# Why are higher frequencies amplified less by filling a bottle with warmer water instead of colder water?

Out of curiosity I was trying to figure out what makes the sound of hot water versus cold water filling a bottle different, but I'm unable to find any explanation as to why such differences occur.

The image above is an FFT of a bottle being filled with water at 20°C, and the one below is an FFT of the same bottle being filled with water at 70°C. These are FFTs taken at the halfway point of filling the bottle (synced with a video) but throughout the filling of the bottle, it seems that for the bottle being filled with hot water, higher frequencies are not amplified at all?

I'm aware that the bottle can act as a resonant cavity that amplifies certain frequencies as the bottle is filling up, and those frequencies come from the sound generated by water hitting the bottle/water surface within the bottle. Many people discussing this in forums and articles talk about the lower viscosity of hot water, but I don't understand how this would contribute towards the effect I've observed, and there are even some articles contradicting what I've observed, highlighting that hot water would actually result in a higher pitched sound due to more splashing of water.

From what I've read it seems that in hot water, bubbles produced when hitting the surface of water are generally smaller than bubbles formed the same way in cold water, and intuitively that could imply that the overall amplitude is lower, but I'm not sure if this is useful towards this question.

When using a cup instead of a bottle, I noticed that there was the presence of some higher frequencies, but the lower frequencies when filling the cup with hot water were more prominent still, so I was wondering if the shape of the resonator could have anything to do with this disappearance of the higher frequencies.

Would there be any explanation that could address the attenuation of higher frequencies when using hot water? Thank you!

This is a picture of the bottle used. It's about 22cm in height, and I placed my microphone about 1cm away from the bottle opening when recording the sound. I did not really maintain a constant flow rate, but I did use a funnel fixed at 20cm above the bottle opening and poured water through the funnel in order to fill the bottle. In this case I'm not sure, but I don't think that the flow rate would play a major part in such attenuation of the sound.

• A photo showing the material and shape of the bottle, the location of the microphone, and anything else touching the bottle (like your hands) during the measurement could be helpful. Commented Apr 15, 2019 at 16:17
• There are so many variables involved here in addition to the issue of hot water versus cold water. The experiment would have to be carefully done to make sure that the effect is really due to hot versus cold water rather than some other factor. Were the water flow rates for the two measurements exactly the same? Were the faucet-to-bottle distances the same? Do the two graphs shown correspond to times when the amount of water in each bottle at that instant were the same? If you were writing this in a paper, you would have to show that you considered all these possible factors.
– user93237
Commented Apr 15, 2019 at 20:42
• @Samuel Weir I didn't manage to conduct a very exact experiment as this was out of curiosity and not for a paper. I fixed a funnel exactly at 20cm above the mouth of the bottle and poured the water through the funnel. If I have the time perhaps I'll try something to keep the flow rate more constant. The microphone was also fixed about 1cm from the mouth of the bottle. I used a glass bottle with a relatively narrower neck, like a beer bottle. I left the bottle resting on a tabletop when I recorded the audio, but that shouldn't have too much of an effect? Commented Apr 16, 2019 at 10:49