The effect of wavelength on diffraction I have learnt that the closer the slit width is to a wavelength source, the more it diffracts as it passes through it. Why is this the case? Also, how does this relate to low frequency sounds being heard more clearly over buildings and such compared to high frequency sounds. 
 A: A non-rigorous but conceptually helpful explanation is that the diffraction effect is the result of interference between waves emanating from all across the opening of the slit. The most important effects are those involving the waves from either side of the slit, as the interference effects between the waves from the central area tend to balance each other out. If the slit is much wider than the wavelength, then most of the propagation is from the waves in the wide central area which pass through without interfering with each other. As you reduce the width of the slit the contribution from the central area reduces and the edge effects become more dominant, so the interference effects become more marked.
However, once you reduce the width of the slit much below the wavelength, then the phase differences between the waves emanating across the slit become less and less, so the whole waveform is moving more in step so the edge effects again become reduced.
The propagation of sound across the landscape is more complicated than simple diffraction. The absorption of sound varies by frequency and by the nature of the material the sound encounters, so different frequencies will bounce off different surfaces more than others. Day to day sounds also tend to have spectra in which high frequencies have a relatively low amplitude, so the high-frequency components don't have to be reduced much before you can no longer hear them.
There's a nice analogy with the effect of time on gravestones. If you have an old headstone that started off with a mix of text, some engraved in a fine script and some deeply carved, as the stone gets weathered the fine script is the first to wear away, and over time you are just left with the more deeply carved letters.
