Doppler Effect

With what velocity an observer should move relative to a stationary source so that a sound of double the frequency of source is heard by an observer?

When the source of sound moves towards a stationary observer, the wavelength of sound received by him

The observer is moving with velocity $v_0^{}$ towards the stationary source of sound and then after crossing moves away from the source with velocity $v_0^{}\mathrm{ }.$ Assume that the medium through which the sound waves travel is at rest. If $v$ is the velocity of sound and $n$ is the frequency emitted by the source then the difference between apparent frequencies heard by the observer is