The crack (or hole) acts like a new source, the dispersion on the other side of the crack is described by:
sin x = W/D
x = angle from "straight through" where minimum sound pressure occurs
W = wavelength of the sound
D = width of the crack
for round openings the equation becomes
sin x = 1.22(W/D)
The picture below shows two different frequency waves being diffracted by a slot. You can see the lower frequency (longer wavelength) wave in the lower picture "spreads out" more on the other side of the slot.
From the above equations, as the frequency decreases, W increases, and the angle of diffraction (x) widens. For small values of D (little holes), only the lowest frequencies will diffract appreciably.
The point is, the hole won't change the frequency, but the lower frequencies (big W) will propogate widely on the other side of the crack or hole. The higher freqencies will only be audible right in front of the hole.
The old listening at the keyhole analogy fits. If you want to hear a conversation (relatively high frequency) in another room through the keyhole, you have to put your ear right up in front of the hole.http://www.zainea.com/kdifr.gif