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.