public function toFrequency($tuning = 'equal', $a = 440, $precision = 2) {
        $ap = new Pitch('A', 0, 4);
        $n = $ap->interval($this); // get the interval between our note and A
        $s = pow(2, (1/12)); // the twelfth root of 2
        $p = pow($s, $n); // raised to the power of the interval in semitones
        $f = $p * $a; // multiplied by the frequency of "A"
        $f = round($f, $precision, PHP_ROUND_HALF_UP);
        return $f;
    }