3. an observer is at the midpoint between two coherent in-phase sources of sound of frequency 780 hz. what is the minimum distance the observer must move toward one of the sources in order to be at a point were destructive interference occurs?

Explanation:

wavelength of sound = velocity / frequency

= 340 / 780

= .4359 m

Let the observer be at equal distance d from in phase source .

let it moves by distance x for destructive interference .

path difference from source

= d + x - (d - x )

=  2x

for destructive interference

path difference = wave length / 2

2x = .4359 / 2 m

x = .4359 / 4

= .108975 m

10.9 cm

so observer must move by distance 10.9 cm towards on of the centers.