Oral and nasal airflow

Air can exit the vocal tract through the nose or the mouth. This is controlled by the position of the velum. The velum is a sort of valve that controls airflow through the nose. If the velum is raised, then the nasal cavities are blocked off. Consequently, air cannot pass through them, and it must exit the vocal tract through the mouth. Sounds with airflow exiting through the mouth only are said to have oral airflow. If the velum is lowered, air flows through the nasal cavities, and out through the nostrils. If the air flows through the nose, the airflow is nasal.

If you say a [s] sound and pinch your nose, you will notice that you can easily continue the [s] sound. This is because [s] is oral: the velum is raised and makes a tight seal, preventing escape of air through the nose. On the other hand, if you say a [m] sound and pinch your nose, you will notice that you can only continue the [m] sound for a very short time. This is because the lips are closed, making oral escape impossible, but the velum is lowered, so that the airflow is nasal. By pinching your nose, you effectively seal off the only remaining means of escape for the air.

A third possibility exists, where air escapes through the nose and the mouth. For these sounds, the velum is lowered, but there is no complete closure in the oral tract, as we had for [m] (where the complete closure is at the lips). A good example would be a nasalized vowel, as in the French word ‘pain’, [pã], ‘bread’. You might try making a nasalized [s] sound, [s˜], but you will notice that it is much quieter and less hissy than it should be, with as much noise caused by air coming through the nostrils as through the mouth.