Why do voices sound high pitched when sped up?

Have you ever fast-forwarded a video or an audio playback? You might have probably notice that the audio sounds high-pitched when sped up. It sounds squeaky and often hilarious, seemingly like the pitched got elevated. But why does it do so?

Well, it concerns sound waves. However, let’s try to understand what waves are first.

Waves are a disturbance that moves energy from a specific point to another without needing any mass net flow. They can be classified into two types: pulses or periodic waves. Keep in mind that, depending on the displacement of the medium, they can also be divided into transverse or longitudinal waves.

Pulse waves often pertain to some type of a one-time disturbance that moves through a medium. On the other hand, periodic waves come in a repetitive, continuous motion. Irregular repeating sound waves produce noise, while constant recurrent waves create the musical notes.

With that, we can infer that sound waves are periodic waves. Moreover, they can be referred to as longitudinal waves as the medium’s particles swing back and forth towards the direction of the sound waves’ motion.

Therefore, a sound is a wave caused by vibrations in the air. When any object ‘sounds,’ what it does is vibrate and create movement in the air particles. These vibrating particles then collide with the particles surrounding them, causing them to vibrate. The process repeats until they run out of energy. If the vibrations reach your ear, you can hear the sound produced by the source of the sound.

Now, since sound waves are considered as a periodic wave, it possesses specific properties or characteristics of such type of wave. Two of the main properties of sound waves are frequency and amplitude. Frequency refers to the speed of the vibration and determines the pitch of the sound. On the other hand, amplitude refers to the loudness of the sound. The stronger the waves, the louder it is perceived by the ear.

But, let’s focus on frequency. Frequency determines the number of wave cycles produced in a single second. It is measured in Hertz in the SI unit. So, if a given sound wave registered a frequency of 5 Hz, it implies that there are five wave cycles each second. Likewise, if a sound wave has a frequency of 20, there are 20 wave cycles produced. It is a significant property of sound, especially in terms of musical melodies. It determines the ‘pitch’ and is strictly observed or regulated to create incredible tones or sounds.

Meanwhile, the pitch is a perceptual attribute of sound that allows us to judge whether they are low or high, or how deep or shrill a sound is. For example, we commonly refer to the girl’s voices as higher in pitch or sharper. On the other hand, the boy’s voices tend to be more low-pitched or deeper. Pitch correlates with frequency. When the vibrations in the air move fast, it produces a high-pitched sound. If the vibrations are slow, you hear a low note.

When speeding up an audio or video playback, you also force the vibrations to move faster in the air. The ear now interprets that as an increase in the frequency of soundwaves pattern, which in result, escalates the pitch, making the sound shrill or squeaky.

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