Note name to frequency API

Converts a scientific-pitch note (e.g. A4, C#5, Bb3) to its equal-temperament frequency with a configurable A4 reference, plus MIDI number, period and wavelength. Answers 'What frequency is A4?', 'What Hz is C#5 at A4=432?'.

Price$0.02per request

MethodPOST

Route/v1/music/note-frequency

StatusLive

MIME typeapplication/json

Rate limit120/minute

Cache0s public

musicfrequencypitchnotehzmidituningequal-temperamentaudio

API URLhttps://x402.hexl.dev/v1/music/note-frequency

Integration docs

Example request

{
  "note": "A4"
}

Example response

{
  "note": "A4",
  "midi": 69,
  "a4Reference": 440,
  "frequency": 440,
  "units": "Hz",
  "periodMs": 2.272727,
  "wavelengthM": 0.779545,
  "formula": "f = A4 · 2^((midi-69)/12)"
}

Input schema

{
  "type": "object",
  "required": [
    "note"
  ],
  "properties": {
    "note": {
      "type": "string",
      "description": "Scientific-pitch note, e.g. A4, C#5, Bb3",
      "examples": [
        "A4",
        "C#5",
        "Bb3"
      ]
    },
    "a4": {
      "type": "number",
      "description": "A4 reference Hz (default 440)",
      "examples": [
        440,
        432
      ]
    }
  }
}

Output schema

{
  "type": "object",
  "additionalProperties": true
}