Frequency Modulation sound spectrum

The PAS phenomenon involves the selective absorption of modulated IR radiation by the sample. The selectively absorbed frequencies of IR radiation correspond to the fundamental vibrational frequencies of the sample of interest. Once absorbed, the IR radiation is converted to heat and subsequently escapes from the solid sample and heats a boundary layer of gas. Typically, this conversion from modulated IR radiation to heat involves a small temperature increase at the sample surface ( 10 6oC). Since the sample is placed into a closed cavity cell that is filled with a coupling gas (usually helium), the increase in temperature produces pressure changes in the surrounding gas (sound waves). Since the IR radiation is modulated, the pressure changes in the coupling gas occur at the frequency of the modulated light, and so does the acoustic wave. This acoustical wave is detected by a very sensitive microphone, and the subsequent electrical signal is Fourier processed and a spectrum produced. 

We will establish the basic principles that govern the behavior of aU acousto-optic devices whether of bulk or waveguide (SAW) construction. An acousto-optic modulator is composed of an acoustic medium (such as water, glass, lithium niobate, rutile, etc.) and a transducer. The transducer converts electrical signals into sound waves propagating in the acoustic medium with an acoustic frequency spectrum that is limited by the bandwidth of the transducer that matches the electrical excitation. The sound wave causes a perturbation in the index of refraction of the material, setting up a refractive index grating of the form... 

The tremolo effect may be considered to be a starting point example of amplitude modulation it is achieved by applying a very slow sub-audio rate of amplitude variation to a sound (i.e. less than approximately 18 Hz). If the frequency of the variation is raised to the audible band (i.e. higher than approximately 18 Hz) then additional partials (or sidebands) will be added to the spectrum of the signal. 

Ring modulation may also be achieved by the multiplication of two signals (Figure 2.6). The multiplication of two sounds results in a spectrum containing frequencies that are the sum... 

The spectrum of an FM sound is composed of the offset carrier frequency (/c) and a number of partials on either side of it, spaced at a distance equal to the modulator frequency (/m)-The partials generated on each side of the carrier frequency are usually called sidebands. The sideband pairs are calculated as follows fc + kxf and fc kx/ , where k is an integer,greater than zero, which corresponds to the order of the partial counting from /<. (Figure 2.9). 

Figure 2.9 The spectrum of an FM sound is composed of the offset carrier frequency and a number of partiais on eifher side of if, spaced at a distance equal to the modulator frequency...

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