Title:	Location	K-4	5-8	9-12	Description	Discipline
Scale Illusion	SoundGarden	-	X	X	Do you hear a smooth or jagged melody?

TIP: although you can use the speakers on your multimedia computer, a set of stereo headphones will provide the best way to properly hear these effects. Make sure the volume isn't too loud!

This illusion uses two melodies that sound jagged when played alone. When played together, your brain does something unexpected.

(09") – listen to the left ear jagged melody.

(09") – listen to the right ear jagged melody.

Can you imagine what happens when you listen to them at the same time, in different ears?

(09") – listen to the two jagged melodies played together by two violas.

At some point lift a single headphone off of your ear. What happens?

What happens when you switch the headphones?

If your brain works like our does, then it combined the two jagged melodies into two smooth "phantom" melodies, one rising and one falling.

(09") – listen to the "phantom" higher scale.

(09") – listen to the "phantom" lower scale.

What did you hear this time?

[ 9" - 86K ] – listen to the combined melodies with different instruments, a viola and a metallic drum.

What did you hear this time?

This illusion shows one way that we group similar sound textures together even when they enter our auditory system through separate ears. In the first example, the portion of your brain that perceives pitch and timbre overrides the portion that perceives the direction of origin, combining two jagged melodies into two smooth "phantom" melodies.

In the second example when you heard two different instruments, you perceived two jagged melodies because it's easy for your brain to tell the difference between two distinct sound textures.

Did you know that you process most of your auditory information in the temporal lobes of your brain? That's right and you actually have two of them, one on each side brain, close to your ears. [ They're not really blue – they're grey, we just did that to make it easier to point out. ]

In this illusion, the point is that your brain's temporal lobes can easily tell a metallic sound apart from a viola (i.e. two different timbres) when both are playing in the same frequency range. However, your brain has difficulty telling the difference between two violas (i.e. two similar timbres) when they are playing in the same frequency range.

What will happen if you speed up the melodies?

(09") – listen to faster melodies, two violas, 240 beats per minute.

(09") – listen to faster melodies, viola and metallic drum, 240 beats per minute.

What will happen if you slow down the melodies?

(08") – listen to slower melodies, two violas, 60 beats per minute.

(08") – listen to slower melodies, viola and metallic drum, 60 beats per minute.

What can you conclude about the effect of speed on this illusion? What other experiments could you try?