Note Freq

{
  "a4Hz": 440,
  "asOf": "2026-10-11T14:26:14.258Z",
  "degraded": false,
  "disclaimer": "Screening signal computed from the named public source. Not financial, legal or investment advice; provided as is. Verify at the source before acting.",
  "enharmonicNames": [
    "C4"
  ],
  "frequencyHz": 261.6256,
  "midi": 60,
  "notChecked": [
    "historical or non-equal temperaments (just intonation, meantone, well temperaments)",
    "stretch tuning of real pianos",
    "…1 more"
  ],
  "note": "C4",
  "octaveConvention": "C4 = middle C = MIDI 60",
  "pitchClass": 0,
  "reasons": [
    "MIDI 60 is middle C (C4) in scientific pitch notation.",
    "Frequency = A4 x 2^((midi - 69) / 12)."
  ],
  "source": "computed locally (12-tone equal temperament)",
  "summary": "C4 = MIDI 60 = 261.6256 Hz (A4 = 440 Hz).",
  "verdict": "COMPUTED",
  "wavelengthCm": 131.1034,
  "wavelengthNote": "wavelength in air at about 20 C (343 m/s)"
}

# bash or zsh: see the payment challenge. Nothing is charged.
curl -sS -g -i 'https://music-theory.x402tookit.com/note-freq?a4=440&note=C4'

# x402 v2 sends the challenge base64-encoded in the PAYMENT-REQUIRED header (v1 sends it in the body).
# To read the header:
curl -sS -g -o /dev/null -D - 'https://music-theory.x402tookit.com/note-freq?a4=440&note=C4' \
  | grep -i '^payment-required:' | cut -d' ' -f2 | tr -d '\r' | base64 --decode

// npm i @x402/fetch @x402/evm viem
import { wrapFetchWithPayment, x402Client } from "@x402/fetch";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

// Pays at most $0.02 per call.
const client = new x402Client().setSpendControls({ maxAmountPerPayment: "$0.02" });
registerExactEvmScheme(client, { signer: privateKeyToAccount(process.env.EVM_PRIVATE_KEY) });
const pay = wrapFetchWithPayment(fetch, client);

// Not sure it's safe to pay? ToolVet checks it first for $0.005. Pay for that from a separate
// client: this one is capped at $0.02 per payment. GET https://toolvet.app/api/v1/check?url=https%3A%2F%2Fmusic-theory.x402tookit.com%2Fnote-freq
const res = await pay("https://music-theory.x402tookit.com/note-freq?a4=440&note=C4");
console.log(res.status, await res.text());

# pip install "x402[requests,evm]"
import os
from eth_account import Account
from x402 import x402ClientSync
from x402.http.clients import x402_requests
from x402.mechanisms.evm import EthAccountSigner
from x402.mechanisms.evm.exact.register import register_exact_evm_client

# Pays at most $0.02 per call.
client = x402ClientSync().set_spend_controls({"max_amount_per_payment": "$0.02"})
register_exact_evm_client(client, EthAccountSigner(Account.from_key(os.environ["EVM_PRIVATE_KEY"])))

with x402_requests(client) as session:
    res = session.get("https://music-theory.x402tookit.com/note-freq?a4=440&note=C4")
    print(res.status_code, res.text)