why does pitch go up when there is more water in a bottle?

Answer 1

The size of the air chamber determines the pitch. As you add water you are reducing the size of the air chamber raising the pitch

Answer 2

Are you asking about whistling across the top of a bottle?

The sound pitch is determined by the air space in the bottle. The predominate tone is a function of the distance from the top of the bottle to the surface of the water. As you blow across the top of the bottle, the air disturbance creates sound waves. The disturbance is creating many waves, but the ones you hear are the waves with just the right length that bounce off the surface of the water. The shorter the sound wave, the higher the pitch. So as you put more water in the bottle, the distance is smaller and you hear a different sound wave - a shorter wave - a higher pitch.

Answer 3

Actually, the pitch is related more to the VOLUME of the container than the LENGTH. The size and shape of the opening also matter.

Basically, the air in the container acts like a "spring" and the air in the neck acts as a "mass" resting on the "spring". The smaller the container, the stiffer this "spring" becomes and the higher the pitch. Similarly, the wider or longer the neck, the heavier the "mass" and the lower the pitch.

This is known as a "Helmholtz Resonator", and much info is available in the internet on the subject, such as the one listed below.

Answer 4

The more full the bottle, the smaller the room for air, which is the medium in which the sound will be generated.

The pitch of the sound is determined by the resonance condition. In this case, we expect that the largest sound wave that fits into the bottle must satisfy:
wavelength/4 = airspace

So in general,
wavelength < airspace * 4

Since the frequency is
frequency = speed_of_sound/wavelength
> (s_o_s)/(4*airspace)

you see that the smaller the airspace gets, the higher the resonant frequencies have to be.

Answer 5

The water bottle will produce a standing wave which is the sound you will hear. When the size of the air space is reduced, the wavelength decreases and so the frequency of the sound wave increases. The pitch is directly linked to the frequency, so as the frequency increases, the pitch goes up.

Answer 6

The reason that the frequency changes relative to a medium is due to a phenomena called wave dispersion. The molecular disturbanceof air travels in a shorter time as the bottle fills up ;hence ,a higher frequency.