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Frequency response is the measure of any system's response at the output to a signal of varying frequency (but constant amplitude) at its input. It is usually referred to in connection with electronic amplifiers, loudspeakers and similar systems. The frequency response is typically characterized by the magnitude of the system's response, measured in dB, and the phase, measured in radians, versus frequency. The frequency response of a system can be measured by:

• applying an impulse to the system and measuring its response (see impulse response)
• sweeping a constant-amplitude pure tone through the bandwidth of interest and measuring the output level and phase shift relative to the input
• applying a signal with a wide frequency spectrum (e.g., maximum length sequence, white noise, or pink noise), and calculating the impulse response by deconvolution of this input signal and the output signal of the system.

Once a frequency response has been measured (e.g., as an impulse response), providing the system is linear and 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 or analog filter can be applied to the signals prior to their reproduction to compensate for these deficiencies.

Frequency response curves 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 kHz ±1 dB. This means that the system amplifies all frequencies within that range within the limits quoted. 'Good frequency response' therefore does not guarantee a specific fidelity, but only indicates that a piece of equipment meets the basic frequency response requirements.

• Transfer function
• Bode plot
• Bandwidth
• Audio system measurements
• Transient response & steady-state response