There are two sources of sound of equal intensity with frequencies `400Hz` and `408Hz` are vibrated together. The number of beats heard per second is

Options

A. `0`
B. `1`
C. `8`
D. `10`

Explanation

To find the number of beats heard per second when two sound sources of frequencies 400 Hz and 408 Hz are vibrated together, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Frequencies**:
We have two frequencies:
- \( f_1 = 400 \, \text{Hz} \)
- \( f_2 = 408 \, \text{Hz} \)

2. **Calculate the Difference in Frequencies**:
The number of beats per second is determined by the absolute difference between the two frequencies.
\[ \Delta f = |f_2 - f_1| = |408 \, \text{Hz} - 400 \, \text{Hz}| \]

3. **Perform the Calculation**:
\[ \Delta f = |408 - 400| = 8 \, \text{Hz} \]

4. **Interpret the Result**:
The number of beats heard per second is equal to the difference in frequencies. Thus, the number of beats is:
\[ \text{Number of beats per second} = \Delta f = 8 \, \text{beats/second} \]

### Final Answer:
The number of beats heard per second is **8 beats/second**.

---