To solve the problem, we need to analyze the statements provided in the question regarding the behavior of sound waves when the source and observer are in motion relative to each other. We will evaluate each option step by step.
### Step-by-Step Solution:
1. **Understanding the Doppler Effect**:
- The Doppler Effect describes how the frequency (and wavelength) of sound changes for an observer moving relative to a source of sound. When the source moves towards the observer, the frequency increases, and when it moves away, the frequency decreases.
2. **Analyzing Option A**:
- **Statement**: When a source of sound moves towards a stationary observer, the wavelength of the sound heard by the observer is less than the original wavelength of the sound.
- **Analysis**:
- The formula for the apparent frequency \( f' \) when the source is moving towards a stationary observer is:
\[
f' = \frac{v}{v - v_s} f
\]
- Since \( v - v_s < v \), the ratio \( \frac{v}{v - v_s} > 1 \), which means \( f' > f \).
- Since frequency and wavelength are inversely related (\( v = f \lambda \)), if \( f' > f \), then \( \lambda' < \lambda \).
- **Conclusion**: Option A is correct.
3. **Analyzing Option B**:
- **Statement**: When both the observer and source of sound move towards each other, the wavelength of sound heard by the observer is less than the original wavelength.
- **Analysis**:
- The formula for the apparent frequency in this case is:
\[
f' = \frac{v + v_o}{v - v_s} f
\]
- Here, both \( v + v_o \) and \( v - v_s \) are greater than \( v \) and less than \( v \) respectively, leading to \( f' > f \).
- Thus, \( \lambda' < \lambda \).
- **Conclusion**: Option B is correct.
4. **Analyzing Option C**:
- **Statement**: When both the observer and source of sound are moving away from each other, the wavelength of sound heard by the observer is less than the original wavelength.
- **Analysis**:
- The formula for the apparent frequency in this case is:
\[
f' = \frac{v - v_o}{v + v_s} f
\]
- Here, \( v - v_o < v \) and \( v + v_s > v \), leading to \( f' < f \).
- Therefore, \( \lambda' > \lambda \).
- **Conclusion**: Option C is incorrect.
5. **Analyzing Option D**:
- **Statement**: When an observer moves away from a stationary source, the wavelength of sound heard by the observer is less than the original wavelength.
- **Analysis**:
- The formula for the apparent frequency in this case is:
\[
f' = \frac{v - v_o}{v} f
\]
- Since \( v - v_o < v \), we have \( f' < f \).
- Thus, \( \lambda' > \lambda \).
- **Conclusion**: Option D is incorrect.
### Final Answer:
The correct options are **A and B**.
