How is speech different from voice?

Mr. Heimler, at 70 years old, seems older and less intelligent to his family since he was hospitalized for a stroke about two months ago. They come to the appointment with him and help him into the exam chair. When I ask Mr. Heimler what is going on with him, the long pause before his strained answer begins makes his family uncomfortable. His daughter interjects, “We are having a very difficult time understanding him.” I learn that the right side of his body is still not completely under his control. He is on blood thinners to prevent another blood clot from floating though his system, plugging up another part of his brain. In rehabilitation, physical therapists teach him to walk and speech therapists coach him to improve his swallowing and his speaking.

When I address him again, and he responds, his voice is loud enough, but his words are slurred and unclear. I notice that his lips at the side of his mouth droop. His smile is out of kilter. I look further. One side of his tongue has little writhing movements (fasciculations) and when he sticks his tongue out, it always moves off toward one side.

Some people ask, aren’t voice and speech the same thing? While they are related, they are not the same. However, in practice, speech problems and voice problems can be confused.

Let’s distinguish them with a mental image where we separate speech from voice with a line, which we will call the “Speech Line,” across the neck above the Adam’s apple. This Speech Line roughly separates speech production above from voice production below. The two systems interact and to some degree overlap. Yet in general, below the Speech Line, sound is created, above the Speech Line, sound is modified into language. 

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The Speech line is an imaginary line just above the thyroid cartilage prominence. Sound is created below this line. Sound is modified into speech above this line.

Below the Speech Line, the larynx produces audible vibrations – sound – in the normal human system. Above the Speech Line sound is modified. More specifically, vowels are the modification of the resonance cavities (mostly the pharynx and tongue modify the shape of the resonating cavities) and consonants are the interruptions or restrictions of the airflow. The interrupters include the palate, the tongue and the lips. These alterations in the airflow, when combined, coalesce to form words, then phrases, then sentences and we begin to communicate. We achieve speech.

Since Mr. Heimler’s problem is with difficulty producing words clearly, the issue is most likely in the upper half of this system. He has dysarthria. His problem is not with sound production because when we check, his volume is good, and any single sound is quite clear on its own. When I look at his vocal cords, they are indeed vibrating well. 

However, when I focus my examination above the Speech Line; on his face he has difficulty moving his lips; in his mouth he has difficulty controlling the movement of his tongue. These motion impairments garble his speech. In adults, most new problems with speech are neurologic problems. His stroke has impaired the neural input to his tongue and his lips so his speech lacks clarity. It is even possible that his intelligence is entirely unaffected by his stroke. The slurred words merely give his family the perception of diminished intelligence.

For the most part, I am not going to cover speech disorders in this book with the exception that a few impairments to speech are caused by an interruption of airflow right at the level of the vocal cords. Completely interrupting the production of sound ends up impairing speech since without sound, movements of the tongue and lips are not heard. Speech is completely dependent on having a voice so with no voice, there is no speech.

On the other hand you can use a voice without speaking. For example, you can sing without word production. A sound can be generated which is entirely musical in quality and carries no significant cognitive information even though it may carry a great deal of emotional information. A baby’s cry is mostly pure sound, carrying some very basic information – and most of that information is emotional and the meaning must be inferred. 

We can imagine speech (at an unvarying pitch and volume) as a package of cognitive information. Voice is the wire used for the transmission of speech. Voice then typically carries information from one location to another from one location to another. With voice, we alter volume or change pitch to carry our words some distance to penetrate through background noise. And to some extent we also alter our volume and pitch to directly transmit emotion.

Mr. Heimler makes sound, and makes it loudly enough. He just cannot deliver clear content because of his tongue and lip weakness. He does not vary his pitch or volume very much so he comes across as having not much emotion. He can be heard, but not understood. He has a problem that the physician will find above the Speech Line. Problems above the Speech Line are not voice problems.

Speech problems can still be assessed, but the focus of the assessment will likely be in the mouth and nasopharynx. For example, a person with a cleft palate cannot close the palate completely against the back of the throat. Closing the palate completely against the back is required to make certain consonant sounds, otherwise air leaks out the nose and those sounds cannot be made. /p/, /t/, /k/ as well as /s/ and /sh/ are typical sounds requiring complete closure of the palate, channeling all of your air out the mouth. If you try to say “pa, pa, pa” with the palate relaxed, air leaks out the nose and you sound to others like you have a cleft palate. From a physician standpoint, when I hear this nasal air leakage, I know to look at the palate. I may not even need to look at the vocal cords. So discerning whethet the problem is with speech of voice directs the subsequent exam.


What is speech?

  • Speech is the modification of sound (voice), typically into vowels and consonants that are ultimately understood as words.
  • Speech occurs largely above the larynx or after the vocal cords create sound.