The reinforcement causes an increase in the amplitude of the frequencies involved while interference attenuates the frequencies. The graph of the pattern of interference and reinforcement which is a plot of the magnitude of energy present in different frequencies inside the auditorium is the transfer function or the transfer spectrum . The size and shape of the auditorium determines which frequencies in the source spectrum reinforce and which frequencies attenuate. In this type of tube, resonances occur at even numbered and odd numbered multiples of the first resonance frequency. In air-filled cavities and tubes, the bandwidth of the resonances is primarily determined by the degree of hardness of the walls.
Hard walls result in sharper resonances and narrower bandwidths.
The transfer function will then be a plot of the frequencies which are resonated inside the vocal tract. The transfer function will be determined by the mass and stiffness of the bodies of air in the vocal tract which, in turn, are determined by the size and shape of the cavities in the vocal tract. In regions within the vocal tract where the sound waves are interfering with each other and, therefore, produce minimal amplitude are called the nodes or antiformants. At other points in the cavity, where the sound waves are reinforcing each other and, therefore, produce a large amplitude are called the antinodes or formants. Formants, which are far more important for the perception of speech than antiformants, are simply vocal tract resonances.