voice tracing

Stephen Hawking, the famous theoretical physicist and author of "A Brief History of Time" has been using an electronic voice synthesizer to communicate ever since his tracheostomy following severe pneumonia. An electronic voice synthesizer is a system implemented by software and hardware to artificially produce speech. Stephen Hawking uses a TTS, Text-to-Speech system that communicates his text input into artificially derived speech. This system is one of many current solutions created by the field of Biomedical Engineering to help patients with speech disorders.

The Human Voice

How deos it work?
How is good voice produced?
What can go wrong with it?

The human voice is one of the most important instrument of communication, at one moment intimate and caressing, the next commanding and resolute. We use it for
talking, laughing, screaming, crying or for singing.  We vary the tone of voice to plead, persuade question, or to display our emotions: excitement, anger, surprise, happiness or sorrow.

How does it work?

To truly appreciate the intricacies of the human voice, it is helpful to understand just how it works.

“Voice” is the sound made by vibration of the vocal folds caused by air passing out through the larynx. The vocal folds in combination with the teeth, the tongue, and the lips make it possible to produce highly intricate arrays of sound.

The sound is produced in the following manner:

  1. Air pressure from the lungs creates a steady flow of air through the trachea (windpipe), larynx (voice box) and pharynx (back of the throat).
  2. The vocal folds in the larynx vibrate, creating fluctuations in air pressure that are known as sound waves.
  3. Resonances in the vocal tract modify these waves according to the position and shape of the lips, jaw, tongue, soft palate, and teeth, creating different qualities of sound.
  4. Mouth and nose openings radiate the sound waves into the environment.
The Vocal Cord

The vocal folds, also known as vocal cords, are composed of twin infoldings of mucous membranes stretched horizontally across the human larynx. They vibrate, modulating the flow of air being expelled from the lungs during phonation. The folds vibrate when they are closed to obstruct the airflow through the glottis, the space between the folds: they are forced open by increased air pressure in the lungs, and closed again as the air rushes past the folds, lowering the pressure (Bernoulli’s principle). In singing, the vibration frequency of the vocal folds determines the pitch of the sound produced.

The larynx

The larynx, or voicebox, is a cylindrical framework of cartilage bounded by ligaments and muscles that serves to protect the trachea and anchor the vocal folds for sound production. When the muscles of the vocal folds contract, the airflow from the lungs is impeded until the vocal folds are forced apart again by the increasing air pressure from the lungs. This process continues in a periodic cycle that is felt as a vibration.

How is a good voice produced?

To produce a good voice, many things must come together: there must be enough air in the lungs to make the vocal folds vibrate-inadequate airflow will leave the voice weak or strained. The layers of the vocal folds must be flexible with just the right amount of tension applied to them so that they vibrate optimally. Vocal folds that are too tense or too lax will not vibrate well, and the voice will suffer. And both vocal folds must move symmetrically, meeting each other completely at the center of the larynx, and then vibrating open.
                                                 recording of normal vocal cord motion

The above video was taken using a videostroboscopy at Johns Hopkins. Videostroboscopy is a principal diagnostic tool for vocal disorders. It is a device with a stroboscopic light and camera on the end of a probe that allows vocal cord vibrations to be "freeze-framed" on a television monitor.

What can go wrong with the human voice?

While most normal people take the human voice production process for granted, reduction or loss of the ability to produce voice can disrupt or preclude normal oral communication and thus have far-reaching social, professional, and personal consequences. A relatively large segment of the population suffers from voice problems: it has been estimated that anywhere from 3-9% of the general population has some type of voice abnormality at any given moment in time.

Disordered voice production can result from a wide variety of pathological conditions; with effects ranging from mild disturbances in vocal quality (e.g., slight hoarseness) to complete loss of the ability to produce laryngeal voicing (e.g., laryngectomy to treat cancer).

Symptoms of Voice problems

Voice problems occur with a change in the voice, often described as hoarseness, roughness, or a raspy quality. People with voice problems often complain about or notice changes in pitch, loss of voice, loss of endurance, and sometimes a sharp or dull pain associated with voice use. Other voice problems may accompany a change in singing ability that is most notable in the upper singing range. A more serious problem is indicated by spitting up blood or when blood is present in the mucus.

Behavioral and Functional Disorders

Many individuals that have voice problems have nothing wrong with their vocal cords. Instead, they have what is referred to as "vocal misuse syndrome"-an umbrella term for an array of behavioral and functional disorders.

Some of the causes of vocal misuse and abuse include excessive talking, strained or excessive voice use during periods of inflammation (e.g. allergy, infection, sinusitis, reflux), excessive coughing and throat clearing. Additional causes are sports requiring excessive intrathoracic pressure (e.g. weight lifting), environmental irritants such as smoke and noxious fumes, and uncoordinated or inadequate breath support for speech.

Since surgery cannot solve the underlying problem in misuse and abuse voice problems, patients with such functional voice disorders must be treated with behavioral therapy.

Speech disorders

Speech impediments as they are also called, are a type of communication disorders where 'normal' speech is disrupted. This can mean stuttering, lisps, or hoarseness. They can be caused by hearing loss, neurological disorders, brain injury, mental retardation, drug abuse, physical impairments such as cleft lip or palate, and vocal abuse or misuse.  Many famous people have speech disorders: they include Stephen Hawking, Thomas Jefferson, Winston Churchill, James Earl Jones, Bruce Willis. They are usually treated by speech therapy or psychotherapy.

Website was designed by Payton Lin, 6/12/2006, for the course BME 240

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           Stephen Hawking using TTS

how voice works
       Air passage for sound production

vocal cord
                       The vocal cord

           Anatomy of the Larynx

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