The source does not have to be the larynx and it does not have to be periodic. For example, the source for unvoiced fricatives is produced in the vocal tract rather than the larynx and is (roughly) white noise. The pictures below show SF models for /s/
https://www.powershow.com/view4/5a2bad-Nzc5N/The_source_does_not_have_to_be_the_larynx_and_it_does_not_have_to_be_periodic_For_example_the_source_for_unvoiced_fricatives_is_produced_in_the_vocal_tract_rather_than_the_larynx_and_is_roughly_white_noise_The_pictures_below_show_SF_models_for_s_powerpoint_ppt_presentation
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