I took my inspiration from this film. I realised that I could do the experiment here by prising the front off my stereo speaker and playing sine waves tones generated in the Audacity app. I wasn't expecting anything given the rudimentary nature of the equipment but I was amazed to get patterns that look like those in Nigel John Stanford's film.
Above is 25 Hertz. Below is 50 Hertz. I'm not sure the photographs look much different but it seemed different looking at it.
The circular ripples in the film turn out to be a transient feature that appears just at the beginning. Thank you to Kay for telling me about
cymatics. Although I've done experiments covered by the term, I'd never heard that word. It is about finding ways to make sound waves visible. I'm keen to try the corn starch and water mix next!
