Experimental demonstration of interference of sound
The phenomenon of interference between two longitudinal waves in air can be demonstrated by Quincke?s tube shown in Fig..
Quincke?s tube consists of U shaped glass tubes A and B. The tube SAR has two openings at S and R. The other tube B can slide over the tube A. A sound wave from S travels along both the paths SAR and SBR in opposite directions and meet at R.
If the path difference between the two waves (i.e) SAR ~ SBR is an integral multiple of wavelength, intensity of sound will be maximum due to constructive interference.
i.e SAR ~ SBR = mλ The corresponding phase difference φ between the two waves is even multiples of π. (i.e) φ = m 2π where m = 0, 1, 2, 3 ....
If the tube B is gradually slided over A, a stage is reached when the intensity of sound is zero at R due to destructive interference. Then no sound will be heard at R.
If the path difference between the waves is odd multiples of λ/2 , intensity of sound will be minimum.
i.e. SAR ~ SBR = (2m + 1) λ/2
The corresponding phase difference φ between the two waves is odd multiples of π. (i.e) φ = (2m + 1)π where m = 0, 1, 2, 3 ....