What does the critical frequency in a filter circuit indicate?

The critical frequency in a filter circuit refers to the specific frequency at which the output signal power is reduced to half of the input signal power. This is a significant point known as the cutoff frequency, often denoted as the -3 dB point. At this frequency, the filter begins to significantly attenuate signals, marking the boundary between the passband (where signals are allowed to pass with little or no attenuation) and the stopband (where signals are significantly attenuated).

In analog filters, reaching the critical frequency indicates that the output power is reduced to half compared to the input, equivalently represented as a decrease of 3 decibels (dB) in signal strength. This characteristic is fundamental in filter design and analysis, as it helps engineers understand how a filter will behave in real-world applications with varying signal frequencies.