Compare voiced/unvoiced th to t/d
unvoiced/voiced th and
t/d sound illustration
The unvoiced and voiced th and the t sound and d sound sounds deserve special attention by nearly all non-native speakers. The relative closeness in vocal tract position between all four of these sounds coupled with the fact that few languages include a sound similar to the English unvoiced and voiced th make the t sound and d sound a common substitution used by non-native English speakers.
Fricatives versus stops
The unvoiced and voiced th sounds are fricatives and the t sound and d sound are stops. Fricatives are created when the vocal tract is restricted and air is forced through a small opening. This important aspect means that the air is not completely restricted and that it is the turbulence of the air that causes the sound. This is a sharp contrast to stop sounds, which require the vocal tract to be closed, briefly yet entirely prohibiting air from leaving the mouth. With a stop sound, the majority of the sound is created when the air is released.
Sound voicing and articulation
The unvoiced th and t sound are alike in that both sounds are unvoiced. This means that the vocal cords do not vibrate during the production of either sound. In contrast, the voiced th and d sound are voiced and do include vibration of the vocal cords during the sounds. Since non-native speakers often substitute one voiced sound for another voiced sound, or one unvoiced sound for another unvoiced sound, it is often helpful to practice sounds in those pairs.
The unvoiced and voiced th sounds are articulated when the tip of the tongue is placed close to the back of the top front teeth. The friction occurs between the top of the front of the tongue and the tooth ridge and between the tip of the tongue and the top front teeth. The t sound and d sound are articulated by stopping the air with the tip of the tongue close to the tooth ridge (directly behind the top front teeth), and then quickly releasing it.
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