Think about the last time you heard a test of the Emergency Broadcast System. A couple of things to note: first, they say "For the next 60 seconds..." but they play the tone for a lot less than 60 seconds. And second, the tone that they play is very annoying. How do you make such an annoying tone? The answer: beats!
Below, we have a little Shockwave animation to allow you to mix tones. First click the blue buttons to play the pure tones: C, C#, and E. Play them each separately.
Next, click the green buttons to play mixed tones: C & C#, or C & E. Listen carefully to the sounds.
Now, try them again, and consider:
¥ When you play C & C#, do you hear the tone pulse on and off? This is a beat.. The two sounds sometimes reinforce each other, and sometimes cancel - so the sound goes on and off. The closer the two notes are to each other, the longer the time between on and off.
¥ The farther away the two tones, the shorter the time between on and off. C and E are pretty far apart, so when you play these two notes, the on and off cycle is very fast. In fact, it is so fast that you don't hear it as an on-off; you will hear it as another note: a bass note, in fact. Play the C & E tone again, and listen carefully. You should hear a high note, which is a mix between the C and E, and a low note - which is the beat! Listen for it; it is subtle, but it is there!!!
Let's extend this a bit. Suppose you mix two notes that are very, very close to each other. The on-off cycle will be very slow, so you will be able to hear it easily. Try the blue buttons below to see the notes, and the green buttons to mix them. What do you hear?
Now, let's do two notes that are pretty close, but not too close. Then, the tone will go on and off not quite fast enough to be a tone, but fast enough to be pretty annoying. Try the two tones using the blue buttons, then mix them using the blue.
And there you have it! Another test of the Emergency Broadcast System! That's how they make the tone so annoying.