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[Phys-l] sound demo/question



I am puzzled about something. I use two 256-Hz resonant tuning forks and
an oscilloscope for some sound activities. When I hit both forks, the
sound appears louder that just one oscillating. The mic picks this up, and
the scope shows a higher (double) amplitude -- constructive interference..
This always works. However, shouldn't there be places in between the forks
that show destructive interference? And why doesn't the scope show this?
No matter when I hit the forks and no matter where I position the forks,
the waves always add up constructively. How can this be?

Using the wave equation with v = 343 and f = 256, the wavelength is abut
1.34 m. So half of this would be 0.67 m for destructive interference to
occur (crest meeting trough).

Do I have to move the forks farther apart and not put the mic exactly in
between the two? Or will the scope always show the amplitudes adding up?
Or I am I misunderstanding something basic about this situation?