Question

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?

Short answer

The air column between water and top of the glass is source of the sound. Pitch (frequency) is inversely proportional to length of the air column so as water fills, frequency of sound increases.

Step-by-step solution

Write the given information

Glass is being filled with water,

Frequency of sound increases with water rising.

Source of Sound.

Water falling and splashing contains a number of non coherent frequencies which mostly gets damped. That means the sound that we hear is due to the resonating frequencies from the air column between water and top of the glass.

Why does pitch (frequency) increases.

Now a narrow glass acts as an open-closed pipe. And we know, frequency of an open closed pipe can be expressed as

f = mv/4L

where, L = length of the air column.

Thus frequency is inversely proportional to the length of the air column. Now as water fills up this length of air column decreases and therefore frequency of the sound increases.