Sine waves and ultrasound

Sine Wave

I remember one thing that always baffled me when I was trying to get my head around ultrasound physics was the concept of transverse and longitudinal waves. As simple as it may be to you – and I don’t know how many people suffer the same confusion that I did – for me, it was one of those things that as soon as I had it clear in my mind, something else would come along to disrupt it all and I’d be confused again. It reminded me of repeatedly failing to be able to spell “because” on successions of spelling tests at the age of around 7, or having to keep looking up “effect vs affect” at the age of 14. Ok, I admit it, 24. Or possibly into my 30s.

I think what made it so confusing was that every ultrasound book I’ve ever seen is structured like this:

  • They’ll begin with “ultrasound is a longitudinal wave.” They explain that it travels in the direction of propagation, in a series of compressions and rarefactions. That’s what it is; a compression wave.
  • What all these books then immediately go on to do, however, is graph the frequency or the pressure oscillations of ultrasound waves, and in doing so, they draw a sine wave.

So ultrasound is not a sine wave, but it can be represented as one. Am I the only person on the planet that finds that confusing?

I don’t think I am, because a browse through diagrams on Google Images shows many pictures of an ultrasound transducer with wiggly lines coming out of them.

In case you, too, found this confusing, I’ve made a short video to demonstrate where this sine wave diagram of ultrasound originates from. I’m also making sure that, in my new book on canine pregnancy scanning (due out in a couple of months – I’ll post a signup link here and on social media soon, so that you will receive an alert when it’s available), there is no mention of sine waves until several chapters in, giving readers the chance to cement the idea of ultrasound as a transverse wave in their heads well before this new concept is introduced.



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