Science Buddies: "Ask an Expert"

Hi there,
I’m planning on doing a science experiment on whether different liquids in wine glasses affect the frequency created. The wine glasses and the amount of liquid will all be the same throughout all my tests. The liquids will be milk, water and vegetable oil. I’ve done some background research into how the glasses make their “sing” and whether the density of the liquids affect the frequency (I understand that the milk is denser than the water and oil and that the water is denser than the vegetable oil), but I’m struggling to find a straightforward explanation (or whether the densities really do effect the frequency).
So is there a way to explain to every one of all ages the relation between the densities of the liquids if there even is one?
Also, thanks Science Buddies with all the awesome science experiments through the years. I LOVE the website. Keep up the good work!
Thanks!

Hi sciencebudbud,

The general concept you are dealing with here is resonant frequency. In layman's terms: bigger, heavier things generally have a lower resonant frequency, and smaller, lighter things generally have a higher resonant frequency. Roughly speaking, to humans this generally means bigger/heavier things sound lower pitched, and smaller/lighter things sound higher pitched. For example, with musical instruments (a tuba sounds deeper than a flute, thicker guitar strings make a lower sound than smaller ones), with animals (an elephant makes deeper noises than a songbird), or people (adult's voice vs a baby), etc. Hopefully those are some common examples that people of all ages can understand, and you can probably think of more.

For the wine glasses, this video has a good explanation of the physics and how the depth of the liquid in the glass affects the frequency: https://youtu.be/VN3K9WhzOSs. When you add more liquid to the glass, you don't change its size or shape, but you increase its mass, resulting in a deeper sound (lower frequency). So at least in theory, if you keep the liquid level constant and use a denser liquid, that is also making the glass heavier, and should also result in a deeper sound. I have not tried this myself and am not sure if the difference will be detectable to the human ear with liquids that are fairly close in density, but it's certainly a good experiment! There are smartphone apps you can use to measure pitch that may give you a more accurate reading.

Hope this helps,

Ben

Hi Ben,
Thanks for the explanation! I (and my family) have definitely learnt something new. This experiment is an awesome boredom buster, especially with everyone being home more often. The video was extremely useful too.

Cheers,
Sciencebudbud

Happy to help and glad to hear that it worked out!