As mentioned in this lesson, sound is generated by making a speaker diaphram pulse inward and outward. For this reason, the mathematical function $sin(x)$ is often use to generate signals for a speaker, since the $\sin$ function goes up and down, which can be thought of as an "in and out" for a speaker.

Here we introduce a function called play_sines() that will play the results of one or more sine-waves added together, out to your speaker. What is important for sound is the amplitude of the sine-wave, (or how loud it'll be), and it's frequency (or how quickly the speaker diaphram is pushed in and out). Amplitudes are between 0 and 1 and frequencies are generally between 100 and 25,000.

If you want to play one sine-wave, it'll resemble $A\sin(2\pi f t)$, where $A$ is the amplitude and $f$ is the frequency. So if you call play_sines() as play_sines({1,500},3), a 500 Hz frequency will be played at full amplitude for 3 seconds. Playing the two sines at full and half-amplitude of 500 Hz and 1000 Hz, for 3 seconds, would be play_sines({1,500,0.5,1000},3).

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Mathematically, what gets played is $\Sigma A_n\sin(2\pi f_n t)$, where $A_n$ is the amplitude of the nth sin-wave and $f_n$ is its frequency.