400Hz

400 Hz is a frequency that corresponds to four hundred cycles per second. It is a common reference point in audio engineering, where the standard pitch A above middle C (A4) is tuned to 440 Hz. The frequency 400 Hz is slightly lower than this standard, falling in the mid‑range of the audible spectrum for humans.

In music, 400 Hz lies near the middle of the piano’s range, roughly between the notes G3 (196 Hz) and F4 (349 Hz). Some synthesizers and samplers use 400 Hz as a neutral tuning base or as a marker for equal‑loudness adjustment curves. In acoustic analysis, 400 Hz is considered a critical frequency for speech intelligibility, as it lies near the vowel formants that carry significant energy for spoken English. Consequently, speech codecs and hearing aids often focus on preserving clarity around this frequency.

In electronics, 400 Hz is used as a test tone or as a reference in audio equipment calibration. Filters designed to reject audio above or below this frequency are common in radio receivers and telephone circuits. The steady 400‑hertz sine wave also serves as a simple tone for tuning oscilloscopes and audio meters.

The frequency is well within the range of human hearing, which spans roughly 20 Hz to 20 kHz for young adults. Exposure to sustained 400 Hz tones at high decibel levels can cause auditory fatigue or discomfort, underscoring the importance of appropriate volume limits in both commercial and regulatory settings.