SCIENCE DIY
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Binaural Beat ( 10 mins.)
“It only exists in your brain.”
When 2 tones of slightly different frequencies
play together, they create a beat frequency
equal to the difference in their frequencies.
This is because the 2 waves periodically align
and then go out of phase, synchronizing additively and then opposing each other. When
you tune a guitar, you listen for and eliminate
the beat frequencies between strings.
When you use headphones to play a slightly
different tone in each ear, the beat frequency
between them doesn’t exist anywhere, but
your brain re-creates and perceives it anyway.
To see how this works, here’s how to create
440Hz and 450Hz tone tracks, play them
together to generate a 10Hz beat, then play
them separately in each ear and perceive a
subtler version of the same beat frequency.
In Audacity, bring up the Tone Generator
box (Figure B) by selecting Generate → Tone,
then change the values to create a sine wave
at 440Hz, 0.4 amplitude, 5 seconds long, and
click the Generate Tone button (Figure C). Hit
the Play button to hear 5 glorious seconds
of a pure “concert A” or A4, as musicians call
this pitch. Click outside the track pane and
create another tone track that’s 450Hz but
otherwise the same. Hit Play to hear both
tracks, and the 10Hz beat.
Shift-click in the control area to select both
tracks, and choose Tracks → Mix and Render
(or Project → Quick Mix) to combine them.
While the separate tracks showed smooth
tones, the resulting combined track will show
beats (Figure D). Play this track and it will
sound the same as the 2 sounded together.
Back out with the Undo button, Ctrl+Z, or
Edit → Undo, so the 2 mono tracks display
again. Pull down the Audio Track Control
Panel menu at the top left of each track pane
and select Left Channel for one track and
Right Channel for the other. Hold the Shift
key and click the Play button; this will play the
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