function getDualBandFilterCoefs(crossoverFrequency, sampleRate) {
  var k = Math.tan(Math.PI * crossoverFrequency / sampleRate),
      k2 = k * k,
      denominator = k2 + 2 * k + 1;

  return {
    lowpassA: [1, 2 * (k2 - 1) / denominator, (k2 - 2 * k + 1) / denominator],
    lowpassB: [k2 / denominator, 2 * k2 / denominator, k2 / denominator],
    hipassA: [1, 2 * (k2 - 1) / denominator, (k2 - 2 * k + 1) / denominator],
    hipassB: [1 / denominator, -2 * 1 / denominator, 1 / denominator]
  };
}
Generate the filter coefficients for the phase matched dual band filter.
@param  {NUmber} crossoverFrequency Filter crossover frequency.
@param  {NUmber} sampleRate         Operating sample rate.
@return {Object}                    Filter coefficients.
                                    { lowpassA, lowpassB, hipassA, hipassB }
                                    (where B is feedforward, A is feedback.)