Fundamental Frequency

We were reading “17 Equations that Changed the World” late last night, the chapter about the wave equation. Simon got all excited about timbres (shapes of sound waves), that are essentially sine waves. He said he knew an alternative way to look for the fundamental frequency (the sin x wave):

“The smallest number that’s divisible by all the numbers in a sequence is the product of all those numbers divided by the greatest common divisor/factor of all those numbers. That’s the Chinese remainder theorem (or rather, a generalisation of it). 

If you took a rational frequency and an irrational one and made them into waves, the waves would never ever ever meet, except for one point. So sometimes there’s no fundamental frequency. Because we need at least two points where the waves meet up to define a fundamental frequency. 

Sadly, this happens most of the time. In fact, not even most. 100 percent of the time there’s no fundamental frequency. Technically, it’s an infinitely small chance that any number you come up with at random is rational! But fortunately for us, we can approximate the fundamental frequency here: use the two points that are closest to the waves meeting to get an approximate fundamental frequency. And it always works!

This is incredible! We’ve found a connection between a discrete problem, of what’s the smallest number that divides all of the numbers in a given sequence, to a continuous problem, of what is the fundamental frequency of a combination of sine waves. In other words, we found a discrete solution to a continuous problem!”

Simon, what does discrete mean?

“I’ll give you an example. The natural numbers, even though they are infinite, they are still discrete, because there are gaps between them. And a number in between those gaps is not a natural number anymore. A continuous thing however is for example like the real numbers. There’re no gaps. Because if there were gaps, any number in between those gaps was another real number”. 

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