To produce voiceless sounds, the glottis should be more open and tense, while for voiced sounds, it should be narrower and less tense. The vocal cords play a crucial role in sound production, as they vibrate for voiced sounds and remain still for voiceless sounds. This can be demonstrated by placing a hand on the larynx while pronouncing vowels; vibration indicates voiced sounds, while whispering shows a lack of vibration, indicating voicelessness. The pairs of sounds discussed—'b' vs. 'p', 'd' vs. 't', 'g' vs. 'k', and 'z' vs. 's'—highlight the difference between voiced and voiceless consonants, and this distinction is present in many languages, including Vietnamese. Understanding and controlling vocal cord vibration is essential for mastering these sound contrasts.