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Ms Nina, You must not be hearing any voice at all which means you must be hearing the super position of the reflected wave and the original wave at a place where destructive interference happens.For destructive interference, the condition is
path difference = (n+1/2) wavelength of the wave. As the reflected wave is at a rigid boundary an additional path difference of lambda/2 must also be taken into account. Iam not able to put in any diagrams / equations here. Please mail exact question to usha_kasi at rediffmail dot com
will help you to the best of my knowledge.