PASSAGE-III : The human voice can produce sounds with a frequency between 60 Hz and 1300 Hz. It is interesting to note that a normal human ear can hear sound of frequency between 20 Hz to 20,000 Hz. The sound of frequency greater than 20000 Hz is called ultrasonic and the sound of frequency below 20 Hz is known as infrasonic.
Frequency of infrasonic sound is

Options

A. below 20 Hz
B. above 20 Hz
C. below 20000 Hz
D. above 2000 Hz

Explanation

To determine the frequency of infrasonic sound based on the provided passage, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Definition of Infrasonic Sound**:
- Infrasonic sound is defined as sound with a frequency below 20 Hz.

2. **Identify the Frequency Range**:
- According to the passage, the human ear can hear sounds in the frequency range of 20 Hz to 20,000 Hz. Sounds below 20 Hz are categorized as infrasonic.

3. **Analyze the Options Provided**:
- The options given are:
- Option 1: Below 20 Hertz
- Option 2: Above 20 Hertz
- Option 3: Below 20 Thousand Hertz
- Option 4: Above 200 Hertz

4. **Select the Correct Option**:
- Since infrasonic sound is defined as having a frequency below 20 Hz, the correct option is **Option 1: Below 20 Hertz**.

5. **Conclusion**:
- Therefore, the frequency of infrasonic sound is below 20 Hz.

### Final Answer:
The frequency of infrasonic sound is **Option 1: Below 20 Hertz**.