Which of the following will remain un changed when a sound wave travels in air or in water?

Options

A. Amplitude
B. Wavelength
C. Frequency
D. Speed

Explanation

To determine which property of sound waves remains unchanged when they travel in different mediums such as air and water, we need to consider the characteristics of sound waves. The key properties of sound waves include:

1. **Frequency**: This is the number of vibrations or cycles per second, measured in Hertz (Hz). The frequency of a sound wave is determined by the source of the sound and does not change when the sound travels from one medium to another.

2. **Wavelength**: This is the distance between successive crests or troughs of a wave. Wavelength can change when sound travels from one medium to another because it is dependent on the speed of sound in that medium.

3. **Speed**: The speed of sound varies between different mediums. For example, sound travels faster in water than in air due to the different densities and elastic properties of the two mediums.

4. **Amplitude**: This refers to the maximum displacement of particles in the medium from their rest position. While amplitude can change due to energy loss or gain, it is not a property that remains unchanged when sound travels through different mediums.

Now, we can conclude that the property that remains unchanged when a sound wave travels from air to water is the **frequency** of the sound wave.

### Step-by-Step Solution:
1. **Identify the properties of sound waves**: Frequency, wavelength, speed, and amplitude.
2. **Analyze how each property behaves when sound travels through different mediums**:
- Frequency remains constant.
- Wavelength changes based on the speed of sound in the medium.
- Speed of sound varies between air and water.
- Amplitude may change due to energy loss.
3. **Determine which property does not change**: Frequency is the only property that remains unchanged.

### Final Answer:
The frequency of the sound wave remains unchanged when it travels in air or in water.