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# Physics Interference Question Help

Here is the original question and the first part:

Two speakers are set up 13.5m apart in an auditorium, pointing at each other. A pure sound of frequency 256Hz is being played through them. You may assume that the phase difference of the signals as they arrive at the speakers is 0∘. A person is standing on the line joining the speakers, 0.25m from the mid point. The speed of sound in air is 330ms −1. Calculate the phase difference as it would be detected by the person.

I got the correct answer for that part of 2.4, and the wavelength(in case needed for the second part of the question) was 165/128m.

I’m unsure however as to how to complete the second half of this isaac physics question:

The person moves to the mid point between the speakers (where the sound is loudest due to constructive interference), and then walks towards one speaker until the sound waves cancel out. How far do they walk until they find this point of near silence?

I know it will be destructive interference and so the phase difference should be 180degrees but when I used this in the equation Phase Diff. = (pi2 x Path Diff.)/Wavelength to be able get the path difference, my answer of 0.64m was incorrect and it said “Think carefully about how the path difference changes as the person moves - the distance travelled by the sound from one speaker increases while the distance travelled by the sound from the other speaker decreases.
Remember that you are now looking for destructive interference, not constructive interference.“ But I thought I had done destructive interference.

Any help would be really appreciated.
Can you give me the link to the IsaacPhysics question please or the name? I'd like to try it out myself to see if I can do it so that I can help you.
If you think that you have a speaker at point A

You have another speaker at Point B

At the mid point 6.75m the waves would be in phase - as stated. But if you walk to the right 0.25m (closer to point B) then the wave from Point A has to travel 0.25m further and the wave from B has to travel 0.25m LESS so this adds up to a Path Difference of 0.50m.

If you drew a diagram of a standing wave. Mark in the nodes and antinodes and a wavelength. As a fraction of a wavelength, how far would the person have to walk to have destructive interference?? Take that fraction and multiply it by your wavelength from the previous part of the question and you should be good.
answer will be 0.32 m as its a destructive interference you have to do lamda by 2