Frequency response
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Frequency response is the measure of any system's response to frequency, but is usually used in connection with electronic amplifiers and similar systems, particularly in relation to audio signals. Because the human ear is generally not sensitive to phase, the frequency response is typically characterized by the magnitude of the system's response, measured in dB, versus frequency. The frequency response of a system is typically measured by applying an impulse to the system and measuring its response (see impulse response), sweeping a pure tone through the bandwidth of interest, or by applying a maximum length sequence.
Once a frequency response has been measured (e.g., as an impulse response), providing the system is linear time invariant, its characteristic can be approximated with arbitrary accuracy by a digital filter. Similarly, if a system is demonstrated to have a poor frequency response, a digital filter can be applied to the signals prior to their reproduction to compensate for the problem.
Frequency responses are often used to indicate the accuracy of amplifiers and speakers for reproducing audio. As an example, a high fidelity amplifier may be said to have a frequency response of 20 Hz - 20,000 Hz ±1 dB, which tells you that the system amplifies equally all frequencies within that range and within the limits quoted. Such a measure does not include any other indicators of quality (e.g., non-linear distortions of the signal, signal-to-noise ratio, etc...). Frequency response therefore does not guarantee a given quality of audio reproduction, but only indicates that a piece of equipment meets the basic requirements needed for it.