Written for a non-technical audience, this gently introduces some key concepts in speech signal processing.
Ladefoged (Elements) – Chapter 1 – Sound waves
Very brief, but a reasonable place to start if you have no idea what sound is.
Ladefoged (Elements) – Chapter 2 – Loudness and pitch
These are perceptual phenomena that relate to physical properties of sound.
Ladefoged (Elements) – Chapter 5 – Resonance
Now we turn to speech production, and resonance is how the vocal tract shapes the quality of sound, to produce different vowels, for example.
Ladefoged & Johnson – A course in phonetics – Chapter 8 – Acoustic phonetics
Links the source-filter model to spectrograms and acoustic analysis of speech.