A sonic mystery... Most mornings at home, I make a cup of tea for myself and a cup of coffee for Debbie. We both use the same kind of mug: a 16 ounce double-walled stainless steel mug. A long time ago I noticed something odd: tapping a spoon on the rim of the mug makes a distinctly different sound on the tea mug than it does on the coffee mug. The tea mug sounds higher pitched and crisper; the coffee mug lower pitched and sort of a thud.
Why would this be so?
I've been trying various experiments over the past couple of weeks to try and narrow down the cause. First I kept track of the mugs we used, and switched which one was for tea. That made no discernible difference. Then I compared the tea and coffee with no additives (cream, sugar, etc.). This time the two mugs sounded very similar, though the coffee mug was still slightly lower pitched and not quite as crisp. Then I added sugar to the tea – and the pitch went up (not what I expected at all!). I added Equal to Debbie's coffee and heard no difference. I added milk to my tea and the sound pitch went down slightly and got slightly less crisp. Then I added collagen powder to Debbie's coffee, and the sound changed quite dramatically: lower pitch, much thuddier. Ah ha! Then when I added milk to her coffee and the pitch went down a bit more.
So ... collagen powder made the biggest difference. Why? What does it do to the liquid that affects the sound of tapping the mug's rim?
I have two theories. First theory is that the collagen powder increases the viscosity of the liquid, and thereby increases the damping effect it has on the “ringing” caused by the tapping. Second theory is that the collagen powder, when dissolved, changes the speed of sound in the liquid, which changes the resonating characteristics.
Do any of my readers have any knowledge of this phenomenon?