Why does sound change continuously when a vessel is filled up with tap water?

Have you noticed how the sound of water filling into a vessel keeps changing as the level rises? To begin with it is bass and gradually becomes shrill as the vessel gets filled up. Do you know why this happens? We know that the sound is a physiological sensation experienced when some vibrations are received by the ear. It is so produced by vibrating objects such as guitar strings, the column of air in an organ pipe or the membrane of a drum. Similarly, when in a metallic vessel the water falls from a running tap, the metal starts vibrating and produces sound. Apart from this metallic sound, there is another one that occurs due to the vibrations of the air column above the water surface. This air column is formed between the water surface and the mouth of the vessel. As already explained, the sound coming from the vessel is bass in the beginning and becomes shrill as the vessel gets filled up. This means, that in the beginning sound has low frequency and later on it gets higher. The frequency of the sound produced depends upon the length of the air column. A longer air column produces sound of low frequency. As the vessel gets filled up with water, the length of air column becomes shorter and produces sharp sounds of high frequency. In addition, the metal of the vessel also produces sound. These vibrations get suppressed as water fills in due to the decreasing air column. Materials with good sound absorbing properties are often fitted on walls and ceilings in buildings such as conference rooms, cinema or concert halls and public auditoriums, in order to reduce or control sound levels. A musical instrument named ‘jaltarang’ is based on the same principle and can produce a variety of musical notes.

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