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What is the Source-Filter Theory of Speech Production?

Amy T. Neel, PhD, CCC-SLP

February 1, 2023



What is the source-filter theory of speech production?


The source-filter (or acoustic) theory of speech production is an explanation of the two-stage process of creating speech sounds. For voiced phonemes (vowels, liquids, nasals, and glides), the source is the complex glottal tone produced by the vibration of the vocal folds. The vocal tract, which consists of the throat, mouth, and nose, filters or shapes the tone produced by the vocal folds into speech sounds. The vocal tract filter allows some frequencies of that multi-frequency glottal tone pass through to the outside air and reduces the intensity of other frequencies. When you change the shape of your vocal tract by moving your tongue, lips, and jaw, you change the frequencies that pass through or are filtered out. For fricative, stop, and affricate consonants, the source created by turbulent airflow at the site of vocal tract constriction is also filtered by the vocal tract.

The source-filter theory is important for understanding differences among vowel sounds. Different vocal tract shapes result in different patterns of formant frequencies, the bands of acoustic energy that we use to perceive vowel phonemes. For example, the vowel /i/ as in “heat” is produced with the tongue positioned high (close to the roof of the mouth) and relatively forward in the mouth. It has a low-frequency first formant, around 300 Hz, and a high-frequency second formant, around 2500 Hz. If you keep your tongue high in position but move it back toward the soft palate to produce the vowel /u/ as in “hoot,” the first formant remains low (300 Hz), and the second formant moves lower in frequency (around 900 Hz). Changes in the shape of the vocal tract result in changes in the patterns of acoustic energy that pass through the mouth, and those acoustic patterns are perceived by listeners as different vowel sounds.

It's important for clinicians to understand the source-filter theory because it can provide the foundation for differential diagnosis of speech disorders and differences. You can think about whether the source or the filter or both components of speech are affected, and that knowledge can guide assessment and treatment considerations. For someone with a hoarse voice whose consonants and vowels are clearly produced, the source component is involved, so you choose assessment techniques that elucidate the function of the larynx and respiratory system. When you hear imprecise consonants accompanied by normal voice quality, you select articulation as your treatment target because the filter is affected. Sometimes speech disorders are complex, especially those associated with neurogenic disorders because several speech systems are affected. The source-filter theory can help you sort out which deficits – those related to producing the laryngeal tone and those responsible for shaping the tone into speech sounds – contribute the most to speech intelligibility. For example, people with hypokinetic dysarthria related to Parkinson's disease can have low-intensity and breathy voice quality, restricted range of pitch and loudness, and distorted consonants. Because both source and filter components of speech are involved, you should consider treatments that address both phonation and articulation, such as loud speech or clear speech.

This Ask the Expert is an excerpt from the course, 20Q: Using Speech Science in Clinical Practice, authored by Amy T. Neel, PhD, CCC-SLP.

amy t neel

Amy T. Neel, PhD, CCC-SLP

Dr. Neel received a bachelor’s degree in Speech-Language Pathology from Texas Christian University, a master’s degree in Speech-Language Pathology from the University of Oklahoma Health Sciences Center, and a Ph.D. in Speech and Hearing Science and Cognitive Science from Indiana University.  Her teaching interests include speech science, phonetics, and motor speech disorders. Her research focuses on speech intelligibility in normal speakers and speakers with dysarthria, including those with Parkinson's disease, Pompe disease and oculopharyngeal muscular dystrophy.  Currently, she is investigating speech biomarkers for repetitive head injury in professional fighters.  She is the Coordinator for ASHA Special Interest Group 19 for Speech Science.

Related Courses

20Q: Using Speech Science in Clinical Practice
Presented by Amy T. Neel, PhD, CCC-SLP


Amy T. Neel, PhD, CCC-SLP
Course: #10326Level: Intermediate1 Hour
  'Great course!'   Read Reviews
Many speech-language pathology clinicians find speech science daunting and not applicable to their clinical practice. This course discusses why and how speech science is useful in understanding speech disorders and differences. Examples are provided of how to assess and treat speech disorders using speech science-based principles and techniques such as the source-filter theory and acoustic, physiologic and biofeedback measures.

20Q: Dynamics of School-Based Speech and Language Therapy Variables
Presented by Kelly Farquharson, PhD, CCC-SLP, Anne Reed, MS, CCC-SLP


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Course: #10002Level: Advanced1 Hour
  'Descriptions for frequency, caseload, and dose'   Read Reviews
This course reviews dynamics of speech and language therapy variables such as session frequency, intervention intensity, and dosage, and how these are impacted by different service delivery models. It discusses how therapy outcomes are related to therapy quality, IEP goals, and SLP-level variables such as job satisfaction and caseload size.

20Q: A Continuum Approach for Sorting Out Processing Disorders
Presented by Gail J. Richard, PhD, CCC-SLP


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There is a good deal of confusion among audiologists and speech-language pathologists when a diagnosis of “processing disorder” is introduced. This course presents a continuum model to differentiate processing disorders into acoustic, phonemic, or linguistic aspects so that assessment and treatment can become more focused and effective. The roles of audiologists and SLPs in relation to processing disorders are described, and compensatory strategies for differing aspects of processing are presented.

20Q: Evaluation and Treatment of Speech/Resonance Disorders and Velopharyngeal Dysfunction
Presented by Ann W. Kummer, PhD, CCC-SLP


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  'Good information on VPI vs velopharyngeal incompetence vs velopharyngeal insufficiency and velopharyngeal mislearning'   Read Reviews
Children with speech and resonance disorders (hypernasality, hyponasality, and cul-de-sac resonance) and/or nasal emission present challenges for speech-language pathologists (SLPs) in all settings. This article will help participants to recognize resonance disorders and the characteristics of velopharyngeal dysfunction, and provide appropriate management.

20Q: Criteria for Diagnosing Apraxia of Speech (AoS) in Adults
Presented by Katarina Haley, PhD, CCC-SLP


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  'This course really got down to the key points quickly and now I have some salient and specific information to use in diagnosis'   Read Reviews
This course reviews the criteria currently used to diagnose AoS after stroke and other adult-onset neurologic conditions. A distinction is made between features that are part of the presentation profile, and features that are useful for differentiating AoS from aphasia with phonemic paraphasia and dysarthria. In addition, challenges of diagnosing AoS and useful assessment tools are discussed.

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