Speech and the Vocal Tract

As described in the earlier lesson, speech sound is created by airflow through the vocal tract. In pulmonic sounds, which are the sounds we will consider here, the lungs push air up into the trachea, through the larynx, and outward through the vocal tract.

So how are different sounds made? In part 1, we discussed that one way to make different sounds is to vary the state of the glottis, making either a voiced or voiceless sound.

Another way is to vary the shape of the vocal tract. Imagine the vocal tract as a tube, through which air passes. If this tube is simply open, the airflow creates a sound. But if you alter the shape of that tube, the airflow moves differently, making a different sound.

Here's an experiment that some of you may have tried. Take an empty bottle and blow air across the top of the bottle. If you can get the airflow just right, you should be able to produce a low sound. Now fill the bottle halfway with water. Blow across the bottle opening again. This time the sound is higher. If you put some more water in the bottle, the sound will get even higher.

What's happening? For a more detailed discussion, you can view the lesson Acoustic Phonetics. However, for now, just understand that if the bottle (vocal tract) is not as filled with water (larger), the sound will be a deep, low sound. If the bottle (vocal tract) is filled with water (smaller), the sound will be a higher sound.

When we make speech sounds, one thing that is happening is that we are varying the shape of the vocal tract, making the sound different. For example, say the sound [t]. To make this sound, you are raising the tip of your tongue behind your teeth and then lowering your tongue. When you do this, the air builds up behind the closure made by your tongue and teeth and is then released. When the air is released by the tongue, the air travels outward through a small area, just from the teeth to outside the mouth.

Now say the sound [k]. To make this sound, you are bringing your tongue up to the velum, closing off the airflow, and then lowering your tongue to release the air. This time, when the air is released, it travels through a larger area before leaving the mouth. This space is from the velum to the lips. Thus, the sound made by the airflow is different from that made by [p].

The following diagrams illustrate the amount of space in the vocal tract available for [t] and [k]:

As the diagrams show, there is more space in the vocal tract for the release of air in the production of [k] than for [t]. Therefore, two distinct sounds are produced.

The point at which the vocal tract is altered is known as the place of articulation. In the next section, we will discuss the major places of articulation in classifying human speech sounds.

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© 2001 The Language Samples Project