Introduction:
In signal processing, the power density spectrum represents the distribution of power of a signal over different frequencies. The power density spectrum provides valuable information about the frequency content of a signal and helps in analyzing its characteristics. Based on the concentration of power in the power density spectrum, signals can be classified into different categories.

Low Frequency Signal:
A low frequency signal refers to a signal that is concentrated around lower frequencies. The power density spectrum of a low frequency signal shows significant power content at frequencies close to zero. This means that the signal has a large portion of its energy distributed in the low-frequency range.

Middle Frequency Signal:
A middle frequency signal refers to a signal that has its power density spectrum concentrated around the middle frequencies. The power content in the spectrum is spread across a wider frequency range compared to low frequency signals, but it is not primarily concentrated around zero frequency.

High Frequency Signal:
A high frequency signal refers to a signal that has its power density spectrum concentrated around higher frequencies. The power content in the spectrum is primarily distributed in the high-frequency range, with little or no power at lower frequencies.

Explanation:
In the given question, it is stated that the power density spectrum of the signal is concentrated about zero frequency. This means that the signal has a significant amount of power at or near zero frequency. Based on this information, we can conclude that the signal is a low frequency signal.

When the power density spectrum is concentrated around zero frequency, it indicates that the signal has a large portion of its energy distributed in the low-frequency range. This could be due to various factors such as the nature of the signal source, the characteristics of the system, or the signal processing techniques applied.

Conclusion:
Therefore, if a power signal has its power density spectrum concentrated about zero frequency, it is classified as a low frequency signal. This classification helps in understanding the frequency content and characteristics of the signal, which can be useful in various applications such as communication systems, audio processing, and image processing.