Question

For commercial telephonic communication, the frequency range for speech signals is

• A. $50 \text{ Hz to } 1000 \text{ Hz}$
• B. $3000 \text{ Hz to } 4500 \text{ Hz}$
• C. $1000 \text{ Hz to } 2000 \text{ Hz}$
• D. $5000 \text{ Hz to } 6500 \text{ Hz}$
• E. $300 \text{ Hz to } 3100 \text{ Hz}$

Hint:

Remember: Telephone bandwidth ≈ 300–3100 Hz (not full audible range).

Answer 1

Answer: (E)

Concept: Bandwidth of speech signals in communication systems
---

Step 1: Understand human hearing range
The human ear can detect sound frequencies approximately in the range: \[ 20 \text{ Hz to } 20,000 \text{ Hz (20 kHz)} \] However, this entire range is not required for speech communication. ---

Step 2: Nature of speech signals
Speech consists of:
• Fundamental frequencies (pitch)
• Harmonics (clarity and tone) Most of the important speech information lies in: \[ 300 \text{ Hz to } 3400 \text{ Hz (approx)} \] ---

Step 3: Why lower frequencies are excluded
Frequencies below 300 Hz:
• Carry very little intelligible speech information
• Mostly contribute to rumbling noise ---

Step 4: Why higher frequencies are excluded
Frequencies above ~3100 Hz:
• Improve quality slightly but not essential
• Increase bandwidth requirement significantly ---

Step 5: Engineering compromise
To:
• Reduce bandwidth
• Reduce transmission cost
• Maintain acceptable clarity The chosen standard is: \[ 300 \text{ Hz to } 3100 \text{ Hz} \] ---

Step 6: Compare with options

• (A) Too narrow → loses speech info
• (B), (C), (D) → wrong ranges
• (E) Matches standard telephony band ✔ --- Final Answer: \[ \boxed{300 \text{ Hz to } 3100 \text{ Hz}} \]