Original Q: If you pour liquid into a tall, narrow glass, you may hear sound with a steadily rising pitch. What is the source of the sound? And why does the pitch rise as the glass fills?
Additional question from me: What does it mean by the source of the sound? like the tapping on the glass?
The pouring of liquid into the glass can excite weak "organ pipe" vibrations of the air column above the liquid. As the column of air gets shorter, the frequency that you hear gets higher.
It has nothing to do with tapping the glass, but a similar thing can happen with the vibrations of the glass itself if you do; in that case the frequency gets lower as the glass is filled.
Plunger Jihads for the win!
The source of the sound in this scenario is the vibration of the glass itself. When liquid is poured into a tall, narrow glass, the pitch of the sound increases because the level of the liquid affects the resonant frequency at which the glass vibrates.
To understand why the pitch rises as the glass fills, we need to consider the concept of resonant frequency. Resonance occurs when an object is subjected to a vibrating force at its natural frequency. In the case of the glass, its natural frequency is determined by its shape, size, and material properties.
When the glass is empty or partially filled, the air inside the glass allows the vibrations to occur more freely, resulting in a lower resonant frequency and a lower pitch. As more liquid is poured into the glass, the air volume decreases, causing the resonant frequency to rise, resulting in a higher pitch.
The tapping on the glass can also produce sound, but it is a different source of sound compared to the rising pitch phenomenon. Tapping the glass causes it to vibrate momentarily, which generates sound waves. However, the rising pitch phenomenon is specifically referring to the sound produced as the liquid is poured into the glass, not the sound generated by tapping on it.
The source of the sound in this scenario is the resonance created by the vibrations of the glass as it is being filled with liquid. When you pour liquid into a tall, narrow glass, it creates a column of liquid that acts as a cylindrical tube.
As you continue pouring liquid into the glass, the column of liquid starts to rise and fills the glass. This causes the volume of air trapped in the glass to decrease.
Now, the pitch of a sound is determined by its frequency or the number of vibrations per second. When you tap on the glass, it creates sound waves inside the glass that bounce back and forth between the surface of the liquid and the bottom of the glass.
As the column of liquid rises and the volume of air decreases, the distance between the surface of the liquid and the bottom of the glass becomes shorter. This leads to the sound waves bouncing back and forth at a faster rate, resulting in a higher frequency or pitch. Therefore, the pitch of the sound rises as the glass fills with liquid.
So, the tapping on the glass is the initial source of the sound, and the changing volume of air inside the glass as it fills causes the pitch to rise.