How come the pitch decreases when as you pour more water into the glass?

Question

When i pour water into the glass and tap it repeatedly as the water fills it, it decreases the pitch. Why is this and how come this contradicts the idea that if you shorten the length of the vibrating column the pitch/frequency goes up?

Answer 1

The velocity of sound in air or the frequency of a tuning fork is determined using the principle of resonance of air column.

In that experiment the length of air column is adjusted and the tuning fork is used to vibrate the air column; the water or glass is not vibrating so much and can be considered as having no vibration at all.

When the frequency of tuning fork is high the resonating length of air column is less.

Here in your case, the water and hence the glass are vibrating, and sound is passed through the air. No stationary sound wave is formed above the water level.

This is a case of forced vibration and the vibration frequency depends only upon the vibration of water particles, the glass vibrates with the frequency of the water particles; the glass is not left free to vibrate with its natural frequency.

If you have water to some level in the glass and strike the glass with some hammer and leave it to vibrate then only the glass will vibrate with its natural frequency.

Answer 2

because the sound waves are longer. Shortening the length is the same as having less water, not the other way around.

I guess your asking in a way how the notes came to be. The ancient greeks took a string, and called it A. Then they halved it, and called it B, and repeat the process when an octave was reached. Thost became the Ionian, Phyrgian, Dorian and all the other scales that start on each note.

In any event, there is an inverse relationship between wave length and pitch. As the the wave shortens, pitch is higher. If you put more water in a glass, you are increasing the wave length as there is less glass to resonate above the water.

Pitch and frequency are not the same thing. Your perception is wrong, what your saying is that if I have a full glass of water the pitch will be higher than having a small amount of water in it, and that is not correct. You may even be a bit tone deaf. In music, being able to write down the tones you hear usually takes at least two years practice.

Answer 3

The sound does not come from a standing wave in the air in the glass. It comes from a standing wave around the circumference of the glass. That is, it's the cylinder of glass that is resonating, not the column of air.

Water is higher density than air, so it interferes more with the resonance of the glass. The effect is as of increasing the damping constant on a damped harmonic oscillator. Increasing the damping lowers the frequency. It also means that the system loses energy more quickly, so the lower frequency sounds are probably also shorter in duration.

Answer 4

When the water level is low there is a lot of area for the glass to vibrate. As the water level rises the vibrations occur on less area and the tone gets slower & deeper.

Answer 5

That is strange. My experience is that the pitch increases. Make sure you use a more or less cylindrical glass without significant texturing and reasonably thin walls too, oh, and that you are tapping the part of the glass **above** the level of the water!

Answer 6

Less water in the glass = more resistance to resonation = lower frequency = lower pitch

Answer 7

BY FILLING THE GLASS YOU ARE SHORTENING THE LENGTH OF THE VIBRATING COLUMN!!

Answer 8

Because the sound is passing through the water, and the water has more density then sound.