The sea shell
If you put a large sea shell to your ear do you really hear the ocean? In reality, what you hear is the ambient sound around you being altered by the resonances of the air in the sea shell. This listening device is less poetic, but does exactly the same thing. In a busy science fair or museum, you can alter the hubbub by wearing these drilled ear defenders to create a strange sound.
How to make it
- 1x cheap pair of hearing defenders (e.g. only £6)
- 1x large drill bit (e.g. 20mm)
Pull the foam insert out of the ear defender. Carefully drill a hole in each of the ear defender cups. That is all! Try experimenting with different sized holes.
The drilled ear defender acts like a resonator.
Blow across a beer bottle and you hear a tone, which is the resonant frequency of the air inside the bottle. The air in the neck of the bottle vibrates up and down against the spring created by the air in the main body of the bottle. Look at the drilled ear defender and you have something very similar. There is the drilled hole that is an opening like the neck of the bottle, and behind it the ‘cup’ creates a cavity of air to act like a spring.
The ambient sound has to pass through this resonator to reach your ear. Parts of the sound that are close to the resonant frequency are amplified. The reason it sounds odd is because it’s unusual to hear just some frequencies from the hubbub of a crowd.
The picture below shows what we measured on the microphones in the ears of the manikin (see video above). The blue plot is without the ear defenders, the red plot with the drilled ear defenders. The is a strong amplification of sound around the resonance at 700 Hz, a frequency right in the middle of the bandwidth important for speech.
Spectrum for sound of a crowd with (red) and without (blue) drilled ear defenders