The sound waves, by the virtue of it being a wave, shows diffraction and interference. But in diffraction, I learnt that if the wave is allowed to enter through a small aperture, there is a central maxima, along with several minima (zero intensity) on the sides.

Why then in everyday life do we not encounter a situation in which we are at one of those minima and fail to here any sound? For example,in a soundproof studio room, suppose the narrow open door is the only entry point for sound, can we then find a spot where we hear nothing going on outside but by shifting to either side, we can hear it (although with different intensities)?

Why aren't such phenomenon encountered in daily life, not being able to hear a source because of diffraction (or interference)? My question also relates to radio waves whose wavelengths is comparable to that of sound waves.


These things do happen. The phenomena is called multipath interference. You don't notice it much in ordinary life becase there is rarely a situation where sound isn't being scattered from lots of different surfaces and that the frequency is pure enough so that there exists a single null point for all the sound. Note that the nodes and anti-nodes caused by the multipath interference move around as a function of frequency. Ordinary sounds contains a wide range of frequencies such that even if one specific tone is partly nulled out, we usually don't notice.

You can create situations without a fancy lab where you can observe interference effects yourself. For example, in college we put a speaker at one end of a long hallway and drove it with a frequency generator. At the right frequency, which was 43 Hz for that hallway if I remember right, there were nodes and anti-nodes dues to standing waves. You could of course still hear the tone at the anti-nodes, but it was significantly softer than at the nodes. The effect was strong enough that if you were at one of the nodes and started walking toward the speaker, the sound would get softer and you'd think it was coming from the other direction. It was great fun to see peoples' reactions that couldn't figure out where the sound was coming from.

With radio waves you can experience this more easily without a deliberate setup because any one radio transmission is in a very narrow frequency band. Particularly with commerical FM (about 3 meter wavelength) you can notice this driving around in a car. There will be spots where the reception is very poor, even though a few meters in any direction it will be better.

  • $\begingroup$ "multipath interference" is manifestation of interference of waves by linear superposition not of diffraction; radio wave diffraction is the interaction of the wave with the boundary by which the wave is confined. The FM reception holes that you experience are really standing waves between reflectors, no diffraction is needed to explain them. $\endgroup$ – hyportnex Dec 29 '13 at 17:09
  • $\begingroup$ @user: Multipath is only about coherent waves from different directions interfering. They can come from different directions due to reflections, diffraction around objects, and in some very unusual circumstances refraction thru objects. A radio signal could, for example, be nulled in a particular location by the superposition of signals diffracted around the horizon and reflected off of a nearby building. $\endgroup$ – Olin Lathrop Dec 29 '13 at 18:27

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