Concept:
Speech signal frequency range (telephone band):
\[
300 \text{ Hz to } 3100 \text{ Hz}
\]
Step 1: Human audible range.
\[
20 \text{ Hz to } 20 \text{ kHz}
\]
Step 2: Speech signal range.
Narrower band used for communication.
\[
300 - 3100 \text{ Hz}
\]