Phase difference between two points

[songoku]

Homework Statement

A sound wave of frequency 1100 Hz is travelling in air at the speed of 330 m s^(−1). What is
the phase difference between two points on the wave 1.0 m apart in the direction of the
wave?
A 0.60 π rad
B 0.67 π rad
C 0.80 π rad
D 1.3 π rad

Relevant Equations

phase difference = 2π . x / λ

v = λ.f

I find λ first and I get λ = 3/10 m

phase difference = $\dfrac{2 \pi (1) }{\dfrac{3}{10}} = 6.7 \pi$

Answer key = B

What is my mistake?

Thanks

 

[haruspex]

Homework Statement: A sound wave of frequency 1100 Hz is traveling in air at the speed of 330 m s^(−1). What is
the phase difference between two points on the wave 1.0 m apart in the direction of the
wave?
A 0.60 π rad
B 0.67 π rad
C 0.80 π rad
D 1.3 π rad
Homework Equations: phase difference = 2π . x / λ

v = λ.f

I find λ first and I get λ = 3/10 m

phase difference = $\dfrac{2 \pi (1) }{\dfrac{3}{10}} = 6.7 \pi$

Answer key = B

What is my mistake?

Thanks

What does a phase difference of 2π look like?

 

[songoku]

What does a phase difference of 2π look like?

One wave?

 

[haruspex]

One wave?

Just consider the two waveforms. Does it look any different from no phase difference?

 

[songoku]

Just consider the two waveforms. Does it look any different from no phase difference?

No difference. Phase difference of 2π is the same as a wave translated by horizontal distance equals to the wavelength

 

[haruspex]

No difference. Phase difference of 2π is the same as a wave translated by horizontal distance equals to the wavelength

So how else might you write a phase difference of 6.7π?

 

[songoku]

So how else might you write a phase difference of 6.7π?

I understand. Thank you very much haruspex