Frequency response describes how a device (microphone, headphone, speaker, or amplifier) changes the relative level of different frequencies in an audio signal. It is usually represented as a graph with frequency (Hz) on the horizontal axis and level (dB) on the vertical axis.
A perfectly flat frequency response means every frequency is reproduced at equal volume — what goes in comes out at the same relative level. In practice, no device is perfectly flat, but the goal of studio-grade equipment is to come as close as possible.
Consumer audio equipment (earbuds, Bluetooth speakers, DJ headphones) typically has an exaggerated V-shaped frequency response — boosted bass and treble, reduced midrange — to make music sound exciting. This makes them unreliable for recording or mixing decisions.
Microphones often have deliberately shaped frequency responses. A slight presence peak at 10–12 kHz adds air and detail to vocals. A low-frequency rolloff (high-pass) removes handling noise and rumble. Manufacturers publish frequency response graphs in their specifications.