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/

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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/ and /S/.
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/s/ and /S/ SF Models Overlaid
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Previous examples Static i.e., unvarying
source, unvarying filter. Everyday speech
Dynamic i.e., source changes in pitch, loudness,
and quality over time filter varies over time as
the articulators move. Source-filter theory can
accommodate the dynamic nature of speech simply
by allowing the pitch, loudness, and detailed
shape of the source to vary over time and by
allowing the vocal tract FR curve (i.e., the v.t.
filter) to vary over time as the articulators
move from one position to the next.
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Time-Varying Source Spectrum
This is what the 1st part of time-varying source
for Where were you looks like in the frequency
domain.
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Time-Varying Source Waveform (i.e., time domain)
...
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Time-Varying Vocal Tract Filter
Source Only Source Modified by Time-Varying Filter
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Sinewave Speech Demonstration
Sinewave speech examples (from HINT sentence
intelligibility test)
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Sinewave Speech Example