Why does the pitch of water going into a cup rise as the cup gets fuller?
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The sound has less area to reverberate in, resulting in shorter (higher pitched) sound waves bouncing out.
What you’re mainly hearing the the sound/vibrations going up the cup, and as the space left gets smaller, less of the cup is vibrating making a higher pitched sound.
Kind of like a guitar string, when the whole string is vibrating it is a deep noise, but if you put your finger lower on it it is higher pitched.
Most real things, including the splashing of water, producs a range of sounds called broadband noise, which is a random mix of a lot of different frequencies (pitches).
As you may know, sound is waves in the air. If you think back to your childhood, maybe you remember making waves in the bath, and you’ll know that if you make a new wave at the exact right time it’ll combine with the reflected previous wave and get much larger, allowing you to eventually create a huge wave from just a bit of sloshing.
This phenomenon is called resonance, and it the frequency at which you have to make waves to get them to resonate is called resonant frequency, and it depends on the size of space where the waves are moving, the smaller the space, the higher the resonant frequency.
Resonane also happens to sound waves, and when you pour water in a glass, the sound waves bounce back and forth across the glass and the water. And the soundwaves that are close to the resonant frequency of the empty air in the glass get amplified by resonance, and so you hear frequencies around the resonant one much louder and they drown out the rest of the random noise. And as you fill the glass, the empty portion gets smaller and smaller, and as such the resonant frequency gets higher and higher, so higher and higher frequencies are amplified, creating the rise in pitch you hear.
Turn a big bass drum so it’s facing downwards and hit the bottom. Then do the same with a smaller drum. The smaller drum will sound higher. The water going into the glass and hitting other water is like a drum that gets smaller as it fills up, making the noise higher.
Then you get into filling up the cup with water that gets hotter in time with the amount you fill so the pitch stays the same.
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