fer for analyser output
        let buffer = new Uint8Array(analyser.frequencyBinCount);

        // helper functions
        const frequencyBinValue = (f) => {
          const hzPerBin = (audioContext.sampleRate) / (2*analyser.frequencyBinCount);
          const index = parseInt((f + hzPerBin/2) / hzPerBin);
          return buffer[index];
        };
        const isActive = (value) => {
          let threshold = +document.querySelector('#bin-value-threshold').value;
          return value > threshold;
   