Audio Principles 21The result is that the harmonic structure [ Figure 1.15(c) ] has changed, and with it the
timbre. Clearly a fundamental requirement for quality sound reproduction is that the
response to all frequencies should be equal.
Frequency response is easily tested using sine waves of constant amplitude at various
frequencies as an input and noting the output level for each frequency.
Figure 1.16 shows that another way in which timbre can be changed is by nonlinearity.
All audio equipment has a transfer function between the input and the output, which
form the two axes of a graph. Unless the transfer function is exactly straight or linear ,
the output waveform will differ from the input. A nonlinear transfer function will cause
distortion, which changes the distribution of harmonics and changes timbre.
At a real microphone placed before an orchestra a multiplicity of sounds may arrive
simultaneously. Because the microphone diaphragm can only be in one place at a
Figure 1.16 : Nonlinearity of the transfer function creates harmonies by distorting the
waveform. Linearity is extremely important in audio equipment.