Final answer:
Increasing the tension in the vocal folds leads to an increased vibration frequency, and consequently, a higher pitch of the voice due to the quicker vibrations of the tighter vocal folds.
Step-by-step explanation:
If you increase the tension in your vocal folds, the vibration frequency of the folds will increase. This is because similar to strings on a musical instrument, tighter vocal folds vibrate faster, producing a higher pitch. To change the pitch of our voice, we often unconsciously relax or tighten the muscles around the larynx which affects the tension of the vocal folds. This affects the pitch in a way analogous to how the tension in the strings of a guitar controls the pitch.
If we refer to wave mechanics, we can recall that when the period of a wave increases, meaning the time for one cycle to occur becomes longer, its frequency (how many cycles occur in a given period of time) decreases. Similarly, when a source of sound moves towards you, the pitch you hear increases if the speed of the source increases due to the Doppler Effect.