Heavy Water: D2O is H2O with every H (regular hydrogen, main isotope nucleus=proton) with a D (rare isotope hydrogen nucleus=proton+neutron). So D, deuterium, has close to twice the mass of hydrogen. Each vial has exactly 10 grams of each liquid- note there is less D2O because it has more neutrons which makes it 11% more dense than regular water.
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One of the physical phenomena of acoustic waves is intereference, caused when sound waves interact with each other.
They can cause resonance and feedback when the waves' peaks and troughs are in phase. But when two notes/frequencies are close in pitch, but not identical, it can cause interference that is known as 'beating'. You can hear it in this video beneath the notes I'm actually playing; sometimes as ascending/descending bass 'combination tone', or where the interference causes the wobbling or 'beating' sound similar to tremolo effect.
The further the two pitches/frequencies are from each other, the quicker the period of the beats. As the frequencies get closer this beating slows to the point of being imperceptable to human ears. .