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Sounds from an oven shelf

Introduction
Get very strange sounds from an oven shelf.
Useful information
Kit List: 

Oven Shelf
String
Something to hit the shelf with (your finger nails will do!)

You could also use a slinky instead of an oven shelf
In this case a plastic cup works as an amplifier

Packing Away: 

Lives in a flat light blue box of floating experiments.

Frequency of use: 
4
Explanation
Explanation: 

Get very strange sounds from an oven shelf. Hang it from two pieces of string, wrap string around your fingers, put your fingers in your ears, hit the oven shelf. It will sound like big ben!

Why does it happen?

First of all we have to understand that sound is all about vibrations. When somebody speaks, their voice box vibrates, and this makes the air around it vibrate. These vibrations carry information about what someone has just said. When these vibrations reach your ear, they make your eardrum vibrate and this is processed by your brain as sound. The amount of energy (or the volume of the sound) that manages to make the journey from voice box to ear depends on what the sound is travelling through and what kind of sound it is.

In the case of speaking to a friend or when you listen to the oven shelf, the vibrations must travel through air. Air is really sloppy, fluid and not very stiff. Water is quite similar - if you put your hand in water and slowly move it around, the water feels very soft and fluidic. However, if you slap the water then it suddenly feels very hard and stiff. This is because the water doesn't have time to get out of the way so it has to form waves. Although it is not quite so obvious, this is the same for air. If you move something through it very quickly, the air feels stiffer and it's much harder to move through it, so high frequency vibrations will transfer more energy into the air.

The ability of sound to reach someone's ear also depends on the ability of the air next to the ear drum to vibrate. In the same way as a high frequency (pitch) vibration can transfer more energy to the air from the oven shelf because it has less time to get out of the way, a high frequency sound will transfer more energy from the air to your eardrum so the sound is loud. Low frequency sounds vibrate the air much more slowly, and so the air seems relatively more sloppy and doesn't transfer energy so well so they sound much quieter. So both low and high frequency sounds are produced by the oven shelf but it's only the high frequencies that vibrate the air by your ear drum much so the shelf sounds tinny and high pitched.

In order to hear the low frequencies, you need to create a stiff connection between the oven shelf and your ears. The string wrapped around your fingers provides this connection. The string is taut and stiff and can transmit both high and low frequencies. When you add the high and low frequencies together, the oven shelf suddenly sounds like a gong.
What about in the real world?

This is why your voice sounds different to everyone else and when you hear it recorded. Everyone else just hears you though the air, but you hear yourself through the bones in your skull as well, so different pitches will reach your ears than other people's.

For more see:
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/the-se...

Extra information
In case someone of a more mathematical background is asking, it's the impedance mismatch between the shelf and the air that filters out the low sounds (acting as a high pass filter). We could get those sounds to the air better if we passed them through a series of media with different impedances instead of a big jump. The string has an impedance much closer to that of the shelf (being a solid and all) so there is less of a mismatch and better transmission.

One of the most familiar forms of impedance matching from school is the use of special gels with ultrasound scanning to ensure that more of the sound passes into the body rather than being reflected from the skin.
In fact, there are even more everyday occurrences. Some instruments such as acoustic guitars have a "soundboard" - essentially the front face of the instrument - and air cavity, connected to the strings via a bridge. The structure of this system amplifies the vibrations, particularly low frequencies, not by adding any energy (as in an electric guitar, which use electromagnets to sense the vibrations), but by a kind of impedance matching. If I understand correctly, the larger area of the soundboard, as well as the ability of the air in the cavity to resonate, are key to this.

Why labour this?
Well, you can get the same oven-shelf effect from a slinky, and the listener can excite the vibrations by nodding their head.
The extra cool part is that rather than sounding like a boring old gong or clock tower, the complex pulses reverberating up and down sound like a Star Wars laser battle.
If you listen very carefully without the strings you can hear the low laser battle sounds.
But, if you put a plastic cup in the top of the slinky, it couples to its vibrations and the cup is able to act like a soundboard of a guitar and everyone can hear the effect without the need for string!

Risk Assessment
Date risk assesment last checked: 
Sat, 04/01/2020
Risk assesment checked by: 
Andrew Sellek
Date risk assesment double checked: 
Fri, 10/01/2020
Risk assesment double-checked by: 
Esmae
Risk Assessment: 

Hanging a vibrating oven shelf from two pieces of string wrapped around your knuckles, which are put in your ears:

Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Fingers If fingers are pushed into ears with long nails/too far it could cause some minor damage. Public 3 3 9 Encourage children to put their knuckles, not their fingers, in their ears.
Call a first aider in the case of an injury.
1 3 3
Fingers Head banging - with the slinky version, over-vigorous nodding not in a clear space may cause someone to bang their head. Public 3 3 9 Make sure the participants have sufficient space to take part.
Call a first aider in the case of an injury.
2 3 6
0
0
This experiment is sometimes run outside during CBS!, see separate risk assessment.
Images
Experiment photos: