The Java source code of this applet and its documentation are freely available.
f = v / 2 * (p2/L2 + q2/W2 + r2/H2)1/2
v is speed of sound (344m/s).
L, W, H are length, width and height of a room.
p, q, r are integers (0, 1, 2...).
When two of the parameters (p, q, r) are zeros, the frequency is axial; when one is zero, the frequency is tangential; when none of them is zero, the frequency is oblique.