Search Front Door Kitchen SoundGarden CoffeeShop Resource Room Kids Create! Gallery Go Back Catching Waves: How do we perceive sound?

Title: Location K-4 5-8 9-12 Description Discipline
Pitch Streaming SoundGarden
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What happens when the pattern speeds up?

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!


Say the words "college ecology" aloud ten times. If anyone notices, give them a confident look and tell them that you're performing an experiment. Go ahead and try it........................

After a while you find yourself saying "collegey, collegey, collegey." What's going on? Your brain grouped the sounds according to rules that we haven't quite figured out yet.

The illusion you're about to listen to does the same thing.

Sound: (09") a marimba plays a three-note pattern the left ear.

Vibraphone and Marimba Three Note Pattern

Sound: (09") a vibraphone plays a similar one in the right ear.


Now each instrument will alternate tones creating a single, combined pattern. As the speed increases what happens?

Descending Three Note Pattern Emerges at Fast Speed

Sound: (40") listen to pitch streams, marimba and vibraphone.

How many speed increases did you hear?

At what point did the instruments begin to separate into two melodic patterns?

Listen to the combined streams a second and third time. Do the instruments separate at the same place every time?

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What's the point?

Brain with Temporal Lobe Highlighted
Human brain with temporal lobe highlighted.

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. ]

The point here is at the slow speed most people can hear a single rising pattern created by a combination of the two instruments. This is called temporal coherence and happens because your brain normally groups sound in similar frequency ranges together into single patterns.

Descending Three Note Pattern Emerges at Fast Speed

By the final speed increase, you should hear two descending patterns emerge, one unique to each instrument. Both may be difficult to listen to at the same time, try listening to one ear and then the other. This is called fission and is the result of your brain's ability to distinguish different sound textures within similar frequency ranges.


Just for Fun!

What do you think will happen if we use two instruments that have very different sound textures, say a muted trumpet and a marimba?

Sound: (40") listen to pitch streams, marimba and muted trumpet.

What can you conclude about the effect of timbre on pitch streaming?

What other experiments might you try?

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"'I weep for you,' the Walrus said:
'I deeply sympathize.'
With sobs and tears he sorted out
Those of the largest size,
Holding his pocket-handkerchief
Before his streaming eyes."

Lewis Carroll


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