Two speakers are driven by the same oscillator of frequency $f$. They are located a distance $d$ from each other on a vertical pole. An observer far away walks slowly, straight towards the lower speaker and his ears are at the same level as the lower speaker. Let $v$ represent the speed of sound and assume that there is no reflection from the ground. Let $n_{\text {max }}$ be the number of times the observer hears the minimum in sound intensity and let $\lambda=v / f$ be wavelength of the sound emitted. Choose the correct alternative(s).

• Correct answer
  A

  $n_{\max } \leq \frac{d}{\lambda}+0.5$
• B

  A minimum occurs at $\frac{4 d^2+9 \lambda^2}{12 \lambda}$.
• C

  $n_{\max } \leq \frac{d}{\lambda}-0.5$
• Correct answer
  D

  A minimum occurs at $\frac{4 d^2-\lambda^2}{4 \lambda}$.

Hence, the correct answer is options: 'A', 'D'.