660Hz

660Hz is a specific frequency in the audible range, lying roughly in the middle of the human hearing spectrum. It is commonly used as a reference point in audio engineering because it matches the pitch of a middle C for a 12-tone equal temperament system in many Western musical contexts. The frequency corresponds to a period of approximately 1.52 milliseconds and is one octave higher than 330Hz.

In acoustic theory, 660Hz falls within the mid‑range, an area that carries most musical harmonics and vocal content. This makes it useful for setting equalization targets and evaluating speaker performance. Many testing signals, such as calibration tones, include 660Hz to assess clarity and absence of distortion. Audio equipment manufacturers often provide 660Hz test signals to verify frequency response profiles, especially in professional audio gear.

The human ear perceives 660Hz as a bright yet neutral tone. Studies on psychoacoustics show that this frequency lies near the peak of average hearing sensitivity, making it a convenient benchmark for critical listening. In tuning environments, 660Hz can be sounded together with other key frequencies (e.g., 440Hz) to check instrument tuning stability and blend.

In digital audio processing, 660Hz may serve as a cut‑off frequency for certain band‑pass filters used in psychoacoustic modeling and automatic mix balancing. It is also employed in some diagnostic applications, such as determining the frequency response of microphones, headphones, and loudspeakers, because its measurable size aids precise calibration.