Kina as a Helmholtz resonator

Dr Craig Radford from the Auckland University Leigh Marine Laboratory explains his research into how kina make sound and the effects of the kina shell size on the resonant frequency

Point of interest: Like many scientists, Craig’s research came out of the work of prior scientists – in this case, Craig was looking to quantify a model proposed by scientist Malcolm Castle.

Transcript

DR CRAIG RADFORD
Kina make noise through their feeding mechanism. They have what is called an ‘Aristotle’s lantern’, which is 5 calcareous teeth. The feeding structure sits underneath the animal in like that, and then when they eat, this pops out and they scrape it on the rocks, and they make noise by the shell acting as a Helmholtz resonator

The Helmholtz resonator is basically an enclosed volume of water or air, and when that water oscillates inside that chamber, it reaches a certain frequency called the resonant frequency, and that’s what you hear. The easiest example of that is a bottle. You fill a bottle up with water, and you blow across the top and it hums.

In 1970, a guy called Malcolm Castle started recording reef noise, and they modelled the urchin shell on a Helmholtz resonator, and via the model, they figured that a large urchin would produce a low-frequency sound.

I had a size range of urchins – the smallest one was 30 millimetres and the largest one was about 92 millimetres. I divided that range into 10 millimetre bandwidths and then recorded a series of urchin sizes within those bandwidths. So theory would say that the larger one would have a lower resonant frequency and the smaller one a higher resonant frequency. So the larger ones did have a lower frequency, once we analysed the data, and the smaller ones had a higher frequency, so we could plot this on a graph and basically draw a straight line through it.

Acknowledgement:
123RF
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