shows the effect of using a different source function while retaining the same filter functions in this case the fundamental frequency of the glottal source is 200 hz with harmonics at integer multiples of the fundamental 400 hz 600 hz etc the effect of applying a filter to a signal is to modify the shape of the signal 39 s spectrum in the frequency domain the effect of applying a filter to a signal is to multiply the spectrum of the signal by that of the filter the result is a spectrum that combines the features of those of the input signal and the filter the spectrum of the glottal source is made up of a number of frequency spikes corresponding to the harmonics of the fundamental frequency of vibration of the vocal folds the spectrum decreases in amplitude with increasing frequency at a rate of around 12 db per octave that is for each doubling in frequency the amplitude of the spectrum decreases by around 12 db sound is converted to electrical current using a microphone continuous oscillations of air pressure become continuous oscillations of voltage in an electrical circuit if we represent the intensity of a sound by numbers proportionally related to the intensity the analog value of the intensity has been represented digitally the accuracy of the digital conversion depends upon the number of.