an incompetent VP valve” and also mentioned, without explanation, that the term “nasal snort” is the same as “nasal turbulence” and “nasal rustle.” This term is used to describe the obligatory form of the distortion but is rarely used in more recent literature | Obligatory distortion | VPI or misarticulation | Inconsistent on all pressure sounds or is phoneme-specific on /s/blends | Release of air through a small sphincter |
| Velar flutter | Trost (1981) first suggested this term when she observed blurring of the VP port in radiographic images from participants with repaired cleft palate who were diagnosed with compensatory misarticulations for oral fricatives (eg, posterior nasal fricative). She attributed the blurring to velar vibrations (flutter), and later noted that similar sound can occur as obligatory distortion due to VPI (Peterson-Falzone et al., 2006). According to this theory, the flow through the small VP openings can excite the margins of the velar tissue to vibrate, thus producing sound by a mechanism similar to vocal fold vibrations. These periodic vibrations can be detected in a nasal microphone during nasometry (Zajac and Preisser, 2016). It should be noted that this theory of velar vibrations during (unvoiced) fricatives was never confirmed through nasopharyngoscopy | Obligatory distortion | VPI only | Inconsistent on all pressure sounds | Vibration of the velum |
| Posterior nasal fricative or pharyngeal fricative | Used to describe a compensatory error that is thought to occur when a voiceless nasal fricative is substitute for high pressure constant. Trost (1981) used radiographic data to show that the oral cavity is blocked by raising the back of the tongue thus forcing all the flow through the VP port. Thus, the fricative sound is thought to be produced in the nasal cavity and has been characterized with and without turbulence sound (Peterson-Falzone, et al., 2010. Posterior nasal fricative is a term that is not commonly used by clinicians because of the difficulty to distinguish this term from terms used with obligatory distortions (Zajac and Vallino, 2017) | Compensatory error | Abnormal articulation to compensate for VPI | Phoneme-specific nasal emission, usually on sibilants | Turbulent airflow when oral placement is substituted by pharyngeal placement |
Abbreviations: VP, velopharyngeal; VPI, velopharyngeal insufficiency.
Summary and Discussion
When there are characteristics of VPI, the audibility of nasal emission can give the examiner clues as to the relative size of the opening. Overall, nasal emission is more audible with a small opening compared to a large opening. The examiner must also determine whether the nasal emission is an obligatory distortion because it is consistent on all pressure-sensitive sounds (plosives, fricatives, affricates) or if it is phoneme-specific. This is because obligatory nasal emission requires surgical management for correction, whereas PSNE can be corrected with speech therapy.
It is generally recognized that the severe form of nasal emission that occurs with a small VP opening can occur due to abnormal structure (obligatory distortion) or abnormal articulation (compensatory). However, what actually causes the undesired sound with the severe form of nasal emission is still debated. It has been hypothesized that the undesired sound is from turbulent airflow in the nasal cavity, bubbling of secretions on the top of the VP valve, and even flutter of the velum. Each one of these terms suggests that the undesired sound in the nasal cavity is created by a very different mechanism. It can also imply that one would expect to find some perceptual differences between these terms although no such evidence exists (eg, nasal turbulence does not sound different than nasal rustle). It should be recognized that when airflow goes through a small VP opening, regardless of whether it is due to abnormal structure or abnormal function, the physics of the flow and the acoustics would be the same.
Further research in this area is therefore needed to determine the actual mechanism that generates the undesired sound in the nasal cavity and how this mechanism changes when the size of the VP opening changes. Determining this mechanism would require a combination of computer modeling data from acoustic recordings (eg, nasometry) and medical imaging (eg, MRI, nasopharyngoscopy, etc). Computer modeling can simulate the motion of the velum and how sound is generated by the airflow and propagated in space. The imaging data can be used to reconstruct the geometries of the model that is used for the computer simulation. Data from acoustic recordings can be used both as input and as validation to the model (eg, by comparing the predicted sound with the recording).
Understanding the etiology of undesired sound in the nasal cavity is important because it can affect treatment options for patients. For example, if surgery is recommended as treatment for audible nasal emission, the surgical outcome must result in complete closure of the VP valve so that no flow can pass through the VP port during oral speech. Merely reducing the size of the opening will only shift or change the flow mechanism that generates sound in the nasal cavity instead of eliminating it. If a tight closure is not achieved post surgery, speech distortion due to VPI will remain but may change from hypernasality to very audible nasal emission.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors acknowledge the support of NIH grant no. K25DC014755.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
References
- Kummer AW, Briggs M, Lee L. The relationship between the characteristics of speech and velopharyngeal gap size. Cleft Palate Craniofac J. 2003;40(6):590–596. [DOI] [PubMed] [Google Scholar]
- Kummer AW, Curtis C, Wiggs M, Lee L, Strife J. Comparison of velopharyngeal gap size in patients with hypernasality, hypernasality and nasal emission, or nasal turbulence (rustle) as the primary speech characteristic. Cleft Palate Craniofac J. 1992;29(2):152–156. [DOI] [PubMed] [Google Scholar]
- Mason RM, Grandstaff HL. Evaluating the velopharyngeal mechanism in hypernasal speakers. Lang Speech Hear Serv Sch. 1971;2(4):53–61. [DOI] [PubMed] [Google Scholar]
- McWilliams B Cleft palate. In: Shames G, Wiig E, eds. Human Communication Disorders. Columbus, OH: CE Merrill; 1982. [Google Scholar]
- Morley ME. Cleft Palate and Speech. 7th ed. Baltimore, MD: Williams & Wilkins; 1970. [Google Scholar]
- Peterson-Falzone SJ, Hardin-Jones MA, Karnell MP. Cleft Palate Speech. 4th ed. St. Louis, MO: Mosby/Elsevier; 2010. [Google Scholar]
- Peterson-Falzone SJ, Trost-Cardamone J, Karnell MP, Hardin-Jones MA. The Clinician’s Guide to Treating Cleft Palate Speech. Amsterdam, the Netherlands: Elsevier Health Sciences; 2006. [Google Scholar]
- Trost JE. Articulatory additions to the classical description of the speech of persons with cleft palate. Cleft Palate J. 1981;18(3):193–203. [PubMed] [Google Scholar]
- Zajac DJ, Preisser J. Age and phonetic influences on velar flutter as a component of nasal turbulence in children with repaired cleft palate. Cleft Palate Craniofac J. 2016;53(6):649–656. [DOI] [PubMed] [Google Scholar]
- Zajac DJ, Vallino LD. Evaluation and Management of Cleft Lip and Palate: A Developmental Perspective. San Diego, CA: Plural Publishing; 2017. [Google Scholar]