at each point. The resultant sound that would be heard would have the same wavelength and frequency as waves 1 & 2 but would have higher amplitude and would therefore be a louder sound. In interference terms waves 1 & 2 have interfered constructively to produce wave 3. In the following example, Graphs 1 (blue) & 2 (green) represent two sound waves of equal wavelength, frequency and amplitude passing through the same medium simultaneously. Note that at all points in the medium the two waves interfere with each other destructively. The result is a series of nodes (points of zero displacement) and no sound would be heard (Graph 3 - brown). To study sound interference effects, the waves must have the same frequencies and wavelengths. To allow complete destruction, as in the case above, the amplitudes must also be the same. When sound sources of different frequencies, wavelengths and amplitudes interfere, the result is just noise. In the special case below, where the two waves have slightly different frequencies, beats are produced. The two interfering waves travelling in the same direction have been drawn to a different scale than the resultant wave, shown below. This complex waveform represents a beat -- a periodic fluctuation in sound intensity or loudness. The graph clearly shows a gradual increase in loudness up to a maximum followed by a gradual decrease to zero. The pattern then repeats. The audible beat frequency is the difference between the frequencies of the interfering waves. F b =